Matrix Isolation of Germacyclopentadienes: Direct Spectroscopic Evidence for Germylene-to-Germene Photochemical Interconversion

Abstract

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 1b 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-d6 produced 1b-d6, 2b-d6, 5b-d6, and 6b-d6. 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 d6 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 GeC stretching frequencies are low and provide evidence for CGeCC and GeCCC π conjugation, which is somewhat weaker than that established earlier in the silacyclopentadienes 5a and 6a.