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

Abstract

The oxidation of [Os(NH3)5Cl]2+ by CeIV 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[(dxy)1(dπ*) 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 ([OsVI≡N]* + [OsVI≡N] → 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)}2N 2]5+. The coupling reaction obeyed the kinetics rate = k2[OsVI][OsV] 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 CeIV in water followed by precipitation with 2 mol dm-3 HCl gave the product [OsN(NH3)4]Cl3 in detectable yield.