New pinch-porphyrin complexes with quantum mixed spin ground state S= [formula omitted], [formula omitted] of iron (III) and their catalytic activity as peroxidase

Abstract

New complexes of the pinch-porphyrin family were obtained from the dimethylester of ( proto-, meso-, and deutero-porphyrinato)iron(III) with the ligand [ N, N′-bis-pyridin-2-ylmethyl-propane-1,3-diamine] 1– 3 and with the ligand [ N-pyridin-2-ylmethyl- N′-{3-[(pyridin-2-ylmethyl)-amino]-propyl}-propane-1,3-diamine] 4– 6. The UV/VIS studies of 1– 6 indicate an increase in the distortion of the ligand field excited state. The 1H NMR spectra of 1– 6 at RT and over the range 223–328 K show iron(III)-complexes with quantum mixed spin state (qms) S= 5 2 , S= 3 2 . The chemical shifts of the meso protons are consistent with qms state S= 3 2 , S= 5 2 , where the S= 3 2 spin state is lowest in energy. For methyl-heme the chemical shifts are also consistent with a qms state but now the S=5/2 ground state is lowest in energy. ESR spectra of 1– 6 show two different species, B and C, of iron(III) with qms, S= 5 2 , S= 3 2 consistent with the 1H NMR results. Species B with 70% of S= 5 2 and species C with 72.5% of S= 3 2 . The catalytic activity as peroxidase of 1– 6 was quantified by guaiacol test; their theoretical maximum rate constants were k cat∼10 2–10 3 M −1 s −1. A quantitative empirical correlation is found: the higher the 3 2 spin contribution to the qms state and the higher proportion of this species into the samples, the higher the peroxidase activity. Such a correlation was also obtained for pinch-porphyrins already reported.