Ruthenium(II) Complexes Containing 8-(Dimethylphosphino)quinoline (Me2Pqn): Preparation, Crystal Structures, and Electrochemical and Spectroscopic Properties of [Ru(bpy or phen)3-n(Me2Pqn)n](PF6)2(bpy = 2,2‘-Bipyridine; phen = 1,10-Phenanthroline;n= 1, 2, or 3)

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Abstract
Several new ruthenium(II) complexes containing 8-(dimethylphosphino)quinoline (Me2Pqn) were synthesized, and their structures and electrochemical/spectroscopic properties have been investigated. In addition to the mono(Me2Pqn) complex [Ru(bpy or phen)2(Me2Pqn)](PF6)2 (1 or 1 ‘; bpy = 2,2‘-bipyridine; phen = 1,10-phenanthroline), the geometrical isomers trans(P)- and C1-[Ru(bpy)(Me2Pqn)2](PF6)2 (tP -2 and C1-2) and mer- and fac-[Ru(Me2Pqn)3](PF6)2 (m -3 and f -3) were also selectively synthesized and isolated. It was found that complexes tP -2 and m -3 were converted quantitatively to the corresponding C1-2 and f -3 isomers, respectively, by irradiation of light corresponding to the MLCT transition energy. The strong trans influence of the Me2P− donor group of Me2Pqn was confirmed by the X-ray structural analyses for 1, tP -2, m -3, and f -3. Cyclic voltammetry of a series of complexes, [Ru(bpy)3](PF6)2, 1, C1-2, and f -3, exhibited a reversible one-electron oxidation wave and two or three one-electron reduction waves. The oxidation potentials of the complexes gave a large positive shift with increasing number of coordinated Me2Pqn molecules, indicating a larger π-acceptability of the Me2P− group compared with bpy or qn. Complex f -3 in EtOH/MeOH (4:1) glass at 77 K exhibited an intense long-lived (τ = 920 μs) emission arising from the quinoline-based 3(π−π*) excited state. In contrast, the mixed-ligand complexes 1, 1‘, and C1-2 showed a characteristic dual emission, giving a double-exponential emission decay, and the dual emission originates from both the bpy-based 3MLCT and the quinoline-based 3(π−π*) emitting states.

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