Redox reactivity of photogenerated osmium(II) complexes

Powerful reductants [Os^(II)(NH_3)_5(L]^(2+) (L = OH_ 2, CH_3, CN can be generated upon ultraviolet excitation of relatively inert [Os^(II)(NH_3)_5(N_2)]^(2+) in aqueous and acetonitrile solutions. Reactions of photogenerated Os(II) complexes with methyl viologen to form methyl viologen radical cation and [Os^(III)(NH_3)_5L]^(3+) were monitored by transient absorption spectroscopy. Rate constants range from 4.9 × 10^4 M^(−1) s^(−1) in acetonitrile solution to 3.2 × 10^7 (pH 3) and 2.5 × 10^8 M^(−1) s^(−1) (pH 12) in aqueous media. Photogeneration of five-coordinate Os(II) complexes opens the way for mechanistic investigations of activation/reduction of CO_2 and other relatively inert molecules.