A 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 νFe=O values of 832 ± 2 cm-1 indicate similar FeIV=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 (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.
Bibliografisk noteFunding Information:
The work was supported by the Danish Council for Independent Research (Grant 9041-00170B to C.J.M.) and The Netherlands Ministry of Education, Culture and Science (W.R.B., Gravity Program 024.001.035). The Carlsberg Foundation grant CF15-0675 is acknowledged for the X-ray diffractometer and The University of Cádiz for the financial support to A.L. E.D.H. acknowledges financial support from the European Commission (MetEmbed, Project No. 745967) and the Villum Foundation (Young Investigator Program, Grant No. 29412).
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