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 Cs conformation, found to be a saddle point and 26.7 kJ mol−1 higher in energy at HF/6-31G*. Important structural parameters (ra) 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 PBut(SiH3)2 and PBut(SiH3)(SiCl3) have also been investigated by ab initio calculations and the distortions analysed in a similar manner to those of PBut(SiCl3)2.