Ligand Redox Activity of Organonickel Radical Complexes Governed by the Geometry

Abstract

Nickel-catalyzed cross-coupling reactions often employ bidentate πacceptor N-ligands to facilitate radical pathways. This report presents the synthesis and characterization of a series of organonickel radical complexes supported by bidentate N-ligands, including bpy, phen, and pyrox, which are commonly proposed and observed intermediates in catalytic reactions. Through a comparison of relevant analogues, we have established an empirical rule governing the electronic structures of these nickel radical complexes. The N-ligands exhibit redox activity in four-coordinate, square-planar nickel radical complexes, leading to the observation of ligand-centered radicals. In contrast, these ligands do not display redox activity when supporting three-coordinate, trigonal planar nickel radical complexes, which are better described as nickel-centered radicals. This trend holds true irrespective of the nature of the actor ligands. These results provide insights into the beneficial effect of coordinating salt additives and solvents in stabilizing nickel radical intermediates during catalytic reactions by modulating the redox activity of the ligands. Understanding the electronic structures of these active intermediates can contribute to the development and optimization of nickel catalysts for cross-coupling reactions.

Nickel-Catalyzed Radical Mechanisms: Informing Cross-Coupling for Synthesizing Non-Canonical Biomolecules

“…Our study on (pyrox)Ni complexes indicates that the redox activity of a ligand can vary among different complexes (Scheme ). This observation remains consistent across a range of reported organonickel radical complexes ligated with bidentate N -ligands . By analyzing data from the literature combined with our results, we established a general framework for understanding the ligand redox activity of nickel radical complexes .…”

“…Soc. 2023 , 145 , 20551–20561 . A collection of well-defined nickel radical complexes that reveals an empirical correlation between coordination geometry and ligand redox activity.…”

“…In the first approach, the reduction of ( t Bu pyrox)­Ni­(CH 2 TMS) 2 36 ( t Bu pyrox = (S)-4-( tert -butyl)-2-(2-pyridyl)­oxazoline) by potassium graphite (KC 8 ) in the presence of 18-crown-6 led to the formation of [K­(18-crown-6)] + [( t Bu pyrox)­Ni­(CH 2 TMS) 2 ] •– 37 (Scheme A) . The second approach involved the comproportionation of (pyrox)­NiCl 2 38 and Ni­(cod) 2 , giving (pyrox)­NiCl 39 in 98% yield (Scheme B) . X-ray crystallography established unsymmetrical Ni–C bond lengths in 37 and a T-shape geometry in 39 , highlighting a pronounced trans-influence of pyridine compared to oxazoline.…”

“…The reduction of (dtbpy)­Ni­(Dipp*)Cl 47 (Dipp* = 2,6-di-Dipp-phenyl) led to the formation of complex 48 , which was characterized as a ligand-centered radical by EPR (Scheme ). Over time, in solution, chloride dissociation led to the formation of the three-coordinate complex 49 , which was identified as a metal-centered radical based on its EPR spectrum. Furthermore, when we reduced [(dtbpy)­Ni­(Dipp)] + [BAr F 24 ] − (BAr F 24 = tetrakis­(3,5-bis­(trifluoromethyl)­phenyl)­borate) 50 in THF, we observed an isotropic, organic radical.…”

Enantioselective C(sp2)–C(sp3) Bond Construction by Ni Catalysis

“…Other sacrificial anode materials, such as Al and Mg, resulted in premature reaction termination. NaI served a dual role as both a reaction additive and optimal electrolyte for the system, − thereby giving rise to superior yields and enantioselectivities compared with other electrolytes like TBAI, TBAPF 6 , and NaPF 6 . When no current was passed, the product was obtained in 2% yield, which suggested that electricity was required to overcome the overpotential of the cell.…”