Alkene-stabilized biradical and Zwitterionic photoproducts of the clusters [Os3(CO)10(α-diimine)]:: A time-resolved transient absorption and infrared study

This work focuses on photochemical Os-Os bond cleavage in the cluster [Os-3(CO)(10)(i-PrAcPy)] (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 mus, are observed for alkene-stabilized biradicals due to pi -backbonding to the alkene. When the Lifetime becomes sufficiently long, conversion of the biradical into the zwitterion [(CO)(4)Os--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 low temperatures. On a much longer time scale the zwitterions also revert to the parent cluster. At even lower temperatures they become virtually stable. The observation of a GO-stabilized zwitterion demonstrates that coordination of a strong two-electron-donating Lewis base is not imperative to drive the intramolecular charge separation reaction, provided the back-reaction of the biradical is significantly hampered. The TRIR spectra of the open-structure biradical photoproducts reflect the changes in electron density distribution caused by variation of L, At the same time the TRIR results confirm that upon formation of zwitterions an electron is transferred from the Os+(L)(CO)(2)(i-PrAcPy.-) site to the Os-.(CO)(4) moiety.