Syntheses of Ruthenium Complexes of Borane-Functionalized 4,5-Diazafluorenide and their Reactivity

toward Carbon Dioxide

by

Adam Nicola Pantaleo

A thesis submitted in conformity with the requirements for the degree of Master of Science

Department of Chemistry University of Toronto

© Copyright by Adam Nicola Pantaleo 2014

ii

Syntheses of Ruthenium Complexes of Borane-Functionalized

4,5-Diazafluorenide and their Reactivity toward Carbon Dioxide

Adam Nicola Pantaleo

Master of Science

Department of Chemistry

University of Toronto

2014

Abstract

Carbon dioxide is an abundant molecule that is present in the earth’s atmosphere and is a

product of fossil fuel combustion. Given the implication of CO2 in global warming and its

potential use as a cheap C1 synthetic feedstock, research into systems that can efficiently

convert CO2 into value-added substrates such as methanol is currently of high interest and

relevance. In fact, many transition metal complexes are known to effect this conversion in the

presence of reducing and oxophilic reagents such as boranes. Herein, the syntheses of several

ruthenium(II) complexes featuring a borane-derivatized 4,5-diazafluorenide ligand are

presented, and their reactivity with boranes, dihydrogen, carbon dioxide, and aromatic solvents

is discussed. In several cases, reactivity occurs exclusively at the actor diazafluorenide ligand as

opposed to the spectator ruthenium centre. Furthermore, these complexes are able to catalyze

the reduction of carbon dioxide by catecholborane to B-methoxycatecholborane, a direct

precursor to methanol.

iii

Acknowledgments

First and foremost, I would like to thank my supervisor, Prof. Datong Song, for his guidance

and support during my time at the University of Toronto. Datong is a brilliant chemist, and my

work is a reflection of his ideas, inspiration, and knowledge. He was always willing to lend a

hand and an ear to my concerns, both in and out of the lab. In particular, I am thankful for his

suggestions and advice as I explored my career options, and I am grateful for his support as I

pursued several teaching and co-curricular opportunities throughout my degree.

I would also like to thank all past and present members of the Song research group whom I had

the privilege of working with over the last year: Shaolong Gong, Tongen Wang; Charlie Kivi,

Trevor Janes, Yanxin Yang, Vince Annibale, Runyu Tan, Yu Li; Rhys Batcup, Celia Gendron-

Herndon, Tara Cho; Andy Yen, Walter Liang, Ellen Yan, Brian Tsui, Maotong (Albert) Xu,

Xhoana Gjergji, Stefan Jevtic, Pavel Zatsepin, and Cindy Ma. In particular, I would like to

sincerely thank my three undergraduate students – Ellen, Brian, and Albert – whom I had the

privilege of mentoring as they completed summer projects in our lab. They may not know this,

but teaching and interacting with them brought me immense joy and was a highlight of my

graduate experience. I am also grateful for the opportunity to serve as a teaching assistant for the

CHM139 course. It is these experiences and these students that have confirmed my love of

teaching and inspired me to pursue a teaching career.

I would not be here today without the tremendous support I received from my family and

friends. I am also indebted to the Toronto Newman Centre and the wonderful people I met there.

They provided me with countless opportunities to strengthen my faith, serve the Church, and

grow closer to God during my journey through graduate school.

“Wait for the Lord; be strong, and let your heart take courage; wait for the Lord!” – Psalm 27.14

iv

Table of Contents

Acknowledgments .................................................................................................................... iii

Table of Contents ..................................................................................................................... iv

List of Figures .......................................................................................................................... vi

List of Schemes ....................................................................................................................... vii

List of Abbreviations................................................................................................................ xi

1 Introduction ..................................................................................................................... 1

1.1 Carbon Dioxide in Nature, Society, and Academia ........................................................ 1

1.2 Carbon Dioxide Reduction by Boranes and Silanes Catalyzed by Homogeneous

Complexes ............................................................................................................................ 2

1.2.1 Silanes as the Reducing Agent ............................................................................... 2

1.2.2 Boranes as the Reducing Agent .............................................................................. 5

1.2.2.1 Metal-Free Systems ........................................................................................ 9

1.3 Actor Ligands and their Carbon Dioxide Chemistry ......................................................12

1.3.1 The Carbon Dioxide Chemistry of 4,5-Diazafluorenide .........................................13

1.4 Research Goals and Scope of this Thesis ......................................................................16

2 Experimental Section .....................................................................................................18

2.1 General Considerations .................................................................................................18

2.2 In Situ Synthesis of [RuHCl(LH–Bcat)(PPh3)2] (2) .......................................................19

2.3 Synthesis of [RuH(L–Bcat)(N2)(PPh3)2] (4) ..................................................................19

2.3.1 Method A: ClBcat as Borane Source .....................................................................19

2.3.2 Method B: HBcat as Borane Source ......................................................................20

2.4 In Situ Synthesis of catB–N(SiMe3)2 (6) .......................................................................21

2.5 Synthesis of cis,trans-[RuHCl(LH)(PPh3)2] (7) .............................................................21

2.6 Synthesis of [RuH(LH)(N2)(PPh3)2][catBcat] (8) ..........................................................21

2.7 In Situ Synthesis of [Ru(C6D5)(L–Bcat)(N2){P(2,6-D2-C6H3}3)2] (9) ............................22

2.8 Synthesis of [Ru(H2Bcat)(L–Bcat)(PPh3)2] (10) ............................................................22

2.8.1 Method A: ClBcat as Borane Source .....................................................................22

2.8.2 Method B: HBcat as Borane Source ......................................................................22

2.9 Synthesis of cis,cis,trans-[RuCl2(LH–Bcat)(PPh3)2] (11) ..............................................23

2.10 In Situ Synthesis of [Ru(H2Bcat)(L)(PPh3)2] (12) ..........................................................23

v

2.11 Synthesis of 9-trimethylsilyl-4,5-diazafluorene (13) ......................................................24

3 Results and Discussions ..................................................................................................25

3.1 Reactivity of 4,5-Diazafluorenide Complexes with Boranes and Silanes .......................25

3.1.1 Reactivity of [RuH(L)(N2)(PPh3)2] (1) with Boranes .............................................25

3.1.1.1 Preliminary Reactivity with ClBcat ...............................................................25

3.1.1.2 Synthesis of Zwitterionic Borane Adduct [RuH(L–Bcat)(N2)(PPh3)2] (4) ......29

3.1.1.2.1 ClBcat as Borane Source ...........................................................................29

3.1.1.2.1.1 In Situ Deprotonation by NaH..............................................................29

3.1.1.2.1.2 In Situ Deprotonation by KN(SiMe3)2 ..................................................30

3.1.1.2.1.3 Stepwise Reactions ..............................................................................33

3.1.1.2.1.4 In Situ Deprotonation by Excess 1 .......................................................33

3.1.1.2.2 HBcat as Borane Source ............................................................................36

3.1.1.3 Reactivity of [RuH(L–Bcat)(N2)(PPh3)2] (4) with Aromatic Solvents ............39

3.1.1.4 Syntheses of Borane-Borohydride Complex [Ru(H2Bcat)(L–Bcat)(PPh3)2] (10)

and Borane Adduct [RuCl2(L–Bcat)(PPh3)2] (11)..........................................................44

3.1.1.5 Reactivity of [RuH(L–Bcat)(N2)(PPh3)2] (4) and [Ru(H2Bcat)(L–Bcat)(PPh3)2]

(10) with Dihydrogen ...................................................................................................47

3.1.2 Reactivity of NaL, MeL, and 1 with Me3SiCl ........................................................49

3.1.2.1 Reaction of NaL with Me3SiCl ......................................................................50

3.1.2.2 Reaction of MeL with Me3SiCl .....................................................................51

3.1.2.3 Reaction of 1 with Me3SiCl ...........................................................................51

3.2 Reactivity of 4,5-Diazafluorenide Complexes with CO2 ...............................................52

3.2.1 Stoichiometric Reaction with CO2.........................................................................52

3.2.2 Catalytic Reaction with CO2 .................................................................................54

4 Conclusion and Future Outlook.....................................................................................56

References ...............................................................................................................................58

vi

List of Figures

Figure 1: Partial 1H NMR spectrum (400 MHz, C6D6) of [RuHCl(LH–Bcat)(PPh3)2] (2),

recorded in situ after 1 and ClBcat were allowed to react for 15 minutes.

Figure 2: Partial 1H NMR spectrum (600 MHz, C6D6) of [RuH(L–Bcat)(N2)(PPh3)2] (4).

Figure 3: Close-up of the 1H NMR resonances centered at 8.58 ppm demonstrating the

conversion of 4 to 9 upon standing in benzene-d6 at room temperature: a) after 1 h in solution, b)

after 12 h in solution.

Figure 4: Partial 2H NMR spectrum (92 MHz, Et2O) of a mixture of [RuH(L–Bcat)(N2)(PPh3)2]

(4) and [Ru(C6D5)(L–Bcat)(N2){P(2,6-D2-C6H3}3)2] (9).

vii

List of Schemes

Scheme 1: Hydrosilation of carbon dioxide to formic acid catalyzed by a copper(I) phosphine

complex.

Scheme 2: Hydrosilation of carbon dioxide to a bis(methoxy)silane derivative catalyzed by an

NHC-carboxylate adduct.

Scheme 3: Generic hydrosilation of a formylsilane to a methoxysilane and a bis(silyl) ether via a

bis(silyl) acetal intermediate. Depending on the system, these reactions can occur with or

without the aid of a catalyst.

Scheme 4: Hydrosilation of carbon dioxide to methane catalyzed by several ion pairs and

transition metal complexes.

Scheme 5: Hydroboration of carbon dioxide to formic acid catalyzed by a copper(I)-NHC

complex. The mechanism involves CO2 insertion into a copper-hydride bond followed by

hydroboration of the formyl intermediate.

Scheme 6: Hydroboration of carbon dioxide catalyzed by a ruthenium(II) polyhydride complex.

The mechanism involves three distinct steps, with formylpinacolborane and formaldehyde as

intermediates.

Scheme 7: Hydroboration of carbon dioxide catalyzed by a nickel(II) PCP pincer complex.

Scheme 8: Hydroboration of carbon dioxide catalyzed by a ruthenium(II) tris(amino)phosphine

frustrated Lewis pair.

Scheme 9: Hydroboration of carbon dioxide catalyzed by a phosphine-borane organocatalyst.

The mechanism involves initial FLP activation of carbon dioxide followed by successive

hydroborations of the activated substrate while it remains coordinated to the catalyst.

Scheme 10: Hydroboration of carbon dioxide catalyzed by the nitrogenous bases TBD and

Me-TBD. TBD directly activates carbon dioxide, while Me-TBD indirectly activates carbon

dioxide via hydroborane activation.

viii

Scheme 11: Generic PNP pincer complex demonstrating concomitant re-aromatization and E–H

bond cleavage (E = H, C, O, N).

Scheme 12: Reversible carbon dioxide activation by a ruthenium(II) PNP pincer complex.

P = PtBu2.

Scheme 13: Carbon dioxide activation by a doubly deprotonated nickel(II) PNP pincer complex.

P = PiPr2.

Scheme 14: Formal insertion of carbon dioxide into the 9-position ligand C–H bond of the

ruthenium(II) diazafluorenide complex 1.

Scheme 15: Formal insertion of carbon dioxide into the 9-position ligand C–H bond of an

electron-rich metal-diazafluorenide complex, followed by in situ deprotonation of the carboxylic

acid and rearrangement to form a dinuclear species with a bridging carboxylate ligand. [M] =

Rh(PPh3)2, Cu(IPr).

Scheme 16: Formal insertion of carbon dioxide into the 9-position ligand C–H bond of N-

methyl-4,5-diazafluorenide.

Scheme 17: Proposed derivatization of the 4,5-diazafluorenide ligand with a boryl moiety,

insertion of carbon dioxide into the C–B bond, and displacement of formylborane.

Scheme 18: Labelling convention for catecholato and 4,5-diazafluorenyl NMR resonances.

Scheme 19: Proposed reactivity between a nucleophilic ruthenium 4,5-diazafluorenide complex

and an electrophilic chloroborane. The 9-position of the ligand is labelled.

Scheme 20: Reaction of 1 (10 mg scale) with ClBcat in benzene-d6.

Scheme 21: Reaction of 1 (50 mg scale) with ClBcat in toluene. [Ru] denotes a ruthenium(II)

species with a protonated LH ligand.

Scheme 22: Proposed series of reactions that occur when 1 and five equivalents of NaH are

combined in THF.

ix

Scheme 23: Reaction of 1 with ClBcat in the presence of KN(SiMe3)2, where 1 and KN(SiMe3)2

were added dropwise to ClBcat.

Scheme 24: Control experiment confirming the formation of the B–N adduct 6 from ClBcat and

KN(SiMe3)2.

Scheme 25: Reaction of 1 with ClBcat in the presence of KN(SiMe3)2, where ClBcat was added

dropwise to complex 1 and KN(SiMe3)2. [Ru] denotes a ruthenium(II) species with a protonated

LH ligand.

Scheme 26: Reaction of ClBcat and 1, followed by reaction of the intermediate product mixture

with KN(SiMe3)2.

Scheme 27: Synthesis of 4 via reaction of two equivalents of 1 with one equivalent of ClBcat.

Scheme 28: Synthesis of 1 via reaction of 7 with KOtBu, supporting the assignment of 7 as an

isomer of [RuHCl(LH)(PPh3)2].

Scheme 29: Proposed reactivity between a nucleophilic ruthenium 4,5-diazafluorenide complex

and an electrophilic borane.

Scheme 30: Synthesis of 4 via reaction of 1 (10 mg scale) with HBcat.

Scheme 31: Reaction of 1 (50 mg scale) with HBcat in toluene.

Scheme 32: Conversion of 4 to 9 by reaction with benzene-d6.

Scheme 33: Proposed reaction of 4 with benzene-d6 via σ-bond metathesis of the aromatic C–D

bond with the Ru–H bond to form a ruthenium phenyl complex.

Scheme 34: Proposed ortho-cyclometalation reaction that leads to the deuteration of the o-PPh3

protons of complex 4. R = H or C6D5, Ar = C6D5.

