Meso-tetrakis(3 ',5 '-di-substituted-phenyl)porphyrins: structural, electrochemical redox and axial ligation properties

A family of meta octa-substituted 5,10,15,20-tetraphenylporphyrin derivatives and their metal (Cu(II) and Zn(II)) complexes were examined by electrochemical studies. These functionalized MTPPs exhibited a dramatic anodic shift (>200 mV) in their first ring redox potentials relative to the para-phenyl substituted-MTPP(X) complexes and follow a fairly linear relationship with the Hammett parameter of the substituents. The extent of Lewis acidity of the core Zn(II) center in these porphyrins was probed by axial ligation of bases of varying pK(a) values. The increase in K-eq values of the electron deficient Zn(II) porphyrins are as high as an order of magnitude and increase with anodic shift of the electrochemical redox potentials of the porphyrin. A crystal structure of the Zn(II) octa(carboxyethylester phenyl)porphyrin shows six-coordination geometry with the coordination through peripheral ester groups to form a supramolecular two-dimensional layer structure while the ZnT(3',5'-DMP)P complex exhibited a five-coordinate structure. Copyright (C) 2005 Society of Porphyrins & Phthalocyanines.