Second-Sphere Aromatic Substitutions That Tune H-Atom Transfer Reactivity Observed for a Series of S = 1 Nonheme FeIV=O Complexes: An Enthalpy-Entropy Compensation Effect.

Synthetic nonheme FeIV═O complexes reported thus far often have S = 1 ground states, demonstrating markedly lower oxidative reactivity in C─H bond breakage compared to their S = 2 counterparts. Recently, we have identified that incorporating cycloalkyl groups into the second coordination sphere of the nitrilotris(benzimidazole) ligand produce the most reactive FeIV═O complexes within the tripodal ligand family, exhibiting an S = 1 ground state. Here, we have synthesized six S = 1 complexes having the NTB ligand framework with either electron-rich or electron-poor aromatic groups in the secondary coordination sphere of the ligand. These complexes have been characterized by Mössbauer data at 4 K and 1H NMR spectra at 193 K with quite similar electronic characteristics despite having distinct electronically tuned ligands. Nevertheless, the complexes show substrate oxidation rates that differ by nearly an order of magnitude from each other, giving rise to a notable linear correlation between activation enthalpies and activation entropies of HAT reactions. These outcomes thus enable further investigation of the Enthalpy-Entropy Compensation Effect (EECE) in HAT reactions involving these oxoiron(IV) complexes. Finally, the role played by steric attributes of the secondary coordination sphere group in regulating the reactivity of this series of complexes has been explored.