Perbrominated 2-nitrotetraphenylporphyrins: electrochemical and axial ligation properties

A new series of perbrominated 2-nitro-5,10,15,20-tetraphenylporphyrins, H2TPPBrn(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 beta -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, K-eq, relative to ZnTPPBr8. Surprisingly, MTPPBrn(NO2) complexes show similar axial ligation and electrochemical redox behaviour. The anodic shift in electrochemical redox potentials and enhanced K-eq of MTPPBrn(NO2) complexes have been attributed to the increased electron deficiency of the porphyrin pi -system.