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}6system

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) (E1/2M) 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.