Intermediate-spin iron(IV)-oxido species with record reactivity

The nonheme iron(IV)-oxido complex trans-N3-[(L¹)Fe^IVO(Cl)]⁺, where L¹ is a derivative of the tetradentate bispidine 2,4-di(pyridine-2-yl)-3,7-diazabicyclo[3.3.1]nonane-1-one, has an S = 1 electronic ground state and is the most reactive nonheme iron model system known so far, of a similar order of reactivity as nonheme iron enzymes (C-H abstraction of cyclohexane, -90 °C (propionitrile), t_1/2 = 3.5 s). The reaction with cyclohexane selectively leads to chlorocyclohexane, but "cage escape" at the [(L¹)Fe^III(OH)(Cl)]⁺/cyclohexyl radical intermediate lowers the productivity. Ligand field theory is used herein to analyze the d-d transitions of [(L¹)Fe^IVO(X)] (X = Cl^-, Br^-, MeCN) in comparison with the thoroughly characterized ferryl complex of tetramethylcyclam (TMC = L²; [(L²)Fe^IVO(MeCN)]²⁺). The ligand field parameters and d-d transition energies are shown to provide important information on the triplet-quintet gap and its correlation with oxidation reactivity.