A new ruthenium nitrosyl species based on a pendant-arm 1,4,8,11-tetraazacyclotetradecane (cyclam) derivative: An experimental and theoretical study

Abstract

The complex cis-[Ru(L py)NO] 3+ ( I) (L py = N-(2-methylpyridyl)1,4,8,11-tetraazacyclotetradecane) was prepared by the stoichiometric reaction between Ru(dmso) 4Cl 2 and L py and an excess of NaNO 2 in ethanolic medium, followed by acidification of the solution. The diamagnetic species was isolated as its hexafluorophosphate salt, and fully characterized by IR ( ν NO = 1917 cm −1) and diverse NMR techniques in combination with theoretical computations based on the density functional theory (DFT). The compound displays strong electronic transitions below 300 nm and weak ones in the visible region of the spectrum, all of them solvent insensitive. The reaction of cis-[Ru(L py)NO] 3+ with OH − generates the strongly colored nitro compound cis-[Ru(L py)NO 2] + ( II) The {RuNO} 6 compound can be interconverted into the one-electron reduced {RuNO} 7 species cis-[Ru(L py)NO] 2+ ( III). The reduction process is completely reversible in the cyclic voltammetry timescale with E 0 (versus Ag/AgCl, 3 M Cl −) = −0.02 V and 0.18 V in water and acetonitrile, respectively. Controlled potential reduction in both solvents yields to the quantitative formation of III, a process which involves significant changes in the electronic spectroscopy. The {RuNO} 7 species proved to be inert against ligand loss, and electrogenerated solutions remained unchanged for several hours if protected from atmospheric oxygen. Electrochemical reoxidation or exposure to air lead to the complete recovery of the starting cis-[Ru(L py)NO] 3+ material, without signs of secondary reactions. The robustness of the coordination sphere appears as a consequence of the multidentate nature of L py.