Photoredox properties of [OsN(NH3)4]3+ and mechanism of formation of [{Os(NH3)4(CH3CN)}2N2]5+ through a nitrido-coupling reaction

Abstract

The oxidation of [Os(NH₃)₅Cl]²⁺ by Ce^IV in water gave [OsN(NH₃)₄]³⁺ quantitatively. Excitation of [OsN(NH₃)₄]³⁺ in the solid state or in solution at 300–400 nm resulted in room-temperature photoluminescence. The low-lying spin orbital E sublevel of the ³E {³[(d_xy)¹(d_π*)¹]} state was assigned as the emission state. The excited state of [OsN(NH₃)₄]³⁺ was quenched by electron donors such as aromatic hydrocarbons, alkoxybenzenes, amines and alcohols. A photoinduced electron-transfer mechanism is proposed for the quenching processes. The emission lifetime of [OsN(NH₃)₄]³⁺ depended on the complex concentration. An excited-state bimolecular self-quenching mechanism ([Os^VIN]*+[Os^VIN]→ product) is likely. Photolysis of [OsN(NH₃)₄]³⁺ in the presence of an electron donor such as 1,4-dimethoxybenzene or C₆Me₆ in acetonitrile led to the formation of the µ-dinitrogen product [{Os(NH₃)₄(CH₃CN)}₂N₂]⁵⁺. The coupling reaction obeyed the kinetics rate =k₂[Os^VI][Os^V] where k₂ was estimated to be (3.75 ± 0.30)× 10⁵ dm³ mol^–1 s^–1 at 298 K. Oxidation of [{Os(NH₃)_4^–(CH₃CN)}₂N₂]⁵⁺ by Ce^IV in water followed by precipitation with 2 mol dm^–3 HCl gave the product [OsN(NH₃)₄]Cl₃ in detectable yield.