HIGHLY COUPLED MIXED-VALENCE DINUCLEAR RUTHENIUM AND OSMIUM COMPLEXES WITH A BIS-CYCLOMETALATING TERPYRIDINE ANALOG AS BRIDGING LIGAND

The dinuclear complexes (tterpy)M(tpbp)M(tterpy)2+ (M = Ru(II) or Os(II)), tterpy = 4',-p-tolyl-2,2':6',2''-terpyridine, tpbpH-2 = 3,3',5,5'-tetrapyridylbiphenyl) have been prepared by oxidative coupling of the corresponding monomers M(tterpy)(dpb)+ (dpbH = 1,3-di-o-pyridylbenzene). Spectrochemical and electrochemical properties of the monomers and dimers in different oxidation states are described. Chemical or electrochemical oxidation of the M(II)-M(II) species leads to the mixed-valence M(II)-M(III) dimer. The comproportionation constants K(c) determined by cyclic voltammetry (600 and 100 for Ru and Os, respectively) reflect the high stability of the M(II)-M(III) states. In addition, the near-infrared spectrum of each mixed-valence ion exhibits a very intense intervalence transfer (IT) transition. Although the two metal centers are separated by a large distance (11 angstrom), the matrix elements V(ab) are relatively large (0.15 and 0.12 eV for Ru and Os, respectively). This demonstrates the surprisingly high coupling ability of the bis-cyclometalating bridging ligand tpbp2-.