Nitro–ruthenium(ii)–arylazoimidazoles: synthesis, spectra, crystal structure and electrochemistry of dinitro-bis{1-alkyl-2-(arylazo)imidazole}ruthenium(ii). Nitro–nitroso derivatives and reactivity of the electrophilic {Ru-NO}6 system

Abstract

Silver ion assisted aquation of blue cis,trans,cis-RuCl₂(RaaiR′)₂ (4–6) leads to solvento species, blue–violet cis,trans,cis-[Ru(OH₂)₂(RaaiR′)₂](ClO₄)₂ [RaaiR′ = p-R–C₆H₄–NN–C₃H₂–NN-1-R′ (1–3), abbreviated as N,N′-chelator where N(imidazole) and N(azo) represent N and N′, respectively; R = H (a), Me (b), Cl (c) and R′ = Me (1/4/7/10), CH₂CH₃(2/5/8/11), CH₂Ph (3/6/9/12)] that have been reacted with NO₂⁻ in warm ethanol resulting violet dinitro complexes of the type, Ru(NO₂)₂(RaaiR′)₂ (7–9). The structure in one case, [Ru(NO₂)₂(HaaiCH₂Ph)₂] (9a), has been established by X-ray diffraction as the cis-Ru(NO₂)₂ motif along with trans-N,N and cis-N′,N′ dispositions of the chelator N atoms around the coordination sphere. The nitrite complexes are useful synthons of electrophilic nitrosyls, and on triturating the compounds 7b–9b with conc. HClO₄ nitro–nitrosyl derivatives, [Ru(NO₂)(NO)(MeaaiR′)₂](ClO₄)₂ (10b–12b), are isolated. The solution structure and stereoretentive transformation in each reaction step have been established by ¹H NMR results. All the complexes exhibit strong MLCT transitions in the visible region. They are redox active and display one metal-centred oxidation and successive ligand-based reductions. The redox potentials of Ru(III)/Ru(II) (E_1/2^M) of 10b–12b are anodically shifted by ∼0.2 V as compared to those of dinitro precursors 7b–9b. The ν(NO) > 1900 cm⁻¹ strongly suggests the presence of linear Ru–N–O bonding. The electrophilic behaviour of metal bound nitrosyl has been proved in one case (12b) by reacting with a bicyclic ketone, camphor, containing an active methylene group and an arylhydrazone with an active methine group, and the heteroleptic tris-chelates thus formed are characterised.