Scheme 35: Formation of the borane-borohydride complex 10 and protonated borane complex

11 from reaction of 1 with two equivalents of ClBcat. [Ru] denotes a ruthenium(II) species with

a protonated LH ligand.

x

Scheme 36: Proposed formation of 10 from reaction of 4 with HBcat.

Scheme 37: Proposed formation of 10 and 11 from reaction of 4 with excess ClBcat.

Scheme 38: Reaction of 4 and 10 with dihydrogen in benzene-d6.

Scheme 39: Summary of proposed and confirmed reactions of 4, 9, 10, and 12 under a

dihydrogen atmosphere in benzene-d6. Solid arrows indicate reactions that have been confirmed

to take place in situ, while dashed arrows indicate proposed reactions.

Scheme 40: Reaction of NaL with Me3SiCl in THF, followed by aqueous workup.

Scheme 41: Reaction of MeL with Me3SiCl in toluene.

Scheme 42: Reaction of 1 with Me3SiCl in benzene-d6.

Scheme 43: Desired reactivity of 4 with carbon dioxide to form a catecholboryl ester moiety at

the 9-position of the diazafluorenide ligand.

Scheme 44: Catalytic reduction of carbon dioxide by 1 in the presence of HBcat to form

H3COBcat and catBOBcat.

xi

List of Abbreviations

LH 4,5-diazafluorene

L or L- 4,5-diazafluorenide

MeL N-methyl-4,5-diazafluorenide

NMR nuclear magnetic resonance

IR infrared

FT Fourier transform

ppm parts per million

atm atmospheres

COSY correlation spectroscopy

HSQC heteronuclear single quantum coherence

HMBC heteronuclear multiple bond correlation

FLP frustrated Lewis pair

NHC N-heterocyclic carbene

THF tetrahydrofuran

tol toluene

DMF N,N-dimethylformamide

Me methyl

iPr iso-propyl

nBu n-butyl

xii

tBu tert-butyl

Bn benzyl

Cy cyclohexyl

dH2O distilled water

IMes 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene

IPr 1,3-bis(2,6-di-iso-propylphenyl)imidazol-2-ylidene

nacnac 1,3-diketimine

Cp cyclopentadienyl

Cp* pentamethylcyclopentadienyl

DPPP 1,3-bis(diphenylphosphino)propane

TMP tetramethylpiperidine

TBD 1,5,7-triazabicyclo[4.4.0]dec-5-ene

Me-TBD 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene

cat catecholato, (C6H4O2)2-

catB catecholboryl

[catBcat] or [catBcat]- bis(catecholato)borate

ClBcat B-chlorocatecholborane

HBcat catecholborane

HBpin pinacolborane

9-BBN 9-borabicyclo[3.3.1]nonane

xiii

LH–Bcat 9-catecholboryl-4,5-diazafluorene

L–Bcat 9-catecholboryl-4,5-diazafluorenide

1 [RuH(L)(N2)(PPh3)2]

2 [RuHCl(LH–Bcat)(PPh3)2]

3 [RuH2(LH)(PPh3)2]

4 [RuH(L–Bcat)(N2)(PPh3)2]

5 cis,cis-[RuHCl(LH)(PPh3)2]

6 catB–N(SiMe3)2

7 cis,trans-[RuHCl(LH)(PPh3)2]

8 [RuH(LH)(N2)(PPh3)2][catBcat]

9 [Ru(C6D5)(L–Bcat)(N2){P(2,6-D2-C6H3}3)2]

10 [Ru(H2Bcat)(L–Bcat)(PPh3)2]

11 cis,cis,trans-[RuCl2(LH–Bcat)(PPh3)2]

12 [Ru(H2Bcat)(L)(PPh3)2]

13 9-trimethylsilyl-4,5-diazafluorene

1

1 Introduction

1.1 Carbon Dioxide in Nature, Society, and Academia

Carbon dioxide is a non-toxic, abundant gas that exists in the earth’s atmosphere and plays an

important role in both nature and society. In nature, carbon dioxide is essential to the proper

functioning of almost every ecosystem and organism. Plants consume carbon dioxide and

convert it to useful forms of energy using photosynthesis, while many organisms release carbon

dioxide as a by-product of cellular respiration. In society, carbon dioxide is a product of the

combustion of carbon-containing fuels such as wood, coal, natural gas, and gasoline – fuels that

are the foundation of the current energy infrastructure of the modern world.

Although these carbon-containing fuels have played an integral role in the development and

growth of civilization, their widespread use has come with a price: atmospheric levels of carbon

dioxide have been increasing steadily over the last few centuries. This increase is problematic

because carbon dioxide is a greenhouse gas and is a direct cause of climate change, one of the

most serious environmental concerns of today.1

In light of this reality, a significant body of research has emerged over the last few decades on

the fundamental and applied chemistry of carbon dioxide. In particular, the fields of carbon

dioxide sequestration and derivatization are currently receiving much attention in both academia

and industry. The former field, sequestration, relates to the development of materials that can

selectively absorb carbon dioxide and thereby “sequester” it from the atmosphere. There are

many examples in the literature of materials that are able to sequester carbon dioxide. These

materials include ionic liquids,2 zeolites,3 and metal-organic frameworks.4 The latter field,

derivatization, deals with the goal of directly using atmospheric carbon dioxide as a C1 feedstock

for chemical syntheses. Exploring the field of carbon dioxide derivatization may also lead to the

discovery of novel methods for the synthesis and fabrication of many industrially and

economically relevant chemicals.

Perhaps the biggest obstacle to using carbon dioxide as a chemical reagent is its inertness.

Carbon dioxide contains two very strong C=O double bonds and is both kinetically and

thermodynamically stable, as indicated by its presence in the earth’s atmosphere and its

formation in highly exergonic combustion reactions. In order to effectively use carbon dioxide as

2

a C1 feedstock, it must be activated and derivatized in such a way that the overall reaction is

spontaneous. This often requires the use of forcing conditions, reactive reagents such as epoxides

and acetylenes, or complex organic molecules that are expensive to synthesize. Despite this

obstacle, there are several examples in the literature of synthetic methodologies that are able to

convert carbon dioxide into value-added products such as carbonates, carbamates, ureas,

carboxylic acids, esters, isocyanates, and various polymers.5

In addition to these transformations, the literature contains many examples of homogeneous

transition metal complexes that are able to stoichiometrically or catalytically reduce carbon

dioxide to formyl, acetal, or methoxy derivatives. A number of phosphine and bipyridine metal

complexes, as well as complexes bearing macrocyclic ligands, are known to catalyze the

electrochemical reduction of carbon dioxide to carbon monoxide or formate derivatives in

solution.6 Additionally, a number of transition metal complexes can catalyze the reduction of

carbon dioxide in the presence of a stoichiometric amount of reductant such as a borane or

silane. As this area of research is of particular relevance to this thesis, it is discussed further in

the following sections.

1.2 Carbon Dioxide Reduction by Boranes and Silanes Catalyzed by Homogeneous Complexes

One of the earliest examples of using a stoichiometric amount of reductant to reduce carbon

dioxide was the discovery that simple metal complexes such as [RuCl2(PPh3)3] and

[RuH2(PPh3)4] could facilitate the reduction of carbon dioxide by silanes R3SiH to the

corresponding formylsilanes R3SiOC(O)H.7 Although these systems were inefficient, producing

only a few equivalents of reduced species, their discovery set the stage for further research into

these types of reactions.

1.2.1 Silanes as the Reducing Agent

Several years after this discovery, other systems were found that could effect carbon dioxide

reduction in much higher yields than the ruthenium(II) systems described above. For example,

the ruthenium complexes mer-[RuX3(MeCN)3] and [RuX2(MeCN)4] (X = Cl, Br) are active pre-

catalysts for the reduction of carbon dioxide by silanes and can produce formylsilanes in near

quantitative yields.8 A similar system used [RuCl(MeCN)5][RuCl4(MeCN)2], formed in situ from

RuCl3·nH2O and MeCN, as the catalyst.9 Another system that can reduce carbon dioxide to the

3

formyl level utilized a copper hydride complex bearing a bidentate phosphine ligand.10 The

active copper hydride species was formed in situ from Cu(OAc)2 and the phosphine ligand in the

presence of silane. The formylsilane product could then be hydrolyzed to formic acid and silyl

alcohol after the reaction was complete (Scheme 1). A key step in this catalytic cycle is insertion

of carbon dioxide into the Cu–H bond of the active catalyst, followed by hydrosilation of the

resulting Cu–O bond to form the formylsilane and regenerate the catalyst.

Scheme 1: Hydrosilation of carbon dioxide to formic acid catalyzed by a copper(I)

phosphine complex.10

It is also possible to further reduce carbon dioxide with silanes to methoxysilanes. One of the

first reported catalysts for this transformation was the iridium(I) complex [Ir(CN)(CO)(dppe)],

which facilitated the reduction of carbon dioxide by Me3SiH to the methoxysilane Me3SiOMe.11

Following this initial finding, it was discovered that N-heterocyclic carbenes (NHCs) could also

catalyze this reaction. In particular, the CO2 adduct of the carbene IMes {1,3-bis(2,4,6-

trimethylphenyl)imidazol-2-ylidene} catalyzed the reduction of carbon dioxide by diphenylsilane

to bis(methoxy)diphenylsilane with turnover numbers as high as 1840 and catalyst loadings as

low as 0.05 mol% (Scheme 2).12

Scheme 2: Hydrosilation of carbon dioxide to a bis(methoxy)silane derivative catalyzed by

an NHC-carboxylate adduct.12

4

In these types of systems, the partially reduced formylsilane intermediate can re-enter the

catalytic cycle or undergo hydrosilation of the C=O bond without the aid of the catalyst to form a

bis(silyl) acetal species (R3SiO)2CH2. This species, in turn, can be further reduced to the

methoxysilane R3SiOMe and the corresponding bis(silyl) ether (R3Si)2O (Scheme 3).

Scheme 3: Generic hydrosilation of a formylsilane to a methoxysilane and a bis(silyl) ether

via a bis(silyl) acetal intermediate. Depending on the system, these reactions can occur with

or without the aid of a catalyst.

Furthermore, many complexes are known to catalyze the hydrosilation of carbon dioxide to

methane, the most reduced form of carbon. Some noteworthy examples of pre-catalysts for this

reaction include [TMPH]+[HB(C6F5)3]-, the frustrated Lewis pair (FLP) formed from reaction of

tetramethylpiperidine (TMP), B(C6F5)3, and H213; the Lewis acidic aluminum cation (Et2Al)+,

isolated as the salt [Et2Al][CH6B11I6]14; the scandium contact ion pair [Cp*2Sc][HB(C6F5)3]

(Cp* = pentamethylcyclopentadienyl)15; several palladium(II) and platinum(II) hydridoborate

complexes of a PSiP pincer ligand16; a cationic iridium(III) PCP pincer complex17; and a

zirconium complex bearing a bis(phenoxide) ligand in the presence of B(C6F5)3 (Scheme 4).18

5

Scheme 4: Hydrosilation of carbon dioxide to methane catalyzed by several ion pairs and

transition metal complexes.13-18

1.2.2 Boranes as the Reducing Agent

In addition to hydrosilation as a carbon dioxide reduction methodology, numerous examples of

borane-mediated reduction of carbon dioxide are known. These reactions are generally analogous

to the silane systems: initial catalyst-mediated conversion of carbon dioxide to a formylborane

can be followed by successive hydroboration steps to form bis(boryl) acetal and methoxyborane

species.

Nozaki and co-workers reported the copper(I)-NHC pre-catalyst [Cu(OtBu)(IPr)] {IPr = 1,3-

bis(2,6-di-iso-propylphenyl)imidazol-2-ylidene}, which facilitated the reduction of carbon

dioxide by pinacolborane (HBpin) to formic acid.19 The active catalyst is a copper hydride

species: insertion of CO2 into the Cu–H bond followed by hydroboration of the copper formyl

intermediate liberates formylpinacolborane and regenerates the catalyst (Scheme 5). This is

analogous to the mechanisms of several of the silane-based systems described in the previous

section. The formylborane can then be hydrolyzed to formic acid after the reaction is complete.

6

Scheme 5: Hydroboration of carbon dioxide to formic acid catalyzed by a copper(I)-NHC

complex. The mechanism involves CO2 insertion into a copper-hydride bond followed by

hydroboration of the formyl intermediate.19

Other systems are able to further reduce the formylborane to bis(boryl) acetal and

methoxyborane species. Sabo-Etienne and co-workers were able to observe all three of these

reduced species when a ruthenium(II) polyhydride complex was used as the catalyst with HBpin

as the reductant.20 Interestingly, they also observed the direct reductive coupling of two CO2

units to form pinBOCH2OC(O)H. A subsequent study provided mechanistic details for this

reaction and elucidated the role of formaldehyde as a reactive intermediate that could be isolated

under certain conditions (Scheme 6).21 The first step of the mechanism is the formation of

formylpinacolborane, analogous to the systems discussed previously, followed by hydroboration

of formylpinacolborane to form a bis(boryl) acetal. This intermediate can then release

formaldehyde, which undergoes rapid hydroboration to form methoxypinacolborane, the fully

reduced product. In this system, all three reduction steps are mediated by the catalyst.

7

Scheme 6: Hydroboration of carbon dioxide catalyzed by a ruthenium(II) polyhydride

complex. The mechanism involves three distinct steps, with formylpinacolborane and

formaldehyde as intermediates.21

8

In many systems, only the fully reduced methoxyborane species is observed. Guan and co-

workers reported a nickel(II) PNP pincer catalyst that was active for the reduction of carbon

dioxide by HBcat to MeOBcat (Scheme 7).22 Unlike the ruthenium system described above,

hydroboration of the formylcatecholborane intermediate to formaldehyde and then to

methoxycatecholborane did not require the catalyst. Several derivatives of this species with

varying R groups on the pincer ligand are also active catalysts.23, 24

Scheme 7: Hydroboration of carbon dioxide catalyzed by a nickel(II) PCP pincer

complex.22

Note that for all of the borane systems described above, a key step is insertion of CO2 into a

metal-hydride bond. An alternate method of activating carbon dioxide involves FLPs, which can

polarize CO2 and make it amenable to hydroboration. Using this method, Stephan and co-

workers reported a ruthenium(II) tris(amino)phosphine complex that catalyzed carbon dioxide

reduction by HBpin to MeOBpin (Scheme 8).25 Three successive hydroborations of the CO2

substrate occurred while it remained bound to the FLP at the Lewis acidic ruthenium centre and

the Lewis basic phosphine donor.

