Opening the CoIII,IV2(μ-O)2 Diamond Core by Lewis Bases Leads to Enhanced C-H Bond Cleaving Reactivity

Generation, Characterization and Reactivity of a High-Valent Mononuclear Cobalt(IV)-Diazide Complex

High-valent Fe(IV)=O intermediates of metalloenzymes have inspired numerous efforts to generate synthetic analogs to mimic and understand their substrate oxidation reactivities. However, high-valent M(IV) complexes of late transition metals are rare. We have recently reported a novel Co(IV)-dinitrate complex (1-NO3) that activates sp3 C-H bonds up to 87 kcal/mol. In this work, we have shown that the nitrate ligands in 1-NO3 can be replaced by azide, a more basic coordinating base, resulting in the formation of a more potent Co(IV)-diazide species (1-N3) that reacts with substrates (hydrocarbons and phenols) at faster rate constants and activates stronger C-H bonds than the parent complex 1-NO3. We have characterized 1-N3 employing a combination of spectroscopic and computational approaches. Our results clearly show that the coordination of azide leads to the modulation of the Co(IV) electronic structure and the Co(IV/III) redox potential. Together with the higher basicity of azide, these thermodynamic parameters contribute to the higher driving forces of 1-N3 than 1-NO3 for C-H bond activation. Our discoveries are thus insightful for designing more reactive bio-inspired high-valent late transition metal complexes for activating inert aliphatic hydrocarbons.

Two-State Hydrogen Atom Transfer Reactivity of Unsymmetric [Cu2(O)(NO)]2+ Complexes

We report the temperature-dependent spin switching of dicopper oxo nitrosyl [Cu2(O)(NO)]2+ complexes and their influence on hydrogen atom transfer (HAT) reactivity. Electron paramagnetic resonance (EPR) and Evans method analysis suggest that [Cu2(O)(NO)]2+ complexes transition from the S = 1/2 to the S = 3/2 state around ca. 202 K. At low temperatures (198 K) where S = 3/2 dominates, a strong correlation between the rate of HAT (kHAT) and the population of the S = 1/2 state was identified (R2 = 0.988), suggesting that the HAT by [Cu2(O)(NO)]2+ complexes proceeds by the S = 1/2 isomer. Installation of functional groups that introduce an unsymmetric secondary coordination environment accelerates the HAT rates through perturbation of the spin equilibria. Given the often unsymmetric coordination sphere of bimetallic active sites in natural proteins, we anticipate that similar strategies could be employed by metalloenzymes to control HAT reactions.

Direct Activation of the C(sp3)-NH2 Bond of Primary Aliphatic Alkylamines by a High-Valent CoIII,IV2(μ-O)2 Diamond Core Complex

Aliphatic alkylamines are abundant feedstock and versatile building blocks for many organic transformations. While remarkable progress has been made to construct C-N bonds on aliphatic and aromatic carbon centers, the activation and functionalization of C(sp³)-NH₂ bonds in primary alkylamines remain a challenging process. In the present work, we discovered an unprecedented method to directly activate the C(sp³)-NH₂ bond of primary alkylamines by a high-valent dinuclear Co^III,IV₂(μ-O)₂ diamond core complex. This reaction results in the installation of other functional groups such as halides and alkenes onto the α-carbon center concomitant with the 2-e^- oxidation of the nitrogen atom on the amino group to form NH₂OH. These results shed light on future development enabling versatile functionalization of primary alkylamines based on the dinuclear cobalt system. Moreover, our work suggests that a related high-valent copper-oxo intermediate is likely generated in the ammonia monooxygenase catalytic cycle to affect the oxidation of NH₃ to NH₂OH.