Laser Flash Photolysis Studies of Some Dimethylgermylene Precursors

The laser flash photolysis of dodecamethylcyclohexagermane (1) and dimethylphenyl(trimethylsilyl)germane (5), two known photochemical precursors to dimethylgermylene (GeMe2), has been reinvestigated. Laser flash photolysis of I in hexane solution affords markedly different results than were reported in an earlier study of this compound in cyclohexane. The present study shows it to yield two primary transient products, one exhibiting lambda(max) = 490 nm and lifetime tau <= 10 ns and a second that exhibits lambda(max) = 470 nm and decays on the microsecond time scale with second-order kinetics, with the concomitant growth of absorptions centered at lambda(max) = 370 nm due to tetramethyldigermene (Ge2Me4). The spectrum and absolute rate constants for quenching of the 470 nm species by acetic acid, 2,3-dimethyl-1,3-butadiene (DMB), CCl4, and oxygen in hexane, as well as the transient behavior observed in the presence of millimolar concentrations of THF, are all quite similar to those established recently for GeMe2 using 1-germacyclopent-3-ene derivatives as precursors. Two different transient products dominate the time-resolved spectra obtained from laser flash photolysis of 5 under similar conditions, one exhibiting lambda(max) = 300 nm and tau approximate to 20 mu s and a second exhibiting lambda(max) = 430 nm and tau approximate to 4 mu s. The latter species was assigned to GeMe2 in earlier studies; it is re-assigned in the present work to the conjugated germene (6) derived from 1,3 trimethylsilyl migration into the ortho-position of the phenyl ring in 5, on the basis of comparisons of the spectrum and absolute rate constants for quenching of the species by oxygen, DMB, CCl4, acetic acid, and acetone to those reported previously for the homologous silene derivative (11) and the corresponding 1,1-diphenyl-substituted analogues of the germene (12) and the silene (13). Weak transient absorptions consistent with the postpulse formation of Ge2Me4 are also detectable from 5 in deoxygenated hexanes, but GeMe2 itself cannot be detected.