Toward the Synthesis of More Reactive S = 2 Non-Heme Oxoiron(IV) Complexes

Abstract

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 Effect of Ligand Design on Metal Ion Spin State—Lessons from Spin Crossover Complexes

“…Many non-heme enzymes and synthetic complexes adopt the alternative approach of using weak field ligands to generate a high-spin ferryl active oxidant [10]. Comparisons of synthetic iron(IV) ferryl species with S = 1 and S = 2 ground states have shown that the latter are consistently more reactive towards H • atom abstraction from a substrate [4,11]. Such considerations can also have a bearing on organometallic catalysis, where first-row transition ions are increasingly playing a role [12].…”

Oxidation chemistry of C–H bond by mononuclear iron complexes derived from tridentate ligands containing phenolato function

“…Investigation of literature revealed that homolytic cleavage of oxygen-oxygen bond leads to the formation Fe IV =O species whereas heterolytic cleavage of the same gives rise to Fe V =O species. The reaction condition and ligand frame involved with iron are responsible for homolytic or heterolytic cleavage and concomitant formation of corresponding oxo species as intermediate [15][16][17][18][19][20][21][22][23].…”

Spectroscopic Characterization and Reactivity of Triplet and Quintet Iron(IV) Oxo Complexes in the Gas Phase

“…One of the most promising approaches is based on hypervalent transition‐metal oxo complexes 1, 2. An important advantage of using transition metals and their oxo complexes is the potential they offer to explore multistate reactivity 3, 4, 5, 6.…”