Aggregation and axial ligand exchange behavior of water-soluble pyrrole-β brominated porphyrins

Abstract

Synthesis of two water-soluble pyrrole-β brominated porphyrins, the tetrachloro salt of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(N-methyl-4-pyridinio)porphyrin [tetrakis(N-methyl pyridyl)-β-octabromoporphyrin, (Br₈TMpyP)H₂] and the tetrasodium salt of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(phenyl sulfonate)porphyrin [tetrakis(4-sulfonatophenyl)-β-octabromoporphyrin, (Br₈TPPS)H₂] and their zinc(II) derivatives have been reported. The investigated porphyrins are highly non-planar due to the presence of eight bromo substituents at the pyrrole-β positions and four aryl groups at the meso positions. At I = 0.1 M and T = 23 °C, the two sulfonated derivatives, (Br₈TPPS)H₂ and (Br₈TPPS)Zn show a small tendency to aggregate while such behavior is almost negligible for the N-methyl pyridyl derivatives. Like the unbrominated water-soluble zinc(II) porphyrin derivatives, the pyrrole-β brominated zinc(II) porphyrins, with one or two water molecules as axial ligand(s) in aqueous solution, undergo ligand exchange reactions with nitrogeneous bases. The calculated equilibrium constant, K, for this reaction is found to be smaller in magnitude when compared to that obtained for the corresponding unbrominated zinc(II) porphyrin derivatives. The equilibrium constant values parallel the basicity of the axial ligands.