Structural, electrochemical and UV/VIS/NIR spectroelectrochemical properties of diastereomerically pure dinuclear ruthenium complexes based on the bridging ligand phenanthroline-5,6-diimine, and a mononuclear by-product with a peripheral isoimidazole group

Abstract

Mono- and di-nuclear ruthenium(II) complexes derived from the potentially bridging ligand 5,6-diaminophenanthroline (dap) have been prepared and investigated. Reaction of dap (which has one ‘bipyridyl-like’ and one diamine co-ordination site) with [Ru(bipy)₂Cl₂] afforded [{Ru(bipy)₂}₂(µ-pdi)]⁴⁺, where pdi = phenanthroline-5,6-diimine, in which the diamine site has undergone a two-electron oxidation to give a quinone diimine site. The complex (as its hexafluorophosphate salt) could be separated chromatographically into its two diastereoisomers which were fully characterised independently, including the crystal structure of the homochiral diastereoisomer as its perchlorate salt. Electrochemical studies revealed six reversible reductions, of which the first two were shown by spectroelectrochemical studies to be centred on the bridging ligand (diimine/diiminosemiquinone and diiminosemiquinone/diamide couples), and the rest based on the terminal bipyridyl ligands. Although these six reductions occurred at identical potentials for both diastereoisomers, the separation between the two closely spaced Ru^II–Ru^III couples varied slightly between the diastereoisomers indicating some sensitivity of delocalisation in the Ru^II–Ru^III mixed-valence state to the optical configuration of the metal centres. The mononuclear complex [Ru(bipy)₂(dap)]²⁺, in which the metal is co-ordinated at the bipyridyl site of dap with the diamine site vacant, easily reacts with acetone to give [Ru(bipy)₂L]²⁺ where L is a phenanthroline derivative with a peripheral isoimidazole ring derived from condensation of one equivalent of acetone with the o-diamine group. The electrochemical, spectroscopic and luminescence properties of [Ru(bipy)₂L]²⁺ are only slightly perturbed from those of [Ru(bipy)₃]²⁺.