J. Am. Chem. Soc., submitted.

© American Chemical Society

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

Paul S. Billone, Katie Beleznay, Cameron R. Harrington, Lawrence A. Huck, and William J. Leigh*

Contribution from the Department of Chemistry & Chemical Biology, McMaster University, 1280 Main Street West, Hamilton, ON Canada L8S 4M1.

The photochemistry of 3-methyl-4-phenyl-1-germacyclopent-3-ene (4) and a deuterium-labeled derivative (4-d2) 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 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 H2 in ca. 20% yield, while HD is obtained from 4-d2 under similar conditions. Photolysis of 4 in THF-d8 containing HOAc affords AcOGeH3 and 6 exclusively, indicating that the formal extrusion of GeH2 from 4 really proceeds quite cleanly, and that the trapping reaction with AcOH proceeds via a different mechanism than in cyclohexane. Laser flash photolysis of 4 in hexanes solution yields a promptly-formed transient exhibiting λmax ≈ 460 nm, which decays on the microsecond time scale with the concomitant growth of a second, much longer-lived transient exhibiting λmax ~ 390 nm. The spectrum and reactivity of the 460 nm species toward various germylene substrates are inconsistent with those expected for free GeH2; rather, the transient is assigned to an intramolecular Ge(II)-alkene π-complex of one of the isomeric substituted hydridogermylenes derived from a solvent-cage reaction between GeH2 and its diene (6) co-product, which proceeds 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 π-complexation of GeH2 with 6 and the formation of the isomeric vinylgermiranes and 1,2-hydrogermylation products, and the UV-vis absorption spectra of the various minimum-energy structures. 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 the absolute rate constants for its reaction with AcOH and AcOD.

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