RECENT DEVELOPMENTS IN

CHALCOGEN CHEMISTRY: 4

Tristram Chivers

Department of Chemistry,

University of Calgary,

Calgary, Alberta, Canada

S S

P P h 2

C

P h 2 P 2 -

S e S e

P P h 2

H C

P h 2 P -

E E

P P h 2

C

P h 2 P 2 -

E '

Theme: Carbon-Centred Reactivity – Compare with PNP-Bridged Anions

Outline:

• Formation of stable carbenoids by oxidation of 1

• Synthesis and metal complexes of diseleno monoanion 2

• Formation of complexes of triseleno dianion 3a by Se-H+ exchange

• Redox behaviour of trichalcogeno dianions 3b and 3c

1 2 3a (E = E’ = Se) 3b (E = S, E’ = Se) 3c (E = E’ = S)

2

Chalcogen-Centred PCP-Bridged Ligands

Double Deprotonation: Metal Complexes: (Le Floch et al. 2004-2011)

• Methanediide complexes with TMs, lanthanides and actinides

• All complexes exhibit S,C,S coordination

• M-C bond: Highly polar, ca. single bond for early and late TMs

Significant multiple bonding for Ln and An

Recent review: S. T. Liddle, D. P. Mills, A. J. Wooles, Chem. Soc Rev. 2011, 40, 2164.

Le Floch et al. ACIE, 2004, 43, 6382.

3

A Dithio PCP-Bridged Dianion: Synthesis

S S

P P h 2

C

P h 2 P 2 -

( L i + ) 2 S S

P P h 2

H 2 C

P h 2 P 2 MeLi

toluene, -80 o C

Li 2 1 ( in situ reagent)

..

P h 2 P P P h 2

C

S

C

S

P P S S P h 2 P h 2

(a) Le Floch et al. ACIE, 2007, 46, 5947. (b) Konu and Chivers, Chem. Commun., 2008, 4995.

Li21 (a) (b)

C2Cl6 I2

- LiI

A Dithio PCP-Bridged Dianion: Oxidation

Formation of Stable Carbenoids upon Two-Electron Oxidation • Mild oxidation produces a monomeric carbenoid • Oxidation with I2 gives a dimeric carbenoid and a C2P2S2 ring comprised of two carbenes

4

L i +

S

S P

C

P

L i +

I

S

S P

C

P

I

( E t 2 O ) P h 2 P h 2

P h 2 P h 2

S S

P P h 2

C

P h 2 P

L i ( E t 2 O ) 2

C l

Synthesis: • Metallation first to avoid P=Se cleavage by RLi • iPr derivative (XRD) prepared from Cl2PCH2PCl2 • Deprotonation of Li2 accompanied by P=Se cleavage

Homoleptic Group 12 Complexes:

2 Li2 + MCl2

(M = Zn, Hg)

M = Zn, pale yellow (48%)

M = Hg, pale yellow (77%)

Diseleno PCP-Bridged Monoanion

L i

P P h 2

H C

P h 2 P

( T M E D A )

P P h 2

H 2 C

P h 2 P

MeLi

Et2O, TMEDA

2 Se

toluene

Li2

(XRD)

S e S e

P P h 2

H C

P h 2 P

L i ( T M E D A )

J. Konu, H. M. Tuononen, T. Chivers, Inorg. Chem., 2009, 48, 11788. 5

Oxidation with I2: Produces C-H rather than Se-Se bond, cf. PNP system

(R = Ph, iPr) [Dark red intermediate]

One-electron Oxidation: Hydrogen Abstraction

6

+ ½ I2

THF THF S e S e

P R 2

H C

R 2 P

L i ( T M E D A )

S e S e

P R 2

H C

R 2 P

S e S e

P R 2

H 2 C

R 2 P

J. Konu, H. M. Tuononen, T. Chivers, Inorg. Chem., 2009, 48, 11788.

Oxidation with I2: Produces C-H rather than Se-Se bond, cf. PNP system

(R = Ph, iPr) [Dark red intermediate]

One-electron Oxidation: Hydrogen Abstraction

7

SOMOs of Neutral Radicals

PCP: R = iPr

Significant contribution from p-orbital on C

PNP: R = Me

Localized on chalcogens; No contribution from N atom

+ ½ I2

THF THF S e S e

P R 2

H C

R 2 P

L i ( T M E D A )

