Controlled Single-Electron Transfer via Metal–Ligand Cooperativity Drives Divergent Nickel-Electrocatalyzed Radical Pathways

Abstract

Electrocatalysis enables the construction of C-C bonds under mild conditions via controlled formation of carboncentered radicals. For sequences initiated by alkyl halide reduction, coordinatively-unsaturated Ni complexes commonly serve as single electron transfer agents, giving rise to the foundational question of whether outer-or inner-sphere electron transfer oxidative addition prevails in redox mediation. Indeed, rational design of electrochemical processes requires the discrimination of these two electron transfer pathways, as they can have outsized effects on the rate of substrate bond activation and thus impact radical generation rates and downstream product selectivities. We present results from combined synthetic, electroanalytical, and computational studies that examine the mechanistic differences of single electron transfer to alkyl halides imparted by Ni metal-ligand cooperativity. Electrogenerated reduced Ni species, stabilized by delocalized spin density onto a redox-active tpyPY2Me polypyridyl ligand, activates alkyl iodides via outer-sphere electron transfer, allowing for the selective activation of alkyl iodide substrates over halogen atom donors and the controlled generation and sequestration of electrogenerated radicals. In contrast, the Ni complex possessing a redoxinnocent pentapyridine congener activates the substrates in an inner-sphere fashion owning to a purely metal-localized spin, thereby activating both substrates and halogen atom donors in an indiscriminate fashion, generating a high concentration of radicals and leading to unproductive dimerization. Our data establish that controlled electron transfer via Ni-ligand cooperativity can be used to limit undesired radical recombination products and promote selective radical processes in electrochemical environments, providing a generalizable framework for designing redox mediators with distinct rate and potential requirements.

Ligand-Metal Cooperation Enables C–C Activation Cross-Coupling Reactivity of Cyclopropyl Ketones

“…Consistent with studies by Vicic, 24,51 Chirik, 32 and Weighardt, 52 our DFT studies show that the (tpy)Ni catalyst system exhibits redox changes at both the metal and the ligand. These calculations predict a radical anion terpyridine ligand and formal nickel(I) oxidation state for neutral complexes with L-type ancillary ligands (Figure 4A-C, Figure S19, Figure S26-S28) 50 . In each case, a triplet ground state is favored with unpaired electrons in nickel 𝑑 !…”

“…Initial reduction of the (tpy)Ni(II)Cl2 pre-catalyst by Zn metal affords a neutral nickel intermediate with the proposed electronic structure (tpy •-)Ni I L (I-NMP, L = Nmethylpyrrolidone (NMP)). 50 Putative intermediate II is proposed to form by coordination of the electron-accepting silyl carboxonium cation to the nickel center of I, which induces a transfer of electron density from tpy •to afford partial ketyl-radical character (~50% spin-density) at the carbonyl carbon Cα. II then undergoes concerted, but asynchronous C-C bond activation and Ni-C bond formation via an energetically accessible triplet transition state (see calculations in Figure 4).…”

Exchange Coupling Determines Metal-Dependent Efficiency for Iron- and Cobalt-Catalyzed Photochemical CO2 Reduction

“…Despite the marked differences in electronic structure for the singly-reduced forms of the iron, cobalt, and nickel tpyPY2Me complexes, all three metals share similar electronic character upon introduction of a second reduction event. Comparing DFT results obtained for [Co] 0 (Figure 6, right) to those of [Fe] 0 44 and [Ni] 0 , 53 we observe that each first-row transition complex adopts a high-spin, unreduced formal M 2+ center bound to a doubly-reduced (tpyPY2Me •• ) 2− ligand. Thus, in the case of the second reduction event, the complexes differ only in the extent of d /π* orbital coupling between the metal atom and surrounding terpyridine ligand fragment.…”

“…Moreover, the analogous nickel tpyPY2Me complex, [Ni(tpyPY2Me)] 2+ ([Ni] 2+ ), has found application as an efficient redox mediator for controlling radical pathways by ligand-centered redox reactivity. 53 Against this backdrop, we sought to expand the reactivity of metal tpyPY2Me complexes to photochemical CO2RR. Considering the precedent for cobalt-based CO2RR catalysts, 11,18,23,39,[54][55][56][57] we now report a comparative study between iron and cobalt tpyPY2Me complexes for photochemical CO2RR, including synthesis and characterization of the novel cobalt tpyPY2Me complex, [Co(tpyPY2Me)] 2+ ([Co] 2+ ).…”

Resolving the Multidecade-Long Mystery in MoaA Radical SAM Enzyme Reveals New Opportunities to Tackle Human Health Problems

“…In particular, the electronic coupling between GTP and the AUX cluster is interesting and may have some mechanistic roles. For example, in synthetic Fe 83 and Ni 84 polypyridine complexes, electronic coupling between the metal and ligand induces positive shifts of the reduction potential of the ligand. Therefore, in MoaA, the electronic coupling between GTP and the AUX cluster may further facilitate the aminyl radical reduction and prevent reoxidation of 3′,8-cH 2 GTP.…”