Highly selective hydroxylation of alkanes catalyzed by (mu-oxo)bis(mu-carboxylato)-bridged diiron(III) complexes: involvement of mononuclear iron(III) species in catalysis

A few diiron(III) complexes [Fe-2(O)(OAc)(2)(L1)(2)](ClO4)(2) 1, [Fe-2(O)(OBz)(2)(L1)(2)](ClO4)(2) 2, [Fe-2(O)(OAc)(2)(L2)(2)](ClO4)(2) 3 and [Fe-2(O)(OBz)(2)(L2)(2)](ClO4)(2) 4, where L1 = N, N-bis(pyrid-2-ylmethyl)- iso-butylamine, L2 = N, N-bis(pyrid-2-ylmethyl) benzylamine, AcO = acetate and BzO = benzoate, have been isolated and characterized by means of elemental analysis and spectral and electrochemical methods. The molecular structures of the complexes 2 and 4 have been determined by single-crystal X-ray diffraction analysis and they possess a distorted bioctahedral geometry in which each iron atom is coordinated to the oxygen atom of the mu-oxo bridge, two oxygen atoms of the mu-benzoato bridges and three nitrogen atoms of L1 and L2 ligands capping the two ends of the diiron(III) cluster. The ESI-MS spectral data of the complexes reveal that the complexes remain intact in dichloromethane (DCM) solution. Upon adding one equivalent of Et3N to a mixture of one equivalent of the diiron(III) complexes and excess of m-chloroperbenzoic acid (m-CPBA) in DCM, an intense absorption band (lambda(max), 670-700 nm) appears, which corresponds to the species [Fe-2(O)(OAc)(m-CPBA)(L)(2)](2+) (ESI-MS, m/z 466) suggested as the intermediate involved in the oxygenation reactions. All the present complexes show efficient alkane hydroxylation with 300-400 turn over numbers and good selectivities for cyclohexane (A/K, 10-14) and adamantane (3 degrees/2 degrees, 9-11). Interestingly, the formation of monoiron(III) species has been discerned in the alkane hydroxylation reactions beyond similar to 50 turnovers. The mononuclear 1 : 1 iron(III) complexes of L1 and L2 ligands generated in situ are also found to catalyze the oxygenation reactions with high selectivity and efficiency for cyclohexane (A/K, 10-14). Upon their reaction with m-CPBA in DCM, a characteristic absorption band (lambda(max), 600 nm, epsilon(max), 355 M-1 cm(-1)) appears and decays at room temperature. This spectral feature is consistent with the mononuclear high-valent iron-oxo species suggested as an intermediate in the oxygenation reactions.