Alkene-Stabilized Biradical and Zwitterionic Photoproducts of the Clusters [Os3(CO)10(α-diimine)]: A Time-Resolved Transient Absorption and Infrared Study

Abstract

This work focuses on photochemical Os−Os bond cleavage in the cluster [Os3(CO)10(i-Pr-AcPy)] (i-Pr-AcPy = 2-acetylpyridine-N-isopropylimine), producing the biradical [(CO)4Os•−Os(CO)4−Os+(CO)2(i-Pr-AcPy•-)]. This photoproduct was studied by nanosecond time-resolved transient absorption (TA) and IR (TRIR) spectroscopy. The fast back-reaction of the biradical to the parent cluster can be slowed in weakly coordinating solvents (THF, acetone) that stabilize the coordinatively unsaturated Os+(CO)2(i-Pr-AcPy•-) moiety. Particularly long lifetimes, up to about 10 μs, are observed for alkene-stabilized biradicals due to π-back-bonding to the alkene. When the lifetime becomes sufficiently long, conversion of the biradical into the zwitterion [(CO)4Os-−Os(CO)4−Os+(L)(CO)2(i-Pr-AcPy)] begins to compete with the radical recombination to re-form the parent cluster. This charge separation process occurs for L = 1-octene, styrene at room temperature, while for L = cyclohexene, acetone, THF it occurs only at sufficiently...