The reactivity of (mu-oxo)diferric complexes with (t)BuOOH (TBHP) for
the functionalization of alkanes in CH3CN has been investigated as par
t of our efforts to model dinuclear sites in nonheme iron enzymes. [Fe
2(TPA)2O(OAc)](ClO4)3(1)(TPA = tris(2-pyridylmethyl)amine, OAc = aceta
te) is an efficient catalyst for cyclohexane oxidation, affording cycl
ohexanol (A, 9 equiv), cyclohexanone (K, 11 equiv), and (tert-butylper
oxy)cyclohexane (P, 16 equiv) in 0.25 h at ambient temperature and pre
ssure under an argon atmosphere. The catalyst is remarkably robust, as
indicated by the H-1 NMR and UV-vis spectra of the reaction mixture d
uring the catalytic reaction and by its ability to maintain its turnov
er efficiency with subsequent additions of oxidant. The catalytic mech
anism for TBHP utilization was explored by observing the effects of va
rying the tripodal ligands on the (mu-oxo)(mu-carboxylato) diferric ca
talysts 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 produ
cts between A + K and P, with benzonitrile favoring hydroxylated produ
cts 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 demons
trate that A and K must derive from an oxidant different from that res
ponsible 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 cent
er to provide a site for coordinating the alkyl peroxide ion. Consiste
nt with this notion, the hydrogen abstraction power of the oxidant, as
indicated by isotope effects of cyclohexane hydroxylation, is modulat
ed by the tripodal ligand but is independent of the bridging anion, al
though the affinity of the bridging anion for the (mu-oxo)diferric cen
ter plays a role in determining the efficiency of the catalyst in cons
uming the alkyl hydroperoxide.