Engineering 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

A series of iron(IV) oxo complexes, which differ in the donor (CH(2)py 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 nu(Fe=O) values of 832 +/- 2 cm(-1) indicate similar Fe-IV=O 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 (k(H)/k(D) = 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, [(FeO)-O-IV(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.

Automated summary

A series of iron(IV) oxo complexes with different donors were prepared in water at different pH values. Differences in reactivities towards C-H substrates were observed, with one complex showing higher reactivity at pH 2. Supramolecular assistance by the first and second coordination spheres in activating the substrate was proposed, and it was noted that slow water oxidation acts as a competing background reaction.