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Download fileEngineering the Oxidative Potency of Non-Heme Iron(IV) Oxo Complexes in Water for C–H Oxidation by a cis Donor and Variation of the Second Coordination Sphere
journal contribution
posted on 2021-01-20, 15:19 authored by Christina Wegeberg, Mathias L. Skavenborg, Andrea Liberato, James N. McPherson, Wesley R. Browne, Erik D. Hedegård, Christine J. McKenzieA series
of iron(IV) oxo complexes, which differ in the donor (CH2py or CH2COO–) cis to
the oxo group, three with hemilabile pendant donor/second coordination
sphere base/acid arms (pyH/py or ROH), have been prepared in water
at pH 2 and 7. The νFeO values of 832 ±
2 cm–1 indicate similar FeIVO
bond strengths; however, different reactivities toward C–H
substrates in water are observed. HAT occurs at rates that differ
by 1 order of magnitude with nonclassical KIEs (kH/kD = 30–66) consistent
with hydrogen atom tunneling. Higher KIEs correlate with faster reaction
rates as well as a greater thermodynamic stability of the iron(III)
resting states. A doubling in rate from pH 7 to pH 2 for substrate
C–H oxidation by the most potent complex, that with a cis-carboxylate donor, [FeIVO(Htpena)]2+, is observed. Supramolecular assistance by the first and second
coordination spheres in activating the substrate is proposed. The
lifetime of this complex in the absence of a C–H substrate
is the shortest (at pH 2, 3 h vs up to 1.3 days for the most stable
complex), implying that slow water oxidation is a competing background
reaction. The iron(IV)O complex bearing an alcohol moiety
in the second coordination sphere displays significantly shorter lifetimes
due to a competing selective intramolecular oxidation of the ligand.