Pinch-porphyrins, new spectroscopic and kinetic models of peroxidases

The novel complexes [1,9-bis(2-pyridyl)-2,5,8-triazanonane]-(protoporphyrinato)iron(III) 4, -(mesoporphyrinato)iron(III) 5 and -(deuteroporphyrinato)iron(III) 6 were synthesized from the parent compounds chloro(porphyrinato)iron(III) 1, -(mesoporphyrinato)iron(III) 2 and -(deuteroporphyrinato)iron(III) 3 and 1,9-bis(2-pyridyl)-2,5,8-triazanonane (picdien). The complexes 1-6 were characterized by UV/VIS, ¹H NMR and ESR spectroscopies and their catalytic activity was determined. The measured theoretical maximum rate constant (k_cat) for guaiacol + H₂O₂→ oxidation guaiacol products (guaiacol = 2-methoxyphenol) in the presence of complexes 4-6, were 7.6 × 10⁶, 4.4 × 10⁵ and 9.0 × 10⁴ mol^-1 s^-1, respectively. These peroxidase activities are to our knowledge the largest reported for model complexes. The UV/VIS spectra show Soret and Q bands for all compounds at energies typical of axially co-ordinated complexes with symmetry D_4h or lower. The intensity of the charge-transfer transitions indicates that the presence of the picdien ligand diminishes the distortion of the parent compounds. The ¹H NMR spectra of complexes 4-6 are indicative of six-co-ordinated complexes with different degrees of quantum mixed-spin (qms) state S = 5/2 into S = 3/2. The ESR spectral features are characteristic of qms species, A and B, for each compound. Maltempo's theory for qms states gives the admixture percentage of species A (53-64%) and species B (<8%) for each compound. The area ratio of the ESR B: A signals follows the same order as the peroxidase activity shown by these complexes. A clear correlation is established among the peroxidase activity, the iron(III) out-of-porphyrin plane configuration and the qms state S = 5/2 and S = 3/2.