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 Ru^II–NO⁺. The equilibrium constants K_eq 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 Eu^II 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 E_½ 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.