Organometallics, 31, 3755-3767 (2012).

© American Chemical Society

Electronic and Steric Effects on the Lewis Acidities of Transient Silylenes and Germylenes: Equilibrium Constants for Complexation with Chalcogen and Pnictogen Donors

Svetlana S. Kostina, Tishaan Singh, and William J. Leigh*

Contribution from the Department of Chemistry & Chemical Biology, McMaster University, 1280 Main Street West, Hamilton, ON Canada L8S 4M1.

The Lewis acid−base complexation reactions of dimethyl-, diphenyl-, and dimesitylsilylene (SiMe₂, SiPh₂, and SiMes₂, respectively) and their germanium homologues (GeMe₂, GePh₂, and GeMes₂) with diethyl ether (Et₂O), tetrahydrothiophene (THT), triethylphosphine (Et₃P), and tricyclohexylphosphine (Cy₃P) have been characterized in hydrocarbon solvents at 25 °C by laser flash photolysis methods. Together with previously reported (and some new) data for the complexation of the six transient tetrellylenes with methanol (MeOH), tetrahydrofuran (THF), and di- and triethylamine (Et₂NH and Et₃N, respectively), the results allow the first systematic assessment of the thermodynamics of Lewis acid−base complexation of simple dialkyl- and diarylsilylenes and their germanium homologues with chalcogen and pnictogen donors in solution. The equilibrium constants (K_C) for complexation of the six species with Et₂O span a range of ca. 10⁵ M⁻¹, decreasing in the order SiPh₂ > SiMe₂ > GePh₂ > GeMe₂ ≫ SiMes₂ > GeMes₂. For each homologous MR₂ pair, K_C is consistently 10−40 times larger for the silylene than the germylene, indicating a systematic difference in binding free energy of 1.5−2.2 kcal mol⁻¹. Equilibrium constants have been determined for complexation of SiMes₂ and GeMes₂ with all the donors in the series except Et₃P, for which only a lower limit can be determined. Those for SiMes₂ decrease in the order Et₃P > Cy₃P > Et₂NH > THT > Et₃N > THF > Et₂O; GeMes₂ is consistently less acidic, but its binding constants follow a similar ordering. The experimental data are supplemented with theoretical (Gaussian-4) calculations of thermochemical parameters for the complexation of SiMe₂ and GeMe₂ with 17 O, S, N, and P donors, which are shown to agree with experiment to within 1 kcal mol⁻¹ for the 6 systems that have also been studied experimentally. The calculations confirm that, in general, trialkylphosphines bind most strongly, followed by amines, sulfides, and then ethers and alcohols. SiMe₂ and GeMe₂ are borderline-soft Lewis acids, stronger and softer than trimethylborane in both cases.

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