RECENT DEVELOPMENTS IN

CHALCOGEN CHEMISTRY: 2

Tristram Chivers

Department of Chemistry,

University of Calgary,

Calgary, Alberta, Canada

Lecture 2: New Aspects of Chalcogen-Chalcogen Bonding

Outline

• Historical Background • Alkali Metal Complexes

a) Synthesis b) Structures and NMR Spectra

• Redox Chemistry (E–E bond formation)

a) One-electron Oxidation b) Two-electron Oxidation

– Mixed Chalcogen Systems

2

E = S, Se, Te; R = Ph, alkyl

N

R 2 P

E E

P R 2

Dichalcogeno PNP Bridged Ligands (Up to 2001)

(E = O, S, Se; R = Ph, iPr)

Luminescent

Materials

Industrial

Catalysts Metal Extraction

Processes

Lanthanide Shift

Reagents

C. Silvestru and J. E. Drake, Coord. Chem. Rev. 2001, 223, 117

I. Haiduc, Comprehensive Coord. Chem. II 2003, 323

Metal Complexes

3

Neutral Ligand

Metal Complexes

Diseleno PNP-Bridged Ligands

R = Ph: J. D. Woollins, J. Chem. Soc. Dalton Trans., 1905, 1607

R = iPr: J. D. Woollins, Inorg. Chim. Acta, 1999, 290, 1

Quantum dots: P. O’Brien , Chem. Commun., 2003, 1454. Thin Films: P. O’Brien, Eur. J. Inorg. Chem., 2004,171. 4

H N

R 2 P P R 2

H N

R 2 P

S e S e

P R 2

2 S e

t o l u e n e r e f l u x 6 h

i P r 2 P S e

M

S e i P r 2 P

N

S e P i P r 2

N

P i P r 2 S e

C V D

s o l u t i o n t h e r m o l y s i s

M = C d

M = Z n , C d , H g

C d S e q u a n t u m d o t s

M S e t h i n f i l m s

N P h 2 P

T e

N a T e

P P h 2

N a

T e T e

P h 2 P

N P P h 2

P h 2 P P P h 2

H N P h 2 P P P h 2

H N

T e

N a [ P h 2 P N P P h 2 ]

N a H T H F

2 T e

t o l u e n e r e f l u x

( t m e d a )

( t m e d a ) 2 T e , T M E D A

t o l u e n e , 8 0 ° C

R = Ph: G. G. Briand, T. Chivers, M. Parvez, Angew. Chem. Int. Ed. 2002, 41, 3468.

R = iPr: T. Chivers, D. J. Eisler, J. S. Ritch, Dalton Trans. 2005, 2675.

• Centrosymmetric dimer with Te,Te’-chelation to Na+

• One Te atom per ligand is three-coordinate • Fast exchange even at 235 K

d(P–Te) = 2.370(1), 2.430(1) Å

NMR Data (d8-THF)

δ(31P): 9.5 ppm

δ(125Te): −404 ppm (235 K) 1JTe–P = 1632 Hz

5

Ditelluro PNP-Bridged Ligands

P-H Tautomer of a PNP Monotelluride

P–H hydrogen atom located

d(P=Te) = 2.380(1) Å

IR: (P–H) = 2329 cm−1

δ(1H): P–H 6.34 ppm (d of d, 1JP-H = 443 Hz)

δ(31P){1H}: P=Te 48.2 ppm (d, 1JTe–P = 1654 Hz)

P–H 44.0 ppm (d, 2JP–P = 18 Hz)

proton coupled 31P: PH (ddm, 1JH-P = 442 Hz)

T. Chivers, D. J. Eisler, J. S. Ritch, H. M. Tuononen, Angew. Chem. Int. Ed. 2005, 44, 4953. 6

i P r 2 P P i P r 2

H N i P r 2 P P i P r 2

N

T e

T e

h e x a n e , 2 3 ° C , 1 6 h H

p a l e y e l l o w c r y s t a l s 8 1 % y i e l d

H

i P r 2 P P

i P r 2

N

T e

N i P r 2 P

T e T e

P i P r 2

T e

i P r 2 P

N P

i P r 2

T e p a l e y e l l o w

A i r

n - h e x a n e , 2 3 ° C

d a r k r e d c r y s t a l s, i d e n t i f i e d b y X R D

7

An Unusual Ditelluride: One-electron Oxidation

An Unusual Ditelluride: One-electron Oxidation

8

H

i P r 2 P P

i P r 2

N

T e

N i P r 2 P

T e N a

T e

P i P r 2

N i P r 2 P

T e T e

P i P r 2

T e

i P r 2 P

N P

i P r 2

T e p a l e y e l l o w

( t m e d a )

a ) A i r

b ) 0 . 5 I 2

n - h e x a n e , 2 3 ° C

T H F , - 7 8 ° C d a r k r e d c r y s t a l s : a ) i d e n t i f i e d b y X R D

b ) 8 7 %

Two doublets:

1JTe–P = 1500 Hz

1JTe–P = 1026 Hz

2JP–P = 26 Hz

d (Te1–Te1A) = 2.946(1) Å

(cf. RTeTeR 2.72-2.76 Å)

d(P2–Te2) = 2.394(2) Å

d(P1–Te1) = 2.489(2) Å

τ(P–Te–Te–P) = 180 °

τ(P–N–P–Te) = 24 °

Low T 31P NMR:

An Unusual Ditelluride: One-electron Oxidation

T. Chivers, D. J. Eisler, J. S. Ritch, H. M. Tuononen, Angew. Chem. Int. Ed. 2005, 44, 4953. 9

H

i P r 2 P P

i P r 2

N

T e

N i P r 2 P

T e N a

T e

P i P r 2

N i P r 2 P

T e T e

P i P r 2

T e

i P r 2 P

N P

i P r 2

T e p a l e y e l l o w

( t m e d a )

a ) A i r

b ) 0 . 5 I 2

n - h e x a n e , 2 3 ° C

T H F , - 7 8 ° C d a r k r e d c r y s t a l s : a ) i d e n t i f i e d b y X R D

b ) 8 7 %

DFT Calculations: J. S. Ritch, T. Chivers, D. J. Eisler, H. M. Tuononen, Chem. Eur. J., 2007, 4643.

• SOMO – a linear combination of px and py orbitals on Te

• Weak SOMO-SOMO interaction – spatial orientation of monomers and

multicentred nature of SOMO

10

C. D. Hoff et al., Inorg. Chem., 2005, 44, 3127.

Why is the Te-Te Bond in Dimer Elongated?

N M e 2 P

T e T e

P M e 2

T e

M e 2 P N

P M e 2

T e ΔHdimerization = −80 kJ mol−1 [d(Te–Te) = 2.95 Å] D(Te–Te) = 137 kJ mol−1 in PhTe–TePh [d(Te–Te) = 2.72 Å]

[PBE0/TZVP Basis set (ECP for Te)]

d(Se-Se) = 2.470(1) Å τ(P-Se-Se-P) = 155 ° d(Se-Se) = 2.464(1) Å

τ(P-Se-Se-P) = 180 °

[ c.f. d(Se-Se) = 2.335 Å for ArSe–SeAr ]

Is this Behaviour Unique for Tellurium?

11

N

R 2 P

S e

N a

S e

P R 2

R 2 P

S e S e

P R 2

N

S e

R 2 P

N

P R 2

S e

( t m e d a )

0 . 5 I 2

T H F , - 7 8 ° C

R = i P r , 9 0 %

R = t B u , 8 9 %

d(S-S) = 2.104(2) Å τ(P-S-S-P) = 180 °

τ(S-P-P-S) = 109.8 °

[ cf. d(S–S) = 2.02-2.08 Å for ArS–SAr ]

Influence of Chalcogen – An Unusual Disulfide

12

N t B u 2 P

S

N a

S

P t B u 2

t B u 2 P

S S

P t B u 2

N

S

t B u 2 P

N

P t B u 2

S

( t m e d a )

0 . 5 I 2

T H F , - 7 8 ° C

6 8 % y i e l d

d (Te2–Te3) = 3.102(1) Å

d(Te1–Te2) = 2.981(1) Å

d(Te2–Te4) = 3.253(1) Å

(Te3–Te2–Te1) = 175.46(3) °

31P NMR (298 K): broad resonance (186 K): four broad resonances

• An incipient internal redox process? • Calculated electron transfer ca. 0.2 e−

• Calculated charge on Te(2) is +0.02

13

Influence of Substituents – Te Systems

J. S. Ritch, T. Chivers, D. J. Eisler, H. M. Tuononen, Chem. Eur. J., 2007, 4643.

N t B u 2 P

T e N a

T e

P t B u 2

T e T e

t B u 2 P N

P t B u 2

T e

P t B u 2 N

t B u 2 P

T e

( t m e d a )

