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Parallels between Metal-Ligand Cooperativity and Frustrated Lewis Pairs

Habraken ERM Jupp AR Brands MB Nieger M Ehlers AW Slootweg JCDOI101002ejic201900169Publication date2019Document VersionFinal published versionPublished inEuropean Journal of Inorganic ChemistryLicenseCC BY-NC-ND

Link to publication

Citation for published version (APA)Habraken E R M Jupp A R Brands M B Nieger M Ehlers A W amp Slootweg J C(2019) Parallels between Metal-Ligand Cooperativity and Frustrated Lewis Pairs EuropeanJournal of Inorganic Chemistry 2019(19) 2436-2442 httpsdoiorg101002ejic201900169

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Download date30 Aug 2021

DOI 101002ejic201900169 Full Paper

Lewis Acids and Lewis Bases

Parallels between Metal-Ligand Cooperativity and FrustratedLewis PairsEvi R M Habraken[a] Andrew R Jupp[a] Maria B Brands[a] Martin Nieger[b]

Andreas W Ehlers[ac] and J Chris Slootweg[a]

Abstract Metal ligand cooperativity (MLC) and frustratedLewis pair (FLP) chemistry both feature the cooperative actionof a Lewis acidic and a Lewis basic site on a substrate A lot ofwork has been carried out in the field of FLPs to prevent Lewis

IntroductionOver the past decades catalysis has been dominated by transi-tion metal complexes The partially filled d-orbitals grant themetal centre both donor and acceptor orbitals on a single atomand allow prototypical transition metal reactivity such as oxid-ative addition of dihydrogen shown in Scheme 1i which in-volves an increase on the formal oxidation state of the metalby +2 In these cases the surrounding ligands are crucial fortuning the electronic and steric properties of the metal centrebut they are not directly involved in the reactions Separatingthe donor and acceptor site has led to new reaction pathwaysfor catalysis This reactivity occurs when the ligand actively par-ticipates in substrate activation together with the metal centreand has been termed bifunctionality or metal-ligand coopera-tivity (MLC) shown in Scheme 1ii Noyori first demonstratedthis concept with a ruthenium-phosphine complex bearing anethylenediamine ligand where the amine functionality cooper-ates with the metal[1] In these reactions the formal oxidationstate of the metal is unchanged on activation of the substrateThis topic has grown rapidly and been reviewed on many occa-sions[2] and has important ramifications for catalyst design

Another form of cooperation can be found in transitionmetal-free frustrated Lewis pairs (FLPs) where the acceptor anddonor site are also on separate sites[3] Lewis acids and basestypically form Lewis adducts however incorporation of bulky

[a] Van t Hoff Institute for Molecular Sciences University of AmsterdamScience Park 904 1090 GD Amsterdam The NetherlandsE-mail jcslootweguvanl

[b] Department of Chemistry University of HelsinkiA I Virtasen aukio 1 PO Box 55 Helsinki Finland

[c] Department of Chemistry Science Faculty University of JohannesburgPO Box 254 Auckland Park Johannesburg South AfricaSupporting information and ORCID(s) from the author(s) for this article areavailable on the WWW under httpsdoiorg101002ejic201900169copy 2019 The Authors Published by Wiley-VCH Verlag GmbH amp Co KGaA middotThis is an open access article under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivs License which permits use and distri-bution in any medium provided the original work is properly cited the useis non-commercial and no modifications or adaptations are made

Eur J Inorg Chem 2019 2436ndash2442 copy 2019 The Authors Published by Wiley-VCH Verlag GmbH amp Co KGaA Weinheim2436

adduct formation which often reduces the FLP reactivity Paral-lels are drawn between the two systems by looking at theirreactivity with CO2 and we explore the role of steric bulk inpreventing dimer formation in MLC systems

Scheme 1 Differing modes of dihydrogen activation by transition metal com-plexes and frustrated Lewis pairs (M = transition metal A = Lewis acidic siteB = Lewis basic site L = ligand)

groups on the donor andor acceptor sites can induce frustra-tion and prevent adduct formation The unquenched reactivityof the Lewis acid and base has been exploited for the activationof small molecules such as H2 and CO2 and for the subsequentcatalytic hydrogenation of unsaturated substrates[4] The Lewisacid and base can be tethered to afford an intramolecular FLP(Scheme 1iii) which allows for preorganization of the reactivesite and can reduce the (entropic) energy barrier for such acti-vation reactions[56] The interplay between the electronic andsteric properties of the Lewis acids and bases is of paramountimportance in determining the activity of the FLP system

The fields of FLP and MLC chemistry have both grown rapidlyand in general separately However it is clear that the underly-ing cooperativity for the activation of substrates is similar inboth cases The distinction was further blurred by the adventof transition metal-based FLPs where a transition metal centreis used as one of the Lewis acidic or basic sites in an FLP[7]

Wass and co-workers introduced a cationic zirconocene-phos-phinoaryloxide complex with the zirconium centre acting as aLewis acid and a pendant phosphine acting as a Lewis basefor the activation of dihydrogen (Scheme 2i)[8] Just as with

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traditional main-group FLPs the balance of sterics and electron-ics is important as simply switching the [C5Me5]ndash (Cp) ligandsfor [C5H5]ndash (Cp) resulted in a strong ZrndashP interaction and shutdown the FLP reactivity This reactivity could be equally welldescribed as FLP or MLC chemistry and Wass noted this insightin subsequent articles[9a9b] The analogy has also been notedelsewhere especially with the transition metal-based FLPs[910]

and recently Bullock and co-workers cited guiding principlesfrom main-group and transition metal-based FLPs in the designof bifunctional Mo complexes for the controlled heterolyticcleavage of dihydrogen (Scheme 2ii)[11] Herein we further ex-plore the relationships between archetypal MLC and FLP sys-tems and in particular investigate the dimerization of the activeMLC-monomer by Lewis adduct formation and to consolidatethe knowledge garnered from the two topics

Scheme 2 Transition metal-based FLP reactivity andor MLC reactivity activa-tion of dihydrogen by i) Wasss Zr complex[8] and ii) Bullocks Mo complex[11]

Blue Lewis acid red Lewis base

Results and DiscussionWe chose to investigate the quintessential Ru-based PNP pincersystems developed by Milstein as it is well established thatthese species can undergo an MLC pathway via dearomatiza-tionrearomatization of the pyridine ring[2a] Treatment of theprecursors 1 and 2 which differ by the R group on the phos-phine with base leads to deprotonation of one of the methyl-ene arms and loss of chloride to afford 3 and 4 respectively(Figure 1)[1213] These compounds feature a Lewis basic site onthe carbon and a Lewis acidic site on the Ru centre This notionwas confirmed by our DFT calculations of the frontier molecularorbitals of 3 at the ωB97X-D6-311G(dp) level of theory whichshowed that the highest occupied molecular orbital (HOMO) isprincipally located on the deprotonated carbon and the lowestunoccupied molecular orbital (LUMO) is centred on the ruth-enium In this case the ldquofrustrationrdquo of the Lewis acidic and basicsites is enforced by the rigid ligand framework Otten and co-workers have previously demonstrated the FLP-like reactivity ofa related Ru-based PNN system with nitriles[14] in which theRuC combination added in a cooperative fashion across theCN triple bond Milstein has also noted that the cooperativeaddition of CO2 across these pincer systems bears a resem-blance to FLPs[1015]

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Figure 1 Top Milstein system activation of precursor by deprotonation witha base resulting in the dearomatized species Bottom molecular orbital dia-gram of MLC 3 (isopropyl groups omitted for clarity) and FLP 5 (left HOMOright LUMO) calculated at the ωB97X-D6-311G(dp) level of theory BlueLewis acid red Lewis base

To compare this traditional MLC system with a main-groupFLP we opted to study the intramolecular FLP 5 (Figure 1) Theacidic and basic components are preorganised by the methyl-ene bridge in the ideal orientation to activate a range of smallmolecule substrates including dihydrogen carbon dioxide andisocyanates despite the lack of strong electron-withdrawinggroups on the boron centre[6] The HOMO is the lone pair onphosphorus and the LUMO is predominantly the formally va-cant p orbital on boron The parallels between the orbitals of 3and 5 should bear out in their reactivity so we resorted to DFTcalculations to provide detailed mechanistic insight into themode of activation of carbon dioxide of the two systems

Milstein and co-workers already partially elaborated on theactivation of CO2 for 4[16] which we extended to 3 to investi-gate the influence of the steric bulk and this was compared tothe geminal FLP system 5 (Figure 2) The latter was also investi-gated in the original publication but at a different level of the-ory so all calculations herein were carried out using ωB97X-D6-311G(dp) for ease and relevance of comparison Pertinentbond metric data are included in Table 1 including the bondlengths between the CO2 and the Lewis acidic and basic sitesas well as the bond lengths and angle within the CO2 moietyFor both MLC systems first a van der Waals complex is formedwith long distances between the MLC and CO2 and the CO2

moiety has barely deviated from linearity The complex is ener-getically favourable but the ΔG values are slightly uphill dueto a decrease in entropy This initial complexation is followedby a nucleophilic attack by the ligand-based carbon to thecarbon of CO2 in an asynchronous concerted transition state (3ΔGDagger = 33 4 ΔGDagger = 38 kcal molndash1) In both cases the RundashO andCndashC bonds are still relatively long indicating an early transitionstate Ring closure affords the product with an overall energy

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difference of ΔG = 121 and 104 kcal molndash1 for 3 and 4 respec-tively There is little energetic difference between the isopropylor tert-butyl groups during the reaction profile and the bondlengths (largest difference 003 Aring) and angles (largest difference06deg) are similar in all cases

Figure 2 Relative ωB97X-D6-311G(dp) Gibbs free energies (energy in brack-ets) in kcal molndash1 for the reaction of CO2 and 34

Table 1 Computed bond metric data for the van der Waals complexes (vdW)transition states (TS) and products (P) during the reactions of CO2 with 3 4and 5[a]

LAndashO1 [Aring] LBndashCCO2 [Aring] CndashO1 [Aring] CndashO2 [Aring] OndashCndashO [deg]

vdW 3 255 314 116 115 17594 256 316 116 115 17605 363 346 116 116 1776

TS 3 243 243 118 116 15894 245 242 118 116 15835 300 226 120 119 1478

P 3 226 159 127 122 12934 229 159 127 122 12915 157 188 128 121 1288

[a] LA = Lewis acid (Ru in 3 4 B in 5) LB = Lewis base (C in 3 4 P in 5)

The reaction profile for 5 is similar (Figure 3) First a van derWaals complex is formed with long distances between theFLP and CO2 with an almost linear CO2 The reaction proceedsvia an asynchronous concerted transition state (ΔGDagger =125 kcal molndash1) where the Lewis basic phosphorus centre at-tacks the electrophilic carbon of CO2 and the O1 is stabilisedby interaction with the boron centre This is evidenced by theslightly longer CndashO1 and CndashO2 bond lengths and the smallerbond OndashCndashO bond angle in TS-5 than the analogous metricsin TS-3 and TS-4 and is in good agreement with the previouslyreported bond order data for TS-5[6] The final product isformed by ring closure which is exergonic by 54 kcal molndash1 Inboth the MLC and FLP systems the mechanism is the same anddifferences in energies and bond metrics are readily explainedby the varying electronic nature of the Lewis acidic and basicsites in the molecules

Intramolecular FLPs can quench either via an intramolecularinteraction or dimerization although ldquoquenchrdquo in this case isperhaps a misnomer as it is now well established that completefrustration is not necessary for typical FLP reactivity to occurStephan and co-workers recently showed that even the classicaladduct of B(C6F5)3 and the strongly basic proazaphosphatrane

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Figure 3 Relative ωB97X-D6-311G(dp) Gibbs free energies (energy in brack-ets) in kcal molndash1 for the reaction of CO2 and 5

P[N(Me)CH2CH2]3N is capable of addition to a range of hetero-allenes[17] In any case the Lewis adduct is a resting state disso-ciation into the corresponding Lewis acidbase componentswith the accompanying energy penalty is required to induceFLP reactivity Therefore the control of reactivity via fine-tuningof the steric environment is still a widely employed strategy inFLP chemistry In a similar manner MLC systems are able todimerize via an intermolecular interaction of the Lewis acidicand Lewis basic sites in the complex For example the dearoma-tized Ru-PNS system dimerizes as shown in Figure 4 and subse-quently undergoes a decomposition pathway involving C-Scleavage and loss of isobutene[18]

Figure 4 Two examples of dimeric MLC complexes Left molecular X-raystructure of [3]2 (displacement ellipsoids are set at 50 probability isopropylgroups on the phosphorus are omitted for clarity)[12] Selected bond lengths[Aring] Ru1ndashC7prime 2409(7) Ru1primendashC7 2403(7) P2ndashC7 1797(6) C6ndashC7 1456(8)C1ndashC2 1489(9) C1ndashP1 1842(6) P2primendashC7prime 1803(6) C6primendashC7prime 1449(8) C1primendashC2prime155(1) C1primendashP1prime 1843(6) Right Milsteins Ru(PNS) dimer A[18] Blue Lewisacid red Lewis base

