The reactivity of the active metal oxo and hydroxo intermediates and their implications in oxidations

Abstract

While the significance of the redox metal oxo moieties has been fully acknowledged in versatile oxidation processes, active metal hydroxo moieties are gradually realized to play the key roles in certain biological oxidation events, and their reactivity has also been evidenced by related biomimic models. However, compared with the metal oxo moieties, the significance of the metal hydroxo moieties has not been fully recognized, and their relationships in oxidations remain elusive until recently. This review summarizes the reactivity of the metal oxo and hydroxo moieties in different oxidation processes including hydrogen atom transfer, oxygen atom transfer and electron transfer, and their reactivity similarities and differences have been discussed as well. Particularly, how the physicochemical properties like metal-oxygen bond order, net charge and potential of a redox metal ion affect its reactivity has also been presented based on available data. We hope that this review may provide new clues to understand the origins of the enzyme's choice on them in a specific event, to explain the elusive phenomena occurring in those enzymatic, homogeneous and heterogeneous oxidations, to design selective redox catalysts and control their reactivity.

Synthesis and reactivity of a mononuclear non-haem cobalt(IV)-oxo complex

“…3). Therefore, we attempted to synthesize 2 through more standard methods for the synthesis of metal–oxo complexes using artificial oxidants, such as PhIO, peracids and hydroperoxides789101112. Interestingly, among the examined oxidants, we were able to obtain 2 with a significantly higher stability when PhIO was used as a terminal oxidant in the presence of a small amount of acid; the formation of 2 was not clean in the absence of acid due to the longer formation time and the less stability of 2 .…”

“…On the basis of the large KIE value and the good correlation between the reaction rate and BDEs of substrates, we conclude that hydrogen atom (H-atom) abstraction from the C–H bonds of the substrates by 2 is the rate-determining step (r.d.s.) in the C–H bond activation reactions789101112.…”

“…Similarly, a transient, but not yet isolated, terminal or bridging manganese–oxo intermediate is proposed to be involved in the energy-demanding O–O bond formation step, which is considered to be the most critical part in the oxidation of water to evolve O 2 in Photosystem II (refs 4, 5, 6). Further, recent synthetic advances have led to the isolation and characterization of several terminal metal–oxo complexes of Fe (refs 7, 8, 9) and Mn (refs 10, 11, 12); detailed reactivity studies, including C–H bond activation and oxygen atom transfer reactions, in conjunction with spectroscopy and theory have helped us to understand how the steric and electronic properties of the metal centres modulate their reactivities. In addition, terminal metal–oxo complexes of Cr, V and Mo have been successfully synthesized in an effort to provide an additional chemical basis for understanding the reaction mechanisms of the metalloenzymes and also to develop artificial oxidation catalysts13.…”

Rapid Hydrogen and Oxygen Atom Transfer by a High-Valent Nickel–Oxygen Species

“…[1][2][3][4][5] Many examples of natural and synthetic terminal high-valent Fe-oxygen, and Mn-oxygen species have been reported in the last decade. 6,7 In sharp contrast, and despite some efforts, detection of terminal high-valent metal-oxygen species involved in the mode of action of highly efficient oxidation catalysts based on late-transition metals such as cobalt, nickel or copper are scarce. This is in part related to the "Oxo Wall" concept, which predicts that late transition elements cannot support a terminal oxido ligand in a tetragonal environment.…”

The Role of Nonheme Transition Metal-Oxo, -Peroxo, and -Superoxo Intermediates in Enzyme Catalysis and Reactions of Bioinspired Complexes

“…94,95 In particular, in collaboration with the Goldberg group we investigated the structure and reactivity of a manganese(V)-oxo corrolazine complex [Mn(O)(H 8 Cz)X], H 8 Cz is corrolazine without side chains and X is the axial ligand (X = no ligand or CN -/F -). We initially investigated the axial ligand effect on the dehydrogenation of 9,10-dihydroanthracene and showed that the reaction rate was enhanced by a factor of 2100 upon the addition of F -, while it increased by a factor of 16000 with addition of CN -.…”