Iodide Ion Pairing with Highly Charged Ruthenium Polypyridyl Cations in CH3CN

A series of three highly charged cationic ruthenium­(II) polypyridyl complexes of the general formula [Ru­(deeb)3–x(tmam)x]­(PF6)2x+2, where deeb is 4,4′-diethyl ester-2,2′-bipyridine and tmam is 4,4′-bis­[(trimethylamino)­methyl]-2,2′-bipyridine, were synthesized and characterized and are referred to as 1, 2, or 3 based on the number of tmam ligands. Crystals suitable for X-ray crystallography were obtained for the homoleptic complex 3, which was found to possess D3 symmetry over the entire ruthenium complex. The complexes displayed visible absorption spectra typical of metal-to-ligand charge-transfer (MLCT) transitions. In acetonitrile, quasi-reversible waves were assigned to RuIII/II electron transfer, with formal reduction potentials that shifted negative as the number of tmam ligands was increased. Room temperature photoluminescence was observed in acetonitrile with quantum yields of ϕ ∼ 0.1 and lifetimes of τ ∼ 2 μs. The spectroscopic and electrochemical data were most consistent with excited-state localization on the deeb ligand for 1 and 2 and on the tmam ligand for 3. The addition of tetrabutylammonium iodide to the complexes dissolved in a CH3CN solution led to changes in the UV–vis absorption spectra consistent with ion pairing. A Benesi–Hildebrand-type analysis of these data revealed equilibrium constants that increased with the cationic charge 1 2 3 with K = 4000, 4400, and 7000 M–1. 1H NMR studies in CD3CN also revealed evidence for iodide ion pairs and indicated that they occur predominantly with iodide localization near the tmam ligand(s). The diastereotopic H atoms on the methylene carbon that link the amine to the bipyridine ring were uniquely sensitive to the presence of iodide; analysis revealed that an iodide “binding pocket” exists wherein iodide forms an adduct with the 3 and 3′ bipyridyl H atoms and the quaternized amine. The MLCT excited states were efficiently quenched by iodide. Time-resolved photoluminescence measurements of 1 revealed a static component consistent with rapid electron transfer from iodide in the “binding pocket” to the Ru metal center in the excited state, ket > 108 s–1. The possible relevance of this work to solar energy conversion and dye-sensitized solar cells is discussed.