This work directly compares the spectroscopic and reactivity properties of an oxoiron(IV) and an oxoiron(V) complex that are supported by the same neutral tetradentate N-based PyNMe3 ligand. A complete spectroscopic characterization of the oxoiron(IV) species (2) reveals that this compound exists as a mixture of two isomers. The reactivity of the thermodynamically more stable oxoiron(IV) isomer (2b) is directly compared to that exhibited by the previously reported 1e--oxidized analogue [FeV(O)(OAc)(PyNMe3)]2+ (3). Our data indicates that 2b is 4 to 5 orders of magnitude slower than 3 in hydrogen atom transfer (HAT) from C-H bonds. The origin of this huge difference lies in the strength of the O-H bond formed after HAT by the oxoiron unit, the O-H bond derived from 3 being about 20 kcal·mol-1 stronger than that from 2b. The estimated bond strength of the FeIVO-H bond of 100 kcal·mol-1 is very close to the reported values for highly active synthetic models of compound I of cytochrome P450. In addition, this comparative study provides direct experimental evidence that the lifetime of the carbon-centered radical that forms after the initial HAT by the high valent oxoiron complex depends on the oxidation state of the nascent Fe-OH complex. Complex 2b generates long-lived carbon-centered radicals that freely diffuse in solution, while 3 generates short-lived caged radicals that rapidly form product C-OH bonds, so only 3 engages in stereoretentive hydroxylation reactions. Thus, the oxidation state of the iron center modulates not only the rate of HAT but also the rate of ligand rebound.
Bibliographical noteFunding Information:
The work at the University de Girona was supported by the Spanish Ministry of Science (CTQ2015-70795-P to M.C., CTQ2016-77989-P to A.C.) and Generalitat de Catalunya (ICREA Academia Award to M.C. and 2014 SGR 862). The European Commission is acknowledged for financial support through the NoNoMeCat project (675020-MSCA-ITN-2015-ETN). The work at the University of Minnesota and Carnegie Mellon University was supported by the US National Science Foundation respectively through grants CHE-1665391 to L.Q. and CHE-1654060 to Y.G. XAS data were collected on Beamline 9-3 at the Stanford Synchrotron Radiation Light source, SLAC National Accelerator Laboratory. SLAC is supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. Use of Beamline 9-3 is supported by the DOE Office of Biological and Environmental Research and the National Institutes of Health, National Institute of General Medical Sciences (including P41GM103393). The Bruker Avance III HD nanobay 400 MHz spectrometer used in this study was purchased from funds provided by the Office of the Vice President of Research, the College of Science and Engineering, and the Department of Chemistry at the University of Minnesota. We thank the Pittsburgh Supercomputing Center for granting us computational resources (CHE180020P to R.F. and Y.G.).
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