Theoretical investigation on the oxidative chlorination performed by a biomimetic non-heme iron catalyst

Abstract

The present study is a part of an effort to understand the mechanism of the oxidative chlorination, as performed by a biomimetic non-heme iron complex. This catalytically active complex is generated from a peroxide and [(TPA)Fe(III)Cl(2)]+ [TPA is tris(2-pyridylmethyl)amine]. The reaction catalyzed by [(TPA)FeCl(2)]+/ROOH involves either [(TPA)ClFe(V)=O](2+) or [(TPA)ClFe(IV)=O]+ as an intermediate. On the basis of density functional theory the reaction of these two possible catalysts with cyclohexane is investigated. A question addressed is how the competing hydroxylation of the substrate is avoided. It is demonstrated that the high-valent iron complex [(TPA)Cl-Fe(V)=O](2+) is capable of stereospecific alkane chlorination, based on an ionic rather than on a radical pathway. In contrast, the results found for [(TPA)ClFe(IV)=O]+ cannot explain the experimental findings. In this case the transition states for chlorination and hydroxylation are energetically too close. The exclusive chlorination of the substrate by Cl-Fe(IV)=O may be explained by an indirect or a direct effect, altering the position of the competing rebound barriers.

Iron‐Catalyzed Halogenation of Alkanes: Modeling of Nonheme Halogenases by Experiment and DFT Calculations

“…This interpretation was supported by DFT, and it was also shown computationally that [Fe V O(L)(OH)] 2+ is accessible (albeit less easily)25 and highly reactive 19. Thus, for the halogenation reactions discussed here (see Table 1), pathways based on high‐spin Fe IV O and Fe V O may need to be taken into account (note that the SyrB2 biosystem involves a high‐spin Fe IV O intermediate,5, 6 and an Fe V O‐based mechanism has been proposed for the stoichiometric reaction with the tpa‐based complex10, 11). While a high‐spin Fe IV O complex (five‐coordinate, not very reactive)26 and an Fe V O complex27 have been fully characterized, so far we have not been able to trap and spectroscopically unambiguously characterize our putative high‐spin iron(IV) and iron(V) complexes.…”

“…Once the Fe V O species is formed, the halogenation reaction (Δ G =−386.9 kJ mol −1 ) is a barrierless process (relaxed scans on the PES), that is, hydrogen abstraction is barrierless and a radical intermediate was not found to be a minimum on the PES. Such a reaction would lead to a selectivity for halogenation, because after hydrogen abstraction the lone pair of the coordinated halide and not that of the hydroxo oxygen atom is pointing toward the singly occupied molecular orbital of the cyclohexyl radical, and the PES therefore directly leads to the halogenated alkane 11. The experimentally observed KIEs suggest in general that the hydrogen abstraction step is rate‐determining.…”

Applications of density functional theory to iron-containing molecules of bioinorganic interest

“…Theoretical studies of such biomimetic models may not only identify the key elements that determine their chemical reactivities, but may also provide insight into intermediates and reactivities of parent enzymes (Shaik et al, 2007a; de Visser et al, 2013). To date, DFT calculations have been applied extensively to various types of non-heme iron species (Scheme 1) (Bassan et al, 2002, 2005a,b; Roelfes et al, 2003; Decker and Solomon, 2005; Kumar et al, 2005; Quinonero et al, 2005; Berry et al, 2006; Bernasconi et al, 2007, 2011; de Visser, 2006, 2010; Hirao et al, 2006a, 2008a,b, 2011; Rohde et al, 2006; Decker et al, 2007; de Visser et al, 2007, 2011; Johansson et al, 2007; Noack and Siegbahn, 2007; Sastri et al, 2007; Sicking et al, 2007; Bernasconi and Baerends, 2008, 2013; Comba et al, 2008; Dhuri et al, 2008; Fiedler and Que, 2009; Klinker et al, 2009; Wang et al, 2009a, 2013b; Cho et al, 2010, 2012a, 2013; Geng et al, 2010; Chen et al, 2011; Chung et al, 2011b; Seo et al, 2011; Shaik et al, 2011; Vardhaman et al, 2011; Wong et al, 2011; Ye and Neese, 2011; Gonzalez-Ovalle et al, 2012; Gopakumar et al, 2012; Latifi et al, 2012; Mas-Ballesté et al, 2012; McDonald et al, 2012; Van Heuvelen et al, 2012; Ansari et al, 2013; Kim et al, 2013; Lee et al, 2013; Sahu et al, 2013; Tang et al, 2013; Ye et al, 2013; Hong et al, 2014; Sun et al, 2014). The intriguing reactivity patterns of these complexes are the result of active involvement of electrons in d-type MOs, which gives rise to multi-state scenarios (Shaik et al, 1998; Schröder et al, 2000; Schwarz, 2011).…”

Molecular Iron-Based Oxidants and Their Stoichiometric Reactions

“…18a). In both cases, DFT calculations support these "rebound-type" mechanisms [134,135]. The iron(IV)-oxo species abstracts a hydrogen from one of the substrate carbons, forming an organic radical which in turn abstracts a chloride (or hydroxide) from the iron(III) center.…”