Dual Photoredox Ni/Benzophenone Catalysis: A Study of the Ni^II Precatalyst Photoreduction Step

Abstract

The combination of NiIIX2 salts with a bipyridine-type ligand and aromatic carbonyl-based chromophores has emerged as a benchmark precatalytic system to efficiently conduct cross-couplings mediated by light. Mechanistic studies have led to two scenarios in which Ni0 is proposed as the catalytic species. Nonetheless, in none of these studies has a NiII to Ni0 photoreduction been evidenced. By exploiting UV–visible, nuclear magnetic resonance, resonance Raman, electron paramagnetic resonance, and dynamic light scattering spectroscopies and also transmission electron microscopy, we report that, when photolyzed by UVA in alcohols, the structurally defined [NiII 2(μ-OH2)­(dtbbpy)2(BPCO2)4] complex 1 integrating a benzophenone chromophore is reduced into a diamagnetic NiI dimer of the general formula [NiI 2(dtbbpy)2(BPCO2)2]. In marked contrast, in THF, photolysis led to the fast formation of Ni0, which accumulates in the form of metallic ultrathin Ni nanosheets characterized by a mean size of ∼100 nm and a surface plasmon resonance at 505 nm. Finally, it is shown that 1 combined with UVA irradiation catalyzes cross-couplings, that is, C­(sp3)–H arylation of THF and O-arylation of methanol. These results are discussed in light of the mechanisms proposed for these cross-couplings with a focus on the oxidation state of the catalytic species.

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

“…Photoredox catalysis has captivated the fields of organic and inorganic chemistry, with nickel­(II)–bipyridine (bpy) aryl halide complexes retaining a prominent place as the metal–ligand scaffold of choice for numerous cross-coupling reactivity pathways. − Among these, C–C and C–heteroatom couplings have been facilitated by Ni­(II)–bpy complexes through either the use of an external photosensitizer (e.g., Ir­(ppy) 3 ) or via direct excitation of the Ni­(II)–bpy aryl halide complex. − Due to the diverse reactivity and intriguing photophysics of these complexes, much interest has been placed on understanding the underlying photophysical and thermal processes involved in photoredox-mediated cross-coupling reactivity. − …”

Recent Advances in Dual Triplet Ketone/Transition-Metal Catalysis

“…14 The formed Ni(I) complexes disproportionate into L n Ni(II)X 2 and catalytically active L n Ni(0) species. 15…”

“…14 The formed Ni(I) complexes disproportionate into L n Ni(II)X 2 and catalytically active L n Ni(0) species. 15 In 2018, Martin utilized push-pull ketone 2.3 under compact fluorescent lamp (CFL) irradiation in tandem catalysis with nickel for the C(sp 3 )-H arylation and alkylation of ethers, toluene, and simple cycloalkanes (Scheme 2). 16 The combination of 10 mol% of Ni(acac) 2 , 10 mol% of ketone 2.3 (4-methoxy-4′-(trifluoromethyl)benzophenone), and 10 mol% of ligand 2.…”

Visible-Light-Mediated Diels-Alder Reactions Under Single-Chain Polymer Confinement: Investigating the Role of the Crosslinking Moiety on Catalyst Activity

“…31 The resulting MOF confined the catalytic components within 0.6 nm of each other, thus facilitating electron and radical transfers between the two centers and allowing for C−S bond formation between aryl iodides and thiols, achieving a turnover number that exceeds homogeneous counterparts by an order of magnitude. Other heterogeneous systems based on MOFs, covalent-organic frameworks, and quantum dots have emerged since then; 14,[32][33][34][35][36][37][38][39][40] combined, these achieve high local catalyst concentrations while taking advantage of periodicity and/or confinement to place catalytic species in close proximity.…”