On examination of the crystal structure of 3 as reported inMilsteins original publication[12] we noted that this species isalso a dimer in the solid state The rutheniumndashcarbon inter-atomic distance between the two monomers in the X-ray struc-ture [Ru1ndashC7prime 2409(7) and Ru1primendashC7 2403(7) Aring] lies within thesum of the van der Waals radii (375 Aring)[19] suggesting a bondinginteraction The C6ndashC7 bond lengths (according to the atomlabelling in Figure 1) in [3]2 are 1456(8) and 1449(8) Aring Thesebond lengths are much longer than that found in the gas-phaseDFT optimized monomer 3 (1379 Aring) but shorter than thatfound in the X-ray structure of unactivated complex 1 [1501(2)Aring] which suggests that the deprotonated arm features a CndashCbond with partial double bond character In fact the C6ndashC7bond length is similar to that found in A [1458(4) Aring Fig-ure 4][18]

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To probe the structural changes that occur during dimeriza-tion we examined the aromaticity of the pyridine ring of thecompounds using NICS(0) calculations[2021] As expected theunactivated precursors feature an aromatic ring (1 ndash64 ppm2 ndash65 ppm) whereas in the activated species dearomatizationhas occurred (3 20 ppm 4 13 ppm) These values are consist-ent with previous studies by Gonccedilalves and Huang on similarorganometallic pincer complexes[2223] Interestingly the dimer[3]2 has a value (ndash47 ppm) between that of 1 and 3 indicatingpartial rearomatization of the pyridine ring and a contributingfactor to the stability of the dimer

The bonding situation in [3]2 was further analysed using AIManalyses[2425] which revealed a bond critical point (BCP) be-tween the Lewis acidic Ru site of one monomer and the Lewisbasic C7 site of the other monomer [Figure 5 ρ = 0047 au( = 021) RundashC7 2499 Aring] indicative of a weak interaction Fur-thermore a ring critical point (RCP) is found in the dimer be-tween the two monomers The examination of the Laplacian ofthe electron densities (nabla2ρ) in the C6ndashC7 bond reveals a weakerinteraction for the dimer than the monomer yet still strongerthan for 1 ([3]2 ndash066 au 3 ndash083 au 1 ndash058 au) ETS-NOCV[26] analyses of the dimer which we have used to assessdonorndashacceptor interactions concur with these observationsand revealed an interaction between ruthenium and the carbonin the backbone of both monomers showing orbital interac-tions and specifically σ donations of 244 and 207 kcal molndash1

from C7 to Ru

Figure 5 Computed AIM bond paths of [3]2 a simplified framework is de-picted (isopropyl groups on P and all H atoms omitted for clarity) bondcritical points (BCP) in red ring critical points (RCP) in green

The monomeric pincer complexes are the active species incatalysis therefore the dimer must first be broken before it canreact (Figure 6) DFT calculations at the ωB97X-D6-311G(dp)level of theory reveal it costs ΔG = 154 kcal molndash1 (ΔE =297 kcal molndash1) to break up dimer [3]2 (Table 2 all values givenper monomer) To give a better reflection of the thermodynam-ics of this equilibrium in solution we augmented our computa-tional method by including implicit solvation effects (benzeneand THF) As expected this lowers the amount of energy re-quired to break the dimer the more polar solvent THF does thisto a greater extent (benzene ΔG = 136 kcal molndash1 THF ΔG =124 kcal molndash1) Explicit solvent interactions are also importantespecially for monomeric species such as 3 where the Ru centrehas a vacant coordination site that can be stabilised by solvent

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Including one benzene molecule per monomer in the calcula-tions lowered the energy required very slightly (ΔG =128 kcal molndash1) as benzene only weakly coordinates to the Rucentre in an η2 fashion Once again the more coordinating THFstabilises the monomer to a greater extent making it easierto cleave the dimer (ΔG = 55 kcal molndash1) we anticipate thatcoordinating substrates will have a similar effect

Figure 6 Top cleavage of dimer [3]2 into monomer 3 Bottom optimizedstructures of 3THF (left oxygen coordinates to ruthenium) and 3C6H6 (rightcoordination in an η2 fashion to ruthenium)

Table 2 Energy (ΔE) and Gibbs free energy (ΔG) required to break dimer [3]2

in kcal molndash1 (all values given per monomer)

ΔG [kcal molndash1] ΔE [kcal molndash1]

No solvent added 154 297Implicit THF 124 264Implicit benzene 136 281Explicit THF added implicit THF 55 141Explicit benzene added implicit Benzene 128 226

Interestingly and reminiscent of the tenets of FLP chemistryincreasing the steric bulk of the alkyl substituents on phos-phorus from isopropyl to tert-butyl (ie going from 3 to 4) de-stabilises these Lewis acidLewis base interactions and pre-cludes dimer formation It was not possible to locate a mini-mum on the potential energy surface corresponding to thestructure of [4]2 and all attempts led to regeneration of thetwo monomers during the optimization process

To corroborate these insights on the dimerization of 3 and 4in different solvents we analysed the diagnostic 1H NMR chemi-cal shift of the Ru-bound hydride both computationally andexperimentally (Table 3) The computed shift for monomer 3 isapproximately ndash20 ppm while the corresponding shift for [3]2

is relatively deshielded and is computed to be approximatelyndash10 ppm with little dependence on the identity of the solventExperimentally the hydride in benzene solutions of 3 wasfound to resonate at δ ndash1304 ppm[12] while in THF it is at δndash2005 ppm The latter is a very good match with the predictedmonomeric structure while the former is closer to the dimericspecies and suggests the existence of a monomerdimer equi-librium These data follow the trends predicted by the computa-

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tions above in that the quenching of the MLC is more likely tooccur in less coordinating solvents such as benzene These dataare further supported by the fact that the analogous hydride inA (Figure 4) which is known to rapidly dimerise resonates rela-tively downfield at δ ndash1183 ppm in the non-coordinating sol-vent CD2Cl2[18]

Table 3 Experimental and computational data of the hydride shift of variouscompounds

Compound Experimental Computational data [ppm][a]

data [ppm] THF[b] Benzene[b]

3 ndash2023 ndash19903 in [D8]THF ndash20053 in C6D6 ndash1304[3]2

[c] ndash1016 ndash1019

4 ndash1858 ndash18364 in [D8]THF ndash26174 in C6D6 ndash2578[4]2 na na

A in CD2Cl2[a] ndash1183

[a] Calculations were performed using ωB97X-D6-311G(dp) Ru Def2TZVPlevel of theory [b] Calculated using implicit solvent interactions na = notapplicable as the dimeric structure could not be obtained computationally[c] Both hydrides have the same shift (ndash1016 or ndash1019)

The notion that the difference in the experimental chemicalshift of the hydride of 3 in THF and benzene is related to dimer-ization is reinforced by the fact that the analogous experimentalvalues for 4 a system where dimerization is not possible arevery similar in the two solvents (benzene δ ndash2578 ppm THFδ ndash2617 ppm implicit solvent added) It should be noted thatthe computed values in this case are not very accurate as theypredict the resonance at approximately ndash185 ppm dependingon the solvent and thus caution is advised in analysing theclose correlation between the computed and observed valuesfor 3 in THF above However the fact that the experimentalvalues of 3 are significantly different in the two solvents whilethe corresponding values for 4 are almost identical is evidencefor the presence of monomerdimer equilibrium effects

Finally we wanted to show that consideration of steric bulkis important for regulating the quenching of the Lewis acidLewis base components in other organometallic pincer systemsKirchner[27] and Huang[28] have replaced the methylene bridgesin Milsteins PNP pincer system with the more acidic NH moiety(Figure 7)[29] Calculations at the ωB97X-D6-311G(dp) level oftheory (with no solvent modelled) reveal a dimer is feasible for[(iPrPNNNP)RuH(CO)] and it costs ΔG = 113 kcal molndash1 to breakup the dimer (per monomer) which is slightly lower than forthe Milstein system Once again for the tert-butyl complex[(tBuPNNNP)RuH(CO)] the added steric protection around theacidic and basic sites prevents dimerization However the al-

Figure 7 Representation of the activated [(R-PNNNP)RuH(CO)] complexR = iPr or tBu Blue Lewis acid red Lewis base

Eur J Inorg Chem 2019 2436ndash2442 wwweurjicorg copy 2019 The Authors Published by Wiley-VCH Verlag GmbH amp Co KGaA Weinheim2440

tered electronics of the Lewis basic site in these complexescompared to 3 and 4 have drastic consequences as the activa-tion of CO2 is no longer feasible ΔG = 153 and ΔG =237 kcal molndash1 for [(iPrPNNNP)RuH(CO)] and [(tBuPNNNP)RuH(CO)]respectively

Conclusions

We have shown that FLP and MLC chemistry both involve thecooperative action of a Lewis acid and a Lewis base and thatsteric control to prevent quenching of the reactive sites is justas important in both paradigms There are many reactions inthe literature that have been given one label or the other on afairly arbitrary basis and we believe both schools of thoughtshould be united so that lessons from one field can be used tobenefit the other ndash whether that is using principles and reac-tions from main-group FLPs to broaden the scope of MLC reac-tivity or using the wealth of knowledge on ligand design andproperties to rationally construct new backbones for preorga-nised intramolecular main-group FLPs

Experimental SectionAll manipulations regarding the preparation of air-sensitive com-pounds were carried out under an atmosphere of dry nitrogen us-ing standard Schlenk and drybox techniques Solvents were puri-fied dried and degassed according to standard procedures 1H NMRspectra were recorded on a Bruker AV 400 or on a Bruker AV300-lland internally referenced to the residual solvent resonances([D8]THF 1H δ 358 172 ppm C6D6 1H δ 716 ppm [D8]Tol 1H δ208 697 701 709 ppm) 31P1H NMR spectra were recorded ona Bruker AV 400 or on a Bruker AV300-ll and externally referenced(85 H3PO4) Chemical shifts are reported in ppm High resolutionmass spectra were recorded on a Bruker MicroTOF with ESI nebu-lizer (ESI) at ndash45 degC

Synthesis of Diisopropylphosphine Diisopropylphosphine wasprepared according to a modified literature procedure of A S Glod-man et al[30] A solution of ClPiPr2 (492 g 0032 mol 10 equiv)diethyl ether (55 mL) was added dropwise to a slurry of LiAlH4

(037 g 001 mol 03 equiv) in diethyl ether (30 mL) in an icewater bath The mixture was stirred overnight and conversion waschecked by 31P1H NMR Degassed H2O (20 mL) was added slowlyand the organic layer was dried with MgSO4 The water layer wasextracted with diethyl ether (3 times 15 mL) and dried with the sameMgSO4 The MgSO4 was filtered off (with a cannula filter) and rinsedwith diethyl ether (3 times 15 mL) All volatiles were removed in vacuowhile the Schlenk vessel was cooled in an icewater bath to afforddiisopropylphosphine as a colourless clear liquid in 81 (308 g0026 mol) If some phosphine was oxidized a Schlenk to Schlenkdistillation was performed Note the presence of some diethyl etherdoes not influence the next step 1H NMR (4001 MHz C6D6 291 K)δ = 293 (dt 1JHP = 1923 Hz 3JHH = 59 Hz 1H PH) 177 (m 2HCH(CH3)) 101 (m 12H CH(CH3)) 31P1H NMR (1620 MHz C6D6295 K) δ = ndash165 (s)

Lithiation of Diisopropylphosphine Lithium diisopropylphos-phide was prepared according to a modified literature procedure ofA Jansen and S Pitter[31] n-Butyllithium (16 M in hexanes 52 mL8347 mmol 14 equiv) was added dropwise to a solution of diiso-propylphosphine (7045 mg 5962 mmol 10 equiv) in n-pentane

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(15 mL) at 0 degC with a glass stirring bean and stirred for an addi-tional 30 min after which the solution was warmed to room tem-perature The resulting colourlesspale yellow solution was stirredfor 16 hours during which an off-white solid precipitated The solidswere collected by filtration subsequently washed with n-pentane(2 times 15 mL) and the solvents evaporated to dryness to give lithiumdiisopropylphosphide as an off-white solid in 76 (5627 mg4535 mmol) 1H NMR (4001 MHz [D8]THF 297 K) δ = 225 (dsept2JHP = 68 Hz 3JHH = 47 Hz 2H CH(CH3)2) 107 (dd 2JHP = 113 Hz3JHH = 68 Hz 12H CH(CH3)2) 7Li NMR (1555 MHz [D8]THF 297 K)13 (s) 13C NMR (1006 MHz [D8]THF 297 K) δ = 2693 (d 3JCP =142 Hz CH(CH3)2) 234 (d 2JCP = 253 Hz CH(CH3)2) 31P1H NMR(1620 MHz [D8]THF 297 K) δ = 15 (s)