9

Scheme 8: Hydroboration of carbon dioxide catalyzed by a ruthenium(II)

tris(amino)phosphine frustrated Lewis pair.25

1.2.2.1 Metal-Free Systems

In addition to the ruthenium and nickel catalysts described above, several metal-free catalysts

have recently been reported. Fontaine and co-workers discovered an extremely active

organocatalyst, 1-Bcat-2-PPh2-C6H4, that can catalyze the hydroboration of carbon dioxide to

methoxyboranes with turnover numbers reaching 2950.26 This organocatalyst activates carbon

dioxide via an FLP mechanism involving the Lewis acidic borane and Lewis basic phosphine,

with successive hydroborations occurring while the CO2-derived substrate is bound to the

catalyst (Scheme 9). Another metal-free system involving a phosphine-derived carbene-9-BBN

(9-BBN = 9-borabicyclo[3.3.1]nonane) complex was recently reported by Stephan and co-

workers.27

10

Scheme 9: Hydroboration of carbon dioxide catalyzed by a phosphine-borane

organocatalyst. The mechanism involves initial FLP activation of carbon dioxide followed

by successive hydroborations of the activated substrate while it remains coordinated to the

catalyst.26

Remarkably, Cantat and co-workers reported that simple nitrogenous bases, absent of phosphine

or borane groups, were able to catalyze carbon dioxide reduction by 9-BBN to methoxy-9-BBN

11

(Scheme 10).28 The two most active catalysts, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and 7-

methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (Me-TBD), differed in their mechanisms of carbon

dioxide activation. The former activated carbon dioxide via an FLP mechanism involving a

Lewis basic nitrogen donor and the Lewis acidic borane; the latter activated the borane and

facilitated its hydridic attack on carbon dioxide (Scheme 10). In both cases, the formyl-9-BBN

species re-entered the catalytic cycle and was ultimately reduced to the methoxy level. A similar

complex, the bidentate nitrogenous base 1,8-bis(dimethylamino)naphthalene (also known as

Proton Sponge), is also an active catalyst for the reduction of carbon dioxide by BH3·SMe2 to

(MeOBO)3.29

Scheme 10: Hydroboration of carbon dioxide catalyzed by the nitrogenous bases TBD and

Me-TBD. TBD directly activates carbon dioxide, while Me-TBD indirectly activates carbon

dioxide via hydroborane activation.28

12

Furthermore, several complexes are known to catalyze the reduction of carbon dioxide to carbon

monoxide using a variety of reductants, including in situ carbodiphosphoranes,30 silane-boranes

(R2B–SiR3),31 aromatic aldehydes,32 and diboranes (R2B–BR2).

33

1.3 Actor Ligands and their Carbon Dioxide Chemistry

A careful scrutiny of the metal-based catalysts described in the previous sections reveals that in

some cases, most notably the FLP systems, the reduction and functionalization of carbon dioxide

directly involves a ligand in addition to the metal centre. These ligands, which directly

participate in the chemical reaction, are known as actor ligands. This behaviour is atypical, as

ligands in transition metal chemistry are generally unreactive and serve only to tune the steric

and electronic properties about the metal centre where the reaction takes place.34 Complexes

containing actor ligands exhibit diverse and exciting reactivity toward small molecules such as

dihydrogen, dioxygen, carbon dioxide, silanes, alkenes, and alkynes. Of particular relevance to

this thesis are those systems which are able to activate and reduce carbon dioxide.

A significant class of actor ligand complexes is the PNP and PNN pincer complexes synthesized

by Milstein and co-workers. These pincer ligands contain a benzylic methylene group that can be

deprotonated to form a dearomatized version of the ligand, which can then heterolytically cleave

a variety of E–H bonds (E = H, C, O, N) (Scheme 11). The driving force for these reactions is re-

aromatization of the ligand. Iron(II), ruthenium(II), and nickel(II) complexes of these ligands are

known. In the case of dihydrogen, the reversibility of the aromatization-dearomatization process

makes these complexes efficient hydrogenation catalysts. Indeed, these complexes are reported

to catalyze the hydrogenation of bicarbonates and carbon dioxide;35 ureas;36 and carbonates,

carbamates, and alkyl formates under mild conditions.37

Scheme 11: Generic PNP pincer complex demonstrating concomitant re-aromatization and

E–H bond cleavage (E = H, C, O, N).

13

These pincer complexes are also able to activate carbon dioxide. Indeed, carbon dioxide has been

shown to undergo [1,3] addition to a ruthenium(II) PNP pincer complex, forming new C–C and

Ru–O bonds (Scheme 12).38 Analogous reactivity was observed for a related ruthenium(II) PNN

pincer complex synthesized by Sanford and co-workers.39 Furthermore, Milstein and co-workers

reported a doubly-deprotonated nickel(II) PNP pincer complex that reacted with carbon dioxide

to give a tautomerized C–H insertion product (Scheme 13).40

Scheme 12: Reversible carbon dioxide activation by a ruthenium(II) PNP pincer complex.

P = PtBu2.38

Scheme 13: Carbon dioxide activation by a doubly deprotonated nickel(II) PNP pincer

complex. P = PiPr2.40

Other notable complexes that are able to activate carbon dioxide include titanium-phosphorus

and zirconium-phosphorus FLPs,41 a hafnium-phosphorus FLP,42 and a scandium nacnac (1,3-

diketimine) complex.43 Together, these systems demonstrate the versatility of actor ligands in

carbon dioxide activation chemistry.

1.3.1 The Carbon Dioxide Chemistry of 4,5-Diazafluorenide

Over the last several years, our group has extensively studied the chemistry of 4,5-

diazafluorenide, a versatile actor ligand that displays exciting reactivity toward small molecules.

In 2010, we first reported the synthesis of the ruthenium(II) dinitrogen complex

[RuH(L)(N2)(PPh3)2] (1) (L = 4,5-diazafluorenide) and described its reaction with dihydrogen.44

14

Two years later, we reported the formal insertion of carbon dioxide into the 9-position C–H bond

of this complex to form a carboxylic acid (Scheme 14).45 The first step in this reaction is

believed to be C–H insertion to form a carboxylate moiety, which then deprotonates the 9-

position proton to form the carboxylic acid product. Remarkably, this reversible reaction occurs

exclusively at the diazafluorenide ligand and does not directly involve the ruthenium centre.

Subsequently, the isoelectronic rhodium(III) complex [RhH2(L)(PPh3)2] was synthesized and

shown to react with carbon dioxide in the same way.46 Interestingly, the rhodium(I) and

copper(I) complexes [Rh(L)(PPh3)2] and [Cu(L)(IPr)] behaved differently and formed dinuclear

species with bridging carboxylate ligands upon reaction with carbon dioxide.46 This difference in

reactivity was attributed to the more electron-rich metal centres in these complexes, which

rendered the diazafluorenide ligand more basic and allowed for deprotonation of the intermediate

carboxylic acid species (Scheme 15).

Scheme 14: Formal insertion of carbon dioxide into the 9-position ligand C–H bond of the

ruthenium(II) diazafluorenide complex 1.45

15

Scheme 15: Formal insertion of carbon dioxide into the 9-position ligand C–H bond of an

electron-rich metal-diazafluorenide complex, followed by in situ deprotonation of the

carboxylic acid and rearrangement to form a dinuclear species with a bridging carboxylate

ligand. [M] = Rh(PPh3)2, Cu(IPr).46

Together, these results demonstrate that the metal centre is purely a spectator in these reactions,

serving only to tune the electronic properties of the complex. Remarkably, when the metal was

replaced by a formal positive charge introduced by N-methylation of the diazafluorenide ligand,

the resulting metal-free complex displayed the same reactivity toward carbon dioxide as the

metal-containing complexes (Scheme 16).46

Scheme 16: Formal insertion of carbon dioxide into the 9-position ligand C–H bond of N-

methyl-4,5-diazafluorenide.46

16

1.4 Research Goals and Scope of this Thesis

Given these recent discoveries involving diazafluorenide-mediated carbon dioxide activation, our

group was interested in expanding this exciting chemistry. Aware of the prevalence of borane

reagents in many carbon dioxide reduction systems, we aimed to derivatize the diazafluorenide

ligand by appending a boryl moiety at the 9-position. Then, if carbon dioxide could insert into

the newly formed C–B bond, the product would be a value-added formylborane species.

Additionally, as formyl species are key intermediates in many catalytic carbon dioxide reduction

systems, we were excited to see if these derivatized complexes displayed catalytic activity in the

presence of excess borane (Scheme 17). If successful, this would be a rare example of carbon

dioxide reduction occurring at a carbon centre, as opposed to a nitrogen centre,28 an FLP, or a

metal centre. Furthermore, the presence of ruthenium may enhance the catalytic activity, as

metal-based catalysts are generally more active than their metal-free counterparts.

Scheme 17: Proposed derivatization of the 4,5-diazafluorenide ligand with a boryl moiety,

insertion of carbon dioxide into the C–B bond, and displacement of formylborane.

The previously reported ruthenium(II) dinitrogen complex 1 was used as a starting point for this

chemistry. 1 can be synthesized in two steps from [RuHCl(PPh3)3] and 4,5-diazafluorene (LH) in

good yields and very high purity,44 making it a known and reliable starting material.

17

Furthermore, the properties and reactivity of any new borane complexes derived from 1 can be

directly compared to those of 1, making it easy to study the effects of introducing a boryl moiety.

This thesis reports the syntheses of several ruthenium(II) complexes featuring borane-derivatized

4,5-diazafluorenide ligands and their reactivity toward boranes, dihydrogen, carbon dioxide, and

aromatic solvents. Also briefly reported are attempts to synthesize analogous silane-derivatized

diazafluorenide complexes. The discovery of a catalytic system that reduces carbon dioxide to a

methoxyborane species is also discussed. As this work was mainly an exploratory study, many of

the results are preliminary, but they nevertheless reveal the rich chemistry of these systems and

set the stage for further studies on these and related complexes.

18

2 Experimental Section

2.1 General Considerations

Unless otherwise specified, all operations were performed using Schlenk/vacuum line techniques

under a dinitrogen atmosphere or in a dinitrogen atmosphere glovebox from MBraun. Unless

otherwise stated, all chemicals were purchased from commercial sources and used without

further purification. NaH (60% dispersion in mineral oil) was washed with hexanes to remove

the oil, and the reagent was used in its pure form. KN(SiMe3)2 solution (0.5 M in toluene) was

evaporated to dryness in vacuo, and the reagent was used in its solid form with the composition

{KN(SiMe3)2} (C7H8)0.36. Me3SiCl was distilled under dinitrogen prior to use. NaL was prepared

from LH and NaH according to a literature procedure.47 Complex 1 was prepared according to a

literature procedure.44 MeL was prepared according to a literature procedure.46 All glassware was

dried overnight in a 180 °C oven or flame-dried prior to use, except for NMR tubes and J-Young

NMR tubes, which were dried overnight in a 60 °C oven. Celite was dried under vacuum at

180 °C overnight. Molecular sieves were activated by heating at 300 °C under vacuum for three

days. Dihydrogen (grade 5.0) and carbon dioxide (grade 4.0) were purchased from Linde

Canada. Tetrahydrofuran and benzene-d6 were dried over Na/benzophenone, distilled under

dinitrogen, and stored over activated molecular sieves. Toluene, pentane, hexanes, and diethyl

ether were sparged with dinitrogen, passed through a Pure Solv Innovative Technology Grubbs-

type solvent purification system, degassed through one freeze-pump-thaw cycle, and stored over

activated molecular sieves. Chloroform-d was degassed through three freeze-pump-thaw cycles,

dried over activated molecular sieves, and stored over a fresh batch of activated molecular

sieves.

IR spectra were collected on a Perkin-Elmer Spectrum One FT-IR spectrometer. 1H, 2H, 11B, 13C,

and 31P NMR spectra were recorded on Varian Mercury 300 MHz, Varian Mercury 400 MHz,

Bruker Avance III 400 MHz, or Agilent DD2 600 MHz NMR spectrometers. 1H and 13C

chemical shifts are reported in ppm relative to the residual proteo solvent peaks. 11B and 31P

NMR spectra were referenced externally using 15 v/v% BF3·Et2O in CDCl3 and 85% aqueous

H3PO4, respectively. 2H spectra were not referenced. Where possible, peaks were assigned by

comparison to known complexes as well as 1H-1H COSY, 1H-13C HSQC, and 1H-13C HMBC

correlation experiments. The labels o-L, o-L’, m-L, m-L’, p-L, and p-L’ refer to locations on a

19

4,5-diazafluorenyl moiety; the labels o-cat and m-cat refer to locations on a catecholato moiety

(Scheme 18). The labels o-PPh3, m-PPh3, and p-PPh3 refer to the ortho, meta, and para positions

of the phenyl rings in a triphenylphosphine ligand, respectively. The label 4° L refers to

quaternary 4,5-diazafluorenyl carbon atoms. The labels ipso-PPh3 and ipso-cat refer to ipso

carbon atoms (relative to the phosphorus atom) on a triphenylphosphine ligand and ipso carbon

atoms (relative to the oxygen atoms) on a catecholboryl ligand, respectively. Coupling constants

marked with an asterisk (*) deviate slightly from their expected values because of overlap with

minor impurity peaks.

Scheme 18: Labelling convention for catecholato and 4,5-diazafluorenyl NMR resonances.

2.2 In Situ Synthesis of [RuHCl(LH–Bcat)(PPh3)2] (2)

To a colourless solution of ClBcat (1.9 mg, 0.012 mmol, 1.0 equiv.) in 0.5 mL benzene-d6 was

added dropwise a dark purple solution of 1 (10 mg, 0.012 mmol, 1.0 equiv.) in 0.5 mL

benzene-d6. A rapid colour change to red-orange was observed during addition. The solution was

transferred to a J-Young tube, and NMR spectra were recorded 15 minutes after addition.

