Perbrominated 2-nitrotetraphenylporphyrins: electrochemical and axial ligation properties

Abstract

A new series of perbrominated 2-nitro-5,10,15,20-tetraphenylporphyrins, H₂TPPBr_n(NO₂) (n = 6 and 7) and their metal (Cu^II and Zn^II) 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 ZnTPPBr_n(NO₂) complexes showed facile ligand binding with >50% enhancement in the equilibrium constants, K_eq, relative to ZnTPPBr₈. Surprisingly, MTPPBr_n(NO₂) complexes show similar axial ligation and electrochemical redox behaviour. The anodic shift in electrochemical redox potentials and enhanced K_eq of MTPPBr_n(NO₂) complexes have been attributed to the increased electron deficiency of the porphyrin π-system.