Electrochemical and binding properties of a meso-tetra(4-pyridyl)porphyrinatozinc supermolecule containing four mu(3)-oxo-triruthenium acetate clusters

The synthesis and characterization of a zinc meso-tetra(4-pyridyl)porphyrin containing four coordinated mu (3)-oxo-triruthenium acetate cluster comple xes, ZnTCP, are reported. The coordination of four ruthenium acetate cluste rs to the zinc porphyrin has been confirmed by H-1 and C-13 NMR spectroscop y, including COSY and HETCOR bidimensional correlation techniques. Cyclic v oltammetry and spectroelectrochemistry studies have revealed the presence o f seven redox processes in the -1.5 to 2.5 V range, four of them attributed to cluster-centered (Ru-IV,Ru-IV,Ru-III /Ru-IV,Ru-III,Ru-III/Ru-III,Ru-III ,Ru-III/Ru-III,Ru-III,Ru-III/Ru-III,Ru-II,Ru-II) and three to porphyrin rin g-centered (ZnP+/ZnP/ZnP-/ZnP2-) redox reactions. The axial coordination re action has been monitored by spectrophotometric titrations in dichlorometha ne and acetonitrile, in the 380-480 nm range, using pyridine, imidazole, wa ter and acetonitrile as ligands. The influence of the peripheral ruthenium complexes on the electronic properties of the porphyrin ring has been assay ed by measuring the axial coordination properties of the zinc ion. The calc ulated equilibrium constants for axial coordination of pyridine and imidazo le to ZnTCP in dichloromethane are respectively one and two orders of magni tude higher than for meso-tetraphenylporphyrinatozinc, showing a regular in crease as a function of the basicity of the ligands. The results have demon strated that the Ru(III,III,III) species act as electron-withdrawing groups , decreasing significantly the electronic density on the porphyrin ring. Co pyright (C) 2000 John Wiley & Sons, Ltd.