T H F , - 7 8 ° C

4 7 % y i e l d

2

I 2

E E

R 2 P

N

P R 2

E

P R 2 N

R 2 P

E

R 2 P

E E

P R 2

N

E

R 2 P N

P R 2

E

Relative Stabilities – DFT Calculations (H. M. Tuononen):

• Stability of CIP relative to DC increases from S to Te

• CIP is more stable for E = Te, R= tBu

DC CIP

ΔE(CIP – DC) (kJ mol−1)

14

Structural Isomers – Dichalcogenide vs Contact Ion Pair

R = tBu iPr

S +15 +25

Se ca. 0 +15

Te -20 -2

Synthesis:

15

Two-Electron Oxidation – Dichalcogeno PNP Cations

J. Konu, T. Chivers, H. M. Tuononen, Chem. Commun., 2006, 1634.

N i P r 2 P

E N a

E

P i P r 2 I 2

E E

P i P r 2

N i P r 2 P I

( t m e d a )

T H F , - 7 8 ° C

( E = S e , 9 0 % ; E = T e , 9 2 % )

d a r k r e d a i r - s t a b l e p o w d e r s

A g [ S b F 6 ]

E E

P i P r 2

N i P r 2 P S b F 6

t o l u e n e , - 7 8 ° C

( E = S e , 7 8 % ; d a r k o r a n g e ) ( E = T e , 8 0 % ; p u r p l e - b l u e c r y s t a l s )

Synthesis: Structures (I- salts): • Puckered 5-membered rings • Linked by I− ions infinite chain • E–E bonds longer than single-bond values by 6-7 % for Se, 3-4 % for Te • Normal E–E bond lengths in SbF6

− salts

d(Se-Se) = 2.484(1) Å d(Te-Te) = 2.839(1) Å

16

Two-Electron Oxidation – Dichalcogeno PNP Cations

E1 E2 I1

I1A

P1

N1 P2

J. Konu, T. Chivers, H. M. Tuononen, Chem. Commun., 2006, 1634.

N i P r 2 P

E N a

E

P i P r 2 I 2

E E

P i P r 2

N i P r 2 P I

( t m e d a )

T H F , - 7 8 ° C

( E = S e , 9 0 % ; E = T e , 9 2 % )

d a r k r e d a i r - s t a b l e p o w d e r s

A g [ S b F 6 ]

E E

P i P r 2

N i P r 2 P S b F 6

t o l u e n e , - 7 8 ° C

( E = S e , 7 8 % ; d a r k o r a n g e ) ( E = T e , 8 0 % ; p u r p l e - b l u e c r y s t a l s )

Prediction: A 6π-electron system – R2P (1πe−), N (1πe−), E (2πe−)

• Overall π–bond order is approximately zero

• Long chalcogen-chalcogen bonds: I−(lp) LUMO(σ*)

LUMO HOMO

HOMO–1 HOMO–2

[ PBE0/TZVP Basis Set (ECP for Te) ]

17

Frontier Orbitals of [N(PR2E)2]+ Cations

Challenge: To obtain these anions in good yields with high purity

Questions: • One-electron oxidation

Which chalcogen-chalcogen bond will be formed?

•Two-electron oxidation:

Which chalcogen in the cation will interact with I-?

How will this affect the structures of the salts so formed?

(E = S, Se)

18

Mixed Chalcogen PNP-Bridged Ligands

T e E

P i P r 2

N

i P r 2 P -

S e

P i P r 2

N i P r 2 P

H

T e

i P r 2 P

N

P i P r 2

H

P i P r 2

H N

i P r 2 P

T e S

P i P r 2

N i P r 2 P

L i T M E D A

2) Te

Optimum syntheses of Li derivatives from iPr2PNHPiPr2

(i) Oxidation of one P(III) (ii) Metallation (iii) Oxidation of second P(III)

(E = S, Se)

1) nBuLi, TMEDA

2) 1/8 S8

(99% purity, 56% yield)

(97% purity, 73% yield)

1) nBuLi, TMEDA

Te

Se

Mixed Chalcogen Ligands – Lithium Derivatives

S. D. Robertson, T. Chivers, Dalton Trans., 2008, 1765. 19

T e S e

P i P r 2

N i P r 2 P

L i T M E D A

20

d(S…I) = 3.99 Å (∑rvdW = 4.05 Å)

No Se…I interaction!

