Matrix isolation of germacyclopentadienes: Direct spectroscopic evidence for germylene-to-germene photochemical interconversion

UV irradiation of matrix-isolated 1,1-diazido-1-germacyclopent-3-ene (4b) produced 1H-germole (1-germacyclopenta-2,4-diene; 1b) and 1-germacyclopent-3-ene-1,1-diyl (2b) as major products along with 3H-germole (1-germacyclopenta-1,4-diene; 5b) and 2H-germole (1-germacyclopenta-1,3-diene; 6b) as minor products. During irradiation at selected wavelengths a photoconversion of Ib into 5b and reversible interconversion of 2b and 6b have been observed and provide experimental evidence for a germylene-to-germene rearrangement. Similar experiments were carried out on deuterated analogues, and the precursor 4b-d(6) produced 1b-d(6), 2b-d(6), 5b-d(6), and 6b-d(6). Full vibrational assignment in the IR spectra of 1b, 2b, 5b, and 6b has been proposed on the basis of restricted Hartree-Fock (DZ+d) calculations for those molecules and their d(6) analogues and Literature data on the silicon analogues 1a, 2a, 5a, 6a, and other related molecules. The UV-vis spectra are interpreted by comparison with the experimental and computed transition energies of the silacyclopentadiene isomers 1a, 2a, 5a, and 6a. The Ge=C stretching frequencies are low and provide evidence for C=Ge-C=C and Ge=C-C=C pi conjugation, which is somewhat weaker than that established earlier in the silacyclopentadienes 5a and 6a.