Photogenerated Ni(I)–Bipyridine Halide Complexes: Structure–Function Relationships for Competitive C(sp²)–Cl Oxidative Addition and Dimerization Reactivity Pathways

Abstract

We report the facile photochemical generation of a library of Ni(I)−bpy halide complexes (Ni(I)( R bpy)X (R = t-Bu, H, MeOOC; X = Cl, Br, I) and benchmark their relative reactivity toward competitive oxidative addition and off-cycle dimerization pathways. Structure−function relationships between the ligand set and reactivity are developed, with particular emphasis on rationalizing previously uncharacterized ligand-controlled reactivity toward high energy and challenging C(sp 2 )−Cl bonds. Through a dual Hammett and computational analysis, the mechanism of the formal oxidative addition is found to proceed through an S N Ar-type pathway, consisting of a nucleophilic two-electron transfer between the Ni(I) 3d(z 2 ) orbital and the C aryl −Cl σ* orbital, which contrasts the mechanism previously observed for activation of weaker C(sp 2 )−Br/I bonds. The bpy substituent provides a strong influence on reactivity, ultimately determining whether oxidative addition or dimerization even occurs. Here, we elucidate the origin of this substituent influence as arising from perturbations to the effective nuclear charge (Z eff ) of the Ni(I) center. Electron donation to the metal decreases Z eff , which leads to a significant destabilization of the entire 3d orbital manifold. Decreasing the 3d(z 2 ) electron binding energies leads to a powerful two-electron donor to activate strong C(sp 2 )−Cl bonds. These changes also prove to have an analogous effect on dimerization, with decreases in Z eff leading to more rapid dimerization. Ligand-induced modulation of Z eff and the 3d(z 2 ) orbital energy is thus a tunable target by which the reactivity of Ni(I) complexes can be altered, providing a direct route to stimulate reactivity with even stronger C−X bonds and potentially unveiling new ways to accomplish Ni-mediated photocatalytic cycles.

Synthesis of Nickel(I)–Bromide Complexes via Oxidation and Ligand Displacement: Evaluation of Ligand Effects on Speciation and Reactivity

“…Complexes 17 and 18 are the first structurally characterized monomeric t ‑Bu bpy-bound Ni I complexes. The nuclearity of similar complexes is observed to be crucial to their reactivity: dimeric [( t ‑Bu bpy)­Ni I X] 2 species (X = Cl, Br) cannot kinetically disaggregate and therefore are inert toward aryl halides (Figure A), whereas in situ generated monomeric ( t ‑Bu bpy)­Ni I X species have been shown to activate even aryl chlorides . Indeed, 17 and 18 are capable of activating aryl halide bonds (Figure B).…”

“…The nuclearity of similar complexes is observed to be crucial to their reactivity: dimeric [( t-Bu bpy)Ni I X] 2 species (X = Cl, Br) cannot kinetically disaggregate and therefore are inert toward aryl halides (Figure 8A), 22 whereas in situ generated monomeric ( t-Bu bpy)Ni I X species have been shown to activate even aryl chlorides. 39 Indeed, 17 and 18 are capable of activating aryl halide bonds (Figure 8B). Furthermore, complex 18 is an especially useful Ni I model system for mechanistic and stoichiometric studies: it has a direct Ni 0 analogue, 19, against which its reactivity can be evaluated.…”

“…22,37 Importantly, these complexes do not replicate the behavior of ( t-Bu bpy)Ni I species in catalysis due to irreversible dimerization of reactive ( t-Bu bpy)Ni I X. While in situ generation of monomeric ( t-Bu bpy)Ni I species has clarified the reactivity of important catalytic intermediates, 38,39 these approaches are more challenging to apply to future catalyst design and mechanistic studies than stoichiometric synthesis.…”

“…Moreover, while 4,4′-di- tert -butylbipyridine ( t ‑Bu bpy) is the ligand of choice for many Ni-catalyzed cross-couplings that invoke open-shell intermediates, – only two examples of ( t ‑Bu bpy)­Ni I species have been structurally characterized: Hazari’s [( t ‑Bu bpy)­Ni I Cl] 2 and Nocera’s [( t ‑Bu bpy)­Ni 1.5+ (quinuclidine)­Cl] 2 Cl. , Importantly, these complexes do not replicate the behavior of ( t ‑Bu bpy)­Ni I species in catalysis due to irreversible dimerization of reactive ( t ‑Bu bpy)­Ni I X. While in situ generation of monomeric ( t ‑Bu bpy)­Ni I species has clarified the reactivity of important catalytic intermediates, , these approaches are more challenging to apply to future catalyst design and mechanistic studies than stoichiometric synthesis.…”

Electronic Structures of Nickel(II)-Bis(indanyloxazoline)-dihalide Catalysts: Understanding Ligand Field Contributions That Promote C(sp2)–C(sp3) Cross-Coupling

“…For example, one of our groups has recently provided detailed electronic structural and mechanistic studies of the ground- and excited-state properties of low-spin Ni II bipyridine aryl halide complexes and their Ni I photogenerated intermediates. These studies identified key structure–function relationships relevant to excited-state bond homolysis and oxidative addition reactivity, which allowed for the mechanistic analysis of Ni I -mediated activation of strong C­(sp 2 )–Cl bonds. , Our groups also recently participated in collaborative work that utilized cyclic voltammetry under catalytic conditions and UV–vis–NIR spectroelectrochemistry to better understand the interplay of Ni and Cr-oxidation states for Nozaki–Hiyama–Kishi coupling . Another study by Neidig et al utilized a combination of Mössbauer spectroscopy, magnetic circular dichroism, and computations to demonstrate that the kinetic competency of four-coordinate organoiron toward halogen abstraction is dependent on the accessibility and relative energy of iron-based orbitals …”