Synthesis and molecular structure of fluoro(triphosphanyl)silane and attempts to synthesize a silylidyne–phosphane

Abstract

The synthesis, isolation, spectroscopic characterization (IR, multi-nuclear NMR) and single-crystal X-ray diffraction analysis of FSi(PH 2) 3 ( 1a), the first isolable fluorophosphanylsilane, is reported along with the gas phase decomposition of 1a, MeSi(PH 2) 3 ( 1b) and EtSi(PH 2) 3 ( 1c) under flash vacuum or pulsed pyrolysis conditions and matrix isolation of the products. The title compound is formed quantitatively by PH 2/F-ligand exchange reaction of tetraphosphanylsilane Si(PH 2) 4 with the difluorodiarylstannane Is 2SnF 2 (Is=2,4,6-triisopropylphenyl) in the molar ratio of 1:1 in benzene as solvent. Since 1a cannot be separated from the solvent by fractional condensation its isolation was achieved by means of preparative GC. A single crystal of 1a (triclinic, P1̄) suitable for X-ray diffraction analysis was grown by in situ crystallization on a diffractometer at 175 K through miniaturized zone melting with focused infra-red radiation. Interestingly, the Si atom is remarkably distorted tetrahedral coordinated with F–Si–P angles of 120.4(7), 110.4(7), 106.3(1)° and normal Si–F (1.60(2) Å) and Si–P distances (av. 2.241(2) Å). According to ab initio (MP2/6-31G(d,p); MP2/6-311G(2d,p)) and DFT calculations (BLYP, B3LYP, B3PW91 functionals), the distortion is not an intrinsic property of the molecule but due to crystal packing forces. The best agreement between the experimental versus calculated geometrical and vibrational data is achieved at the B3PW91/6-311G(2d,p) level of theory. Since 1a– c appeared as potential precursors for the respective silylidyne–phosphanes (‘silaphospha–acetylene’) RSiP through stepwise extrusion of PH 3, some thermodynamical data for the decomposition and the relative energies of linear RSiP versus bent :SiPR isomers (R=H, Me, Et, Pr, Ph, CF 3, OMe, halogen and SiH 3) were also calculated. The latter revealed that electronegative substituents R favor the Si–P triple bond in RSiP (except for R=CF 3 which stabilizes the :SiPR form) while strong σ-donating substituents R (H, SiH 3) favor the :SiPR isomer with SiP double bond. Although elimination of PH 3 and other fragmentation products could be detected by controlled thermal decomposition and matrix isolation, neither flash vacuum experiments of 1a, 1b and 1c (400–600 °C) nor pulsed pyrolyses of 1a at (1100 °C) did provide any direct evidence for the formation of the desired species with Si–P multiple bonds.