Phosphine and Diphosphine Complexes of Silicon(IV) Halides

The reaction of SiX4 (X
= Cl or Br) with PMe3 in anhydrous CH2Cl2 forms trans-[SiX4(PMe3)2], while the diphosphines, Me2P­(CH2)2PMe2, Et2P­(CH2)2PEt2, and o-C6H4(PMe2)2 form cis-[SiX4(diphosphine)], all containing six-coordinate silicon centers.
With Me2PCH2PMe2 the product was trans-[SiCl4(κ1-Me2PCH2PMe2)2]. The complexes have
been characterized by X-ray crystallography, microanalysis, IR, and
multinuclear (1H, 13C­{1H}, and 31P­{1H}) NMR spectroscopies. The complexes are stable
solids and not significantly dissociated in nondonor solvents, although
they are very moisture and oxygen sensitive. This stability conflicts
with the predictions of recent density functional theory (DFT) calculations
(Wilson et al. Inorg. Chem. 2012, 51, 7657–7668) which suggested six-coordinate silicon
phosphines would be unstable, and also contrasts with the failure
to isolate complexes with SiF4 (George et al. Dalton
Trans. 2011, 40, 1584–1593).
No reaction occurred between phosphines and SiI4, or with
SiX4 and arsine ligands including AsMe3 and o-C6H4(AsMe2)2. Attempts to make five-coordinate [SiX4(PR3)] using the sterically bulky phosphines, PtBu3, PiPr3, or PCy3 failed, with no
apparent reaction occurring, consistent with predictions (Wilson et
al. Inorg. Chem. 2012, 51, 7657–7668) that such compounds would be very endothermic,
while the large cone angles of the phosphines presumably preclude
formation of six-coordination at the small silicon center. The reaction
of Si2Cl6 with PMe3 or the diphosphines
in CH2Cl2 results in instant disproportionation
to the SiCl4 adducts and polychlorosilanes, but from hexane
solution very unstable white [Si2Cl6(PMe3)2] and [Si2Cl6(diphosphine)]
(diphosphine = Me2P­(CH2)2PMe2 or o-C6H4(PMe2)2) precipitate. The reactions of SiHCl3 with PMe3 and Me2P­(CH2)2PMe2 also produce the SiCl4 adducts, but using
Et2P­(CH2)2PEt2, colorless
[SiHCl3{Et2P­(CH2)2PEt2}] was isolated, which was characterized by an X-ray structure
which showed a pseudo-octahedral complex with the Si–H trans to P. Attempts to reduce the silicon­(IV) phosphine
complexes to silicon­(II) were unsuccessful, contrasting with the isolation
of stable N-heterocyclic carbene adducts of Si­(II).