Does geometry matter? Effect of the ligand position in bimetallic ruthenium polypyridine siblings

In this work, we present the preparation of a complex [(tpy)(bpy)Ru(mu-CN)Ru(py)(4)(OH2)](PF6)(3) (tpy = 2,2 ',6 ',2 ''-terpyridine; bpy = 2,2 '-bipyridine; py = pyridine) that combines a ruthenium chromophore linked to another ruthenium ion that bears a labile position trans to the bridge. Substitution in this position is very attractive, as it allows us to place a quencher trans to the chromophore maximizing the separation between them. This complex allowed us to prepare a family of cyanide-bridged ruthenium polypyridines of general formula [Ru(tpy)(bpy)(mu-CN)Ru(py)(4)(L)](2/3+) (L = Cl-, NCS-, 4-dimethylaminopyridine or acetonitrile) and compare them with the related complexes [Ru(tpy)(bpy)(mu-CN)Ru(bpy)(2)(L)](2/3+) where the L ligand lies cis to the bridge. The mixed-valence form of these complexes shows evidence of strong coupling between the ruthenium ions and enhanced delocalization as the redox potential of the {Ru(py)(4)L} fragment increases. (TD)DFT calculations reproduce very well the experimental spectra of these complexes and indicate that when L = acetonitrile, the hole in the mixed-valence complex is almost equally distributed between both ruthenium ions. For L = DMAP and NCS- the pi orbitals of the ligands are mixed with d pi orbitals of the Ru ions, resulting in partial delocalization of the charge on the ligands. The latter result illustrates that the trans configuration of these complexes is well-suited to extend the interaction beyond the bridged ruthenium ions.