Electrochemical and Chemical Reactivity of Octahalogenated Meso-Tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)Porphyrins

Abstract

Two highly non-planar octahalogenated nickel(II) porphyrins were characterized for the first time as to their electrochemical and spectroscopic properties as well as their chemical reactivity in nonaqueous media. The complexes have the structures shown in Chart 1. Chart 1. Structures of investigated porphyrins. The UV-vis spectra of NiPorCl8 and NiPorBr8 vary with solvent and degree of axial coordination and are substantially different than spectra of the unhalogenated NiPor parent porphyrin. The NiPorBr8 and NiPorCl8 spectra are stable with time and interpreted in terms of four or six-coordinated derivatives in 14 different nonaqueous solvents but this is not the case for NiPorCl8 in DMF or DMSO where a transient 6-coordinate solvated complex is initially formed upon dissolving the compound followed by formation of a porphodimethene in solution. The combination of eight β-halogens and four meso-t-butyl-hydroxyphenyl groups on the NiPorX8 complexes exerts a powerful influence on the electrochemistry which is characterized by up to five redox reactions, the first 2 or 3 of which are irreversible at room temperature and assigned as involving the easily reducible meso-hydroxyphenyl groups followed by reduction of the π-conjugated system at more negative potentials. A rapid homogeneous chemical reaction occurs after the first electron transfer giving as the final product a nickel(II) porphodimethane at the electrode surface. This was confirmed by comparisons between redox potentials and UV-visible spectra of the in situ generated electroreduction products with that of chemically synthesized porphodimethenes formed in a reaction between NiPorCl8 and the added anions F-, CN-, OAc- or H2PO4 -.