S e S e

P R 2

H C

R 2 P

S e S e

P R 2

H 2 C

R 2 P

2 SnCl2 4

- 2 - [Sn0]

NMR (1H, 31P, 77Se, cf. Group 12 complexes)

red crystals (51%)

- 4 LiCl

NMR: • Monoselenide (P=Se) is eliminated

XRD:

• Octahedral tin(IV) complex • Two tridentate [SeC(Ph2PSe)2]2- ligands • Se-H+ exchange • Redox process [Sn(II) → Sn(IV)]

A Triseleno PCP-Bridged Tin(IV) Complex

S e S e

P P h 2

H C

P h 2 P

L i ( T M E D A )

S e

S e P

H C

P

S n I I

S e

S e P

C H

P

P h 2 P h 2

P h 2 P h 2

S e

P P h 2

H 2 C

P h 2 P

S n S e

S e S e

S e

S e

S e

C P h 2 P

P h 2 P

C

P P h 2

P P h 2

IV

J. Konu, T. Chivers, Chem. Commun., 2010, 46, 1431. 8

2 TeCl2•TMTU

4 - 2 [H2C(PPh2)(PPh2Se)]

- 4 LiCl

dark red crystals (47%)

IV

C

P P h 2

S e

T e

S e S e

S e S e

P h 2 P C

P h 2 P

S e

P P h 2

S e S e

P P h 2

H C

P h 2 P

L i ( T M E D A )

NMR: • Monoselenide H2C(PPh2)PPh2Se detected • No evidence for intermediate Te(II) complex

XRD: • Te(IV) complex • Two bidentate [SeC(Ph2PSe)2]2- ligands (cf. Sn complex) • Stereochemically active LP on Te: See-saw geometry • Se-H+ exchange • Redox process [Te(II) → Te(IV)]

A Triseleno PCP-Bridged Tellurium(IV) Complex

[J. Konu, T. Chivers, Chem. Commun., 2010, 46, 1431]

9

• Homoleptic Tl(I) complex formed initially • Dark purple Tl(I)/Tl(III) complex on heating

XRD: • Mixed-valent Complex: Octahedral Tl(III) anions and Tl+ cations

• cf. Isovalent neutral Sn(IV) complex

• Polymeric chain in solid state via weak Tl···Se interactions

M. Risto, T. Chivers, J. Konu, Dalton Trans. 2011, 40, 8238.

A Mixed-Valent Tl(I)/Tl(III) Complex

LiOEt

TlOEt

S e S e

P P h 2

H C

P h 2 P

L i ( T M E D A )

S e S e

P P h 2

H C

P h 2 P

Tl ( )

T l S e

S e S e

S e

S e

S e

C P h 2 P

P h 2 P

C

P P h 2

P P h 2

60 °C

toluene

–

Tl+

10

• Is redox capability of metal centre necessary for Se-H+ exchange?

XRD: • Two Hg(II) centres linked by two [SeC(Ph2PSe)2]2- ligands

• Hg···Hg = 3.105(1) Å (cf. 2.49-2.53 Å in Hg2X2)

Conclusion: • Se-H+ exchange can occur without oxidation at metal centre

J. Konu, T. Chivers, Chem. Commun., 2010, 46, 1431. 11

A Triseleno PCP-Bridged Binuclear Hg(II) Complex

THF or toluene,

65 oC, 48h

- 2 [H2C(PPh2)(PPh2Se)] 2 II

II H g

S e

S e S e

S e

S e

S e

P h 2 P

P h 2 P

C

P P h 2

P P h 2

H g

C

S e

S e P

H C

P

H g I I

S e

S e P

C H

P

P h 2 P h 2

P h 2 P h 2

Summary: Proton-Selenium Exchange

• Redox disproportionation [MII → MIV + M0 (M = Sn, Te)]

• Oxidation (TlI → TlIII)

• No change in oxidation state (HgII)

• Mechanism???

