Axial Ligand Effects on Electronic Structures and H-Atom Abstraction by Oxidoiron(IV) Complexes Featuring Triplet-Only Reactivity

Abstract

Oxidoiron(IV) species are crucial intermediates for the functionalization of C–H bonds by biological oxidases and oxygenases, and understanding the relations between ligation, electronic structure, and reactivity of the Fe^IV═O unit is key to the rational design of bioinspired catalysts. Here, a series of Fe^IV═O complexes based on a macrocyclic tetracarbene ligand and with different anionic trans-axial ligands (L^ax = CF₃COO^–, ^tBuS^–, AdaS^–) has been comprehensively characterized, including X-ray crystallography in two cases, as well as zero-field and applied-field ⁵⁷Fe Mössbauer spectroscopy, magnetic susceptibility measurements, magnetic circular dichroism (MCD), and helium-tagging infrared photodissociation (IRPD). This establishes a detailed picture of the electronic structures of these ferryl complexes, featuring a well-separated S = 1 ground state, and the influence of the trans-ligand. The confinement to triplet-only pathways in C–H bond activations and comparison with the parent Fe^IV═O complex (L^ax = MeCN) allow uncovering trends in hydrogen atom abstraction (HAA) ability without perturbations from different extents of two-state reactivity (TSR). Kinetic studies on HAA for C–H substrates with BDEs up to 78 kcal mol^–1 show relatively minor effects of the axial ligands, with rates following an electrophilic trend in the order L^ax = MeCN > CF₃COO^– ≈ RS^–. This contrasts the antielectrophilic trend found for a related series of tetramethylcyclam (TMC)-based Fe^IV═O complexes for which TSR is dominant. For the present systems, temperature-dependent KIEs, in combination with computations at the DFT and ab initio levels of theory, suggest a semiclassical model with modest tunneling contributions that are most pronounced in the case of a strongly donating trans-thiolate. This study thus provides a combined experimental and computational basis for understanding intrinsic axial ligand effects and the effect of a trans-axial thiolate in S = 1 ferryl intermediates.