Molecular wires built from binuclear cyclometalated complexes.

Binuclear complexes with cyclometalated ends of the [Ru(bpy)(2)(ppH)](+) type (bpy = 2,2'-bipyridine, ppH = 2-phenylpyridine), linked by various spacers, have been prepared. These spacers are made of one or two triple bonds, or bis-ethynyl aryl groups, with aryl = benzene, thiophene, or anthracene. The complexes with bis-ethynyl aryl spacers are obtained by Sonogashira couplings with suitable bis-alkynes, starting from the [Ru(bpy)(2)(ppBr)](+) synthon. Complexes with one or two triple bonds are obtained from the true alkyne [Ru(bpy)(2)(pp-CCH)](+) cyclometalated precursor, using respectively a Sonogashira coupling with the iodo derivative [Ru(bpy)(2)(ppI)](+), or an oxidative homocoupling. Some complexes with tert-butyl-substituted bipyridine ancillary ligands have also been obtained. Oxidation of the binuclear complexes occurs near 0.5 V, i.e., more easily than with [Ru(bpy)(3)](2+)-based complexes. A single anodic wave is observed, with almost no detectable splitting, corresponding to two closely spaced one-electron processes. Differential pulse voltammetry allows the determination of the corresponding comproportionation constants involving the mixed valence Ru(II)[bond]Ru(III) forms. Controlled potential electrolysis yields the mixed valence forms in comproportionation equilibrium with homovalent forms. Analysis of the intervalence transitions allows the calculation of the electronic coupling element V(ab). This series of complexes exhibit relatively large couplings when comparing with complexes of similar metal-metal distances, with a special mention for the anthracene-containing spacer, which appears particularly efficient for mediating the metal-metal interaction. The results can be rationalized by theoretical calculations at the extended Hückel level.