• Elongation of E–Te (E = S, Se, Te) bond is determined by strength of Te…I interaction : LUMO polarized towards Te

21

Cyclic Mixed Dichalcogeno PNP Cations

E E’ d(E-E’) (Å) % Elongation d(Te…I)

Te Te 2.840(1) 3.6 3.430(1)

Se Te 2.746(1) 8.1 3.005(1)

S Te 2.710(1) 12.5 2.915(1)

S. D. Robertson, T. Chivers, Inorg. Chem., 2008, 47, 10634.

E = S

i P r 2 P

T e E

P i P r 2

N

L i ( t m e d a )

I 2

I 2 E = S e

( T H F , - 7 8 ° C )

1. Oxidation of Anions

2. Reduction of Cations

3. Anion-Cation Reactions

22

Synthetic Approaches to Heterodichalcogenides

N i P r 2 P

E T e

P i P r 2

T e

i P r 2 P

N P

i P r 2

E

N i P r 2 P

T e E

P i P r 2

T e

i P r 2 P

N P

i P r 2

E

N i P r 2 P

T e E

P i P r 2

E

i P r 2 P

N P

i P r 2

Te

E T e

P i P r 2

N i P r 2 P I 2

N i P r 2 P

E N a

T e

P i P r 2

( t m e d a )

2

N i P r 2 P

E N a

T e

P i P r 2

( t m e d a )

E T e

P i P r 2

N i P r 2 P I +

½ I2

-NaI

-NaI

-MI

Reducing Agent (M)

or

or

• Dimers form central Te–Te bond preferentially • Energetically favored over Te–E or E–E dimers

Te-Te Bond Lengths (Å) in Mixed Chalcogen Systems:

23

Reduction of Cations

E d(Te-Te) (Å) Elongation (%)

Te 2.946(1) 8

Se 2.893(1) 6

S 2.807(1) 3

S. D. Robertson, T. Chivers, H. M. Tuononen, Inorg. Chem., 2008, 47, 10634.

(E = S, Se)

i P r 2 P

E T e

P i P r 2

N

T e

i P r 2 P N

P i P r 2

E

E T e

P i P r 2

N i P r 2 P I

C o C p 2 2

• SOMO of radicals [EPiPr2NPiPr2Te]• polarized towards Te when E = S, Se

24

SOMO-SOMO Interaction in Ditellurides

N

M e 2 P

E T e

P M e 2

T e

M e 2 P

N

P M e 2

E

S. D. Robertson, T. Chivers, H. M. Tuononen, Inorg. Chem., 2008, 47, 10634.

T e T e

P R 2

N

R 2 P I

( E = S , S e )

R 2 P

E

N a

E

P R 2

N

( t m e d a )

t o l u e n e

S. D. Robertson, T. Chivers, H. M. Tuononen, Inorg. Chem., 2009, 48, 6755. 25

• One e− reduction of cations Dichalcogenides (DCs) with identical halves

• Do anion-cation reactions Contact Ion Pairs (CIPs) with different halves?

Anion-Cation Reactions

Monodentate CIP

(E = S, Se; R = iPr) Bidentate CIP (E = Se; R = tBu)

• Two-electron oxidation: Formation of cyclic cations

• E–Te bonds elongated by elongated by donation from anion into LUMO [σ*(Te–E)] of cation

• LUMO is polarized towards Te when E = S, Se

26

Conclusions – Chalcogen-Chalcogen Bonding

i P r 2 P

T e E

P i P r 2

N

L i ( t m e d a )

I 2

E T e

P i P r 2

N i P r 2 P I

- L i I

(E = S, Se)

• Synthesis of Dichalcogenides (DCs) or Contact Ion Pairs (CIPs):

(a) Oxidation of Anions (b) Reduction of Cations (c) Anion-Cation Reactions

• Three structural types:

• Elongation of central Te–Te bond (SOMO-SOMO interaction)

• Te–Te bonds formed preferentially in DCs (polarization of SOMO)

• d(Te-Te) is determined by strength of interaction with anions in CIPs

t B u 2 P

E T e

E P

N

t B u 2 P N

P t B u 2

T e

t B u 2

T e

i P r 2 P N

P i P r 2

T e E

i P r 2 P

N

P i P r

i P r

E

i P r 2 P

E T e

P i P r 2

N

T e

i P r 2 P

N P

i P r 2

E

27

Conclusions – Chalcogen-Chalcogen Bonding

DC Bidentate CIP Monodentate CIP