Crystal Structure and C–H Bond-Cleaving Reactivity of a Mononuclear Co^IV–Dinitrate Complex

Abstract

High-valent FeIVO intermediates with a terminal metal–oxo moiety are key oxidants in many enzymatic and synthetic C–H bond oxidation reactions. While generating stable metal–oxo species for late transition metals remains synthetically challenging, notably, a number of high-valent non-oxo–metal species of late transition metals have been recently described as strong oxidants that activate C–H bonds. In this work, we obtained an unprecedented mononuclear CoIV–dinitrate complex (2) upon one-electron oxidation of its Co­(III) precursor supported by a tridentate dianionic N3 ligand. 2 was structurally characterized by X-ray crystallography, showing a square pyramidal geometry with two coordinated nitrate anions. Furthermore, characterization of 2 using combined spectroscopic and computational methods revealed that 2 is a low-spin (S = 1/2) Co­(IV) species with the unpaired electron located on the cobalt d z2 orbital, which is well positioned for substrate oxidations. Indeed, while having a high thermal stability, 2 is able to cleave sp3 C–H bonds up to 87 kcal/mol to afford rate constants and kinetic isotope effects (KIEs) of 2–6 that are comparable to other high-valent metal oxidants. The ability to oxidize strong C–H bonds has yet to be observed for CoIV–O and CoIIIO species previously reported. Therefore, 2 represents the first high-valent Co­(IV) species that is both structurally characterized by X-ray crystallography and capable of activating strong C–H bonds.

Selective oxygenation of C–H and CC bonds with H2O2 by high-spin cobalt(ii)-carboxylate complexes

“…The cobalt-oxygen species has been proposed to participate in CvC and C-H bond activation, but its selectivity in oxidation reactions has been less explored. [44][45][46][47][48][49] In recent times, some progress has been made in peroxide-dependent catalysis with cobalt complexes to achieve selective oxygenation of substrates. 45,[50][51][52][53][54][55][56] Our group recently reported a series of cobalt(II)-carboxylate complexes of tripodal nitrogen donor ligands to study their reactivity towards dioxygen.…”

A Water‐Soluble Highly Oxidizing Cobalt Molecular Catalyst Designed for Bioinspired Water Oxidation

“…The results provide evidence that in the ligand environment of complex 1 , the Co atom exists in the primary location of unpaired electrons ( S =5/2), which was also validated by magnetization measurements and further confirmed by DFT calculations (see below). This high‐spin sextet ground state is distinct from the previously reported examples of cobalt(IV) complexes, [21] most of which reside in a low‐spin doublet ( S =1/2) ground state [13d, 14, 22] . The prevalence of high‐spin electronic configurations among 3d transition metals is commonly accepted to be particularly important in catalytic reactions due to their increased radical character [23] .…”

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 .…”

“…39,54,61−66 In particular, 4 ox -d 45 displays relatively small hyperfine coupling constants that are more similar to those reported for spin delocalized systems 61,63 than those reported for systems where the spin is highly localized on the 59 Co nucleus. 54 This, in addition to the g-values near 2, suggests that there is likely a significant amount of O-centered spin density in 4 ox -d 45 . IBO analysis predicts a population of 0.44 spins on O, 67,68 consistent with this hypothesis.…”

“…Such high-valent Co complexes are quite rare although they have been invoked as reactive intermediates. 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 .…”