Ruthenium complexes in different oxidation states: synthesis, crystal structure, spectra and redox properties

The reactions of a mother solution of RuCl(3) with benzimidazole derivatives 2-(2'-pyridyl)benzimidazole (2,2'-PyBIm, L(1)) and 2-hydroxymethylbenzimidazole (2-CH(2)OHBIm, L(2)) yielded three novel ruthenium complexes: (H(2)L(1))(2)Ru(III)Cl(4)(CH(3)CN)(2)(2)Ru(IV)Cl(4)(CH(3)CN)(2)·2Cl·6H(2)O (1), mer-Ru(III)Cl(3)L(1)(CH(3)CN)·L(1)·3H(2)O (2), and (HL(2))(4)Ru(IV)Cl(6)·2Cl·4H(2)O (3). The isolated compounds were characterised by elemental analyses, UV-Vis and IR spectroscopy, and magnetic measurements. The nature of the ligands bound to the metal ions of these compounds and the experimental conditions significantly influenced the ruthenium complexes in different oxidation states. The N,N-donor ligand bound to the metal centre is a recognised stabiliser of the +III state of ruthenium, whereas the lack of ligand coordination promotes the formation of a mixed (Ru(III)/Ru(IV)) complex. In the case of complex 3, the absence of a N,O-donor ligand in the coordinate sphere facilitates the formation of the compound in a higher oxidation state. X-ray single crystal analyses revealed an octahedral geometry in each of the complexes. The crystal structure of ruthenium complexes is formed by a network of intermolecular classical and unconventional (C-H···π) hydrogen bonds. The most interesting feature of the supramolecular architecture of complexes is the existence of a very rare Cl(-)···π interaction and π···π stacking, which also contribute to structural stabilisation. Ruthenium compounds 2 and 3 behave as paramagnets with an octahedral geometry, corresponding to the presence of one or two unpaired electrons, respectively. The cyclic voltammetric data of complex 2 show three one-electron redox processes. The first redox couple is reversible, whereas the two other couples have a quasi-reversible nature. In the case of complex 3, two redox couples are reversible and the electrode processes are connected with exchange of one electron.