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

Abstract

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.