Preparation of 26-Bis(diisopropylphosphino-methyl)-pyridine26-Bis(diisopropylphosphino-methyl)-pyridine was prepared ac-cording to a modified literature procedure of A Jansen andS Pitter[31] A solution of 26-bis(chloromethyl)pyridine (026 g1477 mmol 10 equiv) in THF (20 mL) as added slowly to a solutionof the lithiated phosphine (040 g 3223 mmol 22 equiv) using aglass stirring bean in THF (40 mL) at ndash78 degC and was stirred for anadditional 15 min at the same temperature During the additionthe solution changed colour from yellow to orange and subse-quently was warmed to room temperature in 16 h Addition of de-gassed water (02 mL) resulted in a colour change to yellow andthe mixture was dried with Na2SO4 The solution was filtered andthe Na2SO4 was washed with THF (3 times 4 mL) The combined solu-tion was dried in vacuo extracted with pentane (3 times 5 mL) and thesolvents evaporated to dryness to give iPrPNP as a yellow oil in88 (044 mg 1296 mmol 95 pure) Note some remainingdiisopropylphosphine can be observed 1H NMR (4001 MHz C6D6298 K) δ = 713ndash704 (m 1H p-PyH) 702ndash695 (m 2H m-PyH) 297(d 2JHP = 15 Hz 4H CH2) 171 (dsept 2JHP = 71 17 Hz 4HCH(CH3)2) 105 (m 24H CH(CH3)2) 31P1H NMR (1620 MHz C6D6297 K) δ = 114 (s product 95 ) ndash121 (s impurity presumablyiPr2PH 5 )

Synthesis of 1 [(iPrPNP)RuHCl(CO)] was prepared according to aliterature procedure[32] X-ray quality crystals were grown at ndash20 degCfrom a saturated solution of [(iPrPNP)RuHCl(CO)] in THF layeredwith n-pentane

Synthesis of 2 [(tBuPNP)RuHCl(CO)] was prepared according to aliterature procedure[13]

Synthesis of 3 [(iPrPNP)RuH(CO)] was prepared according to aslightly modified literature procedure[12] To a solution of complex[(iPrPNP)RuHCl(CO)] 1 (50 mg 0099 mmol) in THF (5 mL) was addedKOtBu (111 mg 0099 mmol) at ndash30 to ndash35 degC Subsequently themixture was stirred at room temperature for 4 h then filtered Theorange filtrate was dried under vacuum and washed with n-pentane(3 times 3 mL) and dried under vacuum to afford a yellowish powderin 56 yield (26 mg 0055 mmol)

Synthesis of 4 [(tBuPNP)RuH(CO)] was prepared according to aliterature procedure[13]

X-ray Crystal Structure Determination The single-crystal X-raydiffraction study (see Figure 8) was carried out on a Bruker D8 Ven-ture diffractometer with Photon100 detector at 123(2) K usingMo-Kα radiation (l = 071073 Aring) Dual space (intrinsic) methods(SHELXT)[33] were used for structure solution and refinement wascarried out using SHELXL-2014 (full-matrix least-squares on F2)[34]

Hydrogen atoms were localized by difference electron density de-termination and refined using a riding model [H(Ru) free] A semi-empirical absorption correction and an extinction correction wereapplied

Eur J Inorg Chem 2019 2436ndash2442 wwweurjicorg copy 2019 The Authors Published by Wiley-VCH Verlag GmbH amp Co KGaA Weinheim2441

Figure 8 Molecular structure of 1 [(iPrPNP)RuHCl(CO)] (displacement ellip-soids are set at 50 probability hydrogen atoms omitted for clarity) Se-lected bond lengths [Aring] RundashP1 23221(4) RundashP2 23061(4) P1ndashC1 18415(14)P2ndashC7 18435(15) C1ndashC2 15051(19) C6ndashC7 1501(2)

1 Colourless blocks C20H36ClNOP2Ru Mr = 50496 crystal size024 times 018 times 010 mm monoclinic space group P21c (No 14) a =137461(6) Aring b = 117625(5) Aring c = 144290(7) Aring = 95022(2)deg V =232405(18) Aring3 Z = 4 ρ = 1443 Mgmndash3 μ(Mo-Kα) = 094 mmndash1F(000) = 1048 2θmax = 552deg 50402 reflections of which 5365 wereindependent (Rint = 0034) 239 parameters R1 = 0019 [for 4830I gt 2σ(I)] wR2 = 0044 (all data) S = 106 largest diff peakhole =035ndash049 e Aringndash3

CCDC 1884053 (for 1) contains the supplementary crystallographicdata for this paper These data can be obtained free of charge fromThe Cambridge Crystallographic Data Centre

Computational Details Density functional calculations were per-formed at the ωB97X-D[35] level of theory using Gaussian09 revisionD01[36] Geometry optimizations were performed using the 6-311G(dp)[37] basis set for atoms C H N O P Cl in combinationwith the Def2TZVP[38] basis set for Ru ZPE and Gibbs free energies(Gdeg) were obtained from frequency analyses performed at the samelevel of theory Calculations of large dimer systems with solvation(SCRF THF or benzene) were obtained from frequency analysesafter optimization without solvation The NICS[39] analyses was per-formed at the ωB97X-D6-311G(dp) Ru Def2TZVPB3LYP[40]6-311G(dp) Ru Def2TZVP level of theory The AIM[41] and ETS-NOCV[42] analyses were performed at the ZORA-BP86TZ2P[42] levelof theory using ADF2016102[42] Structures were optimized usingthe same functional and basis set prior to the analysis using ZORA-BP86TZ2P NMR calculations were performed at ωB97X-D6-311G(dp) Ru Def2TZVP level of theory using solvation (THF orbenzene) and are corrected for the TMS value[3943] and obtainedwith the Gauge-Independent Atomic Orbital (GIAO)[44]

AcknowledgmentsThis work was supported by the Council for Chemical Sciencesof The Netherlands Organization for Scientific Research (NWOCW) by a VIDI grant (J C S) and a VENI grant (A R J) Wegratefully acknowledge Ed Zuidinga for mass spectrometricanalyses

Keywords Lewis acids middot Lewis bases middot Frustrated Lewispairs middot Metal ligand cooperativity middot Density functionalcalculations

[1] T Ohkuma H Ooka S Hashiguchi T Ikariya R Noyori J Am Chem Soc1995 117 2675ndash2676

[2] a) J R Khusnutdinova D Milstein Angew Chem Int Ed 2015 54 12236ndash12273 Angew Chem 2015 127 12406 b) M Trincado H Gruumltzmacher

Full Paper

difference of ΔG = 121 and 104 kcal molndash1 for 3 and 4 respec-tively There is little energetic difference between the isopropylor tert-butyl groups during the reaction profile and the bondlengths (largest difference 003 Aring) and angles (largest difference06deg) are similar in all cases

Figure 2 Relative ωB97X-D6-311G(dp) Gibbs free energies (energy in brack-ets) in kcal molndash1 for the reaction of CO2 and 34

Table 1 Computed bond metric data for the van der Waals complexes (vdW)transition states (TS) and products (P) during the reactions of CO2 with 3 4and 5[a]

LAndashO1 [Aring] LBndashCCO2 [Aring] CndashO1 [Aring] CndashO2 [Aring] OndashCndashO [deg]

vdW 3 255 314 116 115 17594 256 316 116 115 17605 363 346 116 116 1776

TS 3 243 243 118 116 15894 245 242 118 116 15835 300 226 120 119 1478

P 3 226 159 127 122 12934 229 159 127 122 12915 157 188 128 121 1288

[a] LA = Lewis acid (Ru in 3 4 B in 5) LB = Lewis base (C in 3 4 P in 5)

The reaction profile for 5 is similar (Figure 3) First a van derWaals complex is formed with long distances between theFLP and CO2 with an almost linear CO2 The reaction proceedsvia an asynchronous concerted transition state (ΔGDagger =125 kcal molndash1) where the Lewis basic phosphorus centre at-tacks the electrophilic carbon of CO2 and the O1 is stabilisedby interaction with the boron centre This is evidenced by theslightly longer CndashO1 and CndashO2 bond lengths and the smallerbond OndashCndashO bond angle in TS-5 than the analogous metricsin TS-3 and TS-4 and is in good agreement with the previouslyreported bond order data for TS-5[6] The final product isformed by ring closure which is exergonic by 54 kcal molndash1 Inboth the MLC and FLP systems the mechanism is the same anddifferences in energies and bond metrics are readily explainedby the varying electronic nature of the Lewis acidic and basicsites in the molecules

Intramolecular FLPs can quench either via an intramolecularinteraction or dimerization although ldquoquenchrdquo in this case isperhaps a misnomer as it is now well established that completefrustration is not necessary for typical FLP reactivity to occurStephan and co-workers recently showed that even the classicaladduct of B(C6F5)3 and the strongly basic proazaphosphatrane

Eur J Inorg Chem 2019 2436ndash2442 wwweurjicorg copy 2019 The Authors Published by Wiley-VCH Verlag GmbH amp Co KGaA Weinheim2438

Figure 3 Relative ωB97X-D6-311G(dp) Gibbs free energies (energy in brack-ets) in kcal molndash1 for the reaction of CO2 and 5

P[N(Me)CH2CH2]3N is capable of addition to a range of hetero-allenes[17] In any case the Lewis adduct is a resting state disso-ciation into the corresponding Lewis acidbase componentswith the accompanying energy penalty is required to induceFLP reactivity Therefore the control of reactivity via fine-tuningof the steric environment is still a widely employed strategy inFLP chemistry In a similar manner MLC systems are able todimerize via an intermolecular interaction of the Lewis acidicand Lewis basic sites in the complex For example the dearoma-tized Ru-PNS system dimerizes as shown in Figure 4 and subse-quently undergoes a decomposition pathway involving C-Scleavage and loss of isobutene[18]

Figure 4 Two examples of dimeric MLC complexes Left molecular X-raystructure of [3]2 (displacement ellipsoids are set at 50 probability isopropylgroups on the phosphorus are omitted for clarity)[12] Selected bond lengths[Aring] Ru1ndashC7prime 2409(7) Ru1primendashC7 2403(7) P2ndashC7 1797(6) C6ndashC7 1456(8)C1ndashC2 1489(9) C1ndashP1 1842(6) P2primendashC7prime 1803(6) C6primendashC7prime 1449(8) C1primendashC2prime155(1) C1primendashP1prime 1843(6) Right Milsteins Ru(PNS) dimer A[18] Blue Lewisacid red Lewis base

On examination of the crystal structure of 3 as reported inMilsteins original publication[12] we noted that this species isalso a dimer in the solid state The rutheniumndashcarbon inter-atomic distance between the two monomers in the X-ray struc-ture [Ru1ndashC7prime 2409(7) and Ru1primendashC7 2403(7) Aring] lies within thesum of the van der Waals radii (375 Aring)[19] suggesting a bondinginteraction The C6ndashC7 bond lengths (according to the atomlabelling in Figure 1) in [3]2 are 1456(8) and 1449(8) Aring Thesebond lengths are much longer than that found in the gas-phaseDFT optimized monomer 3 (1379 Aring) but shorter than thatfound in the X-ray structure of unactivated complex 1 [1501(2)Aring] which suggests that the deprotonated arm features a CndashCbond with partial double bond character In fact the C6ndashC7bond length is similar to that found in A [1458(4) Aring Fig-ure 4][18]

Full Paper

To probe the structural changes that occur during dimeriza-tion we examined the aromaticity of the pyridine ring of thecompounds using NICS(0) calculations[2021] As expected theunactivated precursors feature an aromatic ring (1 ndash64 ppm2 ndash65 ppm) whereas in the activated species dearomatizationhas occurred (3 20 ppm 4 13 ppm) These values are consist-ent with previous studies by Gonccedilalves and Huang on similarorganometallic pincer complexes[2223] Interestingly the dimer[3]2 has a value (ndash47 ppm) between that of 1 and 3 indicatingpartial rearomatization of the pyridine ring and a contributingfactor to the stability of the dimer

The bonding situation in [3]2 was further analysed using AIManalyses[2425] which revealed a bond critical point (BCP) be-tween the Lewis acidic Ru site of one monomer and the Lewisbasic C7 site of the other monomer [Figure 5 ρ = 0047 au( = 021) RundashC7 2499 Aring] indicative of a weak interaction Fur-thermore a ring critical point (RCP) is found in the dimer be-tween the two monomers The examination of the Laplacian ofthe electron densities (nabla2ρ) in the C6ndashC7 bond reveals a weakerinteraction for the dimer than the monomer yet still strongerthan for 1 ([3]2 ndash066 au 3 ndash083 au 1 ndash058 au) ETS-NOCV[26] analyses of the dimer which we have used to assessdonorndashacceptor interactions concur with these observationsand revealed an interaction between ruthenium and the carbonin the backbone of both monomers showing orbital interac-tions and specifically σ donations of 244 and 207 kcal molndash1

from C7 to Ru

Figure 5 Computed AIM bond paths of [3]2 a simplified framework is de-picted (isopropyl groups on P and all H atoms omitted for clarity) bondcritical points (BCP) in red ring critical points (RCP) in green

The monomeric pincer complexes are the active species incatalysis therefore the dimer must first be broken before it canreact (Figure 6) DFT calculations at the ωB97X-D6-311G(dp)level of theory reveal it costs ΔG = 154 kcal molndash1 (ΔE =297 kcal molndash1) to break up dimer [3]2 (Table 2 all values givenper monomer) To give a better reflection of the thermodynam-ics of this equilibrium in solution we augmented our computa-tional method by including implicit solvation effects (benzeneand THF) As expected this lowers the amount of energy re-quired to break the dimer the more polar solvent THF does thisto a greater extent (benzene ΔG = 136 kcal molndash1 THF ΔG =124 kcal molndash1) Explicit solvent interactions are also importantespecially for monomeric species such as 3 where the Ru centrehas a vacant coordination site that can be stabilised by solvent

