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

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 FeIVO unit is key to the rational design of bioinspired catalysts. Here, a series of FeIVO complexes based on a macrocyclic tetracarbene ligand and with different anionic trans-axial ligands (Lax = CF3COO-, t BuS-, 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 FeIVO complex (Lax = 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 Lax = MeCN > CF3COO- ≈ RS-. This contrasts the antielectrophilic trend found for a related series of tetramethylcyclam (TMC)-based FeIV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.