Perbrominated 2-nitrotetraphenylporphyrins: electrochemical and axial ligation properties

Abstract

A new series of perbrominated 2-nitro-5,10,15,20-tetraphenylporphyrins, H2TPPBrn(NO2) (n = 6 and 7) and their metal (CuII and ZnII) complexes have been synthesised and characterised. The presence of mixed electron withdrawing (bromo- and nitro-) substituents at the β-pyrrole positions induces interesting electrochemical and axial ligation properties. Perbrominated nitroporphyrins exhibit two successive one electron redox potentials with a positive shift of >100 mV relative to their corresponding octabromotetraphenylporphyrin (MTPPBr8) complexes. Axial ligation of various nitrogenous bases to ZnTPPBrn(NO2) complexes showed facile ligand binding with >50% enhancement in the equilibrium constants, Keq, relative to ZnTPPBr8. Surprisingly, MTPPBrn(NO2) complexes show similar axial ligation and electrochemical redox behaviour. The anodic shift in electrochemical redox potentials and enhanced Keq of MTPPBrn(NO2) complexes have been attributed to the increased electron deficiency of the porphyrin π-system.