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 [{(Me(2)bpy)(2)Ru}(2)(mu-bpm)](4+) (Me(2)bpy = 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, H-1 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 [{(Me,bpy),Ru}(2)(mu-bpm)](4+). Analogous interactions are also evident with mononuclear species, [Ru(pp)(3)](2+), where pp is a bidentate ligand such as Me(2)bpy, 1,10-phenanthroline (phen) or 2,2'-bipyridine (bpy): from H-1 NMR titration studies, the first association with a number of organic anions has a stability constant in the order of 100 dm(3) 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.