High-spin oxoiron(IV) species are often implicated in the mechanisms of nonheme iron oxygenases, their C-H bond cleaving properties being attributed to the quintet spin state. However, the few available synthetic S = 2 Fe(IV)═O complexes supported by polydentate ligands do not cleave strong C-H bonds. Herein we report the characterization of a highly reactive S = 2 complex, [Fe(IV)(O)(TQA)(NCMe)](2+) (2) (TQA = tris(2-quinolylmethyl)amine), which oxidizes both C-H and C═C bonds at -40 °C. The oxidation of cyclohexane by 2 occurs at a rate comparable to that of the oxidation of taurine by the TauD-J enzyme intermediate after adjustment for the different temperatures of measurement. Moreover, compared with other S = 2 complexes characterized to date, the spectroscopic properties of 2 most closely resemble those of TauD-J. Together these features make 2 the best electronic and functional model for TauD-J to date.

Conspectus2003 marked a banner year in the bioinorganic chemistry of mononuclear non-heme iron enzymes. The first non-heme oxoiron(IV) intermediate (called J) was trapped and characterized by Bollinger and Krebs in the catalytic cycle of taurine dioxygenase (TauD), and the first crystal structure of a synthetic non-heme oxoiron(IV) complex was reported by Münck, Nam, and Que. These results stimulated inorganic chemists to synthesize related oxoiron(IV) complexes to shed light on the electronic structures and spectroscopic properties of these novel intermediates and gain mechanistic insights into their function in biology. All of the biological oxoiron(IV) intermediates discovered since 2003 have an S = 2 ground spin state, while over 90% of the 60 or so synthetic oxoiron(IV) complexes reported to date have an S = 1 ground spin state. This difference in electronic structure has fueled an interest to more accurately model these enzymatic intermediates and synthesize S = 2 oxoiron(IV) complexes.This Account follows up on a previous Account (Acc. Chem. Res. 2007, 40, 493) that provided a perspective on the early developments in this field up to 2007 and details our group’s efforts in the development of synthetic strategies to obtain oxoiron(IV) complexes with an S = 2 ground state. Upon inspection of a qualitative d-orbital splitting diagram for a d4 metal–oxo center, it becomes evident that the key to achieving an S = 2 ground state is to decrease the energy gap between the dx2–y2 and dxy orbitals. Described below are two different synthetic strategies we used to accomplish this goal.The first strategy took advantage of the realization that the dx2–y2 and dxy orbitals become degenerate in a C3-symmetric ligand environment. Thus, by employing bulky tripodal ligands, trigonal-bipyramidal S = 2 oxoiron(IV) complexes were obtained. However, substrate access to the oxoiron(IV) center was hindered by the bulky ligands, and the complexes showed limited ability to cleave substrate C–H bonds. The second strategy entailed introducing weaker-field equatorial ligands in six-coordinate oxoiron(IV) complexes to decrease the dx2–y2/dxy energy gap to the point where the S = 2 ground state is favored. These pseudo-octahedral S = 2 oxoiron(IV) complexes exhibit high H-atom transfer reactivity relative to their S = 1 counterparts and shed light on the role that the spin state may play in these reactions. Among these complexes is a highly reactive species that to date represents the closest electronic and functional model of the enzymatic intermediate, TauD-J.

The non-heme iron halogenases CytC3 and SyrB2 catalyze C-H bond halogenation in the biosynthesis of some natural products via S = 2 oxoiron(IV)-halide intermediates. These oxidants abstract a hydrogen atom from a substrate C-H bond to generate an alkyl radical that reacts with the bound halide to form a C-X bond chemoselectively. The origin of this selectivity has been explored in biological systems but has not yet been investigated with synthetic models. Here we report the characterization of S = 2 [Fe(IV)(O)(TQA)(Cl/Br)](+) (TQA = tris(quinolyl-2-methyl)amine) complexes that can preferentially halogenate cyclohexane. These are the first synthetic oxoiron(IV)-halide complexes that serve as spectroscopic and functional models for the halogenase intermediates. Interestingly, the nascent substrate radicals generated by these synthetic complexes are not as short-lived as those obtained from heme-based oxidants and can be intercepted by O2 to prevent halogenation, supporting an emerging notion that rapid rebound may not necessarily occur in non-heme oxoiron(IV) oxidations.

Chiral semiconductor nanocrystals, or quantum dots (QDs), are promising materials for applications in biological sensing, photonics, and spin-polarized devices. Many of these applications rely on large dissymmetry, or g-factors, the difference in absorbance between left- and right-handed circularly polarized light compared to the unpolarized absorbance. The majority of chiral QDs, specifically CdSe, reported to date have used thiolated amino acid ligands to introduce chirality onto the nanoparticles, but these systems have ultimately reported small g-factors of ∼2 × 10. In an effort to realize chiral CdSe QDs with higher g-factors and to expand the set of designer chiral semiconductor nanocrystals, we have employed chiral carboxylic acids as a distinct class of ligands for chiral CdSe nanoparticles. Through this family of chiral carboxylic acid ligands, we performed a direct comparison between carboxylate-bound and thiolate-bound chiral CdSe QDs. Spectral analysis revealed that the resulting circular dichroism shifts originate from the splitting of the exciton by the ligand-nanocrystal interaction. Subsequent examination of a series of chiral carboxylic acid ligands revealed a 30-fold range in g-factor through relatively small changes in the structure of the ligand. Finally, we showed that increasing the number of stereocenters on the ligand can further enhance the dissymmetry factors. This versatile and tunable combination of nanocrystals and ligands will inform future designs of chiral nanomaterials and their applications.

Terminal non-heme iron(IV)−oxo compounds are among the most powerful and best studied oxidants of strong C−H bonds. In contrast to the increasing number of such complexes (>80 thus far), corresponding one-electron-reduced derivatives are much rarer and presumably less stable, and only two iron(III)−oxo complexes have been characterized to date, both of which are stabilized by hydrogen-bonding interactions. Herein we have employed gas-phase techniques to generate and identify a series of terminal iron(III)−oxo complexes, all without built-in hydrogen bonding. Some of these complexes exhibit ∼70 cm −1 decrease in ν(Fe−O) frequencies expected for a half-order decrease in bond order upon one-electron reduction to an S = 5/2 center, while others have ν(Fe−O) frequencies essentially unchanged from those of their parent iron(IV)−oxo complexes. The latter result suggests that the added electron does not occupy a d orbital with FeO antibonding character, requiring an S = 3/2 spin assignment for the nascent Fe III −O − species. In the latter cases, water is found to hydrogen bond to the Fe III −O − unit, resulting in a change from quartet to sextet spin state. Reactivity studies also demonstrate the extraordinary basicity of these iron(III)−oxo complexes. Our observations show that metal−oxo species at the boundary of the "Oxo Wall" are accessible, and the data provide a lead to detect iron(III)−oxo intermediates in biological and biomimetic reactions.