Kinetics and Mechanisms of the Reactions of Diaryl- and Dialkylgermylenes with Alcohols and Ethers in Solution

The reactions of methanol, tert -butanol, and THF with a series of simple germylene derivatives dimethyl- (GeMe 2 ), diphenyl- (GePh 2 ), and dimesitylgermylene (GeMes 2; Mes = 2,4,6-trimethylphenyl) have been studied in hexane solution at 25 °C by laser flash photolysis methods. The results are consistent with rapid, reversible reaction to form weakly stabilized Lewis acid−base complexes, which exhibit UV absorption maxima in the range 290−360 nm and decay with mixed order kinetics with concomitant formation of the corresponding digermene, Ge 2 R 4 (R = Me, Ph, or Mes). Absolute rate constants for formation of the complexes could be measured for GePh 2 with all three substrates and for GeMe 2 with THF and were found to vary over the range (4−10) × 10 9 M -1 s -1 . Equilibrium constants were measured in all cases; they vary between 1.2 and 15 M -1 for GeMes 2, decreasing in the order MeOH > t-BuOH > THF, while those for GeMe 2 and GePh 2 are 2−4 orders of magnitude larger and decrease in the order THF > MeOH > t-BuOH. For a given substrate, the equilibrium constants are consistently larger for GePh 2 than for GeMe 2, reflecting the greater ability of phenyl compared to methyl substituents to stabilize negative charge at germanium in the zwitterionic complexes. In spite of the differences in the stabilities of the complexes, the rate constant for complexation with THF is almost twice as large for GeMe 2 than for GePh 2 . The results indicate that the O−H insertion reaction of germylenes with alcohols proceeds via initial, reversible Lewis acid−base complexation, followed by slow proton transfer from oxygen to germanium, most likely via a catalytic route.