Abstract
The oxidative C-H bond activation mediated by heme and nonheme enzymes and related biomimetics is one of the most interesting processes in bioinorganic and oxidative chemistry. However, the mechanisms of these reactions are still elusive and controversy due to the involvement of highly reactive metal-oxo intermediates with multiple spin states, despite extensive experimental efforts, especially for the N-dealkylation of N,N-dialkyalinines. In this work, we employed multistate density functional theory (MSDFT) and the Kohn-Sham DFT to investigate the mechanism of N-demethylation of N,N-dimethyalinines oxidized by the reaction intermediate FeIV(O)(N4Py)(ClO4)2. The Kohn-Sham DFT study demonstrated that the reaction proceeds via a rate-limiting hydrogen atom transfer (HAT) step and a subsequent barrier-free oxygen rebound step to form the carbinol product. The MSDFT investigation on the first C-H activation further showed that this step is an initial hydrogen atom abstraction that is highly correlated between CEPT and HAT, i.e., both CEPT and HAT processes make significant contributions to the mechanism before reaching the diabatic crossing point, then the valence bond character of the adiabatic ground state is switched to the CEPT product configuration. The findings from this work may be applicable to other hydrogen abstraction process.
Highlights
Heme and nonheme iron enzymes mediate a variety of fundamental biochemical transformations which are vital to biological processes
The energetic features of the hydrogen atom transfer (HAT) states prior to the diabatic crossing points, and the dominantly concerted-asynchronous proton-electron transfer (CEPT) character afterwards are, fully consistent with the intuitive perspective of CH activation reaction. Both the transferring electron and proton originate from an identical site in the reactant state, but they end up in different locations in the product. Both Kohn-Sham density functional theory (DFT) and multistate density functional theory (MSDFT) calculations have been performed in the present study to investigate the mechanism of C-H activation, which transpires in the catalytic oxidation of N,N-dimethyalinines by the reactive species
Kohn-Sham DFT calculations reveal that the CH bond activation occurs via a HAT mechanism, in accord with the recent predictions of proton-coupled electron transfer (PCET) reactivity in analogous N-H and O-H bond activation reactions
Summary
Heme and nonheme iron enzymes mediate a variety of fundamental biochemical transformations which are vital to biological processes. Using a local determinant representation of individual Lewis structures, which effectively contracts many VB configurations into a single determinant approximation, we introduced a mixed molecular orbital and valence bond (MOVB) model, in which the block-localized wave (BLW) function method is used to define diabatic electronic states (Song et al, 2009; Gao et al, 2010; Mo et al, 2011; Cembran et al, 2012) This idea has been extended to density functional theory, and the general approach is called multistate density functional theory (MSDFT), and it can be used to study the mechanisms of PCET processes (Song et al, 2009; Gao et al, 2010; Mo et al, 2011; Cembran et al, 2012). Since the individual reaction steps can be separately defined using MSDFT, it is possible to provide a definitive answer to the mechanistic debate about the hydrogen abstraction reaction between N,N-dimethylaniline and a heme or a synthetic nonheme oxo-iron(IV) complexes
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