Abstract:The spectroscopic and kinetic characterization of two intermediates from the H 2 O 2 oxidation of three dimethyl ester [(proto), (meso), (deuteroporphyrinato) (picdien)]Fe(III) complexes ([FePPPic], [FeMPPic] and [FeDPPic], respectively) pinch-porphyrin peroxidase enzyme models, with s = 5/2 and 3/2 Fe(III) quantum mixed spin (qms) ground states is described herein. The kinetic study by UV/Vis at λ max = 465 nm showed two different types of kinetics during the oxidation process in the guaiacol test for peroxidases (1-3 + guaiacol + H 2 O 2 Ñ oxidation guaiacol products). The first intermediate was observed during the first 24 s of the reaction. When the reaction conditions were changed to higher concentration of pinch-porphyrins and hydrogen peroxide only one type of kinetics was observed. Next, the reaction was performed only between pinch-porphyrins-Fe(III) and H 2 O 2 , resulting in only two types of kinetics that were developed during the first 0-4 s. After this time a self-oxidation process was observed. Our hypotheses state that the formation of the π-cation radicals, reaction intermediates of the pinch-porphyrin-Fe(III) family with the ligand picdien [N,N'-bis-pyridin-2-ylmethyl-propane-1,3-diamine], occurred with unique kinetics that are different from the overall process and was involved in the oxidation pathway. UV-Vis, 1 H-NMR and ESR spectra confirmed the formation of such intermediates. The results in this paper highlight the link between different spectroscopic techniques that positively depict the kinetic traits of artificial compounds with enzyme-like activity.

In the present manuscript, we report the kinetic and spectroscopic analysis of six new pinch-porphyrins: protoporphyrin-picpenta 1, mesoporphyrin-picpenta 2, deuteroporphyrin-picpenta 3, protoporphyrin-picocta 4, mesoporphyrin-picocta 5, and deuteroporphyrin-picocta 6. The Michaelis–Menten enzymatic pathway and the guaiacol test confirmed the ability of the compounds to function like new peroxidase models. UV–vis, 1H NMR, and electron spin resonance studies are in accordance with porphyrin-Fe(III) molecules with the quantum phenomena called quantum mixed spin (qms, s = 3/2, s = 5/2). Importantly, the influence of the presence of the s = 3/2 spin state in the compounds and its critical role for the catalytic capacity is proven here, which was the original hypothesis in our research group. The compounds with higher populations of the s = 3/2 spin state have increased peroxidase activity.

The new diamagnetic complex, [Zn2(N3)4(C6H6N2O)2] or [Zn2(pca)2(μ1,1-N3)2(N3)2] was synthesized using pyridine-2-carboxamide (pca) and azido ligands, and characterized using various techniques: IR spectroscopy and single-crystal X-ray diffraction in the solid state, and nuclear magnetic resonance (NMR) in solution. The molecule is placed on an inversion centre in space group P\overline{1}. The pca ligand chelates the metal centre via the pyridine N atom and the carbonyl O atom. One azido ligand bridges the two symmetry-related Zn2+ cations in the end-on coordination mode, while the other independent azido anion occupies the fifth coordination site, as a terminal ligand. The resulting five-coordinate Zn centres have a coordination geometry intermediate between trigonal bipyramidal and square pyramidal. The behaviour of the title complex in DMSO solution suggests that it is a suitable NMR probe for similar or isostructural complexes including other transition-metal ions. The diamagnetic nature of the complex is reflected in similar 1H and 13C NMR chemical shifts for the free ligand pca as for the Zn complex.