One-electron oxidation of an oxoiron(IV) complex to form an [O═Fe ^V ═NR] ⁺ center

Abstract

Oxoiron(V) species are postulated to be involved in the mechanisms of the arene cis-dihydroxylating Rieske dioxygenases and of bioinspired nonheme iron catalysts for alkane hydroxylation, olefin cis-dihydroxylation, and water oxidation. In an effort to obtain a synthetic oxoiron(V) complex, we report herein the oneelectron oxidation of the S ¼ 1 complex ½Fe IV ðOÞðTMCÞðNCCH 3 Þ 2þ (1, where TMC is tetramethylcyclam) by treatment with tert -butyl hydroperoxide and strong base in acetonitrile to generate a metastable S ¼ 1 2 complex 2 at −44°C, which has been characterized by UV-visible, resonance Raman, Mössbauer, and EPR methods. The defining spectroscopic characteristic of 2 is the unusual x∕y anisotropy observed for the 57 Fe and 17 O A tensors associated with the high-valent Fe═O unit and for the 14 N A tensor of a ligand derived from acetonitrile. As shown by detailed density functional theory (DFT) calculations, the unusual x∕y anisotropy observed can only arise from an iron center with substantially different spin populations in the d xz and d yz orbitals, which cannot correspond to an Fe IV ═O unit but is fully consistent with an S ¼ 1 2 Fe V center, like that found for ½Fe V ðOÞðTAMLÞ − (where TAML is tetraamido macrocyclic ligand), the only well-characterized oxoiron(V) complex reported. Mass spectral analysis shows that the generation of 2 entails the addition of an oxygen atom to 1 and the loss of one positive charge. Taken together, the spectroscopic data and DFT calculations support the formulation of 2 as an iron(V) complex having axial oxo and acetylimido ligands, namely ½Fe V ðOÞðTMCÞðNCðOÞCH 3 Þ þ .high-valent iron-oxo | bioinorganic chemistry F ormally oxoiron(V) oxidants are postulated in the catalytic cycles of several iron enzymes that carry out difficult oxidations. Most prominent of these are cytochromes P450, which can hydroxylate strong C─H bonds (1-3), even those of methane (1, 2). Recent evidence has demonstrated that the active oxidant can be best described as having an oxoiron(IV) unit supported by a porphyrin cation radical (4). On the other hand, the Rieske dioxygenases activate O 2 at an active site with a 2-His-1-carboxylate facial triad motif to effect the cis-dihydroxylation of C═C bonds in the biodegradation of aromatic complexes (5). For these nonheme iron enzymes, the proposed Fe V ═O oxidant is as yet unobserved. However, unlike the porphyrin ligand in cytochrome P450, none of the ligands in the nonheme iron active site appears likely to undergo one-electron oxidation to stabilize the high-valent state. Fe V ═O oxidants are also implicated in alkane hydroxylation (6, 7), olefin epoxidation and cis-dihydroxylation (8-10), and water oxidation (11) by bioinspired nonheme iron catalysts supported by neutral tetradentate ligands, and direct evidence for the formation of oxoiron(V) oxidants has been obtained in two cases by mass spectrometry (10, 12). Despite the wealth of synthetic oxoiron(IV) complexes identified during the last decade (13), to date there is only one spectrosco...

Equilibrating (L)FeIII–OOAc and (L)FeV(O) Species in Hydrocarbon Oxidations by Bio-Inspired Nonheme Iron Catalysts Using H2O2 and AcOH

“…If these UV-vis features belong to 5a entirely, then respective extinction coefficients of 9000, 5000 and 2000 M −1 cm −1 can be estimated. These electronic absorption features are in the same spectroscopic region and of similar intensity as those of related g2.07 species 5b (L = PyNMe 3 ), 54 which exhibits visible bands at 490 nm (ε = 7000 M −1 cm −1 ) and 680 nm (ε = 1100 M −1 cm −1 ), and [(TMC)Fe V (O)(NC(O) t Bu)] + , 61 which exhibits bands at 425 nm (ε = 4100 M −1 cm −1 ), 600 nm (ε = 680 M −1 cm −1 ) and 750 nm (ε = 530 M −1 cm −1 ). The similarity in the EPR (see Table 3) and electronic absorption parameters of these g2.07 species indicates that 5a and 5a * may be electronically related to 5b and [(TMC)Fe V (O)(NC(O) t Bu)] + , the EPR properties of which strongly argue for an Fe V (O) assignment.…”

“…Similarly, the EPR signals of 6a and 6a * show an identical hyperfine splitting along the g min = g x = 1.94 feature corresponding to |A x ( 57 Fe)| = 60 MHz, with the other two g -values exhibiting smaller A-values. The large A-tensor anisotropy observed, with the largest A-value along the x or y- axis, has previously been used to argue against an (L •+ )Fe IV (O) electronic structure, 54, 61 which should have comparable A x and A y values because of the d xz 1 d yz 1 configuration. Indeed, S = ½ (L •+ )Fe IV (O) species such as the Compounds I of horseradish peroxidase, chloroperoxidase, and cytochrome P450 exhibit axial 57 Fe A-tensors with |A x | ≈ |A y | ≫ |A z | (Table 3), 62, 63 which reflect the d xy 2 (d xz ) 1 (d yz ) 1 electronic configuration associated with an S = 1 Fe IV (O) unit.…”

“…The similarity in the EPR (see Table 3) and electronic absorption parameters of these g2.07 species indicates that 5a and 5a * may be electronically related to 5b and [(TMC)Fe V (O)(NC(O) t Bu)] + , the EPR properties of which strongly argue for an Fe V (O) assignment. 54, 61 …”

Identification of a low-spin acylperoxoiron(III) intermediate in bio-inspired non-heme iron-catalysed oxidations

“…At T ¼ 160 K, however, the electronic spin relaxes fast so that the magnetic hyperfine interactions are averaged out, yielding a quadrupole doublet for the S ¼ 1/2 species (Fig. 5c, red line) 33 .…”

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).…”