Schiff bases of nickel(II), copper(II) porphyrins and dibenzo-18-crown-6 interspersedbis-metal porphyrins. Protonation studies

Abstract

Schiff-base (SB) derivatives of Ni(II) and Cu(II) porphyrins endowed with various amine functions (R−NH2),n-butylamine,p-anisidine andm-nitroaniline have been prepared from corresponding formyl porphyrins. Protonation studies of these SB derivatives reveal a marked red shift of the optical absorption bands in the visible region relative to the unprotonated imines. The magnitude of the observed red shifts in the protonated derivatives, (SBH+) are found to depend on the electron-withdrawing or electron-donating nature of the R group of the amines. The results of the optical absorption,1H NMR, EPR, and cyclicovoltammetric studies are illustrative of the fact that protonation of the SB derivatives results in a localized positive charge,\(P - C(H) = \bar N(H) - R\) in the periphery of the porphyrin (p) system. The dibenzo-18-crown-6 interspersedbisporphyrin schiff bases have been prepared fromtrans 4,4′-diamino dibenzo-18-crown-6 and formyl porphyrins. The protonation of these SB derivatives is found to proceed in a concerted fashion. The cation complexation studies by the crown ether entity in thebisporphyrin systems have been investigated using optical absorption, magnetic resonance and electrochemical methods. The redox characteristics of the protonated dimeric SB porphyrins reveal that the first oxidation step involves a two-electron transfer reaction. This is important in view of their possible usage in multielectron transfer reactions of biological and catalytic interest.