Does Geometry Matter? Effect of Ligand Position in Bimetallic Ruthenium Polypyridines Siblings

In this work, we present the preparation of the complex [(tpy)(bpy)Ru(μ-CN)Ru(py)4(OH2)](PF6)3, (tpy = 2,2’,6’,2’’‐terpyridine; bpy = 2,2’-bypyridine; 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 the family of cyanide-bridged ruthenium polypyridines of general formula [Ru(tpy)(bpy)(µ-CN)Ru(py)4(L)]2/3+ (L= Cl-, NCS-, 4-dimethylaminopyridine or acetonitrile) and compare them with the related complexes [Ru(tpy)(bpy)(µ-CN)Ru(bpy)2(L)]2/3+ where the L ligand is cis to the bridge. The mixed-valence form of these complexes shows evidence of strong coupling between the ruthenium ions and enhancing delocalization as the redox potential of the {Ru(py)4L} fragment increases. TD(DFT) calculations reproduce very well the spectroscopy 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 π orbitals of the ligands are mixed with dπ orbitals of the Ru ions, resulting in partial delocalization of the charge on the ligands. The latter result illustrates that the trans configuration in these complexes is well-suited to extend the interaction beyond the bridged ruthenium ions.