The Oxo-Wall Remains Intact: A Tetrahedrally Distorted Co(IV)–Oxo Complex

Abstract

In this paper, we report the preparation, spectroscopic and theoretical characterization, and reactivity studies of a Co(IV)−oxo complex bearing an N4-macrocyclic coligand, 12-TBC (12-TBC = 1,4,7,10-tetrabenzyl-1,4,7,10-tetraazacyclododecane). On the basis of the ligand and the structure of the Co(II) precursor, [Co II (12-TBC)(CF 3 SO 3 ) 2 ], one would assume that this species corresponds to a tetragonal Co(IV)−oxo complex, but the spectroscopic data do not support this notion. Co K-edge XAS data show that the treatment of the Co(II) precursor with iodosylbenzene (PhIO) as an oxidant at −40 °C in the presence of a proton source leads to a distinct shift in the Co K-edge, in agreement with the formation of a Co(IV) intermediate. The presence of the oxo group is further demonstrated by resonance Raman (rRaman) spectroscopy. Interestingly, the EPR data of this complex show a high degree of rhombicity, indicating structural distortion. This is further supported by the EXAFS data. Using DFT calculations, a structural model is developed for this complex with a ligand-protonated structure that features a CoO•••HN hydrogen bond and a four-coordinate Co center in a seesaw-shaped coordination geometry. Magnetic circular dichroism (MCD) spectroscopy further supports this finding. The hydrogen bond leads to an interesting polarization of the Co−oxo π-bonds, where one O(p) lone-pair is stabilized and leads to a regular Co(d) interaction, whereas the other π-bond shows an inverted ligand field. The reactivity of this complex in hydrogen atom and oxygen atom transfer reactions is discussed as well.

C–H Bond Activation by a Mononuclear Nickel(IV)-Nitrate Complex

“…The discoveries of high-valent metal-oxo species as key intermediates in a variety of enzymatic transformations have inspired numerous biomimetic and bio-inspired efforts trying to understand the structural and spectroscopic properties and substrate oxidation reactivities of these species. , While a large inventory of synthetic M­(IV)O complexes for first-row early and middle transition metal elements (Cr, Mn, and Fe) have become available, high-valent metal-oxo complexes for late transition metals (Co, Ni, and Cu) are rare, as predicted by the classic bonding theory. , Alternatively, diverging the synthetic effort from the metal-oxo territory to develop non-oxo-metal complexes of late transition metals appears to be an effective strategy that provides more flexibility to study the rich chemistry of their high-valent derivatives . For example, a number of groups, including us, have described oxo- and non-oxo-Co­(IV) species capable of oxidizing sp 3 C–H bonds. − For Ni and Cu, the +3 oxidation state is typically considered as “high valent”. A number of Ni­(III)- and Cu­(III)-hydroxo, carboxylate, and halide complexes are strong oxidants and hydrogen atom transfer (HAT) reagents; − however, examples of these two metals having formal +4 oxidation states are rare.…”

Enzyme-Like Hydroxylation of Aliphatic C–H Bonds From an Isolable Co-Oxo Complex

“…The reaction mechanism shown in Scheme A invokes a key Co IV -oxo intermediate. Such high-valent Co complexes are quite rare although they have been invoked as reactive intermediates. ,,,, While there are two examples of spectroscopically characterized terminal Co IV -oxo species, they do not engage in the activation of aliphatic C–H bonds. , Therefore, we set out to observe any transient Co IV complex in order to support the proposed agency of 4 ox . We first monitored the addition of FcBF 4 to solutions of 4 by UV–vis spectroscopy at temperatures lower than −80 °C in the hopes of slowing down the reaction enough to observe 4 ox .…”

“…32,39,40,53,54 While there are two examples of spectroscopically characterized terminal Co IV -oxo species, they do not engage in the activation of aliphatic C−H bonds. 40,42 Therefore, we set out to observe any transient Co IV complex in order to support the proposed agency of 4 ox . We first monitored the addition of FcBF C−H activation reactions mediated by transition metal-oxo and hydroxide complexes frequently have large deuterium kinetic isotope effects (KIEs), especially at low temperatures.…”

“…This muted reactivity with strong C–H bonds by well-defined systems is often attributed to free energy considerations. These relatively stable transition metal-oxo intermediates frequently have BDFE O–H < BDE C–H , which, as illustrated by the Bell–Evans–Polanyi relationship and extensive literature precedent, results in inhibited reactivity. − The dichotomy of stabilization at the expense of reactivity results in a scarcity of metal-oxo complexes that activate strong C–H bonds where the thermodynamics are known with fidelity, − and in these limited examples, the rates of reaction are slow when BDE C–H > BDFE O–H . − In particular, late transition metal-oxo complexes, which are frequently invoked as potent oxidants, have not displayed this reactivity from well-characterized examples. − The activation of strong tert -butyl C–H bonds in Co complexes has been observed, but hydroxyl radicals are invoked in these reactions and it is unclear whether this reactivity stems from bona fide metal-oxo intermediates. ,, Gas phase C–H activation of cyclohexane (BDE = 99.5 kcal/mol) by a transient Co IV -oxo complex has been reported, but the relevant reaction free energies are unknown . Thus, while it has been proposed that endergonic C–H activation followed by exergonic rebound is central to selective catalysis by both molecular and enzymatic systems, examples of well-characterized transition metal-oxo systems where these steps can be studied are rare.…”

Asymmetric Epoxidation of Alkenes Catalyzed by a Cobalt Complex

“…They suggested that formally, Co–O complex should be best described as a Co­(III)-oxyl tautomer by calculations. Besides, a Lewis acid or the hydrogen bond could stabilize a metal–oxo unit by reducing the electron density of the terminal oxo donor and allow for the characterization of a Co­(IV)–oxo species. − These studies shed light on the potential to develop chiral multidentate ligand-based Co–O complexes for asymmetric epoxidation.…”