2,2'-AZOBIS(PYRIDINE) (ABPY) AS A MULTIPLY REDUCIBLE TETRADENTATE LIGAND - EPR EVIDENCE FOR THE CONFIGURATIONAL DEPENDENCE OF INTRAMOLECULAR ELECTRON-TRANSFER IN THE STEREOISOMERIC TRIS-CHELATE COMPLEXES [RU(ABPY)N(BPY)3-N]M (N=2, 3, M=2-)( TO 3)

The mononuclear title complexes can exist as stereoisomers due to the unsymmetrically chelating alpha-azimino functionality of the abpy liga nd. Both fac and mer isomers of [Ru(abpy)3]2+ and two out of the three possible diastereoisomers of [Ru(abpY)2(bpy)]2+ were isolated via HPL C and identified by virtue of their H-1 NMR spectra. Electrochemical p otentials for several one-electron reduction steps and UV/vis spectroe lectrochemical features vary to a small but detectable degree for the respective pairs of isomers. The absence of a room-temperature EPR sig nal for one of the two isolated isomers of [Ru(II)(abpy-)(abpy)(bpy)]. + demonstrates the structural requirements for intramolecular electron transfer. A Ru-99, Ru-101 EPR hyperfine coupling of about 0.8 mT was established for the previously reported radical ion [Ru(abpy)(bpy)2].. The free alpha-azimino chelate sites in mononuclear complexes of abp y render these systems suitable for the construction of polynuclear co mpounds. The exceptional pi-acceptor capability of abpy is illustrated by the reversible acceptance of at least eight electrons by the dinuc lear complex {[Ru(bpy)2]2(mu,eta4-abpy)}n.