Cis- and trans-nitrosyltetraammineruthenium(II). Spectral and electrochemical properties and reactivity

Abstract

A synthetic route was developed for the preparation of trans-[Ru(NH3)4(NO)X]n+, where X = isonicotinamide (isn), pyrazine (pyz) or sulfite, and cis-[Ru(NH3)4(NO)(NO2)]2+. The complexes have been characterized by elemental analysis, UV/VIS, infrared, 1H NMR and ESR spectroscopies, molar conductance measurements and cyclic voltammetry. All showed ν(NO) in the range characteristic of metal-co-ordinated NO+ and do not exhibit any ESR signal, consistent with the formulation of RuII–NO+. The equilibrium constants Keq for the reaction trans-[Ru(NH3)4(NO)X]3+ + 2OH– ⇌ trans-[Ru(NH3)4(NO2)X]+ + H2O are 2.5 × 108 and 6 × 108 dm6 mol–2 for X = isn or pyz. Cyclic voltammograms of the complexes in aqueous solution exhibited reversible one-electron waveforms in the potential range –0.13 to –0.38 V vs. SCE, which were assigned to the [Ru(NH3)4(NO)X]n+ → [Ru(NH3)4(NO)X](n–1)+ process. Nitric oxide and trans-[Ru(NH3)4(H2O)X]2+ are the final products of the reaction between EuII and trans-[Ru(NH3)4(NO)X]3+, L = isn or pyz. Ab initio molecular orbital calculations performed for trans-[Ru(NH3)4(NO)(pyz)]3+ and trans-[Ru(NH3)4(NO)(pyz)]2+, and the products of the trans-[Ru(NH3)4(NO)(pyz)]3+ one-electron electrochemical or chemical reduction, strongly suggest the added electron is localized mainly on the nitrosyl ligand. A correlation was observed between ν(NO) and E½ for the reversible reduction wave. These results indicate that the nitric oxide reduction is facilitated by strong π-acceptor ligands trans to the NO. Nitric oxide and trans-[Ru(NH3)4(H2O)X]3+ were formed when solutions containing trans-[Ru(NH3)4(NO)X]3+ were irradiated in the range 310–370 nm.