Phosphido-Borane

Stabilised Tetrylenes

By Alexander Craig

Stabilisation of Group 14 (II) Oxidation States

Heavier Group 14 elements favour (II) oxidation state versus (IV) oxidation state, E = Pb > Sn > (Ge), eg PbI2

Heteroatom (NR2, PR2) ligands: Thermodynamic stabilisation

Efficient pπ-pπ orbital overlap in planar configuration

P: Higher energetic barrier to planarization (planar only seen once in tetrylene systems)

Bulky hydrocarbon ligands: Kinetic stabilisationSnP P

PhPh

Armes2 Armes2

(1) P. P. Power et al., Inorg. Chim. Acta., 2007, 360, 1278-1286.

Phosphine-Borane Stabilised Carbanion Tetrylenes Sn(II) centre with two carbanion ligands Agostic-type interactions between B-H…E contacts, (E =

Sn, Pb) Delocalisation of electron density from B-H σ-orbitals

into vacant pπ-orbital on E

EPMe2RMe2Si

SiMe2RMe2P

H2B

BH2

H

H

(1) K. Izod et al., Organometallics, 2009, 28, 2211-2217.(2) K. Izod et al., Organometallics, 2009, 28, 5661-5668.

E

SiMe2

Me2Si

Me3Si

Me3Si

Me2P

PMe2

BH3

BH3

Project Aims

Synthesis and characterisation of [R2P(BH3)]- ligands Investigate [R2P(BH3)]- ligands in the stabilisation of low

valent Sn(II) centres Isolation and characterisation of novel bis-phosphido-

borane stannylenes, [R2P(BH3)]2Sn

R2PLi2SnCl2

THF, -78 oC SnP P

H2B BH2

H H

R RR R

2 LiClBH3

-105-95-85-75-65-55-45-35-25-15-5f1 (ppm)

Synthesis of Phosphido-Borane Precursor LigandPCl3

2 MesMgBrEt2O, -78 oC

Mes2PClLiAlH4 Mes2PH

Et2O, 0 oC

Mes2PHBH3.SMe2THF, RT Mes2PH

BH3

Mes2PHBH3 n-BuLi

THF, RT Mes2PLiBH3

31P NMR spectra in CDCl3 or THF-d8/toluene-d8

Mes2P [Li(THF)2]BH3BH3

Mes2PLiBH3 BH3.SMe2

THF, RT

Crystal Structure Analysis

η2-B…Li = 2.426(3) Å

η2-B2…Li = 2.455(4) Å

η1-B2…Li = 2.505(4) Å

R3PB H

H H Li R2PB2

B1

H HH

HHH Li

Attempted Synthesis of [Mes2P(BH3)]2Sn

Rapid decomposition: P-Sn bond

weaker vs C-Sn No pπ-pπ

overlap Reductive

phosphorus centre

Mes2PLiBH3

2SnCl2

THF, -78 oC Decomposition Products

-95-90-85-80-75-70-65-60-55-50-45-40-35-30-25-20-15-10-5f1 (ppm)

Mes2P [Li(THF)2]BH3BH3

Mes2PHBH3

Mes2PH

31P NMR spectrum in THF

Attempted Synthesis of [Dipp2P(BH3)]2Sn

Dipp: Increased kinetic stabilisation

Rapid decomposition

-100-95-90-85-80-75-70-65-60-55-50-45-40-35-30-25-20-15f1 (ppm)

Dipp2PLi2SnCl2

THF, -78 oC Decomposition ProductsBH3

Dipp2P [Li(THF)2]BH3BH3

Dipp2PHBH3

Dipp2PH

31P NMR spectrum in THF

Synthesis of [Ph2P(BH3)]Li in situ

-60-55-50-45-40-35-30-25-20-15-10-505101520f1 (ppm)

Ph2PHBH3.SMe2THF, RT Ph2PH

BH3

Ph2PH n-BuLiTHF, RT Ph2PLi

BH3BH3

Ph2PH

31P NMR spectra in CDCl3 or THF-d8/toluene-d8

Attempted Synthesis of [Ph2P(BH3)]2Sn

-55-45-35-25-15-505101520253035404550f1 (ppm)

[{Ph2P(BH3)}3Sn]Li(THF)

JSnP = 1630 Hz

Ph2PLiBH32

SnCl2THF, -78 oC [{Ph2(BH3)}3Sn]Li(THF) X

The stannate, [{Ph2P(BH3)}3Sn]Li(THF), was isolated by crystallisation from toluene

X

31P NMR spectrum in THF

Crystal Structure of [{Ph2P(BH3)}3Sn]Li(THF)

η2-B1…Li = 2.470(13) Å

η1-B2…Li = 2.798(14) Å

η2-B3…Li = 2.477(14) Å

Attempted Synthesis of [Ph2P(BH3)]2Sn

-29-27-25-23-21-19-17-15-13-11-9-8-7-6-5f1 (ppm)

31P NMR spectrum in THF-d8/toluene-d8

Ph2PLiBH32

SnCl2THF, -78 oC [{Ph2(BH3)}3Sn]Li(THF)

[{Ph2P(BH3)}3Sn]Li(THF)

JSnP = 1630 Hz

-16.5-15.5-14.5-13.5-12.5-11.5-10.5-9.5-8.5-7.5-6.5f1 (ppm)

Direct Synthesis of [{Ph2P(BH3)}3Sn]Li(THF)

Ph2PLi3SnCl2

THF, -78 oC [{Ph2P(BH3)}3Sn]Li(THF)BH3

JSnP = 1630 Hz

-85-80-75-70-65-60-55-50-45-40-35f1 (ppm)

JSnP = 1630 Hz

31P NMR spectrum in THF-d8/toluene-d8119Sn NMR spectrum in THF-d8/toluene-d8

Summary Isolated & characterised novel

dimesitylphosphido-borane & bis-(borane) lithium salts Agostic-type interactions between BH3 and lithium

Rapid decomposition of bis-phosphido-borane stabilised stannylenes, [R2P(BH3)]2Sn, where R = Mes, Dipp

Isolated & characterised the novel tris-phosphido-borane stannate, [{Ph2P(BH3)}3Sn]Li(THF) No agostic-type interactions between BH3 and Sn(II)

Acknowledgements

I would like to thank: Dr Keith Izod Peter Evans Claire Jones Dr Paul Waddell Richard Wardle

Questions ?