Abstract
Ferric hangman porphyrins are bioinspired models for haem hydroperoxidase enzymes featuring an acid/base group in close vicinity to the metal center, which results in improved catalytic activity for reactions requiring O-O bond activation. These functional biomimics are examined herein with a combination of EPR techniques to determine the effects of the hanging group on the electronics of the ferric center. These results are compared to those for ferric octaethylporphyrin chloride [Fe(OEP)Cl], tetramesitylporphyrin chloride [Fe(TMP)Cl], and the pentafluorophenyl derivative [Fe(TPFPP)Cl], which were also examined herein to study the electronic effects of various substituents. Frequency-domain Fourier-transform THz-EPR combined with field domain EPR in a broad frequency range from 9.5 to 629 GHz allowed the determination of zero-field splitting parameters, revealing minor rhombicity E/D and D values in a narrow range of 6.24(8) to 6.85(5) cm-1. Thus, the hangman porphyrins display D values in the expected range for ferric porphyrin chlorides, though D appears to be correlated with the Fe-Cl bond length. Extrapolating this trend to the ferric hangman porphyrin chlorides, for which no crystal structure has been reported, indicates a slightly elongated Fe-Cl bond length compared to the non-hangman equivalent.
Highlights
Enzymatic catalysis typically utilizes a metal center (Lewis acid) to coordinate a substrate, with Brønsted acid−base groups often present to assist substrate positioning and orientation
[Fe(OEP)Cl] was determined to have a D value of 6.85(5) cm−1, which is comparable to the literature value for hemin (FeIII protoporphyrin IX chloride) of 6.90(1) cm−1.22 These values are significantly higher than D for [Fe(TPP)Cl], which has been examined in great detail using complementary techniques, with 6.465(3) cm−1 having been determined by FD-FT THz-electron paramagnetic resonance (EPR).[23]
A series of ferric porphyrin chlorides was studied with FD-FT THz-EPR and field domain EPR at frequencies between 9.3 and 629 GHz
Summary
Enzymatic catalysis typically utilizes a metal center (Lewis acid) to coordinate a substrate, with Brønsted acid−base groups often present to assist substrate positioning and orientation. These groups abstract or supply protons for proton coupled electron transfer steps (PCET). Biomimetic models aim to resemble the active site by the similar redox potential of the metal center and by proximal sites for proton delivery/abstraction. These are essential for small molecule activation and PCET reactions, which are a foundation of efficient energy storage and utilization.[1]. The linking pillars of second generation Pacman biporphyrins,[1] diporphyrin xanthene (DPX)[7] and diporphyrin dibenzofuran (DPD),[8] allowed greater flexibility in the angle between the two porphyrin units, Received: August 2, 2019 Published: October 10, 2019
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