C–H Bond Cleavage by Bioinspired Nonheme Metal Complexes

Abstract

The functionalization of C−H bonds is one of the most challenging transformations in synthetic chemistry. In biology, these processes are well-known and are achieved with a variety of metalloenzymes, many of which contain a single metal center within their active sites. The most well studied are those with Fe centers, and the emerging experimental data show that high-valent iron oxido species are the intermediates responsible for cleaving the C−H bond. This Forum Article describes the state of this field with an emphasis on nonheme Fe enzymes and current experimental results that provide insights into the properties that make these species capable of C−H bond cleavage. These parameters are also briefly considered in regard to manganese oxido complexes and Cu-containing metalloenzymes. Synthetic iron oxido complexes are discussed to highlight their utility as spectroscopic and mechanistic probes and reagents for C−H bond functionalization. Avenues for future research are also examined.

Reactivity vs. Selectivity of Biomimetic Catalyst Systems of the Fe(PDP) Family through the Nature and Spin State of the Active Iron‐Oxygen Species

“…At the same time, the current level of fundamental understanding of the molecular/electronic properties of the, until recently, elusive Fe V =O intermediates, in relation to their oxidation reactivity, is reminiscent of where the field was 20 years ago in the pursuit of Fe IV =O complexes [12h] . Still, much is to be scrutinized using combined computational and experimental approaches before the consensus on the Fe V =O systems can approach the convincingness comparable to that achieved for heme and nonheme Fe IV =O complexes.…”

“…9 f, j). In any case, the central ion is predicted to have significant (perhaps even dominant) Fe IV character, which underscores the vacillating and formal character of the current consensus around the assignment of the above intermediates to formally Fe V =O species [12h,i] …”

“…The calculated Fe−O distances of 1.63−1.64 Å fall within the narrow range of values from 1.58 to 1.70 Å that are associated with high‐valent terminal Fe=O complexes, [12c–e] also being similar to those measured by EXAFS for the [( MeO PyNMe 3 )Fe=O(OC(O)Cy)] 2+ (1.63±0.02 Å), [11c] and [(PyTACN)Fe=O(OH)] 2+ (1.60 Å) complexes [9f] bearing neutral N4 donor ligands, and somewhat longer than for the [(TAML)Fe=O] − complex (1.58 Å) [6a] which represents the low end of the above range [12c–f] . One has to notice, however, that the oxidation state of the metal center in the metal−oxygen species is not directly related to the M−O bond length [12g] (Fe−O distances in the above range are also considered as characteristic of Fe IV −oxido species [12h] ), so these figures could not be taken as a kind of indication of the actual valence state of the metal center.…”

Oxidative versus basic asynchronous hydrogen atom transfer reactions of Mn(iii)-hydroxo and Mn(iii)-aqua complexes

“…Mechanisms of hydrogen atom transfer (HAT) reactions from organic substrates (SH) to metal–oxygen complexes, such as metal-oxo, 1–23 metal-hydroxo, 24–39 metal-(hydro)peroxo 40–44 and metal-superoxo, 45–54 have been investigated extensively, as summarized in Scheme 1. HAT proceeds via the transfer of both a proton and an electron which can be transferred simultaneously in a coupled fashion (concerted proton–electron transfer, CPET) or a stepwise fashion (proton transfer–electron transfer, PT/ET, or electron transfer–proton transfer, ET/PT) (Scheme 1).…”

Iron(iv) complexes with tetraazaadamantane-based ligands: synthesis, structure, applications in dioxygen activation and labeling of biomolecules

“…[1][2][3][4] The efficiency of these natural enzymes prompted scientists to develop bioinspired artificial catalysts for efficient oxidation of organic substrates preferably with atmospheric oxygen as the oxidant. [5][6][7][8][9][10] Although this goal remains elusive at the moment, much research has been done in the field of high-valent, and in particular, Fe(IV) complexes. 11,12 In recent years, numerous high-valent iron complexes, including heme-and non-heme Fe (IV) and Fe(IV)-oxo/nitride complexes, that mimic the structure and activity of natural oxidases have been generated and characterized.…”