Spectroscopic and Reactivity Comparisons between Nonheme Oxoiron(IV) and Oxoiron(V) Species Bearing the Same Ancillary Ligand

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.