An Electron‐Rich {RuNO} 6 Complex: trans ‐[Ru(DMAP) 4 (NO)(OH)] 2+ – Structure and Reactivity

Abstract

The pseudo-octahedral nitrosyl species trans-[Ru(DMAP)4- (NO)(OH)]2+ [12+, DMAP = 4-(dimethylamino)pyridine] was prepared by the reaction between [Ru(DMAP)5(H2O)]2+ and NaNO2 under mild conditions (room temperature, pH = 6–8), and precipitated with NaBF4 or NaPF6. Single-crystal X-ray diffraction data of 1(BF4)2·2H2O point to a {RuNO}6 electronic configuration of the cation, although there is a significant deviation from the expected linear arrangement of the RuNO moiety (Ru–N–O angle: 169.3°). The remarkably low wavenumber of the NO stretching band (νNO) of 1832 cm–1 in the solid state corresponds to an electron-rich ligand environment with strong backbonding interactions between dπ metal orbitals and π*NO orbitals of the (formal) NO+ (nitrosonium) ligand. Consistently, the complex acts as a poor electrophile: it is extremely unreactive toward OH– and toward the stronger nucleophile cysteine. Cyclic voltammetry (CV) experiments show three reduction processes in acetonitrile. The first one at –0.50 V [vs. Ag/AgCl, KCl(s)] is reversible on the CV time scale. Spectroelectrochemical experiments (IR and UV/Vis) suggest that the reduced complex has a {RuNO}7 configuration. The νNO wavenumber of 1603 cm–1 agrees with a reduction mostly centered at the nitrosyl ligand. This species appears to be inert toward substitution in the coordination sphere, and the original {RuNO}6 ion can be quantitatively recovered upon oxidation. In contrast, further reduction of this species is completely irreversible, even on the CV timescale, which is probably due to the lability of the presumably formed nitroxyl (NO– ) ligand.