Reactivities of Fe(IV) Complexes with Oxo, Hydroxo, and Alkylperoxo Ligands: An Experimental and Computational Study

Abstract

In a previous paper (Jensen et al., J. Am. Chem. Soc. 2005, 127, 10512), we reported the synthesis of the turquoise-colored intermediate [Fe IV (β-BPMCN)(OO t Bu)(OH)] 2+ (Tq; BPMCN = N,N′-bis (2-pyridylmethyl)-N,N′-dimethyl-trans-1,2-diaminocyclohexane). The structure of Tq is unprecedented, as it represents the only synthetic example to date of a non-heme Fe IV complex with both alkylperoxo and hydroxide ligands. Given the significance of similar high-valent Fe intermediates in the mechanisms of oxygenase enzymes, we have explored the reactivity of Tq at −70 °C, a temperature at which it is stable, and found that it is capable of activating weak X-H bonds (X = C, O) with bond dissociation energies ≤~80 kcal/mol. The Fe IV -OH unit of Tq, and not the alkylperoxo moiety, performs the initial H-atom abstraction. However at −45 °C, Tq decays at a rate that is independent of substrate identity and concentration, forming a species capable of oxidizing substrates with stronger C-H bonds. Parallel reactivity studies were also conducted with the related oxoiron(IV) complexes [Fe IV (β-BPMCN)(O)(X)] 2+ (3-X; X = pyridine or nitrile), thereby permitting a direct comparison of the reactivity of Fe IV centers with oxo and hydroxide ligands. We found that the H-atom abstracting ability of the Fe IV =O species greatly exceeds that of the Fe IV -OH species, generally by greater than 100-fold. Examination of the electronic structures of Tq and 3-X with density functional theory (DFT) provides a rationale for their differing reactivities.

Dioxygenase‐Like Reactivity of an Isolable Superoxo–Nickel(II) Complex

“…Indeed, substituted phenols are usually the substrates of choice in order to probe the H-atom abstracting ability of transition-metal complexes. [26,27] Overall, from our initial substrate screening, it seems clear that 1 cannot perform oxygen-atom transfer to alkenes, sulfides or alkanes but it readily activates and oxidizes O À H and N À H bonds.…”

“…Similar KIE values for the oxidation of TTBP have been measured in other metal systems in which a hydrogen-atom abstraction mechanism is postulated. [27,30] Surprisingly, 1 H NMR analysis of the oxidized product from the reaction with TTBP failed to show the characteristic singlet at d = 6.7 ppm expected for tBu PPQ, [31] which suggests the formation of a structurally different oxidized species. By means of 1D and 2D we could determine that, indeed, the oxidized species corresponds to compound 2 tBu (Figure 3), which is a structural isomer of tBu PPQ.…”

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).…”

Distinct Reactivity Differences of Metal Oxo and Its Corresponding Hydroxo Moieties in Oxidations: Implications from a Manganese(IV) Complex Having Dihydroxide Ligand

“…[6,7] Interestingly, using the method introduced by Bordwell et al and Mayer, [8] it was found that the hydrogen-abstraction ability of a metal oxo moiety and the related metal hydroxo in those models are very similar. [7,9,10] [12] Using Mn IV (salen) complexes (salen = N,N'-bis(salicylidene)ethylenediamine anion), Fujii and co-workers also found that Mn IV = O demonstrates much faster rate than its protonated Mn 2 )] can be investigated under three different pH conditions. Hydrogen abstraction reactions from 9,10-dihydroanthracene substrate were performed in acetone/ water (4:1) under nitrogen, and the product distributions are summarized in Table 1 3+ at pH 1.5 gives a 3.0 % yield of anthracene and an 8.3 % yield of anthraquinone, a surprisingly similar product distribution to that for the Mn IV =O moiety at pH 13.4.…”