A study of the asymmetry of ligands in bis(trichlorosilyl)tert-butylphosphine, PBut(SiCl3)2: molecular structure by gas-phase electron diffraction and ab initio calculations

Abstract

The molecular structure of bis(trichlorosilyl)tert-butylphosphine has been determined in the gas phase by electron diffraction (GED) and high level ab initio molecular-orbital calculations. The C₁ global minimum on the potential energy surface could be reached by twisting all three groups in the same sense by 15–20° from the fully staggered C_s conformation, found to be a saddle point and 26.7 kJ mol⁻¹ higher in energy at HF/6-31G*. Important structural parameters (r_a) are: (distances) P–C 190.6(6), P–Si (mean) 221.0(5), C–C (mean) 156.5(6) and Si–Cl (mean) 203.2(1) pm; (bond angles) P–C–C (mean) 109.7(6), C–P–Si 105.0(7) and 104.5(7), Si–P–Si 99.9(6), and P–Si–Cl (mean) 111.2°; (dihedral angles) Si(7)–P–C–C 87.0(17), Si(8)–P–C–C 168.3(17), Cl(17)–Si–P–C 68.3(17) and Cl(18)–Si–P–C 80.0(10)°. Theoretical predictions at the MP2(fc)/6-31G* level were used to restrain some of the refining parameters using the SARACEN method. The angles at silicon and carbon (tert-butyl) were found to vary from 105.9(7) to 117.4(5)°, indicating that the three groups are greatly distorted from regular tetrahedral geometry. The axial and equatorial components of the tilts of these groups have been investigated and the tert-butyl group was found to tilt in the direction of the phosphorus lone pair with virtually no equatorial tilt. One SiCl₃ group is tilted towards the tert-butyl group and one tilted away, both with axial and equatorial components. The structures of the related molecules PBu^t(SiH₃)₂ and PBu^t(SiH₃)(SiCl₃) have also been investigated by ab initio calculations and the distortions analysed in a similar manner to those of PBu^t(SiCl₃)₂.