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

The nonheme iron(IV)-oxido complex trans -N3-[(L 1 )Fe IV O(Cl)] + , where L 1 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 [(L 1 )Fe III (OH)(Cl)] + /cyclohexyl radical intermediate lowers the productivity. Ligand field theory is used herein to analyze the d-d transitions of [(L 1 )Fe IV O(X)] n + (X = Cl - , Br - , MeCN) in comparison with the thoroughly characterized ferryl complex of tetramethylcyclam (TMC = L 2 ; [(L 2 )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.