Chloride Ion-Pairing with Ru(II) Polypyridyl Compounds in Dichloromethane

Abstract

Chloride ion-pairing with a series of four dicationic Ru­(II) polypyridyl compounds of the general form [Ru­(bpy)_3–<i>x</i>(deeb)_<i>x</i>]­(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 ¹H 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