Alkene-stabilized biradical and Zwitterionic photoproducts of the clusters[Os-3(CO)(10)(alpha-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), produc ing the biradical [(CO)4Os.Os(CO)(4)-Os+(CO)(2)(i-Pr-AcPy.-)]. This photopr oduct was studied by nanosecond time-resolved transient absorption (TA) and IR (TRIR) spectroscopy. The fast back-reaction of the biradical to the par ent cluster can be slowed in weakly coordinating solvents (THF, acetone) th at stabilize the coordinatively unsaturated Os+(CO)(2)(i-Pr-AcPy.-) moiety, Particularly long lifetimes, up to about 10 mus, are observed for alkene-s tabilized biradicals due to pi -backbonding to the alkene. When the Lifetim e becomes sufficiently long, conversion of the biradical into the zwitterio n [(CO)(4)Os--Os(CO)(4)-Os+(L)(CO)(2)(i-Pr-AcPy)] begins to compete with th e radical recombination to re-form the parent cluster. This charge separati on process occurs for L = 1-octene, styrene at room temperature, while for L = cyclohexene, acetone, THF it occurs only at sufficiently low temperatur es. On a much longer time scale the zwitterions also revert to the parent c luster. At even lower temperatures they become virtually stable. The observ ation of a GO-stabilized zwitterion demonstrates that coordination of a str ong two-electron-donating Lewis base is not imperative to drive the intramo lecular charge separation reaction, provided the back-reaction of the birad ical is significantly hampered. The TRIR spectra of the open-structure bira dical photoproducts reflect the changes in electron density distribution ca used 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.