Compounds containing B–O–X bonds (X = Si, Ge, Sn, Pb). Part 4.—Crystal structures of B(OSiPh3)3, PhB(OSiPh3)2 and PhB(OGePh3)2

Abstract

This paper provides both experimental and theoretical evidence of the remarkable flexibility of B–O–X bond angles (X = Si or Ge). Single-crystal X-ray diffraction analyses of PhB(OSiPh₃)₂, PhB(OGePh₃)₂ and B(OSiPh₃)₃ are reported. In the compound B(OSiPh₃)₃, the B–O–SiPh₃ units are sterically constrained and the three B–O–Si angles are approximately equal [138.5(2), 139.2(2) and 142.5(2)°]. In both PhB(OSiPh₃)₂ and PhB(OGePh₃)₂, different angles are observed for the two chemically equivalent B–O–X fragments [for X = Si, 139.8(4) and 157.9(3)° and for X = Ge, 130.4(5) and 149.0(5)°]. GAUSSIAN-86 calculations (STO-3G and STO-3G*) were performed on the model compounds H₂B–O–XH₃(X = Si or Ge), [(HO)H₂B–SiH₃]^–, (HO)₂B–O–Si(OH)₃ and [(HO)₃B–O–Si(OH)₃]^– to ascertain their minimum-energy geometries. The B–O–X angles in H₂B–O–XH₃(X = Si or Ge) and [(HO)H₂B–O–SiH₃]^–, could be varied by 22°(36°), 24°(22°) or 16°(28°), respectively [data from STO-3G (and STO-3G*) calculations] without changing the compounds' minimum energies by >5 kJ mol^–1. From the calculations, B–O–X angles appear to be as flexible in tetracoordinated boron species as in tricoordinated ones.