Eur J Inorg Chem 2019 2436ndash2442 wwweurjicorg copy 2019 The Authors Published by Wiley-VCH Verlag GmbH amp Co KGaA Weinheim2439

Including one benzene molecule per monomer in the calcula-tions lowered the energy required very slightly (ΔG =128 kcal molndash1) as benzene only weakly coordinates to the Rucentre in an η2 fashion Once again the more coordinating THFstabilises the monomer to a greater extent making it easierto cleave the dimer (ΔG = 55 kcal molndash1) we anticipate thatcoordinating substrates will have a similar effect

Figure 6 Top cleavage of dimer [3]2 into monomer 3 Bottom optimizedstructures of 3THF (left oxygen coordinates to ruthenium) and 3C6H6 (rightcoordination in an η2 fashion to ruthenium)

Table 2 Energy (ΔE) and Gibbs free energy (ΔG) required to break dimer [3]2

in kcal molndash1 (all values given per monomer)

ΔG [kcal molndash1] ΔE [kcal molndash1]

No solvent added 154 297Implicit THF 124 264Implicit benzene 136 281Explicit THF added implicit THF 55 141Explicit benzene added implicit Benzene 128 226

Interestingly and reminiscent of the tenets of FLP chemistryincreasing the steric bulk of the alkyl substituents on phos-phorus from isopropyl to tert-butyl (ie going from 3 to 4) de-stabilises these Lewis acidLewis base interactions and pre-cludes dimer formation It was not possible to locate a mini-mum on the potential energy surface corresponding to thestructure of [4]2 and all attempts led to regeneration of thetwo monomers during the optimization process

To corroborate these insights on the dimerization of 3 and 4in different solvents we analysed the diagnostic 1H NMR chemi-cal shift of the Ru-bound hydride both computationally andexperimentally (Table 3) The computed shift for monomer 3 isapproximately ndash20 ppm while the corresponding shift for [3]2

is relatively deshielded and is computed to be approximatelyndash10 ppm with little dependence on the identity of the solventExperimentally the hydride in benzene solutions of 3 wasfound to resonate at δ ndash1304 ppm[12] while in THF it is at δndash2005 ppm The latter is a very good match with the predictedmonomeric structure while the former is closer to the dimericspecies and suggests the existence of a monomerdimer equi-librium These data follow the trends predicted by the computa-

Full Paper

tions above in that the quenching of the MLC is more likely tooccur in less coordinating solvents such as benzene These dataare further supported by the fact that the analogous hydride inA (Figure 4) which is known to rapidly dimerise resonates rela-tively downfield at δ ndash1183 ppm in the non-coordinating sol-vent CD2Cl2[18]

Table 3 Experimental and computational data of the hydride shift of variouscompounds

Compound Experimental Computational data [ppm][a]

data [ppm] THF[b] Benzene[b]

3 ndash2023 ndash19903 in [D8]THF ndash20053 in C6D6 ndash1304[3]2

[c] ndash1016 ndash1019

4 ndash1858 ndash18364 in [D8]THF ndash26174 in C6D6 ndash2578[4]2 na na

A in CD2Cl2[a] ndash1183

[a] Calculations were performed using ωB97X-D6-311G(dp) Ru Def2TZVPlevel of theory [b] Calculated using implicit solvent interactions na = notapplicable as the dimeric structure could not be obtained computationally[c] Both hydrides have the same shift (ndash1016 or ndash1019)

The notion that the difference in the experimental chemicalshift of the hydride of 3 in THF and benzene is related to dimer-ization is reinforced by the fact that the analogous experimentalvalues for 4 a system where dimerization is not possible arevery similar in the two solvents (benzene δ ndash2578 ppm THFδ ndash2617 ppm implicit solvent added) It should be noted thatthe computed values in this case are not very accurate as theypredict the resonance at approximately ndash185 ppm dependingon the solvent and thus caution is advised in analysing theclose correlation between the computed and observed valuesfor 3 in THF above However the fact that the experimentalvalues of 3 are significantly different in the two solvents whilethe corresponding values for 4 are almost identical is evidencefor the presence of monomerdimer equilibrium effects

Finally we wanted to show that consideration of steric bulkis important for regulating the quenching of the Lewis acidLewis base components in other organometallic pincer systemsKirchner[27] and Huang[28] have replaced the methylene bridgesin Milsteins PNP pincer system with the more acidic NH moiety(Figure 7)[29] Calculations at the ωB97X-D6-311G(dp) level oftheory (with no solvent modelled) reveal a dimer is feasible for[(iPrPNNNP)RuH(CO)] and it costs ΔG = 113 kcal molndash1 to breakup the dimer (per monomer) which is slightly lower than forthe Milstein system Once again for the tert-butyl complex[(tBuPNNNP)RuH(CO)] the added steric protection around theacidic and basic sites prevents dimerization However the al-

Figure 7 Representation of the activated [(R-PNNNP)RuH(CO)] complexR = iPr or tBu Blue Lewis acid red Lewis base

Eur J Inorg Chem 2019 2436ndash2442 wwweurjicorg copy 2019 The Authors Published by Wiley-VCH Verlag GmbH amp Co KGaA Weinheim2440

tered electronics of the Lewis basic site in these complexescompared to 3 and 4 have drastic consequences as the activa-tion of CO2 is no longer feasible ΔG = 153 and ΔG =237 kcal molndash1 for [(iPrPNNNP)RuH(CO)] and [(tBuPNNNP)RuH(CO)]respectively

Conclusions

We have shown that FLP and MLC chemistry both involve thecooperative action of a Lewis acid and a Lewis base and thatsteric control to prevent quenching of the reactive sites is justas important in both paradigms There are many reactions inthe literature that have been given one label or the other on afairly arbitrary basis and we believe both schools of thoughtshould be united so that lessons from one field can be used tobenefit the other ndash whether that is using principles and reac-tions from main-group FLPs to broaden the scope of MLC reac-tivity or using the wealth of knowledge on ligand design andproperties to rationally construct new backbones for preorga-nised intramolecular main-group FLPs

Experimental SectionAll manipulations regarding the preparation of air-sensitive com-pounds were carried out under an atmosphere of dry nitrogen us-ing standard Schlenk and drybox techniques Solvents were puri-fied dried and degassed according to standard procedures 1H NMRspectra were recorded on a Bruker AV 400 or on a Bruker AV300-lland internally referenced to the residual solvent resonances([D8]THF 1H δ 358 172 ppm C6D6 1H δ 716 ppm [D8]Tol 1H δ208 697 701 709 ppm) 31P1H NMR spectra were recorded ona Bruker AV 400 or on a Bruker AV300-ll and externally referenced(85 H3PO4) Chemical shifts are reported in ppm High resolutionmass spectra were recorded on a Bruker MicroTOF with ESI nebu-lizer (ESI) at ndash45 degC

Synthesis of Diisopropylphosphine Diisopropylphosphine wasprepared according to a modified literature procedure of A S Glod-man et al[30] A solution of ClPiPr2 (492 g 0032 mol 10 equiv)diethyl ether (55 mL) was added dropwise to a slurry of LiAlH4

(037 g 001 mol 03 equiv) in diethyl ether (30 mL) in an icewater bath The mixture was stirred overnight and conversion waschecked by 31P1H NMR Degassed H2O (20 mL) was added slowlyand the organic layer was dried with MgSO4 The water layer wasextracted with diethyl ether (3 times 15 mL) and dried with the sameMgSO4 The MgSO4 was filtered off (with a cannula filter) and rinsedwith diethyl ether (3 times 15 mL) All volatiles were removed in vacuowhile the Schlenk vessel was cooled in an icewater bath to afforddiisopropylphosphine as a colourless clear liquid in 81 (308 g0026 mol) If some phosphine was oxidized a Schlenk to Schlenkdistillation was performed Note the presence of some diethyl etherdoes not influence the next step 1H NMR (4001 MHz C6D6 291 K)δ = 293 (dt 1JHP = 1923 Hz 3JHH = 59 Hz 1H PH) 177 (m 2HCH(CH3)) 101 (m 12H CH(CH3)) 31P1H NMR (1620 MHz C6D6295 K) δ = ndash165 (s)

Lithiation of Diisopropylphosphine Lithium diisopropylphos-phide was prepared according to a modified literature procedure ofA Jansen and S Pitter[31] n-Butyllithium (16 M in hexanes 52 mL8347 mmol 14 equiv) was added dropwise to a solution of diiso-propylphosphine (7045 mg 5962 mmol 10 equiv) in n-pentane

Full Paper

(15 mL) at 0 degC with a glass stirring bean and stirred for an addi-tional 30 min after which the solution was warmed to room tem-perature The resulting colourlesspale yellow solution was stirredfor 16 hours during which an off-white solid precipitated The solidswere collected by filtration subsequently washed with n-pentane(2 times 15 mL) and the solvents evaporated to dryness to give lithiumdiisopropylphosphide as an off-white solid in 76 (5627 mg4535 mmol) 1H NMR (4001 MHz [D8]THF 297 K) δ = 225 (dsept2JHP = 68 Hz 3JHH = 47 Hz 2H CH(CH3)2) 107 (dd 2JHP = 113 Hz3JHH = 68 Hz 12H CH(CH3)2) 7Li NMR (1555 MHz [D8]THF 297 K)13 (s) 13C NMR (1006 MHz [D8]THF 297 K) δ = 2693 (d 3JCP =142 Hz CH(CH3)2) 234 (d 2JCP = 253 Hz CH(CH3)2) 31P1H NMR(1620 MHz [D8]THF 297 K) δ = 15 (s)

Preparation of 26-Bis(diisopropylphosphino-methyl)-pyridine26-Bis(diisopropylphosphino-methyl)-pyridine was prepared ac-cording to a modified literature procedure of A Jansen andS Pitter[31] A solution of 26-bis(chloromethyl)pyridine (026 g1477 mmol 10 equiv) in THF (20 mL) as added slowly to a solutionof the lithiated phosphine (040 g 3223 mmol 22 equiv) using aglass stirring bean in THF (40 mL) at ndash78 degC and was stirred for anadditional 15 min at the same temperature During the additionthe solution changed colour from yellow to orange and subse-quently was warmed to room temperature in 16 h Addition of de-gassed water (02 mL) resulted in a colour change to yellow andthe mixture was dried with Na2SO4 The solution was filtered andthe Na2SO4 was washed with THF (3 times 4 mL) The combined solu-tion was dried in vacuo extracted with pentane (3 times 5 mL) and thesolvents evaporated to dryness to give iPrPNP as a yellow oil in88 (044 mg 1296 mmol 95 pure) Note some remainingdiisopropylphosphine can be observed 1H NMR (4001 MHz C6D6298 K) δ = 713ndash704 (m 1H p-PyH) 702ndash695 (m 2H m-PyH) 297(d 2JHP = 15 Hz 4H CH2) 171 (dsept 2JHP = 71 17 Hz 4HCH(CH3)2) 105 (m 24H CH(CH3)2) 31P1H NMR (1620 MHz C6D6297 K) δ = 114 (s product 95 ) ndash121 (s impurity presumablyiPr2PH 5 )

Synthesis of 1 [(iPrPNP)RuHCl(CO)] was prepared according to aliterature procedure[32] X-ray quality crystals were grown at ndash20 degCfrom a saturated solution of [(iPrPNP)RuHCl(CO)] in THF layeredwith n-pentane

Synthesis of 2 [(tBuPNP)RuHCl(CO)] was prepared according to aliterature procedure[13]

Synthesis of 3 [(iPrPNP)RuH(CO)] was prepared according to aslightly modified literature procedure[12] To a solution of complex[(iPrPNP)RuHCl(CO)] 1 (50 mg 0099 mmol) in THF (5 mL) was addedKOtBu (111 mg 0099 mmol) at ndash30 to ndash35 degC Subsequently themixture was stirred at room temperature for 4 h then filtered Theorange filtrate was dried under vacuum and washed with n-pentane(3 times 3 mL) and dried under vacuum to afford a yellowish powderin 56 yield (26 mg 0055 mmol)

Synthesis of 4 [(tBuPNP)RuH(CO)] was prepared according to aliterature procedure[13]

X-ray Crystal Structure Determination The single-crystal X-raydiffraction study (see Figure 8) was carried out on a Bruker D8 Ven-ture diffractometer with Photon100 detector at 123(2) K usingMo-Kα radiation (l = 071073 Aring) Dual space (intrinsic) methods(SHELXT)[33] were used for structure solution and refinement wascarried out using SHELXL-2014 (full-matrix least-squares on F2)[34]

Hydrogen atoms were localized by difference electron density de-termination and refined using a riding model [H(Ru) free] A semi-empirical absorption correction and an extinction correction wereapplied

Eur J Inorg Chem 2019 2436ndash2442 wwweurjicorg copy 2019 The Authors Published by Wiley-VCH Verlag GmbH amp Co KGaA Weinheim2441

Figure 8 Molecular structure of 1 [(iPrPNP)RuHCl(CO)] (displacement ellip-soids are set at 50 probability hydrogen atoms omitted for clarity) Se-lected bond lengths [Aring] RundashP1 23221(4) RundashP2 23061(4) P1ndashC1 18415(14)P2ndashC7 18435(15) C1ndashC2 15051(19) C6ndashC7 1501(2)

