TIME-RESOLVED INFRARED OBSERVATION OF A LONG-LIVED TRANSIENT IN THE PHOTO-DECARBONYLATION OF MN(CO)5C(O)CH3 - POTENTIAL RELEVANCE TO THE CO MIGRATORY INSERTION MECHANISM

Flash photolysis experiments have been carried out at ambient temperature to probe potential reaction intermediates in the carbonylation of the methylmanganese complex CH3Mn(CO)5 (M) to give the acyl product CH3C(O)Mn(CO)5 (A). Photo-decarbonylation of A gives a long-lived transient C, whose time-resolved infrared spectrum shows but modest sensitivity to the coordinating ability of the solvent such as cyclohexane, perfluoromethylcyclohexane, tetrahydrofuran, and 2,5-dimethyl-THF. Notably, C has an unexpectedly long lifetime (exceeding milliseconds) in each of the solvents investigated. Kinetics studies in cyclohexane show the competing trapping by CO to re-form A and isomerization to M occurs with the respective rate constants k(CO) = 3.8 x 10(3) M-1 s-1 and 10 s-1. The former value is 3 orders of magnitude less than expected for a solvento complex under these conditions. Thus, it is concluded that TRIR spectra and reactivity patterns of C reflect stabilization of this intermediate by eta2-coordination of the acyl group.