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

Abstract

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 (KC) 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, KC 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.