Fast Kinetic Studies of the Reactivities of Transient Germylenes in Methanol and Tetrahydrofuran Solution

Laser flash photolysis techniques have been employed to study the reactivities of dimethylgermylene (GeMe 2 ), diphenylgermylene (GePh 2 ), and a series of ring-substituted diarylgermylenes (GeAr 2 ) 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 GeMe 2 −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 GeAr 2 −MeOH complexes decay with pseudo-first-order kinetics at low laser intensities and lifetimes in the 20−50 μs range; the GeMe 2 −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 GePh 2 −MeOH complex with rate constants of ca. 3 × 10 9 M −1 s −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 Ar 2 (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 Ar 2 (MeO)Ge − anions and determination of rate coefficients for their protonation by MeOH, for all four of the diaryl systems that were studied. The reactivity of the GeMe 2 −THF complex toward sodium methoxide, methanesulfonic and acetic acid, CCl 4, oxygen, isoprene, and 4,4-dimethyl-1-pentene has also been examined.