Among the many types of tetrapyrrole ligand, the corrole has elicited extreme interest due to its ability to support metal ions in high formal oxidation states. Among the many transition metal (d block) ions that are coordinated by corroles, iron is of particular interest due to the role of high oxidation state Fe in heme oxidase enzymes. An important question regarding high oxidation state iron tetrapyrroles is: what is the best description of the complex, namely, are all of the oxidizing equivalents being on the Fe, or shared between the Fe and the ligand? For example, cytochrome P450 Compound I is best described as Fe(IV) with a porphyrin radical rather than formally Fe(V). In Fe corroles, a complex could be described either as Fe(IV) with the corrole in its standard 3- charge state, or Fe(III) with a corrole radical – the two paramagnetic centers being antiferromagnetically coupled to give a net spin triplet (S = 1). An important parameter in characterizing such complexes with S &gt; 1/2 is their zero-field splitting (zfs), usually given by the axial parameter D and the rhombic parameter, E. These parameters can be definitively extracted by electron paramagnetic resonance (EPR) spectroscopy in both the field-domain (using high-frequencies and fields, HFEPR), and in the frequency-domain (FD-FT THz-EPR). We describe here the use of both of these paramagnetic resonance techniques to extract the complete spin Hamiltonian parameters of two, five-coordinate, formally Fe(IV) corrole complexes, which have spin triplet ground states: Fe(TPC)Ph and Fe(TPC)Cl (TPC = 5,10,15-triphenylcorrole; Ph = phenyl). The electronic structure of these two seemingly similar complexes is quite different as reflected in the magnitude of their zfs. This difference is a consequence of the former being best described as FeIV(TPC3-), while the latter is more accurately given as FeIII(TPC●2-). Mössbauer effect spectroscopic studies on these complexes will also be described. Computational support for these results will also be given and related to studies on Fe corroles that address the question of valence description using other experimental techniques (e.g., optical and X-ray absorption spectroscopies).

Advanced paramagnetic resonance studies on high-oxidation state iron corroles

2019 Spring National Meeting

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