Modeling C–H Abstraction Reactivity of Nonheme Fe(IV)O Oxidants with Alkanes: What Role Do Counter Ions Play?

Abstract

b S Supporting Information T here is a surge of interest in synthetic models of mononuclear nonheme iron enzymes, 1À6 which perform CÀH activation and lead to the formation of alcohols and alkenes. Both the enzymes 1 and the synthetic models utilize high-valent iron-(IV)-oxo complexes as the active species. 2À5 One of the potent synthetic complexes is [N4PyFe IV O] 2+ (N4Py: N,N-bis (2-pyridylmethyl)-bis(2-pyridyl) methylamine), which is depicted in Scheme 1 and which is capable of activating even cyclohexane. 6,7 However, unlike the enzymatic complexes that have high-spin quintet (S = 2) ground states, the synthetic variants are generally characterized by triplet ground states (S = 1) 4 and low-lying quintet excited states (S = 2) and as such have a more complex reactivity behavior. Density functional theory (DFT) has contributed to the understanding of this reactivity, which was characterized as two-state reactivity (TSR), 8 wherein the S = 2 state cuts through the larger S = 1 barrier and mediates the reaction. 9À13 However, because most of the synthetic complexes carry a high positive charge, usually 2+, the gas-phase calculations have resulted in some nonphysical anomalies, such as barrier-free S = 2 surfaces, 10,11 electron transfer processes, artificial charge delocalization, 14 formation of charged organic intermediates due to hydride abstractions instead of the experimentally observed hydrogen atom abstraction (HAT), 6 and discontinuities in the potential energy profiles. 10,15 These anomalies were invincibly shown 14,16,17 to originate in the self-interaction error inherent in DFT. As such, an important class of these bioinorganic reactions cannot be confidently studied with DFT unless these anomalies can be evaded. Siegbahn et al. 14,16 have suggested that the anomalies can be muted by masking the charge of the iron oxo reagent, for example, by using counterions. 14 In this Letter, we report the boon of incorporating counterions in the UB3LYP calculations of the reactions, depicted in Scheme 1, of the synthetic complex 6 [N4PyFe IV O] 2+ with cyclohexadiene, with which all of the above anomalies manifest with the bare oxidant (model 1), and cyclohexane, for which the anomalies appear after the first HAT, in the follow-up steps in Scheme 1a. As shall be shown, adding the two ClO 4À counterions (model 2) as in the [N4PyFe II (CH 3 CN)](ClO 4 ) 2 crystal structure 18 removes the anomalies and creates smooth energy profiles that enable one to study the entire stepwise processes in Scheme 1a, explore various reaction trajectories (σ/π) 9b,19À21 of the rate-limiting HAT step, offer unequivocal characterization of the reaction intermediates, assess and derive coherent reactivity Scheme 1. (a) Reactions Studied Using (b) Oxidant Models 1, 1-solv, and 2 (1-solv Signifies That All Species Are Optimized in the Solvent) and (c) Two Substrates a a Reb and 2H are abbreviated processes.

Nonheme Fe(IV) Oxo Complexes of Two New Pentadentate Ligands and Their Hydrogen-Atom and Oxygen-Atom Transfer Reactions

“…For equatorial donors an increase of the donor strength of the ligand will lower the redox potential of the Fe IV O moiety and increase the pK a of the Fe III OH product formed upon hydrogen-atom transfer. It has been found that ligand donor moieties in cis-position to the oxo unit (equatorial donors) exert less influence on the reactivity of Fe IV O (S = 1) octahedral complexes as compared to those ligated trans (axial) 48 as "trans-ligands can interact with both σ-and π-orbitals involved in the FeO bond but cis-ligands cannot". 53 However, in the present case, the effect of replacing equatorial pyridyl donors with (Nmethyl)benzimidazolyl units is quite significant.…”

Dichotomous Hydrogen Atom Transfer vs Proton-Coupled Electron Transfer During Activation of X–H Bonds (X = C, N, O) by Nonheme Iron–Oxo Complexes of Variable Basicity

“…The dipositive charge of 4 was neutralized using two perchlorate anions (ClO 4 ) − . 28 The counter ions were positioned as found in the crystal structures of 1 , 3 17d and 4 . 29 During geometry optimization and throughout the reaction path, the position of the counter ions essentially remained constant.…”

“…24,28 Spin densities and natural bond order (NBO) charges of the optimized structures at B2+Solv level (Table S14–S19) were also analyzed for the identification of the electronic states of stationary points.…”

A potential role of a substrate as a base for the deprotonation pathway in Rh-catalysed C–H amination of heteroarenes: DFT insights

“…The recently reported explicit use of counter-ions in the non-heme iron catalyst reaction mechanism suggests that it helps to avoid the irregularities associated with self-interaction problem inherent in DFT. [37][38][39] Hence, we have also used SbF6 ─ ions along with all intermediates and transition states (after extensive site sampling) to get rid of the charge inconsistency associated with the reaction steps as well as to account for the effect of non-bonding interactions induced by the counter ions in the reaction environment.…”