• Can the dianion [SeC(Ph2PSe)2]2- be generated by direct synthesis? 12

-H+, +Se

+

S e S e

P P h 2

H C

P h 2 P -

Se

Se P P h 2

C

P P h 2

Se

S e

P P h 2

H 2 C

P h 2 P

Formation of Triseleno PCP-Bridged Dianions

Direct Synthesis: • Diseleno PCP-bridged dianion inaccessible (P=Se cleavage)

• Use Le Floch’s dithio PCP-bridged dianion

XRD (E = Se): • S,Se-chelation of both Li+ ions

• Approx. planar C atom (Σ <C = 355°)

• d(C-Se) = 1.970(3) Å (single bond = 1.97 Å; double bond = 1.74 Å)

E, 2 TMEDA, toluene

(85-90%)

-80 oC (E = S)

0 oC (E = Se)

J. Konu, T. Chivers, H. M Tuononen, Chem. Eur. J., 2010, 16, 12977. 13

Trichalcogeno PCP-Bridged Dianions: Lithium Derivatives

S S

P P h 2

C

P h 2 P 2 -

( L i + ) 2 E

C

P h 2 P

S

L i L i

S

P h 2 P

( T M E D A ) ( T M E D A )

Expected products of One and Two-Electron Oxidation: Questions: • Can the anion radicals (B) be stabilized or do they dimerize rapidly? • What is the nature of the chalcogen-chalcogen bonding in these dimers? • Can neutral chalcogenocarbonyls (C) be isolated?

- e- - e-

A (dianion) B (radical anion) C (neutral species)

14

S S

P P h 2

C

P h 2 P

E

S S

P P h 2

C

P h 2 P

E

S S

P P h 2

C

P h 2 P

E

Trichalcogeno PCP-Bridged Dianions: Redox Behaviour?

One-electron oxidation:

• Products are dimers of monoanion radicals [EC(Ph2PS)2]•-

• Long chalcogen-chalcogen distances (compared to single bonds):

S–S = 2.222(2) Ǻ (ca. 8 % elongation)

Se–Se = 2.508(1) Ǻ (ca. 8 % elongation)

• Almost planar carbon atoms (Σ <C = 352-357o) 15

Formation of Chalcogen-Chalcogen Bonded Dimers

I2, toluene

- 2 LiI

(E = S, 60%

E = Se, 74%)

[Li+(TMEDA)]2

2

S

S P P h 2

C

P P h 2

E S

S P h 2 P

C

P h 2 P

E E

C

P h 2 P

S

L i L i

S

P h 2 P

( T M E D A ) ( T M E D A )

• SOMO of anion radical is π* orbital of C=E bond

• Net chalcogen-chalcogen bonding in dimers due to overlap of two SOMOs

• Explains long E–E bonds

[EC(PPh2S)2]•− [(SPh2P)2CEEC(PPh2S)2]2−

16

Frontier Orbitals of Radical Anions and Their Dimers

• Attempted 2e- oxidation of all-sulfur system → monoprotonation

• Also occurs upon removal of Li+ from dianion with 12-crown-4

Monoprotonated Dimer:

• S,S-Chelated [Li(TMEDA)]+ derivative [d(S-S) = 2.140(1) Ǻ]

• Ion-separated [Li(12-crown-4)2]+ salt [d(S-S) = 2.112(2) Ǻ]

• [SC(Ph2PS)2]•- anion radical and [S(H)C(Ph2PS)2]• radical linked by S-S bond

17

THF, CH2Cl2,

[Li+(TMEDA)]2

CH3CN or 12-c-4

[Li+(L)]

(L = TMEDA, (12-c-4)2)

S

S P P h 2

C

P P h 2

S S

S P h 2 P

C

P h 2 P

S

S

S P P h 2

C

P P h 2

S

S

S P

C

P

S

H

P h 2

P h 2

Monoprotonation of Dimeric S-S Bonded Dimer

Two-electron Oxidation: •Occurs cleanly for selenodithio PCP-bridged dianion XRD: • d(Se-I) = 2.722(1) Å, ca. 0.2 Å longer than in RSeI • Planar at C(Se): d(C-Se) = 1.815(4) Å (cf. 1.77-1.84 Å for R2C=Se)

Properties: • Free selone cannot be isolated; Removal of LiI → decomposition

I2, toluene

- LiI

- TMEDA

deep red crystals (71%)

18

Formation and Structure of a Selone-LiI Complex

S e

C

P h 2 P

S

L i L i

S

P h 2 P

( T M E D A ) ( T M E D A )

S

S P P h 2

C

P P h 2

S e ( T M E D A ) L i

I

1. Can the anion radicals (B) be stabilized or do they dimerize rapidly?

The anion radicals dimerize rapidly upon one-electron oxidation of dianions

2. What is the nature of the chalcogen-chalcogen bonding in these dimers?

The dimers form elongated chalcogen-chalcogen bonds (π* - π* interaction}

3. Can neutral chalcogenocarbonyls (C) be isolated?

A LiI-stabilized selenocarbonyl can be isolated.

