Organometallics 28, 1484-1494 (2009).
© American Chemical Society
Farahnaz Lollmahomed, William J. Leigh,* Lawrence A. Huck, Saurabh S. Chitnis, and Cameron R. Harrington
Contribution from the Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, ON Canada L8S 4M1.
Laser flash photolysis techniques have been employed to study the reactivities of dimethylgermylene (GeMe2), diphenylgermylene (GePh2) and a series of ring-substituted diarylgermylenes (GeAr2) in methanol (MeOH) and tetrahydrofuran (THF) solution, where the germylenes exist as the corresponding Lewis acid-base complexes with the O-donor solvents. Dimerization to the corresponding digermene is a significant mode of decay in THF solution, particularly for the GeMe2-THF complex, though for the diaryl systems it is slowed dramatically compared to the situation in hexane solution. On the other hand, dimerization is undetectable in MeOH, where the GeAr2-MeOH complexes decay with pseudo-first order kinetics at low laser intensities and lifetimes in the 20-50 μs range; the GeMe2-MeOH complex exhibits a lifetime of ca. 4 μs under similar conditions. Solvent kinetic isotope effects are consistent with solvent-catalyzed proton transfer to yield the corresponding alkoxyhydridogermane as the process responsible for the decay of the germylene-MeOH complexes. Their decay is accelerated by strong acids and bases such as methanesulfonic acid and sodium methoxide, respectively, which quench the GePh2-MeOH complex with rate constants of ca. 3 x 109 M-1s-1 in each case. Kinetic isotope and substituent effects are consistent with rate-determining protonation at germanium in the reactions with acid, while the results for methoxide quenching are consistent with the formation of the corresponding Ar2(MeO)Ge- anion as a discrete intermediate, which has been detected in MeOH solution in one instance. Reaction of sodium methoxide with the germylene-THF complexes also proceeds rapidly, and allows the detection of the corresponding Ar2(MeO)Ge- anions and determination of rate cooefficients for their protonation by MeOH, for all four of the diaryl systems that were studied. The reactivity of the GeMe2-THF complex toward sodium methoxide, methanesulfonic and acetic acid, CCl4, isoprene, and 4,4-dimethyl-1-pentene has also been examined.
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