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

The nonheme iron(IV)-oxido complex trans-N3-[(L1)FeIV=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), t1/2 = 3.5 sec). The reaction with cyclohexane selectively leads to chlorocyclohexane, but “cage escape” at the [(L1)FeIII-OH(Cl)]+ / cyclohexyl radical intermediate lowers the productivity. Ligand field theory is used herein to analyze the d-d transitions of [(L1)FeIV=O(X)]n+ (X = Cl, Br, MeCN) in comparison with the thoroughly characterized ferryl complex of tetramethylcyclam (TMC=L2; [(L2)FeIV=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.