Theoretical Insight on the High Reactivity of Reductive Elimination of Ni^III Based on Energy- and Electron-Transfer Mechanisms

Abstract

Iridium/nickel (Ir/Ni) metallaphotoredox dual catalysis overcomes the challenging reductive elimination (RE) of Ni(II) species and has made a breakthrough progress to construct a wide range of C–X (X = C, N, S, and P) bonds. However, the corresponding reaction mechanisms are still ambiguous and controversial because the systematic research on the nature of this synergistic catalysis is not sufficient. Herein, IrIII/NiII and IrIII/Ni0 metallaphotoredox catalysis have been theoretically explored taking the aryl esterification reaction of benzoic acid and aryl bromide as an example by a combination of density functional theory (DFT), molecular dynamics, and time-dependent DFT computations. It is found that an electron-transfer mechanism is applicable to IrIII/NiII metallaphotoredox catalysis, but an energy-transfer mechanism is applicable to IrIII/Ni0 combination. The IrIII/NiII metallaphotoredox catalysis succeeds to construct a NiI–NiIII catalytic cycle to avoid the challenging RE of Ni(II) species, while the RE occurs from triplet excited-state Ni(II) species in the IrIII/Ni0 metallaphotoredox catalysis. In addition, the lower lowest unoccupied molecular orbital energy level of Ni(III) species than that of Ni(II) species accelerates RE from Ni(III) one. The triplet excited-state Ni(II) species can resemble a Ni(III) center, considering the metal-to-ligand charge transfer character to promote the RE.

Photochemically Driven Nickel‐Catalyzed Carboxylative C−N Coupling: Scope and Mechanism**

“…This is consistent with previous computational studies that indicate the Ni(II)−Ni(0) C−O reductive elimination is challenging [16] whereas the reverse reaction–Ni(0)−Ni(II) oxidative addition of aryl carbamates–was exergonic by more than 25 kcal/mol [17] . Based on previous computational studies by Chen [16a] and Guan [16b] and experimental studies by MacMillan and Scholes [8b] that suggested the C−O(carboxylate) RE of Ni(II)(aryl) acetate complexes can be promoted by an energy transfer (EnT) process with iridium photocatalysts, we performed TD‐DFT calculations to study the excitation‐promoted C−O RE pathway to form aryl carbamate. Our calculations indicated that the adiabatic excitation of arylnickel(II) carbamate 1 9 leads to a metal‐to‐ligand charge transfer (MLCT) complex 3 9 (MLCT)[ 18 ] with an adiabatic excitation energy of 34.0 kcal/mol.…”

Imidazophenothiazine-Based Thermally Activated Delayed Fluorescence Materials with Ultra-Long-Lived Excited States for Energy Transfer Photocatalysis

“…We next applied these materials to five catalytic EnT reactions with varying reaction mechanisms: dearomative [2 + 2] cycloadditions; , EnT-mediated Ni-catalysis involving etherification, ,, esterification, and amination; and the disulfide-ene click reaction (Figure ). IPTZ photocatalysts were also benchmarked against ACR-IMAC and 2CzPN TADF materials that have recently been shown to be efficient EnT triplet sensitizers in dearomative [2 + 2] cycloadditions ,, and are preferable to 4CzIPN due to their higher E T .…”

Mechanism of Ni-Catalyzed Photochemical Halogen Atom-Mediated C(sp3)–H Arylation

“…Surprisingly, while 3.1 equiv of 19 was formed (effective catalyst TON of 3.1), the o-Tol cross-coupled product (8) was not observed, rather 10-Br was evolved in 64% yield. We then subjected the analogous ( t-Bu bpy)Ni II (p-CF 3 Ph)(Br) complex (20) to the standard reaction conditions in the presence of 10 equiv of o-TolBr (10-Br). After 20 min, the inverse crossover experiment afforded one equivalent of the trifluoromethyl cross-coupled product (19) from one equivalent of 20 with only trace p-CF 3 electrophile (18) observed (Figure 5B).…”