New pinch-porphyrin complexes with quantum mixed spin ground state S= 3/2, 5/2 of iron (III) and their catalytic activity as peroxidase

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 ²-10³ M^-1 s^-1. A quantitative empirical correlation is found: the higher the 32 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.