Time-resolved spectroscopic studies of the reactivities of dimethylgermylene and tetramethyldigermene in solution

Dimethylgermylene (GeMe2) has been generated and studied by laser flash photolysis of 1,1-dimethyl-and 1,1,4-trimethyl-3-phenylgermacyclopent-3-ene (3a and 3b, respectively) in hexane solution, where it exhibits lambda(max) = 470 nm and decays on the microsecond time scale with second-order kinetics. In the absence of scavengers the decay of GeMe2 is accompanied by the growth of absorptions due to tetramethyldigermene (Ge2Me4; lambda(max) = 370 nm), which decays over the next ca. 100 mu s. Absolute rate constants for reaction of GeMe2 with several reagents have been determined, including acetic acid, CCl4, oxygen, n-butyl- and triethylamine, a terminal alkene and alkyne, two dienes, and the group 14 trialkymetallyl hydrides R3MH (M = Si, Ge, Sn; R = Et or n-Bu). The rate constants agree closely with gas-phase values for the same or similar substrates. The data are compared to those for reaction of GePh2 with the same set of reagents, showing GeMe2 to be significantly more reactive than its phenylated analogue in every case. Evidence of reversibility is obtained for the reactions with the alkene and dienes, which are thought to yield the corresponding three-membered germanocycles as primary products; the equilibrium constants for these reactions are also considerably higher than those for reaction of GePh2. Comparisons are also made with published data for SiMe2 in solution. Absolute rate constants for reaction of Ge2Me4 have also been measured or assigned upper limits and are compared to those for the tetraphenyl analogue (Ge2Ph4), providing the first quantitative assessment of the effects of substituents on the reactivity of the Ge=Ge bond.