1H NMR (C6D6, 400 MHz, 25 °C): δ 8.21 (d, 3JH–H = 7.6 Hz, 1H), 8.17 (d, 3JH–H = 7.6 Hz, 1H),

7.50 (d, 3JH–H = 5.2 Hz, 1H), 7.45-7.40 (m), 7.26-6.67 (m), 6.42 (dd, 3JH–H = 7.6 Hz, 3JH–H =

5.2 Hz, 1H), 5.85 (dd, 3JH–H = 7.6 Hz, 3JH–H = 5.2 Hz, 1H), 4.36 (s, 1H), -12.89 (dd, 2JP–H =

19.2 Hz, 1H). 31P{1H} NMR (C6D6, 162 MHz, 25 °C): δ 47.36 (d, 2JP–P = 5.2 Hz), 46.71 (d,

2JP–P = 5.2 Hz).

2.3 Synthesis of [RuH(L–Bcat)(N2)(PPh3)2] (4)

2.3.1 Method A: ClBcat as Borane Source

To a dark purple solution/suspension of 1 (50 mg, 0.061 mmol, 2.0 equiv.) in 2 mL toluene was

added dropwise with stirring a colourless solution of ClBcat (4.7 mg, 0.031 mmol, 1.0 equiv.) in

8 mL toluene. A colour change from purple to red was observed during addition, along with the

formation of a precipitate. The reaction was stirred for four hours, after which the tan-coloured

20

precipitate (complex 7) was removed by filtration and the red supernatant was evaporated to

dryness in vacuo, leaving 32 mg of dark red solids. Crystallization was performed by allowing

pentane to slowly diffuse into a solution of the crude mixture in a 2:1 toluene-THF solution at

–35 °C; a purified sample remained in the supernatant, which was decanted from the precipitated

solids, filtered through Celite, evaporated to dryness in vacuo, triturated with hexanes, and once

again evaporated to dryness in vacuo to give light red solids. During NMR acquisition, slow

conversion to complex 9 was observed; thus, the spectra contained some unidentified peaks that

likely belong to this compound. 1H NMR (C6D6, 600 MHz, 25 °C): δ 8.58 (dd, 3JH–H = 7.8 Hz,

4JH–H = 0.6 Hz, 1H, o-L), 8.39 (dd, 3JH–H = 7.8 Hz, 4JH–H = 0.6 Hz, 1H, o-L’), 8.02 (d, 3JH–H =

4.8* Hz, 1H, p-L), 7.36-7.32 (m, 12H, o-PPh3), 7.30 (dd, 3JH–H = 5.4 Hz, 4JH–H = 3.0 Hz, 2H,

o-cat), 6.99 (dd, 3JH–H = 4.8 Hz, 4JH–H = 0.6 Hz, 1H, p-L’), 6.93 (dd, 3JH–H = 8.4* Hz, 3JH–H =

4.8 Hz, 1H, m-L), 6.90 (dd, 3JH–H = 6.0* Hz, 4JH–H = 3.0 Hz, 2H, m-cat), 6.88-6.82 (m, 18H,

m-PPh3 + p-PPh3), 6.23 (dd, 3JH–H = 7.8 Hz, 3JH–H = 4.8 Hz, 1H, m-L’), -12.60 (t, 2JP–H = 19.2 Hz,

1H, Ru–H). 31P{1H} NMR (C6D6, 243 MHz, 25 °C): δ 48.36 (s). 11B NMR (C6D6, 192 MHz,

25 °C): δ 15.54 (br). 13C NMR (C6D6, 150 MHz, 25 °C): δ 150.37 (ipso-cat), 146.66 (4° L),

145.87 (4° L), 140.48 (o-L’), 137.90 (o-L), 135.17 (4° L), 134.86 (4° L), 133.55 (o-PPh3),

129.53 (PPh3), 128.02 (PPh3), 128.01 (p-L), 127.93 (4° L), 127.28 (p-L’), 121.72 (m-cat), 119.87

(m-L’), 119.52 (m-L), 111.86 (o-cat). The ipso-PPh3 resonance lies somewhere within a cluster

of unidentified low-intensity peaks (133.1-132.6). IR(Nujol): Ru–N2) 2109 (s) cm-1.

2.3.2 Method B: HBcat as Borane Source

To a colourless solution of HBcat (1.5 mg, 0.012 mmol, 1.0 equiv.) in 0.5 mL toluene was added

dropwise a dark purple solution/suspension of 1 (10 mg, 0.012 mmol, 1.0 equiv.) in 1 mL

toluene. A colour change from purple to red was observed during addition. The reaction was

allowed to stand for one hour and was then filtered through Celite. The red supernatant was then

evaporated to dryness in vacuo. 7 mg of dark red solids were obtained, which were purified by

dissolution in a minimal amount of toluene followed by addition of pentane, isolation of the

precipitated solids by filtration, and drying the solids in vacuo. 1H and 31P{1H} NMR data agreed

with those obtained using Method A and demonstrated the formation of 4 in >91% purity.

21

2.4 In Situ Synthesis of catB–N(SiMe3)2 (6)

ClBcat (2.0 mg, 0.013 mmol) and {KN(SiMe3)2} (toluene)0.36 {3.0 mg, 0.013 mmol

KN(SiMe3)2} were dissolved in 0.75 mL benzene-d6. A precipitate immediately formed. The

solution was transferred to an NMR tube, and spectra were recorded shortly afterward.

Approximately 35% unreacted ClBcat remained, likely a result of error in the mass

measurement. Several minor impurities were also present (18% by integration). 1H NMR (C6D6,

400 MHz, 25 °C): δ 6.96 (dd, 3JH–H = 6 Hz, 4JH–H = 2 Hz, 2H, cat), 6.76 (dd, 3JH–H = 6 Hz, 4JH–H =

2 Hz, 2H, cat), 0.32 {s, 18H, –N(SiMe3)2}. 11B NMR (C6D6, 128 MHz, 25 °C): δ 26.90 (br).

2.5 Synthesis of cis,trans-[RuHCl(LH)(PPh3)2] (7)

To a dark purple solution/suspension of 1 (50 mg, 0.061 mmol, 2.0 equiv.) in 2 mL toluene was

added dropwise with stirring a colourless solution of ClBcat (4.7 mg, 0.031 mmol, 1.0 equiv.) in

8 mL toluene. A colour change from purple to red was observed during addition, along with the

formation of a precipitate. The reaction was stirred for four hours, after which the tan-coloured

precipitate was isolated by filtration, washed with 1 mL toluene, and dried in vacuo. (The

supernatant contained complex 4.) 24 mg of solids were isolated. 1H NMR (CDCl3, 400 MHz,

25 °C): δ 8.54 (d, 3JH–H = 4 Hz, 1H, o-L), 8.06 (d, 3JH–H = 8 Hz, 1H, p-L), 7.95 (d, 3JH–H = 8 Hz,

1H, o-L’), 7.40 (dd, 3JH–H = 8 Hz, 3JH–H = 4 Hz, 1H, m-L), 7.32-7.18 (m, 31H, PPh3 + p-L’), 6.69

(dd, 3JH–H = 8 Hz, 3JH–H = 4 Hz, 1H, m-L’), 4.05 (s, 2H, LH), -13.35 (t, 2JP–H = 20 Hz, 1H,

Ru–H). 31P{1H} NMR (C6D6, 162 MHz, 25 °C): δ 46.89 (s).

2.6 Synthesis of [RuH(LH)(N2)(PPh3)2][catBcat] (8)

To a colourless solution of HBcat (7.3 mg, 0.061 mmol, 1.0 equiv.) in 2.5 mL toluene was added

dropwise with stirring a dark purple solution/suspension of 1 (50 mg, 0.061 mmol, 1.0 equiv.) in

5 mL toluene. A colour change from purple to red was observed during addition, along with the

formation of a precipitate. The reaction was stirred for two hours, after which the precipitate was

isolated by filtration and dried in vacuo. 23 mg of tan-coloured solids were obtained. (The

supernatant contained complex 4.) 1H NMR (CDCl3, 300 MHz, 25 °C): δ 8.51 (d, 3JH–H = 6 Hz,

1H, o-L), 7.72 (d, 3JH–H = 6 Hz, 1H, p-L), 7.60 (d, 3JH–H = 6 Hz, 1H, o-L’), 7.31-7.15 (m, 32H,

m-L + p-L’ + PPh3), 6.67-6.64 (m, 4H, cat), 6.60-6.55 (m, 5H, cat + m-L’), 3.68 (s, 2H, LH),

22

-13.36 (t, 2JP–H = 21 Hz, 1H, Ru–H). 31P{1H} NMR (CDCl3, 122 MHz, 25 °C): δ 46.94 (s).

11B NMR (CDCl3, 96 MHz, 25 °C): δ 14.49 (s).

2.7 In Situ Synthesis of [Ru(C6D5)(L–Bcat)(N2){P(2,6-D2-C6H3}3)2] (9)

During the characterization of complex 4 in benzene-d6, approximately 50% conversion to 9

occurred after 12 hours at room temperature. 1H NMR (C6D6, 600 MHz, 25 °C): identical to 4

(within 0.01 ppm), with the following resonances being absent: 7.36-7.32 (o-PPh3), -12.60

(Ru–H). 31P{1H} NMR (C6D6, 243 MHz, 25 °C): δ 47.86 (s). 2H NMR (Et2O, 92 MHz, 25 °C):

δ 7.25 (s), 7.09 (br).

2.8 Synthesis of [Ru(H2Bcat)(L–Bcat)(PPh3)2] (10)

2.8.1 Method A: ClBcat as Borane Source

To a colourless solution of ClBcat (38 mg, 0.24 mmol, 2.0 equiv.) in 1 mL toluene was added

dropwise with stirring a dark purple solution/suspension of 1 (100 mg, 0.12 mmol, 1.0 equiv.) in

9 mL toluene. The dark-coloured suspension was stirred overnight at room temperature. The

following day, the suspension was filtered, giving a deep red supernatant and light brown solids

(complex 12). The supernatant was evaporated to dryness in vacuo, leaving dark red solids. A

yield for the reaction could not be obtained as aliquots were periodically removed throughout the

reaction to monitor its progress. The crude solids were purified by crystallization: pentane was

allowed to diffuse into a toluene-THF solution of the crude product. Impurities precipitated out

of solution, leaving relatively pure 10 in the supernatant. NMR data are reported below.

2.8.2 Method B: HBcat as Borane Source

To a colourless solution of HBcat (9.3 mg, 0.078 mmol, 2.0 equiv. relative to 1) in 0.5 mL

toluene was added dropwise with stirring a dark purple solution/suspension of a mixture of 1 and

4 (104 mg, 67% 1 + 33% 4, 0.038 mmol 1, 1.0 equiv. 1) in 8 mL toluene. A colour change from

purple to red was observed during addition. The reaction was stirred for one hour and was then

filtered through Celite to remove a small amount of tan-coloured solids. The red supernatant was

evaporated to dryness in vacuo, and after trituration with (2 x 0.5 mL) hexanes, 93 mg of light

red solids were obtained. NMR showed the presence of 80% complex 4 and 20% complex 10.

1H NMR (C6D6, 400 MHz, 25 °C): δ 8.31 (d, 3JH–H = 8 Hz, 2H, o-L), 8.23 (d, 3JH–H = 8 Hz, 2H,

23

p-L), 7.36-7.31 (m, 14H, o-PPh3 + o-cat), 6.91-6.89 (m, 2H, m-cat), 6.87-6.79 (m, 18H, m-PPh3

+ p-PPh3), 6.69 (dd, 3JH–H = 8 Hz, 3JH–H = 8 Hz, 2H, m-L), 6.53 (dd, 3JH–H = 8 Hz, 4JH–H = 4 Hz,

2H, RuH2Bcat), 6.38 (dd, 3JH–H = 8 Hz, 4JH–H = 4 Hz, 2H, RuH2Bcat), -11.50 (br, 2H, RuH2Bcat).

31P{1H} NMR (C6D6, 162 MHz, 25 °C): δ 49.24 (s). 11B NMR (C6D6, 128 MHz, 25 °C): δ 22.43

(br).

2.9 Synthesis of cis,cis,trans-[RuCl2(LH–Bcat)(PPh3)2] (11)

To a colourless solution of ClBcat (38 mg, 0.24 mmol, 2.0 equiv.) in 1 mL toluene was added

dropwise with stirring a dark purple solution/suspension of 1 (100 mg, 0.12 mmol, 1.0 equiv.) in

9 mL toluene. The dark-coloured suspension was stirred overnight at room temperature. The

following day, the suspension was filtered, giving a deep red supernatant (contained complex 10)

and light brown solids. The solids were washed several times with toluene, then dried in vacuo.

48 mg of solids were isolated. The crude solids were purified by crystallization: pentane was

allowed to diffuse into a THF solution of the crude product. Relatively pure 11 precipitated from

solution. 1H NMR (CDCl3, 300 MHz, 25 °C): δ 7.82 (d, 3JH–H = 6 Hz, 2H), 7.60-7.46 (m),

7.25-6.91 (m), 6.56 (dd, 3JH–H = 6 Hz, 3JH–H = 6 Hz, 2H), 4.50 (s, 1H). 31P{1H} NMR (CDCl3,

122 MHz, 25 °C): δ 24.25 (s). 11B NMR (CDCl3, 96 MHz, 25 °C): δ 22.40 (br).

2.10 In Situ Synthesis of [Ru(H2Bcat)(L)(PPh3)2] (12)

Approximately 5 mg of a mixture of 80% complex 4 and 20% complex 10 in 0.25 mL benzene-

d6 was placed in a 3 mm thick-walled J-Young tube. The tube was freeze-pump-thaw degassed

three times, then backfilled with dihydrogen at –196 °C (4 atm H2) and warmed to room

temperature. The tube was then heated at 40 °C for 18 hours. NMR indicated the presence of

37% complex 10 and 63% complex 12. Partial deuteration of the o-PPh3 protons of 12 was

observed. 1H NMR (C6D6, 600 MHz, 25 °C): δ 8.48 (d, 3JH–H = 6.0 Hz, 2H), 7.79 (d, 3JH–H =

6.0 Hz, 2H), 7.27-7.24 (m, < 12H, o-PPh3), 6.85-6.77 (m, 18H), 6.53 (dd, 3JH–H = 5.4* Hz,

3JH–H = 3.6* Hz, 2H), -9.94 (br, 2H, RuH2Bcat). The singlet for the 9-position L proton could not

be found and may be overlapped by other resonances. 31P{1H} NMR (C6D6, 243 MHz, 25 °C):

δ 51.45 (s).

