A Glimpse at the Chemistry of GeH2 in Solution. Direct Detection of an Intramolecular Germylene-Alkene π-Complex

The photochemistry of 3-methyl-4-phenyl-1-germacyclopent-3-ene (4) and a deuterium-labeled derivative (4-d(2)) has been studied in solution by steady state and laser flash photolysis methods, with the goal of detecting the parent germylene (GeH2) directly and studying its reactivity in solution. Photolysis of 4 in C6D12 containing acetic acid (AcOH) or methanol (MeOH) affords 2-methyl-3-phenyl-1,3-butadiene (6) and the O-H insertion products ROGeH3 (R = Me or Ac) in yields of ca. 60% and 15-30%, respectively, along with numerous minor products which the deuterium-labeling studies suggest are mainly derived from hydrogermylation processes involving GeH2 and diene 6. The reaction with AcOH also affords H-2 in ca. 20% yield, while HD is obtained from 4-d2 under similar conditions. Photolysis of 4 in THF-d(8) containing AcOH affords AcOGeH3 and 6 exclusively, indicating that the nucleophilic solvent assists the extrusion of GeH2 from 4 and alters the mechanism of the trapping reaction with AcOH compared to that in cyclohexane. Laser flash photolysis of 4 in hexanes yields a promptly formed transient exhibiting lambda(max) approximate to 460 nm, which decays on the microsecond time scale with the concomitant growth of a second, much longer-lived transient exhibiting lambda(max) approximate to 390 nm. The spectrum and reactivity of the 460 nm species toward various germylene trapping agents are inconsistent with those expected for free GeH2; rather, the transient is assigned to an intramolecular Ge(II)-alkene pi-complex of one of the isomeric substituted hydridogermylenes derived from a solvent-cage reaction between GeH2 and its diene (6) coproduct, formed by addition of HGe-H across one of the C=C bonds. These conclusions are supported by the results of DFT calculations of the thermochemistry associated with pi-complexation of GeH2 with 6 and the formation of the isomeric vinylgermiranes and 1,2-hydrogermylation products. A different species is observed upon laser photolysis of 4 in THF solution and is assigned to the GeH2-THF complex on the basis of its UV-vis spectrum and rate constants for its reaction with AcOH and AcOD.