Nonheme iron enzymes utilize S = 2 iron(IV)-oxo intermediates as oxidants in biological oxygenations. In contrast, corresponding synthetic nonheme FeIV=O complexes characterized to date favor the S = 1 ground state that generally show much poorer oxidative reactivity than their S = 2 counterparts. However, one intriguing exception found by Nam a decade ago is the S = 1 2+ complex (Me3NTB = , 1O) with a HAT reactivity that is 70% that of the S = 2 2+ complex (TQA = tris(2-quinolylmethyl)amine, 3O). In our efforts to further explore this direction, we have unexpectedly uncovered a family of new S = 1 complexes with HAT reaction rates beyond currently reported limits in the tripodal ligand family, surpassing oxidation rates found for the S = 2 2+ complex by as much as an order of magnitude. This is achieved simply by replacing the secondary sphere methyl groups of the Me3NTB ligand with larger cycloalkyl-CH2 (R groups in 2OR) moieties ranging from c-propylmethyl to c-hexylmethyl. These 2OR complexes show Mo?ssbauer data at 4 K and 1H NMR spectra at 233 K that reveal S = 1 ground states, in line with DFT calculations. Nevertheless, they give rise to the most reactive synthetic nonheme oxoiron(IV) complexes found to date within the tripodal ligand family. Our DFT study indicates transition state stabilization through entropy effects, similar to enzymatic catalysis.

It has been recently published open access in Journal of the American Chemical Society:

A. Das, N. Pal, J. Xiong, V. G. Young, Y. Guo*, M. Swart* and L. Que*

“10-Fold Increase in Hydrogen Atom Transfer Reactivity for a Series of S = 1 FeIV=O Complexes Over the S = 2 2+ Complex via Entropic Lowering of Reaction Barriers by Secondary Sphere Cycloalkyl Substitution”

J. Am. Chem. Soc., 2025, 147, 292–304

DOI: 10.1021/jacs.4c10120

Girona, January 13th, 2025
For more info: ges.iqcc@udg.edu