Attempts to generate the thiocarbonyl resulted in proton abstraction 19

- e- - e-

A (dianion) B (radical anion) C (neutral species)

S S

P P h 2

C

P h 2 P

E

S S

P P h 2

C

P h 2 P

E

S S

P P h 2

C

P h 2 P

E

Trichalcogeno PCP-Bridged Dianions: Redox Behaviour

• Multidentate Chalcogen-centred Dianionic Ligands

• Elongated E–E Bonds (E = S, Se)

Questions:

• What are the possible coordination modes of this dianionic ligand?

• Does the E–E bond remain intact?

• What is the influence of complexation on E–E bond length?

• Does redox occur to give complexes of the monomeric dianion?

Test by carrying out reactions with coinage metals (Cu, Ag, Au)

20

Dichalcogenide Dianions: Coordination Chemistry

+ 2 CuCl2

+ 2 CuCl

2

(dark red)

XRD: (a) Se-Se bond retained, but elongated by 5-6 % compared to Li derivative

(b) Cu–Se bonding approx. symmetrical (c) First example of η2-Se2 bonding for RSe-SeR 21

Binuclear Cu(I) Complex: Retention of Se–Se Bond

Cu

Cu

C

C

P

P

P

P S

Se

S

S

Se

S

Redox

Metathesis

+ 2 CuCl2

+ 2 CuCl

2

(dark blue)

XRD: • Two independent molecules with S···S contacts elongated by 19 and 27 % compared to Li derivative

DFT: • Some diradical character, cf. flexible S-S bond

Konu, Chivers, Tuononen, Chem. Eur. J., 2011, 17, 11844. 22

Binuclear Cu(I) Complex: Retention of S–S Bond

S P

C

Cu

Cu

S

S

S

S

S

P

P

P

C

Redox

Metathesis

XRD: • Unsymmetrical Ag-η2-Se2 bonding d(Ag−Se) = 2.856(1) Å, 3.200(2) Å

• d(Se−Se) = 2.51 Å (cf. 2.51 Å in Li2 complex)

31P NMR: • VT experiments indicate presence of two

isomers in solution • Rapid Ag-Se exchange process?

+ 2 AgOSO2CF3

(red)

23

Binuclear Ag(I) Complex: Retention of Se–Se Bond

C

Ag

Ag

S S

S

Se Se

P

P

P C

P

S

(Metathesis)

XRD:

• Two-electron reduction (cleavage) of E–E bond to form two monomeric dianions

• One AuI centre oxidized to AuIII (square-planar); the other remains as AuI (linear)

• Tridentate [SeC(Ph2PS)2]2- dianions are S,S’-chelated to the Au2 unit and E-

monodentate towards square-planar AuIII centre

24

Binuclear Au(I)/Au(III) Complexes: Redox Behaviour

+ 2 Au(CO)Cl

S

S P P h 2

C

P P h 2

E S

S P h 2 P

C

P h 2 P

E

[ L i + ( T M E D A ) ] 2

(Redox)

E

u

u S A

S

S

S

E P h

2 P

P h 2 P

C

P P h 2

P P h 2

A

C

(E = S, Se)

Summary:

• E–E bond is retained in Cu and Ag complexes

• Metal-dichalcogenide interaction is stronger for Cu

• Pronounced elongation of S–S bond and some diradical character

• Redox reaction occurs with gold(I) reagent to give Au(I)/Au(III) complex • Very flexibile ligand due to labile E–E bond

• Diradical character - activation of small molecules- further studies 25

Dichalcogenide Dianions: Coordination Chemistry

A new feature of the Se-H+ exchange process

NMR: •Monoselenide H2C(PPh2)PPh2Se detected

XRD: •Dimeric CuI/CuI complex with Se-Se bond (2.689(1) Å), cf. single bond ~ 2.33 Å

Conclusion: •Se-H+ exchange, reduction of metal (CuII → CuI) and oxidation of ligand → diselenide

+ 2 CuIICl2

4 - 2 [H2C(PPh2)(PPh2Se)]

(45%)

26

Binuclear Cu(I) Complex via Se-H+ Exchange

Cu

Cu

C

C

P

P

P

P Se

Se

Se

Se

Se

Se

M. Risto, J. Konu, T. Chivers, Inorg. Chem. 2010, 50, 406