Effects of substituents on the spectral and redox properties of cadmium(II) texaphyrins

Abstract

The synthesis, electronic absorption spectra, and electrochemistry of cadmium(II) texaphyrins substituted with various electron-donating or electron-withdrawing groups are described. The one-electron oxidation and reduction potentials and the energies of the lowest energy absorption band maxima (λ_Q 754-797 nm) are sensitive to the nature of substituents attached to the phenyl ring of these novel pentadentate macrocycles. Cyclic voltammetric data revealed that weakly binding counterions associated with these monopositive complexes (e.g. ClO^4- or NO₃) dissociative upon one-electron reduction; the reduction peaks are irreversible to quasi-reversible in these cases. However, in the presence of strongly binding chloride ion (Cl^-) or nitrogen bases (e.g. pyridine), the reduction process is electrochemically reversible and ligand dissociation does not occur on the cyclic voltammetric time scale. A linear relationship between the frequency of the lowest energy absorption maximum and the different in potentials for the first oxidation and reduction processes (1.54-1.40 V) exists in these systems. A similar relationship was also found for those texaphyrin complexes in which the phenyl group is replaced by a naphthyl, phenanthryl, or dicyano group. These results are discussed on the basis of the known spectral and redox features of metalloporphyrins.