Cyclometalated Ru Complexes of Type RuII(N∧N)2(C∧N)z: Physicochemical Response to Substituents Installed on the Anionic Ligand

The electrochemical and photophysical properties of a series of Ru(II) complexes related to Ru(dcbpyH(2))(2)(ppy)(1+) (1; dcbpyH(2) = 4,4'-dicarboxy-2,2'-bipyridine; ppy = 2-phenylpyridine) were examined to elucidate the effect of modifying the anionic fragment of the C--N ligand with conjugated substituents (R). Included in this study is a family of compounds (2-5) consisting of one or two -NO(2) groups installed meta, ortho, and para to the organometallic bond. A suite of compounds with electron-donating and withdrawing groups (e.g., R = -F (6), -phenyl (7), -4-pyridine (8), -thiophene-2-carbaldehyde (9)) were also evaluated. Deprotonated forms of select compounds were isolated as tetrabutylammonium salts to benefit solution studies. All complexes were structurally characterized by a combination of mass spectrometry, (1)H and (13)C NMR spectroscopy, and/or elemental analysis. The electronic absorption spectra for all of the compounds reveal three broad bands over the 350-700 nm range. The maximum wavelength of the lowest energy absorbance bands for complexes modified with electron-withdrawing groups are hypsochromically shifted up to 45 nm relative to 1; the weakly emitting compounds (i.e., 1, 3, 6-9) display a hypsochromic shift of up to 63 nm compared to 1. Emission was not observed in cases where the -NO(2) group was positioned meta to the Ru-C bond. The sensitivity of the oxidation potentials to the nature, number, and position of the electron-withdrawing/-donating substituents for the entire set of compounds reflect a highest occupied molecular orbital (HOMO) character extended over the metal, the anionic portion of the C--N ligand, and, in the case of 7-9, the conjugated R group. The reduction potentials indicate that the lowest unoccupied molecular orbital (LUMO) is localized to the C--N ligand where R = -NO(2), and on the dcbpyH(2) ligands for all other compounds. This assessment was corroborated by time-dependent density functional theory (TD-DFT) studies.