H-1 NMR and electronic absorption spectroscopy of paramagnetic water-soluble meso-tetraarylsubstituted cationic and anionic metalloporphyrins

The ionization, mu-oxo-dimerization and axial ligation equilibria of free bases, iron(III) and manganese(III) derivatives of meso-tetrakis(p-sulfonatophenyl)porphyrin (TPPS4) and meso-tetrakis(4-N-methyl-pyridiniumyl)porphyrin (TMPyP) in aqueous solution are studied by H-1 NMR and electronic absorption spectroscopy. At physiological pH, Fe(III) complexes of TMPyP and TPPS4 exist predominantly as dimers and may undergo transition to low spin species upon binding to biomolecules, whereas Mn(III) complexes are essentially monomeric. Dicyano and bis-imidazole complexes of FeTMPyP and FeTPPS4 are low spin;;monomer adducts in the pH range 2.0 to 11.2. No low spin dimeric complexes were found. The low spin monocyano and high spin mono-imidazole complexes of FeTMPyP are formed in acidic and alkaline media, respectively. T-1-relaxation enhancement of water protons at 200 MHz induced by FeTPPS4 falls dramatically in the sequence high spin much greater than dimeric > low spin form.