Electrochemical Reduction of [Ni(Mebpy)₃]²⁺: Elucidation of the Redox Mechanism by Cyclic Voltammetry and Steady‐State Voltammetry in Low Ionic Strength Solutions

Abstract

Bipyridine complexes of Ni are used as catalysts in a variety of reductive transformations. Here, the electroreduction of [Ni(Mebpy)3]2+ (Mebpy=4,4′‐dimethyl‐2,2′‐bipyridine) in dimethylformamide is reported, with the aim of determining the redox mechanism and oxidation states of products formed under well‐controlled electrochemical conditions. Results from cyclic voltammetry, steady‐state voltammetry (SSV) and chronoamperometry demonstrate that [Ni(Mebpy)3]2+ undergoes two sequential 1e reductions at closely separated potentials (E10’=−1.06±0.01 V and E20’=−1.15±0.01 V vs Ag/AgCl (3.4 M KCl)). Homogeneous comproportionation to generate [Ni(Mebpy)3]+ is demonstrated in SSV experiments in low ionic strength solutions. The comproportionation rate constant is determined to be >106 M−1 s−1, consistent with rapid outer‐sphere electron transfer. Consequentially, on voltammetric time scales, the 2e reduction of [Ni(Mebpy)3]2+ results in formation of [Ni(Mebpy)3]+ as the predominant species released into bulk solution. We also demonstrate that [Ni(Mebpy)3]0 slowly loses a Mebpy ligand (∼10 s−1).

Nickel-Catalyzed Electro-Reductive Cross-Coupling of Aliphatic N-Acyl Imides with Alkyl Halides as a Strategy for Dialkyl Ketone Synthesis: Scope and Mechanistic Investigations

“…Finally, we thought that the stability of Ni­(II)–acyl complex I would provide an interesting opportunity to get further insights into the reactivity of electroreduced monovalent Ni–acyl complex toward alkyl and acyl electrophiles (Figure ). In particular, Ni­(I) intermediates have been recently proposed to be the most relevant catalytically active species in cross-coupling reactions under reductive conditions, but very less mechanistic information has been gained on the electrophile activation step at Ni­(I). ,,,, …”

“…This has been substantiated with electro-generated Ni­(I)–OAc and Ni­(I)–acyl intermediates; (ii) stoichiometric studies have shown that C–N bond oxidative addition of alkyl N -acyl imides also readily proceeds at Ni(0) species to give isolable Ni­(II)–acyl imidate species. The succinimidate ligand was shown to provide enhanced stability to the Ni­(II)–acyl intermediate, allowing us to evidence its catalytic relevance; (iii) under reductive electrolysis of ( t Bubpy)­Ni­(OAc) 2 , the predominant species responsible for the activation of both electrophiles are Ni­(I) …”

Advances on the Merger of Electrochemistry and Transition Metal Catalysis for Organic Synthesis

“…Recent mechanistic studies using voltammetric studies show the viability of a sequential two-electron reduction of Ni(II) to Ni(I) and Ni(0), thus, providing the possibility of aryl halide activation via oxidative addition of Ni(I) or Ni(0). 12 The ability of metal-catalyzed electrosynthesis in controlling redox states of the catalyst allows the transformation to preclude the use of highly air-sensitive M(0) catalysts and phosphine ligands as well as access to the desired Ni(III) intermediates. As such, the reactions can be performed at room temperature and in air.…”

“…An anodic oxidation of the aryl-Ni­(II)-amine complex generates the reactive aryl-Ni­(III)-amine intermediate that can readily undergo reductive elimination at room temperature. Recent mechanistic studies using voltammetric studies show the viability of a sequential two-electron reduction of Ni­(II) to Ni­(I) and Ni(0), thus, providing the possibility of aryl halide activation via oxidative addition of Ni­(I) or Ni(0) …”

Electrocatalysis with Molecular Transition-Metal Complexes for Reductive Organic Synthesis

“…The electrocatalytically relevant pathways involve the subsequent expulsion of bipyridine (at moderate rate of ca. 10 s –1 for [Ni II (Me 2 bpy) 3 ] 2+ ) 97 that produces a more stable tetracoordinated Ni(0) [Ni 0 (R 2 bpy) 2 ] species ( Figure 12 ).…”