ALKANE FUNCTIONALIZATION AT (MU-OXO)DIIRON(III) CENTERS

The reactivity of (mu-oxo)diferric complexes with (t)BuOOH (TBHP) for the functionalization of alkanes in CH3CN has been investigated as part of our efforts to model dinuclear sites in nonheme iron enzymes. [Fe2(TPA)2O(OAc)](ClO4)3(1)(TPA = tris(2-pyridylmethyl)amine, OAc = acetate) is an efficient catalyst for cyclohexane oxidation, affording cyclohexanol (A, 9 equiv), cyclohexanone (K, 11 equiv), and (tert-butylperoxy)cyclohexane (P, 16 equiv) in 0.25 h at ambient temperature and pressure under an argon atmosphere. The catalyst is remarkably robust, as indicated by the H-1 NMR and UV-vis spectra of the reaction mixture during the catalytic reaction and by its ability to maintain its turnover efficiency with subsequent additions of oxidant. The catalytic mechanism for TBHP utilization was explored by observing the effects of varying the tripodal ligands on the (mu-oxo)(mu-carboxylato) diferric catalysts and varying the bridge on Fe2O(TPA)2 catalysts. The (A + K)/P ratio increased as the ligands became more electron donating. Solvent also played an important role in determining the partitioning of products between A + K and P, with benzonitrile favoring hydroxylated products at the expense of P and pyridine having the opposite effect. Most significantly, the addition of dimethyl sulfide (to trap two-electron oxidants) to this system completely suppressed the formation of A and K but did not affect the amount of P formed. These observations demonstrate that A and K must derive from an oxidant different from that responsible for P production. TBHP is thus decomposed by the catalyst via two mechanisms: a heterolytic process that affords a high-valent iron-oxo species responsible for A and K formation and a homolytic pathway that generates (t)BuO. and (t)BuOO. radicals that are responsible for the formation of P. It is proposed that the heterolytic mechanism is initiated by the dissociation of the bridging anion from one iron center to provide a site for coordinating the alkyl peroxide ion. Consistent with this notion, the hydrogen abstraction power of the oxidant, as indicated by isotope effects of cyclohexane hydroxylation, is modulated by the tripodal ligand but is independent of the bridging anion, although the affinity of the bridging anion for the (mu-oxo)diferric center plays a role in determining the efficiency of the catalyst in consuming the alkyl hydroperoxide.