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

The nonheme iron(IV)-oxido complex -N3-[(L)FeO(Cl)], where L is a derivative of the tetradentate bispidine 2,4-di(pyridine-2-yl)-3,7-diazabicyclo[3.3.1]nonane-1-one, has an = 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), = 3.5 s). The reaction with cyclohexane selectively leads to chlorocyclohexane, but "cage escape" at the [(L)Fe(OH)(Cl)]/cyclohexyl radical intermediate lowers the productivity. Ligand field theory is used herein to analyze the d-d transitions of [(L)FeO(X)] (X = Cl, Br, MeCN) in comparison with the thoroughly characterized ferryl complex of tetramethylcyclam (TMC = L; [(L)FeO(MeCN)]). 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.