Formation and Reactions of Tetracarbonyl Intermediates of the Fischer Carbene Complex (CO)5W=C(OMe)Ph : A Laser Flash Photolysis Study Using Time-Resolved Infrared and UV/Vis Spectroscopy

Abstract

The results of nanosecond laser flash photolysis (XeCl excimer irradiation at 308 nm) of the Fischer carbene complex [(CO)5W=C(OMe)Ph] (CZ) in di-n-butyl ether and n-hexane are reported. Spectrokinetical detection of the intermediates was based on transient absorptions simulaneously recorded in the UV/vis and the IR spectral regions. The primary photoproducts are the unstable isomer CE produced with a quantum yield of about 30% and a tetracarbonyl complex I-S, probably of structure [(CO)4(S)W=C(OMe)Ph], where S is a solvent molecule. The quantum yield of CO photoelimination was estimated as 1.7%. The CE isomer was found to return to the stable CZ isomer within a few microseconds, confirming previous results of McGarvey and co-workers. The tetracarbonyl species reacts with cosolutes present in the solution such as N2, H2O, acetonitrile, and methyl trans-crotonate to form secondary complexes of cis-tetracarbonyl complex structure [(CO)4(L)W=C(OMe)Ph] (I-L). The tetracarbonyl complexes react readily with CO. For I-N2 in n-hexane the rate constant is 3.5 × 108 M-1 s-1. The rate constants of the corresponding reactions with acetonitrile (MeCN) and the olefin are larger by a factor of 2. In the presence of CO the complexes I-MeCN and I-olefin revert to CZ in a dissociative reaction mechanism. The rate constants for the elimination of MeCN or olefin are 5 and 105 s-1, respectively. The I-olefin complex irreversibly decomposes with a rate constant of 8 × 103 s-1.