24

2.11 Synthesis of 9-trimethylsilyl-4,5-diazafluorene (13)

To a colourless solution of Me3SiCl (1.1 mL, 8.9 mmol, 15 equiv.) in 3 mL THF in a Schlenk

bomb was added dropwise with stirring via cannula a dark pink solution of NaL (0.59 mmol,

1.0 equiv.) in 15 mL THF. An immediate colour change to royal blue occurred. The bomb was

sealed and the reaction was stirred overnight at room temperature. The following day, the solvent

and excess Me3SiCl were removed by evaporation in vacuo. In the glovebox, the residues were

re-dissolved in THF and filtered. Evaporation of the supernatant in vacuo gave 124 mg of a

mixture of beige solids and magenta solids. These solids were then exposed to air and subject to

a standard organic workup. The solids were dissolved in 5 mL CH2Cl2 and washed with

(2 x 2 mL) dH2O. The aqueous layers were combined and extracted with (2 x 1 mL) CH2Cl2. The

combined organic layers were dried over anhydrous MgSO4, filtered, and evaporated to dryness

using a Rotary evaporator. 76 mg of beige solids were recovered, which consisted of 65% LH

and 35% 13 according to integration of the 1H spectrum. 1H NMR (CDCl3, 400 MHz, 25 °C):

δ 8.71 (m, 2H), 7.81 (d, 2H), 7.27 (d, 2H), 3.77 (s, 1H), 0.04 (s, 9H).

25

3 Results and Discussions

3.1 Reactivity of 4,5-Diazafluorenide Complexes with Boranes and Silanes

3.1.1 Reactivity of [RuH(L)(N2)(PPh3)2] (1) with Boranes

As a first step in expanding the carbon dioxide chemistry of the 4,5-diazafluorenide (L-) ligand,

attempts were made to append a borane moiety at the 9-position of the ligand in the known

complex [RuH(L)(N2)(PPh3)2] (1). Since the L- ligand in this complex is known to be

nucleophilic, it was hypothesized that reaction with an electrophilic haloborane would lead to the

desired borane adduct via an SN2 mechanism. Subsequent deprotonation of this adduct would

then form a complex analogous to 1, where the 9-position proton has been replaced by a borane

(Scheme 19). Since boranes are regularly used as reductants and oxygen acceptors in carbon

dioxide reduction chemistry, and since the parent complex 1 is known to react with carbon

dioxide, this chemistry is both promising and relevant to the field of carbon dioxide activation.

Scheme 19: Proposed reactivity between a nucleophilic ruthenium 4,5-diazafluorenide

complex and an electrophilic chloroborane. The 9-position of the ligand is labelled.

3.1.1.1 Preliminary Reactivity with ClBcat

Dropwise addition of a dark purple solution of complex 1 (10 mg) in benzene-d6 to a colourless

solution of ClBcat (one equivalent) in benzene-d6 (Scheme 20) resulted in a rapid colour change

to red-orange. NMR spectra recorded approximately 15 minutes after mixing indicated the

complete consumption of 1 and ClBcat and the formation of a new product (90%) along with at

least one minor product. Notably, the singlet at 6.44 ppm arising from the 9-position aromatic

proton of 1 disappeared, and a new singlet at 4.36 ppm that integrated to one proton relative to

the o-L proton appeared (Figure 1). This shift suggests that the central five-membered ring of the

ligand is no longer aromatic, yet the 9-position proton is more deshielded than the protons of free

26

LH. For comparison, the 9-position protons of free 4,5-diazafluorene (LH) give rise to a singlet

at 2.98 ppm.48 There is also an overlapped doublet of doublets at -12.89 ppm that integrates to

one proton and two doublets in the 31P spectrum at 47.36 and 46.71 ppm (2JP–P = 5.2 Hz),

suggesting that two inequivalent PPh3 ligands are bound to ruthenium and adopt a cis geometry.

The 11B spectrum contains several low-intensity, broad peaks that do not correspond to ClBcat.

Given this information, the new product is assigned as [RuHCl(LH–Bcat)(PPh3)2] (2). The

catecholato proton signals are likely buried under the intense resonances from the PPh3 ligands.

An alternate formulation for this product is [RuH(LH–Bcat)(N2)(PPh3)2][Cl]; however, given

that the product is soluble in benzene, the former assignment is more probable.

Scheme 20: Reaction of 1 (10 mg scale) with ClBcat in benzene-d6.

Figure 1: Partial 1H NMR spectrum (400 MHz, C6D6) of [RuHCl(LH–Bcat)(PPh3)2] (2),

recorded in situ after 1 and ClBcat were allowed to react for 15 minutes.

Ru–H

27

Continued monitoring of the reaction by NMR showed the slow decomposition of 2 into at least

four other soluble products over 24 hours along with formation of a precipitate. Interestingly,

dihydrogen was observed in the 1H spectrum after two hours. Complex 1 is known to reversibly

react with dihydrogen to give the complex [RuH2(LH)(PPh3)2] (3), where H2 has been

heterolytically split across the acidic ruthenium centre and the basic 9-position of the L- ligand.44

It is conceivable that similar reactivity could occur with the borane adduct 2, which may explain

the evolution of H2 upon decomposition.

When the same reaction was performed in THF and monitored by 31P NMR, at least two

products formed after 30 minutes. The same pattern of decomposition occurred: at least four

products were observed over the course of the reaction, along with the formation of a precipitate.

Interestingly, the reaction behaved rather differently when performed on a larger scale. A

solution of 1 (50 mg) in toluene was added dropwise to a solution of ClBcat (one equivalent) in

toluene. Aliquots were removed periodically and analyzed by 31P NMR. After one hour, 1 was

consumed and 2 had formed, as expected. However, the spectrum also contained a singlet

corresponding to a new product. Over seven hours, complex 2 was mostly converted to the new

product (Scheme 21), resulting in a deep red solution and formation of a precipitate. After

removing the precipitate by filtration and evaporating the supernatant to dryness in vacuo, dark

red solids were obtained. Based on NMR, these crude solids consisted of a new product (66%)

and several impurities (34%). When the spectra of the new product were compared to those of 2,

three important differences emerged: the 1H singlet at 4.36 ppm had vanished, the hydride

resonance was now a triplet, and only one singlet was present in the 31P spectrum. Based on

these data, the new product was assigned as the zwitterionic borane adduct

[RuH(L–Bcat)(N2)(PPh3)2] (4), where the diazafluorenide ligand has been deprotonated and the

PPh3 ligands have adopted a trans geometry.

28

Scheme 21: Reaction of 1 (50 mg scale) with ClBcat in toluene. [Ru] denotes a

ruthenium(II) species with a protonated LH ligand.

Complex 4 can conceivably be formed by deprotonation of 2, concomitant loss of the chloride

ligand, isomerization, and coordination of N2 to fill the vacant coordination site. Analogous

reactivity is observed in the synthesis of 1 from the precursor cis,cis-[RuHCl(LH)(PPh3)2] (5)

and KOtBu.44 Since no external base such as KOtBu was added to the reaction between 1 and

ClBcat, it is conceivable that complex 1 is acting as a base and deprotonating 2 to form a

ruthenium species with a protonated LH ligand. Indeed, this species may be present in the

precipitate that formed during the reaction. Further experiments, discussed in Section 3.1.1.2.1.4,

confirmed this hypothesis as well as the identity of complex 4. Unlike its protonated analogue 2,

complex 4 showed no signs of decomposition after standing for several hours in toluene.

Isolation of complex 2 was not attempted given its instability and the complexity of the reaction.

Instead, attempts were made to directly synthesize, isolate, and characterize the zwitterionic

borane adduct 4 in high purity. The following sections describe several synthetic pathways that

led to the synthesis of 4: in situ deprotonation with external base, stepwise deprotonation with

external base, in situ deprotonation with 1 as internal base, and direct one-step synthesis from

HBcat.

29

3.1.1.2 Synthesis of Zwitterionic Borane Adduct [RuH(L–Bcat)(N2)(PPh3)2] (4)

3.1.1.2.1 ClBcat as Borane Source

3.1.1.2.1.1 In Situ Deprotonation by NaH

Although complex 1 can be used as a base to deprotonate 2 in situ, this is rather inefficient as 1 is

expensive to synthesize, both in terms of time and cost. An external base may be able to

accomplish the same reaction in a much cheaper and more efficient way.

When a solution of ClBcat in benzene-d6 was added to a mixture containing 1 and NaH (one

equivalent each) in benzene-d6, an orange solution formed. The reaction behaved very similarly

to the reaction performed in the absence of base: NMR monitoring indicated that complex 2

formed initially and began to decompose after several hours with the evolution of dihydrogen.

No evidence for a deprotonated L- ligand was observed, suggesting that NaH was unable to

deprotonate 2. The same result was obtained when five equivalents of NaH were used.

The reaction was then repeated in THF with five equivalents of NaH. An orange solution formed

after addition, but after standing for several minutes, a red colour appeared and the evolution of

gas bubbles was observed. After two hours, the solution was once again purple, the colour of the

starting material 1. At this point, the reaction was evaporated to dryness in vacuo, and the crude

residues were dissolved in benzene-d6. NMR spectroscopy indicated the presence of 1 (75%) and

the hydrogen-splitting product 3 (25%). It is conceivable that the initial orange colour arose from

complex 2, which formed in situ. Then, 2 may have been deprotonated by NaH to form complex

4 along with the evolution of dihydrogen. The evolved dihydrogen may then react with complex

4 to re-form 1, which may further react to form complex 3 (Scheme 22). The reactivity of 4

toward dihydrogen is discussed in Section 3.1.1.5 and lends support to this hypothesis. The

volatile HBcat produced in this proposed reaction scheme would have been removed when the

THF solution was evaporated to dryness prior to recording the NMR spectra. Repeating this

experiment in a closed system and monitoring the intermediates via NMR will provide additional

evidence to support this proposed mechanism.

30

Scheme 22: Proposed series of reactions that occur when 1 and five equivalents of NaH are

combined in THF.

3.1.1.2.1.2 In Situ Deprotonation by KN(SiMe3)2

When KN(SiMe3)2, was used in place of NaH, drastically different results were obtained. A dark

purple solution of 1 and KN(SiMe3)2 (one equivalent each) in toluene was added dropwise to a

colourless solution of ClBcat (one equivalent) in toluene with stirring. The solution became red

after addition and was allowed to stir for three hours. A precipitate formed during the reaction,

which was removed by filtration, and the supernatant was evaporated to dryness in vacuo. NMR

analysis of the residues remaining after evaporation indicated the presence of 1, 4, and the B–N

adduct catB–N(SiMe3)2 (6) in roughly a 1:1:1 ratio (Scheme 23). Several minor impurities were

also present.

31

Scheme 23: Reaction of 1 with ClBcat in the presence of KN(SiMe3)2, where 1 and

KN(SiMe3)2 were added dropwise to ClBcat.

Adduct 6 formed from a side reaction between ClBcat and KN(SiMe3)2; this was verified by

performing a control experiment in the absence of 1 (Scheme 24). Adduct 6 has been previously

reported,49 and its literature NMR data agree well with the experimental data. This side reaction

depleted some of the ClBcat and KN(SiMe3)2, which explains the presence of unreacted 1.

Scheme 24: Control experiment confirming the formation of the B–N adduct 6 from ClBcat

and KN(SiMe3)2.

When the order of addition was reversed – a solution of ClBcat was added dropwise to a solution

of 1 and KN(SiMe3)2 – a roughly 1:1 mixture of 4 and 6 was obtained, with no unreacted 1

remaining (Scheme 25). Based on previous results (see Section 3.1.1.1), some of complex 1

likely acted as a base and deprotonated 2 as it formed in situ.

32

Scheme 25: Reaction of 1 with ClBcat in the presence of KN(SiMe3)2, where ClBcat was

added dropwise to complex 1 and KN(SiMe3)2. [Ru] denotes a ruthenium(II) species with a

protonated LH ligand.

This reaction was also performed using a 1:2:2 ratio of 1, ClBcat, and KN(SiMe3)2. It was hoped

that this stoichiometry would prevent 1 from reacting as a base. This was not the case: the

reaction behaved similarly to the 1:1:1 reaction, but produced significantly more impurities.

Efforts were made to purify these reaction mixtures, but were largely unsuccessful. Washing the

crude solids with pentane or hexanes reduced the amount of 6 present to less than 10%, but could

not completely remove it even after several washes. Attempts to remove 6 by sublimation under

vacuum (boiling point = 70 °C at 60 mTorr)49 were also unsuccessful and led to partial

decomposition of complex 4. Finally, recrystallization of the mixtures was performed using

several different solvent combinations. In most cases, unidentified impurities precipitated from

the solution, leaving 4 and 6 in the supernatant.

Most other common laboratory bases, such as methoxide, tert-butoxide, n-butyllithium, and

benzylpotassium, would likely form the adducts MeOBcat, tBuOBcat, nBuBcat, and BnBcat,

respectively, by reaction with ClBcat in a manner analogous to the formation of 6. Indeed, a

control experiment confirmed that ClBcat and KBn react to form at least two new products, one

of which is likely the B–C adduct BnBcat. Thus, these bases are not feasible reagents for the

synthesis of 4.

33

3.1.1.2.1.3 Stepwise Reactions

Since purifying reaction mixtures containing the adduct 6 proved to be problematic, efforts were

made to suppress or avoid its formation during the synthesis of 4. A stepwise reaction was

attempted, where 1 and ClBcat were allowed to react for a short period of time before addition to

a solution of KN(SiMe3)2 (Scheme 26). The first step should consume all of the ClBcat,

preventing its undesired reaction with KN(SiMe3)2. The order of addition is also important: if

complex 1 (and by analogy, complex 4) is ever in excess, it can act as a base and deprotonate the

reaction intermediates.

Scheme 26: Reaction of ClBcat and 1, followed by reaction of the intermediate product

mixture with KN(SiMe3)2.

When the two steps were separated by five minutes, the reaction did not go to completion, and

36% of complex 1 remained. When the interval was increased to 30 minutes, the same result

occurred, with 22% of 1 remaining. The B–N adduct 6 was observed in both cases, along with

several minor impurities. The time intervals between the two steps were chosen in an attempt to

find a compromise between the time required for the reaction between 1 and ClBcat to go to

completion and the known tendency of 2 to decompose in solution.

