Chloride ion-pairing with Ru(II) polypyridyl compounds in dichloromethane

Chloride ion-pairing with a series of four dicationic Ru(II) polypyridyl compounds of the general form [Ru(bpy)3-x(deeb)x](PF6)2, where bpy is 2,2'-bipyridine and deeb is 4,4'-diethylester-2,2'-bipyridine, was observed in dichloromethane solution. The heteroleptic compounds [Ru(bpy)2(deeb)](2+) and [Ru(bpy)(deeb)2](2+) were found to be far less sensitive to ligand loss photochemistry than were the homoleptic compounds [Ru(bpy)3](2+) and [Ru(deeb)3](2+) and were thus quantified in most detail. X-ray crystal structure and (1)H NMR analysis showed that, when present, the C-3/C-3' position of bpy was the preferred site for adduct formation with chloride. Ion-pairing was manifest in UV-visible absorption spectral changes observed during titrations with TBACl, where TBA is tetrabutyl ammonium. A modified Benesi-Hildebrand analysis yielded equilibrium constants for ion-pairing that ranged from 13 700 to 64 000 M(-1) and increased with the number of deeb ligands present. A Job plot indicated a 2:1 chloride-to-ruthenium complex ratio in the ion-paired state. The chloride ion was found to decrease both the excited state lifetime and the quantum yield for photoluminescence. Nonlinear Stern-Volmer plots were observed that plateaued at high chloride concentrations. The radiative rate constants decreased and the nonradiative rate constants increased with chloride concentration in a manner consistent with theory for radiative rate constants and the energy gap law. Equilibrium constants for excited state ion-pairing abstracted from such data were found to be significantly larger than that measured for the ground state. Photophysical studies of hydroxide and bromide ion-pairing with [Ru(bpy)2(deeb)](2+) are also reported.