Mechanisms of cis/trans isomerization of (η5-C5H5)2Fe2(CO)2(μ-SR)2, where R = methyl or tert-butyl, under photochemical and thermal conditions

Photolysis of cis,syn-(eta(5)-C5H5)(2)Fe-2(CO)(2)(mu-SMe)(2) and mixtures enriched in the trans,anti isomer in frozen Nujol matrixes at ca. 90 K has revealed a complex set of wavelength-dependent photoproducts. These photoproducts include three species with bridging carbonyl groups that appear to be a set of conformers differing in the relative orientation of the S-Me groups, a dicarbonyl species that may be a conformer of the trans,anti isomer, and two species that appear to have a single terminal carbonyl group. Evidence is also presented for the formation of an IR-silent intermediate, (eta(5)-C5H5)(2)Fe-2(mu-SMe)(2). Solution studies have demonstrated that the bridging carbonyl species are sufficiently stable to be observed at room temperature. These bridging isomers readily react with CO to mostly form the trans,anti isomer, providing one mechanism for cis to trans interchange. Photolysis of cis,syn-(eta(5)-C5H5)(2)-Fe-2(CO)(2)(mu-SMe)(2) in petroleum ether under (CO)-C-13 at room temperature results in formation of both labeled and unlabeled trans,anti isomer, suggesting both a CO-loss and an internal non CO-loss pathway for isomer interconversion. Consistent with this, thermal reequilibration of trans,anti-enriched mixtures in petroleum ether under (CO)-C-13 results in formation of labeled trans,anti isomer, labeled (eta(5)-C5H5)Fe(CO)(2)(SMe), and mostly unlabeled cis,syn isomer. Photolysis of cis,syn-(eta(5)-C5H5)(2)Fe-2(CO)(2)(mu-S-t-Bu)(2) in frozen Nujol yields a bridging carbonyl photoproduct and the trans,anti isomer. Photolysis of the tert-butyl derivative in petroleum ether at 0 degrees C results in formation of the trans,anti isomer.