Issue 19, 2009

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

Abstract

A few diiron(III) complexes [Fe2(O)(OAc)2(L1)2](ClO4)21, [Fe2(O)(OBz)2(L1)2](ClO4)22, [Fe2(O)(OAc)2(L2)2](ClO4)23 and [Fe2(O)(OBz)2(L2)2](ClO4)24, 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 μ-oxo bridge, two oxygen atoms of the μ-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 (λmax, 670–700 nm) appears, which corresponds to the species [Fe2(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°/2°, 9–11). Interestingly, the formation of monoiron(III) species has been discerned in the alkane hydroxylation reactions beyond ∼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 (λmax, 600 nm, εmax, 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.

Graphical abstract: Highly selective hydroxylation of alkanes catalyzed by (μ-oxo)bis(μ-carboxylato)-bridged diiron(iii) complexes: involvement of mononuclear iron(iii) species in catalysis

Supplementary files

Article information

Article type
Paper
Submitted
23 Jan 2009
Accepted
02 Mar 2009
First published
24 Mar 2009

Dalton Trans., 2009, 3814-3823

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

K. Visvaganesan, E. Suresh and M. Palaniandavar, Dalton Trans., 2009, 3814 DOI: 10.1039/B901508F

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