Redox reactivity of photogenerated osmium(ii) complexes

Abstract

Powerful reductants [Os^II(NH₃)₅L]²⁺ (L = OH₂, CH₃CN) can be generated upon ultraviolet excitation of relatively inert [Os^II(NH₃)₅(N₂)]²⁺ in aqueous and acetonitrile solutions. Reactions of photogenerated Os(II) complexes with methyl viologen to form methyl viologen radical cation and [Os^III(NH₃)₅L]³⁺ were monitored by transient absorption spectroscopy. Rate constants range from 4.9 × 10⁴ M⁻¹ s⁻¹ in acetonitrile solution to 3.2 × 10⁷ (pH 3) and 2.5 × 10⁸ 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 CO₂ and other relatively inert molecules.