1 Colourless blocks C20H36ClNOP2Ru Mr = 50496 crystal size024 times 018 times 010 mm monoclinic space group P21c (No 14) a =137461(6) Aring b = 117625(5) Aring c = 144290(7) Aring = 95022(2)deg V =232405(18) Aring3 Z = 4 ρ = 1443 Mgmndash3 μ(Mo-Kα) = 094 mmndash1F(000) = 1048 2θmax = 552deg 50402 reflections of which 5365 wereindependent (Rint = 0034) 239 parameters R1 = 0019 [for 4830I gt 2σ(I)] wR2 = 0044 (all data) S = 106 largest diff peakhole =035ndash049 e Aringndash3

CCDC 1884053 (for 1) contains the supplementary crystallographicdata for this paper These data can be obtained free of charge fromThe Cambridge Crystallographic Data Centre

Computational Details Density functional calculations were per-formed at the ωB97X-D[35] level of theory using Gaussian09 revisionD01[36] Geometry optimizations were performed using the 6-311G(dp)[37] basis set for atoms C H N O P Cl in combinationwith the Def2TZVP[38] basis set for Ru ZPE and Gibbs free energies(Gdeg) were obtained from frequency analyses performed at the samelevel of theory Calculations of large dimer systems with solvation(SCRF THF or benzene) were obtained from frequency analysesafter optimization without solvation The NICS[39] analyses was per-formed at the ωB97X-D6-311G(dp) Ru Def2TZVPB3LYP[40]6-311G(dp) Ru Def2TZVP level of theory The AIM[41] and ETS-NOCV[42] analyses were performed at the ZORA-BP86TZ2P[42] levelof theory using ADF2016102[42] Structures were optimized usingthe same functional and basis set prior to the analysis using ZORA-BP86TZ2P NMR calculations were performed at ωB97X-D6-311G(dp) Ru Def2TZVP level of theory using solvation (THF orbenzene) and are corrected for the TMS value[3943] and obtainedwith the Gauge-Independent Atomic Orbital (GIAO)[44]

AcknowledgmentsThis work was supported by the Council for Chemical Sciencesof The Netherlands Organization for Scientific Research (NWOCW) by a VIDI grant (J C S) and a VENI grant (A R J) Wegratefully acknowledge Ed Zuidinga for mass spectrometricanalyses

Keywords Lewis acids middot Lewis bases middot Frustrated Lewispairs middot Metal ligand cooperativity middot Density functionalcalculations

[1] T Ohkuma H Ooka S Hashiguchi T Ikariya R Noyori J Am Chem Soc1995 117 2675ndash2676

[2] a) J R Khusnutdinova D Milstein Angew Chem Int Ed 2015 54 12236ndash12273 Angew Chem 2015 127 12406 b) M Trincado H Gruumltzmacher

Full Paper

To probe the structural changes that occur during dimeriza-tion we examined the aromaticity of the pyridine ring of thecompounds using NICS(0) calculations[2021] As expected theunactivated precursors feature an aromatic ring (1 ndash64 ppm2 ndash65 ppm) whereas in the activated species dearomatizationhas occurred (3 20 ppm 4 13 ppm) These values are consist-ent with previous studies by Gonccedilalves and Huang on similarorganometallic pincer complexes[2223] Interestingly the dimer[3]2 has a value (ndash47 ppm) between that of 1 and 3 indicatingpartial rearomatization of the pyridine ring and a contributingfactor to the stability of the dimer

The bonding situation in [3]2 was further analysed using AIManalyses[2425] which revealed a bond critical point (BCP) be-tween the Lewis acidic Ru site of one monomer and the Lewisbasic C7 site of the other monomer [Figure 5 ρ = 0047 au( = 021) RundashC7 2499 Aring] indicative of a weak interaction Fur-thermore a ring critical point (RCP) is found in the dimer be-tween the two monomers The examination of the Laplacian ofthe electron densities (nabla2ρ) in the C6ndashC7 bond reveals a weakerinteraction for the dimer than the monomer yet still strongerthan for 1 ([3]2 ndash066 au 3 ndash083 au 1 ndash058 au) ETS-NOCV[26] analyses of the dimer which we have used to assessdonorndashacceptor interactions concur with these observationsand revealed an interaction between ruthenium and the carbonin the backbone of both monomers showing orbital interac-tions and specifically σ donations of 244 and 207 kcal molndash1

from C7 to Ru

Figure 5 Computed AIM bond paths of [3]2 a simplified framework is de-picted (isopropyl groups on P and all H atoms omitted for clarity) bondcritical points (BCP) in red ring critical points (RCP) in green

The monomeric pincer complexes are the active species incatalysis therefore the dimer must first be broken before it canreact (Figure 6) DFT calculations at the ωB97X-D6-311G(dp)level of theory reveal it costs ΔG = 154 kcal molndash1 (ΔE =297 kcal molndash1) to break up dimer [3]2 (Table 2 all values givenper monomer) To give a better reflection of the thermodynam-ics of this equilibrium in solution we augmented our computa-tional method by including implicit solvation effects (benzeneand THF) As expected this lowers the amount of energy re-quired to break the dimer the more polar solvent THF does thisto a greater extent (benzene ΔG = 136 kcal molndash1 THF ΔG =124 kcal molndash1) Explicit solvent interactions are also importantespecially for monomeric species such as 3 where the Ru centrehas a vacant coordination site that can be stabilised by solvent

Eur J Inorg Chem 2019 2436ndash2442 wwweurjicorg copy 2019 The Authors Published by Wiley-VCH Verlag GmbH amp Co KGaA Weinheim2439

Including one benzene molecule per monomer in the calcula-tions lowered the energy required very slightly (ΔG =128 kcal molndash1) as benzene only weakly coordinates to the Rucentre in an η2 fashion Once again the more coordinating THFstabilises the monomer to a greater extent making it easierto cleave the dimer (ΔG = 55 kcal molndash1) we anticipate thatcoordinating substrates will have a similar effect

Figure 6 Top cleavage of dimer [3]2 into monomer 3 Bottom optimizedstructures of 3THF (left oxygen coordinates to ruthenium) and 3C6H6 (rightcoordination in an η2 fashion to ruthenium)

Table 2 Energy (ΔE) and Gibbs free energy (ΔG) required to break dimer [3]2

in kcal molndash1 (all values given per monomer)

ΔG [kcal molndash1] ΔE [kcal molndash1]

No solvent added 154 297Implicit THF 124 264Implicit benzene 136 281Explicit THF added implicit THF 55 141Explicit benzene added implicit Benzene 128 226

Interestingly and reminiscent of the tenets of FLP chemistryincreasing the steric bulk of the alkyl substituents on phos-phorus from isopropyl to tert-butyl (ie going from 3 to 4) de-stabilises these Lewis acidLewis base interactions and pre-cludes dimer formation It was not possible to locate a mini-mum on the potential energy surface corresponding to thestructure of [4]2 and all attempts led to regeneration of thetwo monomers during the optimization process

To corroborate these insights on the dimerization of 3 and 4in different solvents we analysed the diagnostic 1H NMR chemi-cal shift of the Ru-bound hydride both computationally andexperimentally (Table 3) The computed shift for monomer 3 isapproximately ndash20 ppm while the corresponding shift for [3]2

is relatively deshielded and is computed to be approximatelyndash10 ppm with little dependence on the identity of the solventExperimentally the hydride in benzene solutions of 3 wasfound to resonate at δ ndash1304 ppm[12] while in THF it is at δndash2005 ppm The latter is a very good match with the predictedmonomeric structure while the former is closer to the dimericspecies and suggests the existence of a monomerdimer equi-librium These data follow the trends predicted by the computa-

Full Paper

tions above in that the quenching of the MLC is more likely tooccur in less coordinating solvents such as benzene These dataare further supported by the fact that the analogous hydride inA (Figure 4) which is known to rapidly dimerise resonates rela-tively downfield at δ ndash1183 ppm in the non-coordinating sol-vent CD2Cl2[18]

Table 3 Experimental and computational data of the hydride shift of variouscompounds

Compound Experimental Computational data [ppm][a]

data [ppm] THF[b] Benzene[b]

3 ndash2023 ndash19903 in [D8]THF ndash20053 in C6D6 ndash1304[3]2

[c] ndash1016 ndash1019

4 ndash1858 ndash18364 in [D8]THF ndash26174 in C6D6 ndash2578[4]2 na na

A in CD2Cl2[a] ndash1183

[a] Calculations were performed using ωB97X-D6-311G(dp) Ru Def2TZVPlevel of theory [b] Calculated using implicit solvent interactions na = notapplicable as the dimeric structure could not be obtained computationally[c] Both hydrides have the same shift (ndash1016 or ndash1019)

The notion that the difference in the experimental chemicalshift of the hydride of 3 in THF and benzene is related to dimer-ization is reinforced by the fact that the analogous experimentalvalues for 4 a system where dimerization is not possible arevery similar in the two solvents (benzene δ ndash2578 ppm THFδ ndash2617 ppm implicit solvent added) It should be noted thatthe computed values in this case are not very accurate as theypredict the resonance at approximately ndash185 ppm dependingon the solvent and thus caution is advised in analysing theclose correlation between the computed and observed valuesfor 3 in THF above However the fact that the experimentalvalues of 3 are significantly different in the two solvents whilethe corresponding values for 4 are almost identical is evidencefor the presence of monomerdimer equilibrium effects

Finally we wanted to show that consideration of steric bulkis important for regulating the quenching of the Lewis acidLewis base components in other organometallic pincer systemsKirchner[27] and Huang[28] have replaced the methylene bridgesin Milsteins PNP pincer system with the more acidic NH moiety(Figure 7)[29] Calculations at the ωB97X-D6-311G(dp) level oftheory (with no solvent modelled) reveal a dimer is feasible for[(iPrPNNNP)RuH(CO)] and it costs ΔG = 113 kcal molndash1 to breakup the dimer (per monomer) which is slightly lower than forthe Milstein system Once again for the tert-butyl complex[(tBuPNNNP)RuH(CO)] the added steric protection around theacidic and basic sites prevents dimerization However the al-

Figure 7 Representation of the activated [(R-PNNNP)RuH(CO)] complexR = iPr or tBu Blue Lewis acid red Lewis base

Eur J Inorg Chem 2019 2436ndash2442 wwweurjicorg copy 2019 The Authors Published by Wiley-VCH Verlag GmbH amp Co KGaA Weinheim2440

tered electronics of the Lewis basic site in these complexescompared to 3 and 4 have drastic consequences as the activa-tion of CO2 is no longer feasible ΔG = 153 and ΔG =237 kcal molndash1 for [(iPrPNNNP)RuH(CO)] and [(tBuPNNNP)RuH(CO)]respectively

Conclusions

We have shown that FLP and MLC chemistry both involve thecooperative action of a Lewis acid and a Lewis base and thatsteric control to prevent quenching of the reactive sites is justas important in both paradigms There are many reactions inthe literature that have been given one label or the other on afairly arbitrary basis and we believe both schools of thoughtshould be united so that lessons from one field can be used tobenefit the other ndash whether that is using principles and reac-tions from main-group FLPs to broaden the scope of MLC reac-tivity or using the wealth of knowledge on ligand design andproperties to rationally construct new backbones for preorga-nised intramolecular main-group FLPs

Experimental SectionAll manipulations regarding the preparation of air-sensitive com-pounds were carried out under an atmosphere of dry nitrogen us-ing standard Schlenk and drybox techniques Solvents were puri-fied dried and degassed according to standard procedures 1H NMRspectra were recorded on a Bruker AV 400 or on a Bruker AV300-lland internally referenced to the residual solvent resonances([D8]THF 1H δ 358 172 ppm C6D6 1H δ 716 ppm [D8]Tol 1H δ208 697 701 709 ppm) 31P1H NMR spectra were recorded ona Bruker AV 400 or on a Bruker AV300-ll and externally referenced(85 H3PO4) Chemical shifts are reported in ppm High resolutionmass spectra were recorded on a Bruker MicroTOF with ESI nebu-lizer (ESI) at ndash45 degC

Synthesis of Diisopropylphosphine Diisopropylphosphine wasprepared according to a modified literature procedure of A S Glod-man et al[30] A solution of ClPiPr2 (492 g 0032 mol 10 equiv)diethyl ether (55 mL) was added dropwise to a slurry of LiAlH4

(037 g 001 mol 03 equiv) in diethyl ether (30 mL) in an icewater bath The mixture was stirred overnight and conversion waschecked by 31P1H NMR Degassed H2O (20 mL) was added slowlyand the organic layer was dried with MgSO4 The water layer wasextracted with diethyl ether (3 times 15 mL) and dried with the sameMgSO4 The MgSO4 was filtered off (with a cannula filter) and rinsedwith diethyl ether (3 times 15 mL) All volatiles were removed in vacuowhile the Schlenk vessel was cooled in an icewater bath to afforddiisopropylphosphine as a colourless clear liquid in 81 (308 g0026 mol) If some phosphine was oxidized a Schlenk to Schlenkdistillation was performed Note the presence of some diethyl etherdoes not influence the next step 1H NMR (4001 MHz C6D6 291 K)δ = 293 (dt 1JHP = 1923 Hz 3JHH = 59 Hz 1H PH) 177 (m 2HCH(CH3)) 101 (m 12H CH(CH3)) 31P1H NMR (1620 MHz C6D6295 K) δ = ndash165 (s)

