Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 141, Issue 38, Pages 15078-15091Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jacs.9b05758
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Funding
- Spanish Ministry of Science [CTQ201S-70795-P, CTQ2016-77989-P]
- Generalitat de Catalunya (ICREA Academia Award)
- European Commission [675020-MSCA-ITN-2015-ETN]
- US National Science Foundation [CHE-1665391, CHE-1654060]
- U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences [DE-AC02-76SF00515]
- DOE Office of Biological and Environmental Research
- National Institutes of Health, National Institute of General Medical Sciences [P41GM103393]
- Office of the Vice President of Research
- College of Science and Engineering
- Department of Chemistry at the University of Minnesota
- Generalitat de Catalunya [2014 SGR 862]
- Pittsburgh Super computing Center [CHE180020P]
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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 [Fe-v(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 (FeO)-O-IV-H bond of 100 kcal.mol(-1) i 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.
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