Computational Insight into the Mechanism of Alkane
Hydroxylation by Non-heme Fe(PyTACN) Iron Complexes. Effects of the
Substrate and Solvent
- Publication date
- Publisher
Abstract
The reaction mechanisms for alkane
hydroxylation catalyzed by non-heme
Fe<sup>V</sup>O complexes presented in the literature vary from rebound
stepwise to concerted highly asynchronous processes. The origin of
these important differences is still not completely understood. Herein,
in order to clarify this apparent inconsistency, the hydroxylation
of a series of alkanes (methane and substrates bearing primary, secondary,
and tertiary C–H bonds) through a Fe<sup>V</sup>O species,
[Fe<sup>V</sup>(O)(OH)(PyTACN)]<sup>2+</sup> (PyTACN = 1-(2′-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane),
has been computationally examined at the gas phase and in acetonitrile
solution. The initial breaking of the C–H bond can occur via
hydrogen atom transfer (HAT), leading to an intermediate where there
is an interaction between the radical substrate and [Fe<sup>IV</sup>(OH)<sub>2</sub>(PyTACN)]<sup>2+</sup>, or through hydride transfer
to form a cationic substrate interacting with the [Fe<sup>III</sup>(OH)<sub>2</sub>(PyTACN)]<sup>+</sup> species. Our calculations show
the following: (i) except for methane in the rest of the alkanes studied,
the intermediate formed by R<sup>+</sup> and [Fe<sup>III</sup>(OH)<sub>2</sub>(PyTACN)]<sup>+</sup> is more stable than that involving the
alkyl radical and the [Fe<sup>IV</sup>(OH)<sub>2</sub>(PyTACN)]<sup>2+</sup> complex; (ii) in spite of (i), the first step of the reaction
mechanism for all substrates is a HAT instead of hydride abstraction;
(iii) the HAT is the rate-determining step for all analyzed cases;
and (iv) the barrier for the HAT decreases along methane →
primary → secondary → tertiary carbon. The second part
of the reaction mechanism corresponds to the rebound process. Therefore,
the stereospecific hydroxylation of alkane C–H bonds by non-heme
Fe<sup>V</sup>(O) species occurs through a rebound stepwise mechanism
that resembles that taking place at heme analogues. Finally, our study
also shows that, to properly describe alkane hydroxylation processes
mediated by Fe<sup>V</sup>O species, it is essential to consider the
solvent effects during geometry optimizations. The use of gas-phase
geometries explains the variety of mechanisms for the hydroxylation
of alkanes reported in the literature