Lithiation of Diisopropylphosphine Lithium diisopropylphos-phide was prepared according to a modified literature procedure ofA Jansen and S Pitter[31] n-Butyllithium (16 M in hexanes 52 mL8347 mmol 14 equiv) was added dropwise to a solution of diiso-propylphosphine (7045 mg 5962 mmol 10 equiv) in n-pentane

Full Paper

(15 mL) at 0 degC with a glass stirring bean and stirred for an addi-tional 30 min after which the solution was warmed to room tem-perature The resulting colourlesspale yellow solution was stirredfor 16 hours during which an off-white solid precipitated The solidswere collected by filtration subsequently washed with n-pentane(2 times 15 mL) and the solvents evaporated to dryness to give lithiumdiisopropylphosphide as an off-white solid in 76 (5627 mg4535 mmol) 1H NMR (4001 MHz [D8]THF 297 K) δ = 225 (dsept2JHP = 68 Hz 3JHH = 47 Hz 2H CH(CH3)2) 107 (dd 2JHP = 113 Hz3JHH = 68 Hz 12H CH(CH3)2) 7Li NMR (1555 MHz [D8]THF 297 K)13 (s) 13C NMR (1006 MHz [D8]THF 297 K) δ = 2693 (d 3JCP =142 Hz CH(CH3)2) 234 (d 2JCP = 253 Hz CH(CH3)2) 31P1H NMR(1620 MHz [D8]THF 297 K) δ = 15 (s)

Preparation of 26-Bis(diisopropylphosphino-methyl)-pyridine26-Bis(diisopropylphosphino-methyl)-pyridine was prepared ac-cording to a modified literature procedure of A Jansen andS Pitter[31] A solution of 26-bis(chloromethyl)pyridine (026 g1477 mmol 10 equiv) in THF (20 mL) as added slowly to a solutionof the lithiated phosphine (040 g 3223 mmol 22 equiv) using aglass stirring bean in THF (40 mL) at ndash78 degC and was stirred for anadditional 15 min at the same temperature During the additionthe solution changed colour from yellow to orange and subse-quently was warmed to room temperature in 16 h Addition of de-gassed water (02 mL) resulted in a colour change to yellow andthe mixture was dried with Na2SO4 The solution was filtered andthe Na2SO4 was washed with THF (3 times 4 mL) The combined solu-tion was dried in vacuo extracted with pentane (3 times 5 mL) and thesolvents evaporated to dryness to give iPrPNP as a yellow oil in88 (044 mg 1296 mmol 95 pure) Note some remainingdiisopropylphosphine can be observed 1H NMR (4001 MHz C6D6298 K) δ = 713ndash704 (m 1H p-PyH) 702ndash695 (m 2H m-PyH) 297(d 2JHP = 15 Hz 4H CH2) 171 (dsept 2JHP = 71 17 Hz 4HCH(CH3)2) 105 (m 24H CH(CH3)2) 31P1H NMR (1620 MHz C6D6297 K) δ = 114 (s product 95 ) ndash121 (s impurity presumablyiPr2PH 5 )

Synthesis of 1 [(iPrPNP)RuHCl(CO)] was prepared according to aliterature procedure[32] X-ray quality crystals were grown at ndash20 degCfrom a saturated solution of [(iPrPNP)RuHCl(CO)] in THF layeredwith n-pentane

Synthesis of 2 [(tBuPNP)RuHCl(CO)] was prepared according to aliterature procedure[13]

Synthesis of 3 [(iPrPNP)RuH(CO)] was prepared according to aslightly modified literature procedure[12] To a solution of complex[(iPrPNP)RuHCl(CO)] 1 (50 mg 0099 mmol) in THF (5 mL) was addedKOtBu (111 mg 0099 mmol) at ndash30 to ndash35 degC Subsequently themixture was stirred at room temperature for 4 h then filtered Theorange filtrate was dried under vacuum and washed with n-pentane(3 times 3 mL) and dried under vacuum to afford a yellowish powderin 56 yield (26 mg 0055 mmol)

Synthesis of 4 [(tBuPNP)RuH(CO)] was prepared according to aliterature procedure[13]

X-ray Crystal Structure Determination The single-crystal X-raydiffraction study (see Figure 8) was carried out on a Bruker D8 Ven-ture diffractometer with Photon100 detector at 123(2) K usingMo-Kα radiation (l = 071073 Aring) Dual space (intrinsic) methods(SHELXT)[33] were used for structure solution and refinement wascarried out using SHELXL-2014 (full-matrix least-squares on F2)[34]

Hydrogen atoms were localized by difference electron density de-termination and refined using a riding model [H(Ru) free] A semi-empirical absorption correction and an extinction correction wereapplied

Eur J Inorg Chem 2019 2436ndash2442 wwweurjicorg copy 2019 The Authors Published by Wiley-VCH Verlag GmbH amp Co KGaA Weinheim2441

Figure 8 Molecular structure of 1 [(iPrPNP)RuHCl(CO)] (displacement ellip-soids are set at 50 probability hydrogen atoms omitted for clarity) Se-lected bond lengths [Aring] RundashP1 23221(4) RundashP2 23061(4) P1ndashC1 18415(14)P2ndashC7 18435(15) C1ndashC2 15051(19) C6ndashC7 1501(2)

1 Colourless blocks C20H36ClNOP2Ru Mr = 50496 crystal size024 times 018 times 010 mm monoclinic space group P21c (No 14) a =137461(6) Aring b = 117625(5) Aring c = 144290(7) Aring = 95022(2)deg V =232405(18) Aring3 Z = 4 ρ = 1443 Mgmndash3 μ(Mo-Kα) = 094 mmndash1F(000) = 1048 2θmax = 552deg 50402 reflections of which 5365 wereindependent (Rint = 0034) 239 parameters R1 = 0019 [for 4830I gt 2σ(I)] wR2 = 0044 (all data) S = 106 largest diff peakhole =035ndash049 e Aringndash3

CCDC 1884053 (for 1) contains the supplementary crystallographicdata for this paper These data can be obtained free of charge fromThe Cambridge Crystallographic Data Centre

Computational Details Density functional calculations were per-formed at the ωB97X-D[35] level of theory using Gaussian09 revisionD01[36] Geometry optimizations were performed using the 6-311G(dp)[37] basis set for atoms C H N O P Cl in combinationwith the Def2TZVP[38] basis set for Ru ZPE and Gibbs free energies(Gdeg) were obtained from frequency analyses performed at the samelevel of theory Calculations of large dimer systems with solvation(SCRF THF or benzene) were obtained from frequency analysesafter optimization without solvation The NICS[39] analyses was per-formed at the ωB97X-D6-311G(dp) Ru Def2TZVPB3LYP[40]6-311G(dp) Ru Def2TZVP level of theory The AIM[41] and ETS-NOCV[42] analyses were performed at the ZORA-BP86TZ2P[42] levelof theory using ADF2016102[42] Structures were optimized usingthe same functional and basis set prior to the analysis using ZORA-BP86TZ2P NMR calculations were performed at ωB97X-D6-311G(dp) Ru Def2TZVP level of theory using solvation (THF orbenzene) and are corrected for the TMS value[3943] and obtainedwith the Gauge-Independent Atomic Orbital (GIAO)[44]

AcknowledgmentsThis work was supported by the Council for Chemical Sciencesof The Netherlands Organization for Scientific Research (NWOCW) by a VIDI grant (J C S) and a VENI grant (A R J) Wegratefully acknowledge Ed Zuidinga for mass spectrometricanalyses

Keywords Lewis acids middot Lewis bases middot Frustrated Lewispairs middot Metal ligand cooperativity middot Density functionalcalculations

[1] T Ohkuma H Ooka S Hashiguchi T Ikariya R Noyori J Am Chem Soc1995 117 2675ndash2676

[2] a) J R Khusnutdinova D Milstein Angew Chem Int Ed 2015 54 12236ndash12273 Angew Chem 2015 127 12406 b) M Trincado H Gruumltzmacher

Full Paper

tions above in that the quenching of the MLC is more likely tooccur in less coordinating solvents such as benzene These dataare further supported by the fact that the analogous hydride inA (Figure 4) which is known to rapidly dimerise resonates rela-tively downfield at δ ndash1183 ppm in the non-coordinating sol-vent CD2Cl2[18]

Table 3 Experimental and computational data of the hydride shift of variouscompounds

Compound Experimental Computational data [ppm][a]

data [ppm] THF[b] Benzene[b]

3 ndash2023 ndash19903 in [D8]THF ndash20053 in C6D6 ndash1304[3]2

[c] ndash1016 ndash1019

4 ndash1858 ndash18364 in [D8]THF ndash26174 in C6D6 ndash2578[4]2 na na

A in CD2Cl2[a] ndash1183

[a] Calculations were performed using ωB97X-D6-311G(dp) Ru Def2TZVPlevel of theory [b] Calculated using implicit solvent interactions na = notapplicable as the dimeric structure could not be obtained computationally[c] Both hydrides have the same shift (ndash1016 or ndash1019)

The notion that the difference in the experimental chemicalshift of the hydride of 3 in THF and benzene is related to dimer-ization is reinforced by the fact that the analogous experimentalvalues for 4 a system where dimerization is not possible arevery similar in the two solvents (benzene δ ndash2578 ppm THFδ ndash2617 ppm implicit solvent added) It should be noted thatthe computed values in this case are not very accurate as theypredict the resonance at approximately ndash185 ppm dependingon the solvent and thus caution is advised in analysing theclose correlation between the computed and observed valuesfor 3 in THF above However the fact that the experimentalvalues of 3 are significantly different in the two solvents whilethe corresponding values for 4 are almost identical is evidencefor the presence of monomerdimer equilibrium effects

Finally we wanted to show that consideration of steric bulkis important for regulating the quenching of the Lewis acidLewis base components in other organometallic pincer systemsKirchner[27] and Huang[28] have replaced the methylene bridgesin Milsteins PNP pincer system with the more acidic NH moiety(Figure 7)[29] Calculations at the ωB97X-D6-311G(dp) level oftheory (with no solvent modelled) reveal a dimer is feasible for[(iPrPNNNP)RuH(CO)] and it costs ΔG = 113 kcal molndash1 to breakup the dimer (per monomer) which is slightly lower than forthe Milstein system Once again for the tert-butyl complex[(tBuPNNNP)RuH(CO)] the added steric protection around theacidic and basic sites prevents dimerization However the al-

Figure 7 Representation of the activated [(R-PNNNP)RuH(CO)] complexR = iPr or tBu Blue Lewis acid red Lewis base

Eur J Inorg Chem 2019 2436ndash2442 wwweurjicorg copy 2019 The Authors Published by Wiley-VCH Verlag GmbH amp Co KGaA Weinheim2440

tered electronics of the Lewis basic site in these complexescompared to 3 and 4 have drastic consequences as the activa-tion of CO2 is no longer feasible ΔG = 153 and ΔG =237 kcal molndash1 for [(iPrPNNNP)RuH(CO)] and [(tBuPNNNP)RuH(CO)]respectively

Conclusions

We have shown that FLP and MLC chemistry both involve thecooperative action of a Lewis acid and a Lewis base and thatsteric control to prevent quenching of the reactive sites is justas important in both paradigms There are many reactions inthe literature that have been given one label or the other on afairly arbitrary basis and we believe both schools of thoughtshould be united so that lessons from one field can be used tobenefit the other ndash whether that is using principles and reac-tions from main-group FLPs to broaden the scope of MLC reac-tivity or using the wealth of knowledge on ligand design andproperties to rationally construct new backbones for preorga-nised intramolecular main-group FLPs

Experimental SectionAll manipulations regarding the preparation of air-sensitive com-pounds were carried out under an atmosphere of dry nitrogen us-ing standard Schlenk and drybox techniques Solvents were puri-fied dried and degassed according to standard procedures 1H NMRspectra were recorded on a Bruker AV 400 or on a Bruker AV300-lland internally referenced to the residual solvent resonances([D8]THF 1H δ 358 172 ppm C6D6 1H δ 716 ppm [D8]Tol 1H δ208 697 701 709 ppm) 31P1H NMR spectra were recorded ona Bruker AV 400 or on a Bruker AV300-ll and externally referenced(85 H3PO4) Chemical shifts are reported in ppm High resolutionmass spectra were recorded on a Bruker MicroTOF with ESI nebu-lizer (ESI) at ndash45 degC

Synthesis of Diisopropylphosphine Diisopropylphosphine wasprepared according to a modified literature procedure of A S Glod-man et al[30] A solution of ClPiPr2 (492 g 0032 mol 10 equiv)diethyl ether (55 mL) was added dropwise to a slurry of LiAlH4

(037 g 001 mol 03 equiv) in diethyl ether (30 mL) in an icewater bath The mixture was stirred overnight and conversion waschecked by 31P1H NMR Degassed H2O (20 mL) was added slowlyand the organic layer was dried with MgSO4 The water layer wasextracted with diethyl ether (3 times 15 mL) and dried with the sameMgSO4 The MgSO4 was filtered off (with a cannula filter) and rinsedwith diethyl ether (3 times 15 mL) All volatiles were removed in vacuowhile the Schlenk vessel was cooled in an icewater bath to afforddiisopropylphosphine as a colourless clear liquid in 81 (308 g0026 mol) If some phosphine was oxidized a Schlenk to Schlenkdistillation was performed Note the presence of some diethyl etherdoes not influence the next step 1H NMR (4001 MHz C6D6 291 K)δ = 293 (dt 1JHP = 1923 Hz 3JHH = 59 Hz 1H PH) 177 (m 2HCH(CH3)) 101 (m 12H CH(CH3)) 31P1H NMR (1620 MHz C6D6295 K) δ = ndash165 (s)