3.1.1.2.1.4 In Situ Deprotonation by Excess 1

Given the difficulties described in the preceding sections, the concept of using 1 as a base to

deprotonate 2 in situ to form 4 was revisited. A dilute solution of 1 (two equivalents) in toluene

was added dropwise to a stirred solution of ClBcat (one equivalent) in toluene (Scheme 27).

34

After stirring for four hours, a considerable amount of tan-coloured precipitate had formed,

which was isolated by filtration from the red supernatant. The precipitate was shown by NMR to

be cis,trans-[RuHCl(LH)(PPh3)2] (7), an isomer of the known complex cis,cis-

[RuHCl(LH)(PPh3)2] (5).44 Further support for this assignment was obtained by reacting 7 with

KOtBu (Scheme 28), which deprotonated the ligand as expected and formed complex 1,

analogous to the known reactivity of the cis,cis isomer 5.44

Scheme 27: Synthesis of 4 via reaction of two equivalents of 1 with one equivalent of

ClBcat.

Scheme 28: Synthesis of 1 via reaction of 7 with KOtBu, supporting the assignment of 7 as

an isomer of [RuHCl(LH)(PPh3)2].

When the red supernatant was evaporated to dryness in vacuo, dark red solids were obtained.

NMR revealed the formation of the desired borane adduct 4 along with several unidentified

impurities. In an effort to purify the crude reaction mixture, crystallization was attempted using a

variety of methods and solvent combinations. When vapour diffusion was used, two solvent

combinations – toluene/pentane and diethyl ether/pentane – were able to increase the proportion

of 4 in the supernatant by selectively precipitating out the impurities. Although most attempts did

35

not completely remove the impurities, a “pure” sample of 4 was obtained from one attempt

where pentane was slowly diffused into a 2:1 toluene-THF solution of the crude reaction

products at –35 °C. The multinuclear NMR spectra (1H, 11B, 13C, and 31P) obtained of this sample

are in excellent agreement with the assigned structure of 4, namely the borane adduct

[RuH(L–Bcat)(N2)(PPh3)2] (Figure 2). The IR spectrum of 4, recorded as a Nujol mull,

confirmed the presence of the dinitrogen ligand (N–N = 2109 cm-1). For comparison, the N–N

stretching frequency for the dinitrogen ligand in 1 is 2092 cm-1.44

Figure 2: Partial 1H NMR spectrum (600 MHz, C6D6) of [RuH(L–Bcat)(N2)(PPh3)2] (4).

Numerous attempts were made to obtain single crystals of 4 for analysis by X-ray

crystallography, but none were successful. Three-coordinate boron species are known to be

difficult to crystallize. Efforts to form a four-coordinate boron centre, which would be expected

to crystallize much more readily, by using coordinating solvents such as THF and acetonitrile for

recrystallization did not appear to work. It is worth exploring whether reaction of complex 4 with

a fluoride anion source can lead to the formation of a salt of the form

[E][RuH{L–B(F)cat}(N2)(PPh3)2] (E = non-coordinating cation) that is more amenable to

characterization by X-ray crystallography.

36

Complex 4 slowly converted to other products when heated at 40 °C for several days (see

Section 3.1.1.5). It also decomposed in THF over several days at room temperature to form

several unidentified products. It was later determined that 4 also reacts slowly with aromatic

solvents such as benzene and toluene to form a ruthenium(II) aryl complex via a proposed C–H

activation pathway. This reactivity is discussed in Section 3.1.1.3.

3.1.1.2.2 HBcat as Borane Source

In addition to the reactions with ClBcat described above, the feasibility of using HBcat as a

borane source for the synthesis of 4 was explored. Conceivably, this reaction would be analogous

to the reaction with ClBcat, but since hydride is not a good leaving group, an intermediate

containing a four-coordinate boron centre might be expected. Then, this intermediate may

eliminate dihydrogen to form the desired borane adduct (Scheme 29).

Scheme 29: Proposed reactivity between a nucleophilic ruthenium 4,5-diazafluorenide

complex and an electrophilic borane.

When a dark purple solution of 1 (10 mg) in toluene was added dropwise to a colourless solution

of HBcat (one equivalent) in toluene, a red colour emerged. 31P NMR indicated the formation of

4 in high purity after one hour (Scheme 30). Presumably, dihydrogen was evolved but could not

be detected because the reaction was performed in an open system. The red solution was filtered

through Celite to remove a very small amount of solids, and the supernatant was evaporated to

dryness in vacuo. The resulting crude residues were purified by dissolution in a minimal amount

of toluene followed by addition of pentane and isolation of the precipitated solids, which were

shown to be relatively pure 4 (> 91%) by NMR. Further crystallization at low temperature should

be able to give an analytically pure sample. At this scale, this method of synthesizing complex 4

is superior to the route involving ClBcat and a twofold excess of 1 (Section 3.1.1.2.1.4).

37

Scheme 30: Synthesis of 4 via reaction of 1 (10 mg scale) with HBcat.

Unfortunately, the reaction behaved differently at larger scales. When 50 mg of 1 was reacted

with one equivalent of HBcat, considerably more precipitate formed during the reaction. The

precipitate was isolated by filtration and dried in vacuo, and 23 mg of tan-coloured solids were

isolated (see below). The red supernatant was evaporated to dryness in vacuo, giving 37 mg of

dark red solids. NMR spectroscopy confirmed the presence of 4 in the supernatant in

approximately 73% purity. Attempts to further purify this sample by crystallization or washing

with solvent were unsuccessful.

Interestingly, the 1H spectrum of the tan-coloured solids in chloroform-d contained two

resonances at 6.65 and 6.58 ppm, attributable to a catecholato moiety that integrated to four

protons each, double the expected intensity. A singlet at 3.68 ppm that integrated to two protons

was also present, indicative of a protonated LH backbone. A single peak in the 31P spectrum

indicated that the two PPh3 ligands were equivalent. Finally, the 11B spectrum contained a fairly

sharp singlet at 14.49 ppm. Based on these data, the product is assigned as the protonated borate

salt [RuH(LH)(N2)(PPh3)2][catBcat] (8) (Scheme 31).

38

Scheme 31: Reaction of 1 (50 mg scale) with HBcat in toluene.

The [catBcat]- anion has been reported previously in the literature as part of several organic and

metal-containing compounds. For instance, the P–B adduct [(Me2PhP)2BH2][catBcat] displays an

11B singlet at 15.2 ppm and a 1H multiplet at 6.57 ppm for the [catBcat]- anion (CD2Cl2).50

Similarly, the rhodium(III) complex [RhH2(PMe3)4][catBcat] displays an 11B singlet at 15.1 ppm

(THF-d8).50 Additionally, the [catBcat]- anion was observed in situ in a D2O solution of catechol

and boric acid and displayed 1H and 11B resonances of 6.81 ppm and 14.3 ppm, respectively.51

These data are in good correlation with the resonances observed in the spectra of 8, supporting

the presence of the [catBcat]- anion.

In their work with ruthenium(II) PNP pincer complexes, Milstein and co-workers observed

partial decomposition to ionic borate complexes containing a protonated PNP ligand and the

[catBcat]- anion in several reactions involving HBcat.52 They hypothesized that this

decomposition was autocatalytic and triggered by adventitious moisture or by trace amounts of

BH3 in the HBcat. HBcat is known to slowly decompose in solution by disproportionation to

BH3 and B2(cat)3, the latter consisting of two Bcat units bridged through the boron atoms by a

catecholato anion.50 The [catBcat]- anion has also been observed to form during the reaction of

metal complexes with HBcat. Indeed, the complexes [RhH2(PMe3)4][catBcat],50

[RhH2(DPPP)2][catBcat] {DPPP = 1,3-bis(diphenylphosphino)propane},50 and

[(RO)3Ti(catBcat)] (R = 2,6-di-iso-propylphenyl)53 have been reported.

This information provides some insight to the formation of complex 8 during the synthesis of 4.

First, trace amounts of BH3 produced from disproportionation of HBcat may lead to similar

autocatalytic decomposition during the synthesis of 4, analogous to what was observed for

39

Milstein’s system. Indeed, boranes such as BH3 are known to react with similar ligands such as

N-heterocyclic carbenes to form borates or B–H activation products.54 Second, trace amounts of

moisture may hydrolyze the ligated Bcat moiety of 4 to the corresponding catecholborinic acid,

HOBcat, and regenerate complex 1. The HOBcat or additional traces of moisture may then act as

proton sources to protonate the ligand backbone of 1 to form the cation [RuH(LH)(N2)(PPh3)2]+.

Third, the [catBcat]- anion may form by the decomposition of HBcat or by its reaction with trace

moisture. Together, these events may lead to the formation of 8.

Although a suitable method to avoid this competing side reaction could not be found, the borate

salt 8 precipitates from solution during the synthesis and can easily be separated from the

supernatant, which contains complex 4. Since BH3 is known to form adducts with monodentate

ligands, it is possible that the addition of a small amount of such a ligand will sequester any BH3

that forms and suppress the undesired formation of complex 8.

3.1.1.3 Reactivity of [RuH(L–Bcat)(N2)(PPh3)2] (4) with Aromatic Solvents

In the process of running full NMR characterization of a “pure” sample of 4 in benzene-d6 (see

Section 3.1.1.2.1.4), an interesting observation was made: over several hours at room

temperature, the 1H resonances attributed to the hydride and o-PPh3 protons of 4 noticeably

decreased in intensity. After 12 hours, these peaks integrated to only half of their original

intensity, while the remainder of the spectrum appeared unchanged. Upon closer examination, it

was found that each resonance in the spectrum was actually composed of two resonances with

nearly identical chemical shifts: one for 4 and one for a new product (9) (Figure 3). The

difference in chemical shifts between these peaks was less than 0.01 ppm and thus could only be

observed on the high-field 600 MHz spectrometer. The 31P spectrum confirmed the slow

conversion of 4 into 9, as the singlet attributed to 4 (48.36 ppm) slowly decreased in intensity

while a new singlet emerged (47.86 ppm). It was later discovered that complete conversion of 4

to 9 in situ could be achieved after two days at room temperature or after heating at 40 °C

overnight.

40

Figure 3: Close-up of the 1H NMR resonances centered at 8.58 ppm demonstrating the

conversion of 4 to 9 upon standing in benzene-d6 at room temperature: a) after 1 h in

solution, b) after 12 h in solution.

Very similar reactivity was observed when 4 was allowed to stand in toluene-d8 overnight at

room temperature: the hydride and o-PPh3 resonances in the 1H spectrum decreased in intensity,

and the 31P resonance of 4 (48.51 ppm, toluene-d8) decreased in intensity as a new resonance

(48.00 ppm) appeared.

Complex 1 is known to react with benzene-d6 upon heating to give a new product (1’) where

deuterium has been incorporated into the hydride and o-PPh3 protons.55 This reactivity was

elucidated by the observation of two resonances in the 2H spectrum of 1’ attributable to the

hydride and o-PPh3 deuterons, as well as the discovery that the reverse reaction to reform 1

occurs when 1’ is heated in proteo-benzene. This reaction proceeds via reversible C–D activation

of benzene-d6 by 1, which replaces the hydride with a deuteride, along with reversible

cyclometalation of the PPh3 ligands, which deuterates their ortho protons.

41

The 2H spectrum of a sample containing roughly 50% each of 4 and 9 in proteo-Et2O contains a

singlet at 7.25 ppm and a broad singlet at 7.09 ppm, but does not contain any resonances in the

hydride region (Figure 4). One of the resonances is likely attributable to the o-PPh3 deuterons of

9, suggesting a reversible cyclometalation process is indeed occurring. However, it appears that 9

does not contain a deuteride ligand. Given the chemical shift of free benzene (7.24 ppm in

proteo-Et2O), it is possible that the second resonance arises from a deuterated phenyl ligand

coordinated to the ruthenium centre in place of the expected deuteride. Based on this

information, the structure of 9 is tentatively assigned as

[Ru(C6D5)(L–Bcat)(N2){P(2,6-D2-C6H3}3)2] (Scheme 32).

Figure 4: Partial 2H NMR spectrum (92 MHz, Et2O) of a mixture of

[RuH(L–Bcat)(N2)(PPh3)2] (4) and [Ru(C6D5)(L–Bcat)(N2){P(2,6-D2-C6H3}3)2] (9).

42

Scheme 32: Conversion of 4 to 9 by reaction with benzene-d6.

Complex 9 may form by the initial displacement of N2 by benzene-d6 followed by σ-bond

metathesis to form a ruthenium phenyl complex with a coordinated HD ligand, analogous to

what occurs for complex 1. Unlike with 1, the HD ligand is then presumably replaced with N2,

which prevents the reverse reaction from occurring (Scheme 33). The derivatized L–Bcat ligand

likely renders the ruthenium centre of 4 more electron-deficient compared to that of 1, which

may make replacement of coordinated HD with N2 favourable. This increased lability of the HD

ligand may be explained by the fact that coordinated H2 (or HD) binds more weakly to electron-

poor metal centres because of reduced π-backdonation.56 Since the reaction was performed in an

open system, there was no opportunity to spectroscopically observe the evolution of HD.

Scheme 33: Proposed reaction of 4 with benzene-d6 via σ-bond metathesis of the aromatic

C–D bond with the Ru–H bond to form a ruthenium phenyl complex.

A concomitant ortho-cyclometalation process (Scheme 34) may then lead to deuteration of the o-

PPh3 protons in a manner analogous to what occurs for complex 1.

43

Scheme 34: Proposed ortho-cyclometalation reaction that leads to the deuteration of the

o-PPh3 protons of complex 4. R = H or C6D5, Ar = C6D5.

Leitner and co-workers reported a similar reversible C–D activation process for a ruthenium(II)

dihydrogen-dihydrido PNP pincer complex.57 Initial replacement of the coordinated dihydrogen

ligand with benzene-d6 was followed by σ-bond metathesis to form a ruthenium phenyl hydrido

complex with a coordinated HD ligand. These reactions were performed under an argon

atmosphere. In the case of 4 reacting with benzene-d6 under a dinitrogen atmosphere, a similar

mechanism may be at work with the additional step of displacement of the HD ligand by N2.

