LONG-RANGE ELECTRONIC COUPLING IN BIS(CYCLOMETALATED) RUTHENIUM COMPLEXES

Symmetrical and unsymmetrical ligands containing terpyridyl coordinati ng units (N, N, N) or a cyclometalating equivalent (N, C, N), connecte d back-to-back either directly or via a p-terphenylene or 1,3-phenylen e spacer, have been used to construct new diruthenium complexes. These compounds incorporate various terdentate chelates as capping ligands, to allow a double control of the electronic properties of each subcom plex and of the ensemble: via the terminal ligand or through the bridg ing fragment. Electronic coupling was studied from the intervalence tr ansitions observed in several bimetallic ruthenium complexes of the bi s-(cyclometalated) type differing by the substitution of a nitrogen at om by carbon in the terminal terpyridyl unit. The largest metal-metal interaction was found in complexes for which the terminal complexing u nit is of the 1,3-di-2-pyridylbenzene type, i.e., with the carbon atom located on the metal-metal C-2 axis of the molecule. Investigations o f the mechanism of interaction by extended Huckel calculations showed that the replacement of nitrogen by carbon raises the filled ligand le vels, increasing the mixing with ligand orbitals and thus the metal-me tal coupling. Finally, the intervalence transition was still observed for a bridging ligand containing three phenylene units as spacers, cor responding to a 24-Angstrom metal-metal distance.