Organometallics, 25, 5424-5434 (2006).
© American Chemical Society
William J. Leigh,* Farahnaz Lollmahomed, Cameron R. Harrington, and Jacklyn M. McDonald
Contribution from the Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, ON Canada L8S 4M1.
The reactions of methanol, tert-butanol, and THF with a series of simple germylene derivatives - dimethyl- (GeMe2), diphenyl- (GePh2) and dimesitylgermylene (GeMes2; Mes = 2,4,6-trimethylphenyl) - have been studied in hexane solution at 25 oC 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 of 290-360 nm and decay with mixed order kinetics with concomitant formation of the corresponding digermene, Ge2R4 (R = Me, Ph, or Mes). Absolute rate constants for formation of the complexes could be measured for GePh2 with all three substrates and for GeMe2 with THF, and were found to vary over the range 4-10 x 109 M-1s-1. Equilibrium constants were measured in all cases; they vary between 1.2 M-1 and 13 M-1 for GeMes2, decreasing in the order MeOH > t-BuOH > THF, while those for GeMe2 and GePh2 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 GePh2 than for GeMe2, 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 larger for GeMe2 than for GePh2. 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.
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