Anion interactions with (polypyridyl)ruthenium complexes, and their importance in the cation-exchange chromatographic separation of stereoisomers of dinuclear species †

The separation of the ligand-bridged dinuclear complex cation [{(Me2bpy)2Ru}2(µ-bpm)]4+ (Me2bpy = 4,4′-dimethyl-2,2′-bipyridine; bpm = 2,2′-bipyrimidine) into its two diastereoisomeric forms, meso and rac, by cation-exchange chromatography has been investigated, using a wide range of organic counter anions (aromatic and aliphatic) in the aqueous eluents. 1H NMR titration studies not only confirm association between the complex cation and the counter anions in solution, but also reveal differential associations with the two diastereoisomers of [{(Me2bpy)2Ru}2(µ-bpm)]4+. Analogous interactions are also evident with mononuclear species, [Ru(pp)3]2+, where pp is a bidentate ligand such as Me2bpy, 1,10-phenanthroline (phen) or 2,2′-bipyridine (bpy): from 1H NMR titration studies, the first association with a number of organic anions has a stability constant in the order of 100 dm3 mol–1 in aqueous solution. This association also affects the emission from the MLCT excited state of the monomers, with up to a 10% enhancement in the luminescence.