Characterization of the Fleeting Hydroxoiron(III) Complex of the Pentadentate TMC-py Ligand

Ching, W.-M.; Zhou, A.; Klein, J. E. M. N.; Fan, R.; Knizia, G.; Cramer, C. J.; Guo, Y.; Que, L., Jr.
Inorg. Chem. 2017, 56, 11129 (doi:10.1021/acs.inorgchem.7b01459).

Non-heme mononuclear hydroxoiron(III) species are important intermediates in biological oxidations, but well characterized examples of synthetic complexes are scarce due to their instability or tendency to form the μ-oxodiiron(III) complexes, which are the thermodynamic sink for such chemistry. Herein, we report the successful stabilization and characterization of a mononuclear hydroxoiron(III) complex, [Fe^III(OH)(TMC-py)]²⁺ (3; TMC-py = 1-(pyridyl-2'-methyl)-4,8,11-trimethyl-1,4,8,11-tetrazacyclotetradecane), which is directly generated from the reaction of [Fe^IV(O)(TMC-py)]²⁺ (2) with 1,4-cyclohexadiene (CHD) at –40 ^oC by H-atom abstraction. Complex 3 exhibits a UV spectrum with a λ_max at 335 nm (ε ~ 3500 M^–1 cm^–1) and a molecular ion in its electrospray ionization mass spectrum at m/z 555 with an isotope distribution pattern consistent with its formulation. EPR and Mössbauer spectroscopy shows 3 to be a high-spin Fe(III) center that is formed in 85% yield. EXAFS analysis reveals an Fe–OH bond distance of 1.84 Å, which is also found in [(TMC-py)Fe^III–O–Cr^III(OTf)₃]⁺ (4) obtained from the reaction of 2 with Cr(OTf)₂. The S = 5/2 spin ground state and the 1.84 Å Fe–OH bond distance are supported computationally. Complex 3 reacts with 1-hydroxy-2,2,6,6-tetramethyl-piperidine (TEMPOH) at –40 ^oC with a second-order rate constant of 7.1 M^–1 s^–1 and an OH/OD KIE value of 6. Based on density functional theory calculations, the reaction between 3 and TEMPOH is classified as a proton-coupled electron transfer as opposed to a hydrogen-atom transfer.