Examination of the Magneto-Structural Effects of Hangman Groups on Ferric Porphyrins by EPR

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.