Lithiation of Diisopropylphosphine Lithium diisopropylphos-phide was prepared according to a modified literature procedure ofA Jansen and S Pitter[31] n-Butyllithium (16 M in hexanes 52 mL8347 mmol 14 equiv) was added dropwise to a solution of diiso-propylphosphine (7045 mg 5962 mmol 10 equiv) in n-pentane

Full Paper

(15 mL) at 0 degC with a glass stirring bean and stirred for an addi-tional 30 min after which the solution was warmed to room tem-perature The resulting colourlesspale yellow solution was stirredfor 16 hours during which an off-white solid precipitated The solidswere collected by filtration subsequently washed with n-pentane(2 times 15 mL) and the solvents evaporated to dryness to give lithiumdiisopropylphosphide as an off-white solid in 76 (5627 mg4535 mmol) 1H NMR (4001 MHz [D8]THF 297 K) δ = 225 (dsept2JHP = 68 Hz 3JHH = 47 Hz 2H CH(CH3)2) 107 (dd 2JHP = 113 Hz3JHH = 68 Hz 12H CH(CH3)2) 7Li NMR (1555 MHz [D8]THF 297 K)13 (s) 13C NMR (1006 MHz [D8]THF 297 K) δ = 2693 (d 3JCP =142 Hz CH(CH3)2) 234 (d 2JCP = 253 Hz CH(CH3)2) 31P1H NMR(1620 MHz [D8]THF 297 K) δ = 15 (s)

Preparation of 26-Bis(diisopropylphosphino-methyl)-pyridine26-Bis(diisopropylphosphino-methyl)-pyridine was prepared ac-cording to a modified literature procedure of A Jansen andS Pitter[31] A solution of 26-bis(chloromethyl)pyridine (026 g1477 mmol 10 equiv) in THF (20 mL) as added slowly to a solutionof the lithiated phosphine (040 g 3223 mmol 22 equiv) using aglass stirring bean in THF (40 mL) at ndash78 degC and was stirred for anadditional 15 min at the same temperature During the additionthe solution changed colour from yellow to orange and subse-quently was warmed to room temperature in 16 h Addition of de-gassed water (02 mL) resulted in a colour change to yellow andthe mixture was dried with Na2SO4 The solution was filtered andthe Na2SO4 was washed with THF (3 times 4 mL) The combined solu-tion was dried in vacuo extracted with pentane (3 times 5 mL) and thesolvents evaporated to dryness to give iPrPNP as a yellow oil in88 (044 mg 1296 mmol 95 pure) Note some remainingdiisopropylphosphine can be observed 1H NMR (4001 MHz C6D6298 K) δ = 713ndash704 (m 1H p-PyH) 702ndash695 (m 2H m-PyH) 297(d 2JHP = 15 Hz 4H CH2) 171 (dsept 2JHP = 71 17 Hz 4HCH(CH3)2) 105 (m 24H CH(CH3)2) 31P1H NMR (1620 MHz C6D6297 K) δ = 114 (s product 95 ) ndash121 (s impurity presumablyiPr2PH 5 )

Synthesis of 1 [(iPrPNP)RuHCl(CO)] was prepared according to aliterature procedure[32] X-ray quality crystals were grown at ndash20 degCfrom a saturated solution of [(iPrPNP)RuHCl(CO)] in THF layeredwith n-pentane

Synthesis of 2 [(tBuPNP)RuHCl(CO)] was prepared according to aliterature procedure[13]

Synthesis of 3 [(iPrPNP)RuH(CO)] was prepared according to aslightly modified literature procedure[12] To a solution of complex[(iPrPNP)RuHCl(CO)] 1 (50 mg 0099 mmol) in THF (5 mL) was addedKOtBu (111 mg 0099 mmol) at ndash30 to ndash35 degC Subsequently themixture was stirred at room temperature for 4 h then filtered Theorange filtrate was dried under vacuum and washed with n-pentane(3 times 3 mL) and dried under vacuum to afford a yellowish powderin 56 yield (26 mg 0055 mmol)

Synthesis of 4 [(tBuPNP)RuH(CO)] was prepared according to aliterature procedure[13]

X-ray Crystal Structure Determination The single-crystal X-raydiffraction study (see Figure 8) was carried out on a Bruker D8 Ven-ture diffractometer with Photon100 detector at 123(2) K usingMo-Kα radiation (l = 071073 Aring) Dual space (intrinsic) methods(SHELXT)[33] were used for structure solution and refinement wascarried out using SHELXL-2014 (full-matrix least-squares on F2)[34]

Hydrogen atoms were localized by difference electron density de-termination and refined using a riding model [H(Ru) free] A semi-empirical absorption correction and an extinction correction wereapplied

Eur J Inorg Chem 2019 2436ndash2442 wwweurjicorg copy 2019 The Authors Published by Wiley-VCH Verlag GmbH amp Co KGaA Weinheim2441

Figure 8 Molecular structure of 1 [(iPrPNP)RuHCl(CO)] (displacement ellip-soids are set at 50 probability hydrogen atoms omitted for clarity) Se-lected bond lengths [Aring] RundashP1 23221(4) RundashP2 23061(4) P1ndashC1 18415(14)P2ndashC7 18435(15) C1ndashC2 15051(19) C6ndashC7 1501(2)

1 Colourless blocks C20H36ClNOP2Ru Mr = 50496 crystal size024 times 018 times 010 mm monoclinic space group P21c (No 14) a =137461(6) Aring b = 117625(5) Aring c = 144290(7) Aring = 95022(2)deg V =232405(18) Aring3 Z = 4 ρ = 1443 Mgmndash3 μ(Mo-Kα) = 094 mmndash1F(000) = 1048 2θmax = 552deg 50402 reflections of which 5365 wereindependent (Rint = 0034) 239 parameters R1 = 0019 [for 4830I gt 2σ(I)] wR2 = 0044 (all data) S = 106 largest diff peakhole =035ndash049 e Aringndash3

CCDC 1884053 (for 1) contains the supplementary crystallographicdata for this paper These data can be obtained free of charge fromThe Cambridge Crystallographic Data Centre

Computational Details Density functional calculations were per-formed at the ωB97X-D[35] level of theory using Gaussian09 revisionD01[36] Geometry optimizations were performed using the 6-311G(dp)[37] basis set for atoms C H N O P Cl in combinationwith the Def2TZVP[38] basis set for Ru ZPE and Gibbs free energies(Gdeg) were obtained from frequency analyses performed at the samelevel of theory Calculations of large dimer systems with solvation(SCRF THF or benzene) were obtained from frequency analysesafter optimization without solvation The NICS[39] analyses was per-formed at the ωB97X-D6-311G(dp) Ru Def2TZVPB3LYP[40]6-311G(dp) Ru Def2TZVP level of theory The AIM[41] and ETS-NOCV[42] analyses were performed at the ZORA-BP86TZ2P[42] levelof theory using ADF2016102[42] Structures were optimized usingthe same functional and basis set prior to the analysis using ZORA-BP86TZ2P NMR calculations were performed at ωB97X-D6-311G(dp) Ru Def2TZVP level of theory using solvation (THF orbenzene) and are corrected for the TMS value[3943] and obtainedwith the Gauge-Independent Atomic Orbital (GIAO)[44]

AcknowledgmentsThis work was supported by the Council for Chemical Sciencesof The Netherlands Organization for Scientific Research (NWOCW) by a VIDI grant (J C S) and a VENI grant (A R J) Wegratefully acknowledge Ed Zuidinga for mass spectrometricanalyses

Keywords Lewis acids middot Lewis bases middot Frustrated Lewispairs middot Metal ligand cooperativity middot Density functionalcalculations

[1] T Ohkuma H Ooka S Hashiguchi T Ikariya R Noyori J Am Chem Soc1995 117 2675ndash2676

[2] a) J R Khusnutdinova D Milstein Angew Chem Int Ed 2015 54 12236ndash12273 Angew Chem 2015 127 12406 b) M Trincado H Gruumltzmacher

Full Paper

(15 mL) at 0 degC with a glass stirring bean and stirred for an addi-tional 30 min after which the solution was warmed to room tem-perature The resulting colourlesspale yellow solution was stirredfor 16 hours during which an off-white solid precipitated The solidswere collected by filtration subsequently washed with n-pentane(2 times 15 mL) and the solvents evaporated to dryness to give lithiumdiisopropylphosphide as an off-white solid in 76 (5627 mg4535 mmol) 1H NMR (4001 MHz [D8]THF 297 K) δ = 225 (dsept2JHP = 68 Hz 3JHH = 47 Hz 2H CH(CH3)2) 107 (dd 2JHP = 113 Hz3JHH = 68 Hz 12H CH(CH3)2) 7Li NMR (1555 MHz [D8]THF 297 K)13 (s) 13C NMR (1006 MHz [D8]THF 297 K) δ = 2693 (d 3JCP =142 Hz CH(CH3)2) 234 (d 2JCP = 253 Hz CH(CH3)2) 31P1H NMR(1620 MHz [D8]THF 297 K) δ = 15 (s)

Preparation of 26-Bis(diisopropylphosphino-methyl)-pyridine26-Bis(diisopropylphosphino-methyl)-pyridine was prepared ac-cording to a modified literature procedure of A Jansen andS Pitter[31] A solution of 26-bis(chloromethyl)pyridine (026 g1477 mmol 10 equiv) in THF (20 mL) as added slowly to a solutionof the lithiated phosphine (040 g 3223 mmol 22 equiv) using aglass stirring bean in THF (40 mL) at ndash78 degC and was stirred for anadditional 15 min at the same temperature During the additionthe solution changed colour from yellow to orange and subse-quently was warmed to room temperature in 16 h Addition of de-gassed water (02 mL) resulted in a colour change to yellow andthe mixture was dried with Na2SO4 The solution was filtered andthe Na2SO4 was washed with THF (3 times 4 mL) The combined solu-tion was dried in vacuo extracted with pentane (3 times 5 mL) and thesolvents evaporated to dryness to give iPrPNP as a yellow oil in88 (044 mg 1296 mmol 95 pure) Note some remainingdiisopropylphosphine can be observed 1H NMR (4001 MHz C6D6298 K) δ = 713ndash704 (m 1H p-PyH) 702ndash695 (m 2H m-PyH) 297(d 2JHP = 15 Hz 4H CH2) 171 (dsept 2JHP = 71 17 Hz 4HCH(CH3)2) 105 (m 24H CH(CH3)2) 31P1H NMR (1620 MHz C6D6297 K) δ = 114 (s product 95 ) ndash121 (s impurity presumablyiPr2PH 5 )

Synthesis of 1 [(iPrPNP)RuHCl(CO)] was prepared according to aliterature procedure[32] X-ray quality crystals were grown at ndash20 degCfrom a saturated solution of [(iPrPNP)RuHCl(CO)] in THF layeredwith n-pentane

Synthesis of 2 [(tBuPNP)RuHCl(CO)] was prepared according to aliterature procedure[13]

Synthesis of 3 [(iPrPNP)RuH(CO)] was prepared according to aslightly modified literature procedure[12] To a solution of complex[(iPrPNP)RuHCl(CO)] 1 (50 mg 0099 mmol) in THF (5 mL) was addedKOtBu (111 mg 0099 mmol) at ndash30 to ndash35 degC Subsequently themixture was stirred at room temperature for 4 h then filtered Theorange filtrate was dried under vacuum and washed with n-pentane(3 times 3 mL) and dried under vacuum to afford a yellowish powderin 56 yield (26 mg 0055 mmol)

Synthesis of 4 [(tBuPNP)RuH(CO)] was prepared according to aliterature procedure[13]

X-ray Crystal Structure Determination The single-crystal X-raydiffraction study (see Figure 8) was carried out on a Bruker D8 Ven-ture diffractometer with Photon100 detector at 123(2) K usingMo-Kα radiation (l = 071073 Aring) Dual space (intrinsic) methods(SHELXT)[33] were used for structure solution and refinement wascarried out using SHELXL-2014 (full-matrix least-squares on F2)[34]

Hydrogen atoms were localized by difference electron density de-termination and refined using a riding model [H(Ru) free] A semi-empirical absorption correction and an extinction correction wereapplied

Eur J Inorg Chem 2019 2436ndash2442 wwweurjicorg copy 2019 The Authors Published by Wiley-VCH Verlag GmbH amp Co KGaA Weinheim2441

Figure 8 Molecular structure of 1 [(iPrPNP)RuHCl(CO)] (displacement ellip-soids are set at 50 probability hydrogen atoms omitted for clarity) Se-lected bond lengths [Aring] RundashP1 23221(4) RundashP2 23061(4) P1ndashC1 18415(14)P2ndashC7 18435(15) C1ndashC2 15051(19) C6ndashC7 1501(2)