Further experiments are necessary to confirm these hypotheses. Performing the reaction in a

closed system would allow for the spectroscopic identification of HD. Additionally, heating

samples of 9 in a non-aromatic solvent under a dinitrogen-dihydrogen atmosphere would probe

the reversibility of the reaction. If the identity of 9 is indeed

[Ru(C6D5)(L–Bcat)(N2){P(2,6-D2-C6H3}3)2], it would presumably revert to 4 under these

conditions.

44

3.1.1.4 Syntheses of Borane-Borohydride Complex [Ru(H2Bcat)(L–Bcat)(PPh3)2] (10) and Borane Adduct [RuCl2(L–Bcat)(PPh3)2] (11)

This section describes the discovery of two new complexes that occurred during an attempted

synthesis of complex 2 from complex 1 and ClBcat. In this synthesis, an excess of ClBcat was

used in an effort to prevent 1 from reacting as a base. When a solution of 1 in toluene was added

dropwise to two equivalents of ClBcat in toluene, two major products were obtained (Scheme

35). The first product, which is soluble in toluene, is assigned as the borane-borohydride

complex [Ru(H2Bcat)(L–Bcat)(PPh3)2] (10), a derivative of 4 in which an additional

catecholboryl moiety is coordinated to the ruthenium centre via two bridging hydride ligands.

This is evidenced by a broad resonance in the 1H NMR spectrum (benzene-d6) at -11.48 ppm that

integrates to two protons relative to the two equivalent meta protons of the L- ligand. A second

set of catecholato resonances is also present in the 1H spectrum, further supporting this

assignment.

Scheme 35: Formation of the borane-borohydride complex 10 and protonated borane

complex 11 from reaction of 1 with two equivalents of ClBcat. [Ru] denotes a ruthenium(II)

species with a protonated LH ligand.

For comparison, an analogous borohydride complex, [(tBu-PNP)FeH(η2-BH4)] (PNP = 2,6-

bis(di-tert-butylphosphinomethyl)pyridine), has a broad 1H resonance at -8.40 ppm (benzene-d6)

for the bridging hydride ligand trans to the N-donor of the PNP ligand.35 Two other analogous

45

complexes, [Cp*Fe(LiPr)(H2Bcat) (LiPr =1,3-di-iso-propyl-4,5-dimethylimidazol-2-ylidene)58

and [RuH2(η2-HBcat)(η2-H2)(PCy3)2]

59, also exhibit broad 1H resonances attributable to the

bridging hydride ligands at -15.6 ppm (benzene-d6) and -8.48 ppm (toluene-d8), respectively. For

complex 10, the presence of an analogous broad resonance in the same general region of the 1H

spectrum reinforces its assignment as a borohydride complex.

The second product, which is insoluble in toluene and precipitates from solution, is tentatively

assigned as cis,cis,trans-[RuCl2(LH–Bcat)(PPh3)2] (11). This assignment is based on two striking

observations: the 1H spectrum lacks a hydride resonance and the 31P spectrum (chloroform-d)

contains a singlet at 24.25 ppm. For comparison, the complex cis,cis,trans-[RuCl2(L’)(PPh3)2]

(L’ = 4,5-diazafluoren-9-one) has a 31P chemical shift of 25.61 ppm (methylene chloride-d2).60

The 1H spectrum of 11 also contains a singlet at 4.50 ppm (chloroform-d) integrating to one

proton relative to the two equivalent meta protons of the diazafluorene ligand. For comparison,

the 9-position proton of complex 2, cis,cis-[RuHCl(LH–Bcat)(PPh3)2], has a chemical shift of

4.36 ppm (benzene-d6).

Interestingly, complex 10 was also observed in a separate synthetic attempt involving HBcat.

When a reaction between 1 and HBcat did not go to completion, two equivalents of additional

HBcat per equivalent of unreacted 1 were added in an effort to drive the reaction to completion.

After the reaction was complete, 4 and 10 (80% and 20%, respectively) were present. However,

no complex 11 was observed under these conditions. These results suggest that complex 10

forms from complex 4 in the presence of excess HBcat (Scheme 36).

46

Scheme 36: Proposed formation of 10 from reaction of 4 with HBcat.

In the case where ClBcat was used as the borane source, the HBcat needed to form 10 may have

arisen from reaction of the excess ClBcat with complex 2 (formed in situ from 1 and ClBcat),

which contains a hydride ligand coordinated to ruthenium. This reaction also leads to the

formation of 11. Complex 2 can also react with 1 to form 4 as described previously, and 4 can

react with the HBcat produced to form 10. These reactions are summarized in Scheme 37.

Indeed, metal-hydride complexes are known to react with haloboranes to produce boranes. For

example, reaction of the compound [K][(MeCp)MnH(CO)2] (Cp = cyclopentadienyl) with

ClBcat yielded [(MeCp)Mn(HBcat)(CO)2] and KCl.61

47

Scheme 37: Proposed formation of 10 and 11 from reaction of 4 with excess ClBcat.

3.1.1.5 Reactivity of [RuH(L–Bcat)(N2)(PPh3)2] (4) and

[Ru(H2Bcat)(L–Bcat)(PPh3)2] (10) with Dihydrogen

As described in the previous section, a mixture of 80% complex 4 and 20% complex 10 was

isolated during a synthetic attempt. In order to study the relationship between 4 and 10 as well as

the conversion of 4 to 9, this mixture was gently heated in benzene-d6 under both dinitrogen and

dihydrogen atmospheres in two separate experiments. Under a dinitrogen atmosphere, 4 fully

converted to 9 after heating overnight at 40 °C. Furthermore, a new minor product 12 appeared,

and the ratio between products changed such that the relative amount of 9 decreased and the

relative amounts of 10 and 12 increased. When this reaction was repeated under a dihydrogen

atmosphere (4 atm), complex 9 was fully depleted and 12 was the major species present after 18

hours. The 1H and 31P NMR spectra of 12 were similar to those of 10, with the notable difference

that only one catechol moiety was present in 12. Since a broad hydride resonance that integrated

to two protons was still present (-9.94 ppm in benzene-d6), complex 12 is tentatively assigned as

[Ru(H2Bcat)(L)(PPh3)2], where the 9-position borane moiety has been lost (Scheme 38).

48

Scheme 38: Reaction of 4 and 10 with dihydrogen in benzene-d6.

Since 12 forms readily under a dihydrogen atmosphere, it is hypothesized that 12 forms from 10

by addition of H2 across the 9-position C–B bond, liberating HBcat in the process. In the absence

of dihydrogen but in the presence of benzene-d6, the HD evolved in the conversion of 4 to 9 may

act as the hydrogen source for this transformation. Neither HD nor HBcat were detected during

the experiment, but this is not surprising since both species are reagents in the proposed reaction

pathways. Scheme 39 summarizes these results and the proposed reactions that may be taking

place. Further experiments involving pure samples of 4, 9, 10, and 12 are needed to confirm

these hypotheses.

49

Scheme 39: Summary of proposed and confirmed reactions of 4, 9, 10, and 12 under a

dihydrogen atmosphere in benzene-d6. Solid arrows indicate reactions that have been

confirmed to take place in situ, while dashed arrows indicate proposed reactions.

3.1.2 Reactivity of NaL, MeL, and 1 with Me3SiCl

To continue expanding the chemistry of the 4,5-diazafluorenide ligand in light of its carbon

dioxide chemistry and to complement the work with boranes described above, attempts were also

made to append a silane moiety at the 9-position of the ligand. Like boranes, silanes are

frequently used as reductants and oxygen acceptors in carbon dioxide reduction chemistry. By

following a similar methodology as described for the borane chemistry (Section 3.1.1), several L-

complexes were reacted with Me3SiCl in an effort to synthesize an analogous silane adduct.

50

3.1.2.1 Reaction of NaL with Me3SiCl

When a dark pink solution of NaL in THF, prepared by reacting LH with NaH according to a

literature procedure,47 was added dropwise with stirring to a colourless solution of excess

Me3SiCl (15 equivalents) in THF, a royal blue colour appeared immediately. The reaction was

stirred overnight at room temperature. The next day, the reaction was filtered to remove a white

precipitate (presumably NaCl), and the royal blue supernatant was evaporated to dryness in

vacuo. The 1H spectrum of the crude residues indicated the presence of at least four products in

addition to free LH, which presumably formed by protonation of the highly reactive NaL by

adventitious moisture. Several intense singlets between 0.0 and 0.4 ppm were present, suggesting

that some of the products contained –SiMe3 groups. A similar result was obtained when the

reaction was repeated using 2.7 equivalents of Me3SiCl and heated at 70 °C for three days.

It is possible that the products contained an –SiMe3 group bonded to the 9-position carbon, a

pyridyl nitrogen, or both. In an attempt to simplify the reaction mixture, the crude solids from the

room temperature reaction were exposed to water by performing an aqueous workup in air. Any

products containing C–Si bonds would be expected to survive aqueous conditions, while

products containing N(pyridyl)→Si interactions would likely be unstable. After the aqueous

workup, the only major products remaining were LH (65%) and a product tentatively assigned as

9-trimethylsilyl-4,5-diazafluorene (13) (35%) (Scheme 40). The 1H spectrum (chloroform-d) of

13 contained three aromatic resonances that integrate to two protons each, a singlet at 3.80 ppm

that integrated to one proton, and an intense singlet at 0.04 ppm that integrated to nine protons.

These resonances can be assigned to the aromatic LH protons, the 9-position proton, and the

–SiMe3 group, respectively. Another resonance was present at -0.05 ppm, but its identity remains

unclear.

Scheme 40: Reaction of NaL with Me3SiCl in THF, followed by aqueous workup.

51

Attempts to separate these two products using preparative thin layer chromatography were

unsuccessful. Additionally, after two weeks in air, the mixture of LH and 13 had converted

entirely to LH and unidentified –SiMe3-containing products, suggesting that 13 is unstable.

3.1.2.2 Reaction of MeL with Me3SiCl

To avoid possible undesired reactivity at the pyridyl nitrogen atoms of the L- ligand, the reaction

described in the previous section was attempted using the zwitterionic species N-methyl-4,5-

diazafluorenide (MeL)46 as the L- source. The N-methyl group should sterically block the pyridyl

nitrogen atoms and prevent them from reacting. When a dark blue solution of MeL in toluene

was added dropwise with stirring to a colourless solution of excess Me3SiCl (15 equivalents) in

toluene, no appreciable colour change occurred. No reaction occurred after stirring overnight at

room temperature. There was also no reaction after heating to 80 °C for three days (Scheme 41).

Scheme 41: Reaction of MeL with Me3SiCl in toluene.

3.1.2.3 Reaction of 1 with Me3SiCl

Given that 1 reacts with ClBcat to form the protonated borane adduct 2 in situ (see Section

3.1.1.1), the analogous reaction with Me3SiCl was attempted. Complex 1 and Me3SiCl were

dissolved in benzene-d6 in a 1:1 ratio in a J-Young tube. No reaction occurred after two hours at

room temperature. When the reaction was heated at 80 °C for three hours, 1H NMR indicated the

formation of a complex mixture of products that could not be identified (Scheme 42).

52

Scheme 42: Reaction of 1 with Me3SiCl in benzene-d6.

3.2 Reactivity of 4,5-Diazafluorenide Complexes with CO2

Section 3.1 described the efforts that were made to append a borane moiety at the 9-position of

the 4,5-diazafluorenide ligand in complex 1. The most important result of these studies was the

isolation of the desired complex 4. The ultimate goal of synthesizing 4 was to test its reactivity

with carbon dioxide. In particular, if 4 could catalyze the reduction of CO2 by boranes, then

value-added methoxyboranes could be produced, which are direct precursors to methanol. This

section summarizes the stoichiometric reaction of 4 with carbon dioxide and presents the

discovery of a novel catalytic system involving complex 1 and HBcat.

3.2.1 Stoichiometric Reaction with CO2

It is known that carbon dioxide readily inserts into the 9-position C–H bond of 1 to form a

carboxylic acid.45 In order to determine if complex 4 demonstrated analogous reactivity, namely

insertion of CO2 into the 9-position C–B bond to form a boryl ester (Scheme 43), a benzene-d6

solution of 4 was exposed to one atmosphere of carbon dioxide at room temperature. Shortly

after exposure, a colour change from dark red to dark yellow-brown occurred. The reaction was

monitored by NMR spectroscopy, which showed the complete consumption of 4 within 15

minutes and the formation of a complex mixture of products. After two hours, a small amount of

precipitate had formed.

53

Scheme 43: Desired reactivity of 4 with carbon dioxide to form a catecholboryl ester moiety

at the 9-position of the diazafluorenide ligand.

After standing at room temperature overnight, the composition of the mixture changed, but

several products were still present. In an effort to drive the reaction to a single species, the

reaction was heated at 45 °C for 20 hours. Afterward, almost no peaks remained in the 1H

spectrum, suggesting that a majority of the ruthenium-containing species had precipitated from

solution. Given the small scale of the reaction, isolation of the precipitated solids was not

feasible. Instead, the benzene-d6 was removed in vacuo and the remaining orange solids were

analyzed by NMR and IR spectroscopy.

The orange solids dissolved completely in bromobenzene-d5, and the 1H spectrum recorded in

this solvent showed the presence of several new species that did not correspond to 1, 4, or free

LH. The IR spectrum of the solids contained a peak at 2120 cm-1, suggesting the presence of a

coordinated dinitrogen ligand. Interestingly, the spectrum also contained a broad peak at

1578 cm-1 that was not present in the spectrum of 4. For comparison, the carbonyl stretching

frequency for the carbon dioxide adduct of complex 1, [RuH(L–CO2H)(N2)(PPh3)2], appears at

1609 cm-1.45 This result suggests that carbon dioxide insertion into the C–B bond of 4 may have

occurred.

An attempt to isolate this new species by performing the reaction on a larger scale using toluene

instead of benzene-d6 was unsuccessful. A complex mixture of products was again observed, and

few solids precipitated from solution. Additionally, attempts to spectroscopically detect a

carbonyl group by 13C NMR were unsuccessful. Further experiments, including the isolation and

54

characterization of the new species, are necessary to confirm that carbon dioxide insertion into

the C–B bond of complex 4 has occurred.

