Reactions of a tungsten alkylidyne complex with mono-dentate phosphines: Thermodynamic and theoretical studies

Abstract

Addition of mono-dentate phosphines PMe3 and PMe2Ph to the W(VI) alkyl alkylidyne complex W(CH2SiMe3)3(CSiMe3) (1) is reversible, each reaching equilibrium. Thermodynamic studies of the equilibria have been conducted, giving ΔH°=−10.0(1.1)kcal/mol and ΔS°=−23(4)eu for the addition of PMe3 and ΔH°′=−3.0 (0.7)kcalmol−1 and ΔS°′=−6(3)eu for the addition of PMe2Ph, indicating that the addition is exothermic. The experimental measurement allows a benchmarking study to select a proper DFT method to describe the current system. Of the DFT methods tested, M06 has demonstrated superior performance in calculating binding energy of a bimolecular reaction. The calculated reaction pathways show that W(CH2SiMe3)3(CSiMe3) (1) reacts with PR3 to form W(CH2SiMe3)3(CSiMe3)(PR3) (PR3=PMe3, 3a; PMe2Ph, 3b), and the adduct then undergoes α-H migration to form W(CH2SiMe3)2(CHSiMe3)2(PR3) (4a, 4b). 4a and 4b are found to be thermodynamically and kinetically stable intermediates. The calculations also suggest a pathway in the formation of the alkyl alkylidene alkylidyne complex W(CH2SiMe3)-(CHSiMe3)(CSiMe3)(PR3)2 (5a).