1 Colourless blocks C20H36ClNOP2Ru Mr = 50496 crystal size024 times 018 times 010 mm monoclinic space group P21c (No 14) a =137461(6) Aring b = 117625(5) Aring c = 144290(7) Aring = 95022(2)deg V =232405(18) Aring3 Z = 4 ρ = 1443 Mgmndash3 μ(Mo-Kα) = 094 mmndash1F(000) = 1048 2θmax = 552deg 50402 reflections of which 5365 wereindependent (Rint = 0034) 239 parameters R1 = 0019 [for 4830I gt 2σ(I)] wR2 = 0044 (all data) S = 106 largest diff peakhole =035ndash049 e Aringndash3

CCDC 1884053 (for 1) contains the supplementary crystallographicdata for this paper These data can be obtained free of charge fromThe Cambridge Crystallographic Data Centre

Computational Details Density functional calculations were per-formed at the ωB97X-D[35] level of theory using Gaussian09 revisionD01[36] Geometry optimizations were performed using the 6-311G(dp)[37] basis set for atoms C H N O P Cl in combinationwith the Def2TZVP[38] basis set for Ru ZPE and Gibbs free energies(Gdeg) were obtained from frequency analyses performed at the samelevel of theory Calculations of large dimer systems with solvation(SCRF THF or benzene) were obtained from frequency analysesafter optimization without solvation The NICS[39] analyses was per-formed at the ωB97X-D6-311G(dp) Ru Def2TZVPB3LYP[40]6-311G(dp) Ru Def2TZVP level of theory The AIM[41] and ETS-NOCV[42] analyses were performed at the ZORA-BP86TZ2P[42] levelof theory using ADF2016102[42] Structures were optimized usingthe same functional and basis set prior to the analysis using ZORA-BP86TZ2P NMR calculations were performed at ωB97X-D6-311G(dp) Ru Def2TZVP level of theory using solvation (THF orbenzene) and are corrected for the TMS value[3943] and obtainedwith the Gauge-Independent Atomic Orbital (GIAO)[44]

AcknowledgmentsThis work was supported by the Council for Chemical Sciencesof The Netherlands Organization for Scientific Research (NWOCW) by a VIDI grant (J C S) and a VENI grant (A R J) Wegratefully acknowledge Ed Zuidinga for mass spectrometricanalyses

Keywords Lewis acids middot Lewis bases middot Frustrated Lewispairs middot Metal ligand cooperativity middot Density functionalcalculations

[1] T Ohkuma H Ooka S Hashiguchi T Ikariya R Noyori J Am Chem Soc1995 117 2675ndash2676

[2] a) J R Khusnutdinova D Milstein Angew Chem Int Ed 2015 54 12236ndash12273 Angew Chem 2015 127 12406 b) M Trincado H Gruumltzmacher

Full Paper

in Cooperative Catalysis Designing Efficient Catalysts for Synthesis (Ed RPeters) Wiley-VCH Weinheim 2015 p 67ndash110 c) H Valdeacutes M A Garciacutea-Eleno D Canseco-Gonzalez D Morales-Morales ChemCatChem 2018 103136-3172 d) C Gunanathan D Milstein Acc Chem Res 2011 44 588ndash602 e) H Gruumltzmacher Angew Chem Int Ed 2008 47 1814ndash1818Angew Chem 2008 120 1838 f ) C Gunanathan D Milstein Chem Rev2014 114 12024ndash12087

[3] G C Welch R R San Juan J D Masuda D W Stephan Science 2006314 1124ndash1126

[4] a) F-G Fontaine D W Stephan Phil Trans R Soc A 2017 375 20170004b) G Kehr S Schwendemann G Erker Top Curr Chem 2012 332 45ndash84 c) D J Scott M J Fuchter A E Ashley Chem Soc Rev 2017 465689ndash5700 d) D W Stephan J Am Chem Soc 2015 137 10018ndash10032

[5] a) P Spies G Erker G Kehr K Bergander R Froumlhlich S Grimme D WStephan Chem Commun 2007 5072ndash5074 b) C M Moumlmming E OttenG Kehr R Froumlhlich S Grimme D W Stephan G Erker Angew ChemInt Ed 2009 48 6643ndash6646 Angew Chem 2009 121 6770

[6] F Bertini V Lyaskovskyy B J J Timmer F J J de Kanter M Lutz A WEhlers J C Slootweg K Lammertsma J Am Chem Soc 2012 134 201ndash204

[7] Examples of complexes where the metal is the Lewis base a) J CamposJ Am Chem Soc 2017 139 2944ndash2947 b) W H Harman J C Peters JAm Chem Soc 2012 134 5080ndash582 c) S J K Forrest J Clifton N FeyP G Pringle H A Sparkes D F Wass Angew Chem Int Ed 2015 542223ndash2227 Angew Chem 2015 127 2251

[8] a) A M Chapman M F Haddow D F Wass J Am Chem Soc 2011 1338826ndash8829 b) A M Chapman M F Haddow D F Wass J Am ChemSoc 2011 133 18463ndash18478

[9] a) S R Flynn D F Wass ACS Catal 2013 3 2574ndash2581 b) D F WassA M Chapman Top Curr Chem 2013 334 261ndash280 c) D W StephanG Erker Angew Chem Int Ed 2015 54 6400ndash6441 Angew Chem 2015127 6498 d) D W Stephan Science 2016 354 aaf7229 e) M T WhitedBeilstein J Org Chem 2012 8 1554ndash1563 f ) R M Bullock G M Cham-bers Phil Trans R Soc A 2017 375 20170002

[10] For more papers drawing an analogy between MLCs and TM-FLPs seea) A T Normand C G Daniliuc B Wibbeling G Kehr P L Gendre GErker J Am Chem Soc 2015 137 10796ndash10808 b) A N Marziale AFriedrich I Klopsch M Drees V R Celinski J S auf der Guumlnne S Schnei-der J Am Chem Soc 2013 135 13342ndash13355 c) B Askevold H WRoesky S Schneider ChemCatChem 2012 4 307ndash320 d) M J Sgro D WStephan Chem Commun 2013 49 2610ndash2612 e) A Pal K Vanka Dal-ton Trans 2013 42 13866ndash13873 f ) S Arndt M Rudolph A S KHashmi Gold Bull 2017 50 267ndash282 g) N P Mankad Chem Commun2018 54 1291ndash1302

[11] S Zhang A M Appel R M Bullock J Am Chem Soc 2017 139 7376ndash7387

[12] J Zhang G Leitus Y Ben-David D Milstein Angew Chem Int Ed 200645 1113ndash1115 Angew Chem 2006 118 1131

[13] B Gnanaprakasam J Zhang D Milstein Angew Chem Int Ed 2010 491468ndash1471 Angew Chem 2010 122 1510

[14] L E Eijsink S C P Perdriau J G de Vries E Otten Dalton Trans 201645 16033ndash16039

[15] M Feller U Gellrich A Anaby Y Diskin-Posner D Milstein J Am ChemSoc 2016 138 6445ndash6454

[16] M Vogt M Gargir M A Iron Y Diskin-Posner Y Ben-David D MilsteinChem Eur J 2012 18 9194ndash9197

[17] T C Johnstone G N J H Wee D Stephan Angew Chem Int Ed 201857 5881ndash5884 Angew Chem 2018 130 5983

[18] M Gargir Y Ben-David G Leitus Y Diskin-Posner L J W Shimon DMilstein Organometallics 2012 31 6207ndash6214

[19] a) S S Batsanov Inorg Mater 2001 37 871ndash885 b) S S Batsanov InorgMater 2001 37 1031ndash1046

[20] Z Chen C S Wannere C Corminboeuf R Puchta P von Ragueacute SchleyerChem Rev 2005 105 3842ndash3888

[21] NICS calculations were performed using ωB97X-D6-311G(dp) RuDef2TZVPB3LYP6-311G(dp) Ru Def2TZVP level of theory whereNICS(0) is calculated in the ring centre Negative values of NICS(0) corre-spond to aromaticity positive values to antiaromaticity and small nega-tive or positive values to non-aromaticity

Eur J Inorg Chem 2019 2436ndash2442 wwweurjicorg copy 2019 The Authors Published by Wiley-VCH Verlag GmbH amp Co KGaA Weinheim2442

[22] T Gonccedilalves K-W Huang J Am Chem Soc 2017 139 13442ndash13449[23] For more aromatizationdearomatization of PNP pincer ligands see a) T

Simler G Frison P Braunstein A A Danopoulos Dalton Trans 2016 452800ndash2804 b) M Feller E Ben-Ari M A Iron Y Diskin-Posner G LeitusL J W Shimon L Konstantinovski D Milstein Inorg Chem 2010 491615ndash1625

[24] R F W Bader Atoms in Molecules Claredon Press Oxford 1994[25] The AIM analysis was performed at BP86TZ2P Ru ZORA using

ADF2016102 see ESI for details[26] M P Mitoraj A Michalak T Ziegler J Chem Theory Comput 2009 5

962ndash975[27] D Benito-Garagorri E Becker J Wiedermann W Lackner M Pollak K

Mereiter J Kisala K Kirchner Organometallics 2006 25 1900ndash1913[28] L-P He T Chen D-X Xue M Eddaoudi K-W Huang J Organomet

Chem 2012 700 202ndash206[29] H Li B Zheng K-W Huang Coord Chem Rev 2015 293ndash294 116ndash138[30] K Zhu P D Achord X Zhang K Krogh-Jespersen A S Goldman J Am

Chem Soc 2004 126 13044ndash13053[31] A Jansen S Pitter Monatsh Chem 1999 130 783ndash794[32] J Zhang G Leitus Y Ben-David D Milstein J Am Chem Soc 2005 127

10840ndash10841[33] G M Sheldrick Acta Crystallogr Sect A 2015 71 3ndash8[34] G M Sheldrick Acta Crystallogr Sect C 2015 71 3ndash8[35] J-Da Chai M Head-Gordon Phys Chem Chem Phys 2008 10 6615ndash

6620[36] M J Frisch G W Trucks H B Schlegel G E Scuseria M A Robb J R

Cheeseman G Scalmani V Barone B Mennucci G A Petersson H Na-katsuji M Caricato X Li H P Hratchian A F Izmaylov J Bloino GZheng J L Sonnenberg M Hada M Ehara K Toyota R Fukuda J Hase-gawa M Ishida T Nakajima Y Honda O Kitao H Nakai T Vreven J AMontgomery Jr J E Peralta F Ogliaro M Bearpark J J Heyd E Broth-ers K N Kudin V N Staroverov R Kobayashi J Normand K Raghava-chari A Rendell J C Burant S S Iyengar J Tomasi M Cossi N RegaJ M Millam M Klene J E Knox J B Cross V Bakken C Adamo JJaramillo R Gomperts R E Stratmann O Yazyev A J Austin R CammiC Pomelli J W Ochterski R L Martin K Morokuma V G ZakrzewskiG A Voth P Salvador J J Dannenberg S Dapprich A D Daniels OumlFarkas J B Foresman J V Ortiz J Cioslowski D J Fox Gaussian 09Revision D01 Gaussian Inc Wallingford CT 2013

[37] a) A D McLean G S Chandler J Chem Phys 1980 72 5639ndash5648 b)K Raghavachari J S Binkley R Seeger J A Pople J Chem Phys 198072 650ndash654 c) J-P Blaudeau M P McGrath L A Curtiss L Radom JChem Phys 1997 107 5016ndash5021 d) A J H Wachters J Chem Phys1970 52 1033ndash1036 e) P J Hay J Chem Phys 1977 66 4377ndash4384 f )K Raghavachari G W Trucks J Chem Phys 1989 91 1062ndash1065 g)R C Binning Jr L A Curtiss J Comput Chem 1990 11 1206ndash1216 h)M P McGrath L Radom J Chem Phys 1991 94 511ndash516 i) L A CurtissM P McGrath J-P Blaudeau N E Davis R C Binning Jr L Radom JChem Phys 1995 103 6104ndash6113

[38] a) F Weigend R Ahlrichs Phys Chem Chem Phys 2005 7 3297ndash3305b) F Weigend Phys Chem Chem Phys 2006 8 1057ndash1065

[39] J Gauss Chem Phys Lett 1992 191 614ndash620[40] A D Becke J Chem Phys 1993 98 5648ndash5652[41] a) J I Rodriacuteguez R F W Bader P W Ayers C Michel A W Goumltz C Bo

Chem Phys Lett 2009 472 149ndash152 b) J I Rodriacuteguez J Comput Chem2013 34 681ndash686

[42] a) G te Velde F M Bickelhaupt S J A van Gisbergen C Fonseca GuerraE J Baerends J G Snijders T Ziegler J Comput Chem 2001 22 931ndash967 b) C Fonseca Guerra J G Snijders G te Velde E J Baerends TheorChem Acc 1998 99 391ndash403 c) ADF2016 SCM Theoretical ChemistryVrije Universiteit Amsterdam The Netherlands httpwwwscmcom

[43] J R Cheeseman G W Trucks T A Keith M J Frisch J Chem Phys1996 104 5497ndash5509

[44] a) F London J Phys Radium 1937 8 397ndash409 b) R McWeeny Phys Rev1962 126 1028ndash1034 c) R Ditchfield Mol Phys 1974 27 789ndash807d) K Wolinski J F Hilton P Pulav J Am Chem Soc 1990 112 8251ndash8260 e) Ref [43]

Received February 13 2019