Mononuclear nonheme iron enzymes perform a wide range of oxidative transformations in biology such as hydroxylation, desaturation, epimerization, and epoxidation. The reactive intermediates in these enzymes are characterized as oxoiron(IV) oxidants with an S = 2 ground spin state. Bio-inorganic chemists have been modeling these high reactive intermediates to shed light on structural and functional properties of the oxoiron(IV) unit in biological systems. Most of these model complexes have S = 1 spin states and show lower oxidative reactivity than the S = 2 oxoiron(IV) oxidants in biological systems. [(TPA)FeIVO]+2 (TPA = tris(pyridyl-2-methyl)amine) is one of the first synthetic analogs for such enzyme oxidants, even though it has an S =1 ground state. The Que group has systematically replaced the three pyridine arms of [(TPA)FeIVO]+2 by bulkier quinolines in order to weaken the equatorial ligand field and support an S =2 ground state in [(TQA)FeIVO]+2, which is one of the most reactive synthetic FeIV=O species that has been reported. In this work, we have synthesized a series of complexes based on the parent complex [(TPA)FeIVO]+2, in which one, two, and three pyridine arms are replaced systematically by quinoline arms in order to estimate where the transition from the S =1 ground state in [(TPA)FeIVO]+2 to the S = 2 ground state in [(TQA)FeIVO]+2 occurs. Typically, the spin states of oxoiron(IV) species would be determined by Mössbauer spectroscopy at 4 K, which is not the same temperature as the reaction conditions used for measuring the oxidative reactivity. So we are utilizing NMR spectroscopy to determine the spin states of these complexes at 233 K, in which we find that S = 2 complexes show a much larger chemical shift range and have different chemical shift patterns. This technique provides the bioinorganic community a direct method to determine the spin state of oxoiron(IV) units at the same temperature at which the oxidative reactivity is investigated. Also, the oxidative reactivity of this series of complexes will be discussed, in which we found that S = 1 complexes are less reactive than their S = 2 counterparts, as predicted by theoretical calculations.

Determining the spin states of nonheme FeIV=O complexes by NMR spectroscopy

ACS Spring Meeting 2021

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