Electrochemical and binding properties of a meso-tetra(4-pyridyl)porphyrinatozinc supermolecule containing four μ3-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 complexes, ZnTCP, are reported. The coordination of four ruthenium acetate clusters to the zinc porphyrin has been confirmed by H-1 and C-13 NMR spectroscopy, including COSY and HETCOR bidimensional correlation techniques. Cyclic voltammetry and spectroelectrochemistry studies have revealed the presence of 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 ring-centered (ZnP+/ZnP/ZnP-/ZnP2-) redox reactions. The axial coordination reaction has been monitored by spectrophotometric titrations in dichloromethane and acetonitrile, in the 380-480 nm range, using pyridine, imidazole, water and acetonitrile as ligands. The influence of the peripheral ruthenium complexes on the electronic properties of the porphyrin ring has been assayed by measuring the axial coordination properties of the zinc ion. The calculated equilibrium constants for axial coordination of pyridine and imidazole to ZnTCP in dichloromethane are respectively one and two orders of magnitude higher than for meso-tetraphenylporphyrinatozinc, showing a regular increase as a function of the basicity of the ligands. The results have demonstrated that the Ru(III,III,III) species act as electron-withdrawing groups, decreasing significantly the electronic density on the porphyrin ring. Copyright (C) 2000 John Wiley & Sons, Ltd.