3.2.2 Catalytic Reaction with CO2

Although the formation of a stoichiometric carbon dioxide adduct of 4 could not be confirmed,

the potential still remained that 1, 4, or both could catalyze carbon dioxide reduction in the

presence of boranes. To test this possibility, a solution of complex 1 and ten equivalents of

HBcat in benzene-d6 was exposed to carbon dioxide (1.5 atmospheres), and the reaction was

monitored by NMR spectroscopy.

After mixing of 1 and HBcat but before addition of carbon dioxide, the 1H spectrum

demonstrated the complete consumption of 1 and the formation of 4 along with a new product

(4’). This new product displayed a hydride resonance as well as several aromatic resonances

consistent with the presence of an L- ligand. A new singlet also appeared in the 31P spectrum.

This product is distinct from the complexes reported in Section 3.1.

After the addition of carbon dioxide, a drastic colour change from dark red to light yellow

occurred, together with the formation of a small amount of brown precipitate. NMR revealed the

complete consumption of 4 within ten minutes. HBcat and 4’ were still present, along with a

second new product (4’’) that displayed a broad 1H hydride resonance and a singlet in the 31P

spectrum. A new singlet in the 1H spectrum at 4.47 ppm also appeared, which may be indicative

of dihydrogen being evolved in the reaction. The broad hydride resonance suggests the presence

of an (H2Bcat)- moiety coordinated to ruthenium in 4’’.

Hardly any change was observed after two hours at room temperature. However, heating the

reaction at 40 °C for 15 hours revealed an exciting result: all of the HBcat was consumed, and

the 1H, 11B, and 13C spectra clearly indicated the presence of H3COBcat and catBOBcat, the

expected products of carbon dioxide reduction. The only other major product remaining was 4’’,

which contains a symmetric L- backbone, equivalent PPh3 groups, and an (H2Bcat)- group

coordinated to ruthenium. It is likely a derivative of the known borohydride complexes 10 and

12. Scheme 44 summarizes this catalytic reaction.

55

Scheme 44: Catalytic reduction of carbon dioxide by 1 in the presence of HBcat to form

H3COBcat and catBOBcat.

Although this result is very preliminary, it confirms the ability of complexes 1 and/or 4 to act as

pre-catalysts or catalysts for the reduction of carbon dioxide to methoxycatecholborane. The

optimization and characterization of this novel catalytic system is currently underway.

56

4 Conclusion and Future Outlook

This thesis reported the syntheses of several ruthenium(II) complexes featuring borane-

derivatized 4,5-diazafluorenide ligands. Starting from the known ruthenium diazafluorenide

complex 1, reaction with 0.5 equivalents of ClBcat or one equivalent of HBcat afforded the

borane adduct 4 with a catecholboryl moiety appended at the 9-positon of the diazafluorenide

ligand. In the presence of excess borane, this complex is believed to convert to the borane-

borohydride complex 10. Furthermore, 4 was shown to react with benzene-d6 to form the

ruthenium(II) phenyl complex 9 via a proposed C–D bond activation mechanism. Complexes 4,

9, and 10 are also believed to react with dihydrogen, and a preliminary study of this reactivity

was reported.

Notably, 4 reacted with carbon dioxide to give a mixture of products, one of which may be the

product of CO2 insertion into the 9-position C–B bond of the diazafluorenide ligand.

Furthermore, complex 1 is a pre-catalyst for the reduction of carbon dioxide by HBcat, and

complex 4 was observed to form in situ during the catalysis. The reduction products were

MeOBcat and (catB)2O, the former being a direct precursor to methanol.

Given the exploratory nature of this work, there are still many aspects of these systems that

require further study. In particular, three short-term goals are isolation and full characterization

of complex 4, isolation and identification of the proposed carbon dioxide adduct of 4, and

optimization of the catalytic system involving complex 1.

The isolation of an analytically pure sample of 4 was precluded by the presence of trace

impurities that were difficult to remove by standard purification methods and by the reaction of 4

with the aromatic solvents used in the synthesis and NMR experiments. The former problem can

likely be overcome by careful and repeated crystallization of the crude reaction mixture. The

latter problem is harder to resolve, but one possible solution is to perform the synthesis of 4 in

proteo-benzene with the goal of fully converting 4 to the phenyl species

[Ru(C6H5)(L–Bcat)(N2)(PPh3)2]. Another possibility involves replacing the labile dinitrogen

ligand in 4 with a strongly bound ligand such as CO, which should prevent the reaction with

aromatic solvents from occurring.

57

Moving forward, our group is interested in expanding the borane-diazafluorenide chemistry

reported in this thesis. Preliminary results by a graduate student in our group indicate that

complex 1 also reacts with HBpin to form complexes analogous to the HBcat adducts reported in

this thesis, although the reactions occur rather slowly and display slightly different selectivity.

Although HBpin is less reactive than HBcat, it has the advantage of being less prone to

decomposition than HBcat. Attempts to synthesize the analogous 9-BBN and BH3 adducts of 1

are also underway.

Furthermore, a tetrahedral zinc nacnac diazafluorenide complex was recently synthesised by an

undergraduate student in our group. This complex has a fixed coordination sphere about the zinc

centre and does not contain any labile ligands, unlike the octahedral coordination sphere of

complexes 1 and 4 that is prone to isomerization and dissociation of the dinitrogen ligand. This

fixed coordination sphere should preclude any metal-based reactivity, thus simplifying the

system. This zinc complex has been shown to react with HBpin to form an adduct analogous to

complex 4. This system also exhibits catalytic activity for the reduction of carbon dioxide, and

efforts to characterize and optimize this system are currently underway.

58

References

1. Kargari, A.; Ravanchi, M. T. Carbon Dioxide: Capturing and Utilization; Ed. Liu, G.; InTech:

Rijeka, Croatia, 2012; pp 3-30.

2. Rees, N. V.; Compton, R. G. Energy Environ. Sci. 2011, 4, 403-408.

3. Phan, A.; Doonan, C. J.; Uribe-Romo, F. J.; Knobler, C. B.; O'Keeffe, M.; Yaghi, O. M. Acc.

Chem. Res. 2010, 43, 58-67.

4. Sabouni, R.; Kazemian, H.; Rohani, S. Environ. Sci. Pollut. Res. 2014, 21, 5427-5449.

5. Sakakura, T.; Choi, J.; Yasuda, H. Chem. Rev. 2007, 107, 2365-2387.

6. Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. Chem. Soc. Rev. 2009, 38, 89-99.

7. Koinuma, H.; Kawakami, F.; Kato, H.; Hirai, H. J. Chem. Soc., Chem. Commun. 1981, 213-

214.

8. Deglmann, P.; Ember, E.; Hofmann, P.; Pitter, S.; Walter, O. Chem. Eur. J. 2007, 13, 2864-

2879.

9. Jansen, A.; Goerls, H.; Pitter, S. Organometallics 2000, 19, 135-138.

10. Motokura, K.; Kashiwame, D.; Miyaji, A.; Baba, T. Org. Lett. 2012, 14, 2642-2645.

11. Eisenschmid, T. C.; Eisenberg, R. Organometallics 1989, 8, 1822-1824.

12. Riduan, S. N.; Zhang, Y.; Ying, J. Y. Angew. Chem. Int. Ed. 2009, 48, 3322-3325.

13. Berkefeld, A.; Piers, W. E.; Parvez, M. J. Am. Chem. Soc. 2010, 132, 10660-10661.

14. Khandelwal, M.; Wehmschulte, R. J. Angew. Chem. Int. Ed. 2012, 51, 7323-7326.

15. Berkefeld, A.; Piers, W. E.; Parvez, M.; Castro, L.; Maron, L.; Eisenstein, O. Chem. Sci.

2013, 4, 2152-2162.

16. Mitton, S. J.; Turculet, L. Chem. Eur. J. 2012, 18, 15258-15262.

59

17. Park, S.; Bezier, D.; Brookhart, M. J. Am. Chem. Soc. 2012, 134, 11404-11407.

18. Matsuo, T.; Kawaguchi, H. J. Am. Chem. Soc. 2006, 128, 12362-12363.

19. Shintani, R.; Nozaki, K. Organometallics 2013, 32, 2459-2462.

20. Bontemps, S.; Vendier, L.; Sabo-Etienne, S. Angew. Chem. Int. Ed. 2012, 51, 1671-1674.

21. Bontemps, S.; Vendier, L.; Sabo-Etienne, S. J. Am. Chem. Soc. 2014, 136, 4419-4425.

22. Chakraborty, S.; Zhang, J.; Krause, J. A.; Guan, H. J. Am. Chem. Soc. 2010, 132, 8872-8873.

23. Chakraborty, S.; Patel, Y. J.; Krause, J. A.; Guan, H. Polyhedron 2012, 32, 30-34.

24. Chakraborty, S.; Zhang, J.; Patel, Y. J.; Krause, J. A.; Guan, H. Inorg. Chem. 2013, 52,

37-47.

25. Sgro, M. J.; Stephan, D. W. Angew. Chem. Int. Ed. 2012, 51, 11343-11345.

26. Courtemanche, M.; Legare, M.; Maron, L.; Fontaine, F. J. Am. Chem. Soc. 2013, 135,

9326-9329.

27. Wang, T.; Stephan, D. W. Chem. Eur. J. 2014, 20, 3036-3039.

28. Das Neves Gomes, C.; Blondiaux, E.; Thuery, P.; Cantat, T. Chem. Eur. J. 2014, 20,

7098-7106.

29. Legare, M.; Courtemanche, M.; Fontaine, F. Chem. Commun. 2014, 50, 11362-11365.

30. Dobrovetsky, R.; Stephan, D. W. Catalytic Angew. Chem. Int. Ed. 2013, 52, 2516-2519.

31. Kleeberg, C.; Cheung, M. S.; Lin, Z.; Marder, T. B. J. Am. Chem. Soc. 2011, 133,

19060-19063.

32. Gu, L.; Zhang, Y. J. Am. Chem. Soc. 2010, 132, 914-915.

33. Laitar, D. S.; Mueller, P.; Sadighi, J. P. J. Am. Chem. Soc. 2005, 127, 17196-17197.

34. Annibale, V. T.; Song, D. RSC Adv. 2013, 3, 11432-11449.

60

35. Langer, R.; Diskin-Posner, Y.; Leitus, G.; Shimon, L. J. W.; Ben-David, Y.; Milstein, D.

Angew. Chem. Int. Ed. 2011, 50, 9948-9952.

36. Balaraman, E.; Ben-David, Y.; Milstein, D. Angew. Chem. Int. Ed. 2011, 50, 11702-11705.

37. Balaraman, E.; Gunanathan, C.; Zhang, J.; Shimon, L. J. W.; Milstein, D. Nat. Chem. 2011,

3, 609-614.

38. Vogt, M.; Gargir, M.; Iron, M. A.; Diskin-Posner, Y.; Ben-David, Y.; Milstein, D. Chem.

Eur. J. 2012, 18, 9194-9197.

39. Huff, C. A.; Kampf, J. W.; Sanford, M. S. Organometallics 2012, 31, 4643-4645.

40. Vogt, M.; Rivada-Wheelaghan, O.; Iron, M. A.; Leitus, G.; Diskin-Posner, Y.; Shimon, L. J.

W.; Ben-David, Y.; Milstein, D. Organometallics 2013, 32, 300-308.

41. Chapman, A. M.; Haddow, M. F.; Wass, D. F. J. Am. Chem. Soc. 2011, 133, 18463-18478.

42. Sgro, M. J.; Stephan, D. W. Chem. Commun. 2013, 49, 2610-2612.

43. LeBlanc, F. A.; Berkefeld, A.; Piers, W. E.; Parvez, M. Organometallics 2012, 31, 810-818.

44. Stepowska, E.; Jiang, H.; Song, D. Chem. Commun. 2010, 46, 556-558.

45. Annibale, V. T.; Song, D. Chem. Commun. 2012, 48, 5416-5418.

46. Annibale, V. T.; Dalessandro, D. A.; Song, D. J. Am. Chem. Soc. 2013, 135, 16175-16183.

47. Jiang, H.; Song, D. Organometallics 2008, 27, 3587-3592.

48. Plater, M. J.; Kemp, S.; Lattmann, E. J. Chem. Soc. Perkin Trans. 1 2000, 971-979.

49. Janik, J. F.; Narula, C. K.; Gulliver, E. G.; Duesler, E. N.; Paine, R. T. Inorg. Chem. 1988,

27, 1222-1227.

50. Westcott, S. A.; Blom, H. P.; Marder, T. B.; Baker, R. T.; Calabrese, J. C. Inorg. Chem.

1993, 32, 2175-2182.

51. Pasdeloup, M.; Brisson, C. Org. Magn. Reson. 1981, 16, 164-167.

61

52. Anaby, A.; Butschke, B.; Ben-David, Y.; Shimon, L. J. W.; Leitus, G.; Feller, M.; Milstein,

D. Organometallics 2014, 33, 3716-3726.

53. Knizek, J.; Noeth, H. Eur. J. Inorg. Chem. 2011, 1888-1900.

54. Molitor, S.; Gessner, V. H. Chem. Eur. J. 2013, 19, 11858-11862.

55. Stepowska, E. Ruthenium 4,5-diazafluorene complexes and their reactivity. M.Sc. Thesis,

University of Toronto, November 2009.

56. Jessop, P. G.; Morris, R. H. Coord. Chem. Rev. 1992, 121, 155-284.

57. Prechtl, M. H. G.; Hoelscher, M.; Ben-David, Y.; Theyssen, N.; Loschen, R.; Milstein, D.;

Leitner, W. Angew. Chem. Int. Ed. 2007, 46, 2269-2272.

58. Ohki, Y.; Hatanaka, T.; Tatsumi, K. J. Am. Chem. Soc. 2008, 130, 17174-17186.

59. Lachaize, S.; Essalah, K.; Montiel-Palma, V.; Vendier, L.; Chaudret, B.; Barthelat, J.; Sabo-

Etienne, S. Organometallics 2005, 24, 2935-2943.

60. Jiang, H.; Stepowska, E.; Song, D. Dalton Trans. 2008, 5879-5881.

61. Schlecht, S.; Hartwig, J. F. J. Am. Chem. Soc. 2000, 122, 9435-9443.