Molecular structure and conformational preferences of trimethylphosphite, P(OCH3)3, as a free molecule and as a ligand in d-block metal complexes

Abstract

Free P(OCH₃)₃ has been studied by gas electron diffraction (GED) and DFT calculations at the B3PW91/6-311+G* level. Each conformer is characterised by three dihedral angles τ(COPlp) where lp denotes the direction of the electron lone pair on the P atom; assumed to lie in a plane containing the P–O bond and bisecting the opposing OPO angle. DFT calculations indicate that the most stable conformer is an anti,gauche⁺,gauche⁺ (ag⁺g⁺) conformer characterised by the angles τ_a = (COPlp) −173, τ_b = 54 and τ_c = 41°. It is followed by an ag⁻g⁺ conformer at ΔE = 6.3 kJ mol⁻¹, an aa⁺g⁺ conformer at ΔE = 6.6 kJ mol⁻¹, and a g⁺g⁺g⁺ conformer at ΔE = 10.4 kJ mol⁻¹. The calculated standard free energies at 298.15 K indicate that the mole fractions in the gas phase at this temperature are χ(±ag⁺g⁺) = 73%, χ(ag⁻g⁺) = 16%, χ(±aa⁺g⁺) = 10% and χ(±ag⁺g⁺g⁺) =1%; GED data indicate that the mole fractions of the more stable conformers at room temperature are χ(±ag⁺g⁺) = 78(13)%, χ(ag⁻g⁺) = 9(11)% and χ(±aa⁺g⁺) = 14(21)%. Natural Bond Orbital (NBO) analysis of the wavefunctions suggest that while bond distances and valence angles are determined by anomeric effects, the relative stabilities of the four conformers are not determined by such effects alone. Examination of the crystal structures of 287 complexes where one or more P(OCH₃)₃ units are coordinated to a d-block transition metal M, shows that in the 523 crystallographically independent MP(OCH₃)₃ fragments 46% of the trimethylphosphite units adopt a ±ag⁺g⁺ conformation, 19% an ag⁻g⁺ conformation and 30% a ±aa⁺g⁺ conformation. It is suggested that the increased number of ±aa⁺g⁺ conformers and decreased number of ±ag⁺g⁺ conformers relative to the gas phase, as well as the folding back of the gauche ligands are due to interligand repulsion in the transition metal complexes. Structure optimisation of F₂POMe by DFT calculations at the B3PW91/6-311+G* level indicate that the most stable conformation is anti, while the energy of a gauche conformer is about 15 kJ mol⁻¹ higher. NBO analysis of the wavefunctions indicate that the relative stabilities of the two conformers as well as the differences between bond distances and valence angles may be determined by anomeric effects.