The Role of Excited States of LNi^II/III(Aryl)(Halide) Complexes in Ni–Halide Bond Homolysis in the Arylation of C_sp3–H Bonds

Abstract

LNi II (Ar)X (L = bipyridine-type ligand; X = Br and Cl) complexes are fundamental intermediates in photoredox-Ni catalysis for the activation of C sp3 −H bonds in C sp3 −C sp2 crosscoupling reactions. Their interaction with a light-excited Irphotocatalyst is assumed to promote the Ni-complex to an excited state from which the Ni−X bond homolysis event occurs. Debate is open on this interaction occurring via single electron transfer (SET) or via energy transfer (EnT), which is the indirect excitation of the Ni-complex by the Ir-photocatalyst. Aiming at addressing this question, we report an electronic structure analysis of these complexes and their 1e − oxidized form using multireference electronic structure methods, which were needed because of the highly multiconfigurational nature of some of the species involved in these reactions. We identified the excited states of LNi II (Ar)X relevant to Ni−X homolysis within the energy range of the irradiating light source, and we propose that they are accessible via triplet Dexter EnT. This is a spin-conserved process with simultaneous exchange of ground-and excited-state electrons between the lightexcited Ir-photocatalyst and the Ni-complex. Conversely, SET from LNi II (Ar)X to the Ir-photocatalyst followed by light excitation of the resulting [LNi III (Ar)X] + complex is unlikely, as no excited state corresponding to Ni−X dissociation was found in the energy range consistent with the irradiating light source.

Enantioselective C(sp3)–H Functionalization of Oxacycles via Photo-HAT/Nickel Dual Catalysis

“…In the nickel catalytic cycle (Scheme b), DFT computation starts from the key Ni­(I) intermediate D Ni I . In contrast to the oxidative addition of 31 through the transition state TS3A , the resulting D Ni I is prior to undergo radical addition to radical A via transition states TS3B- S or TS3B- R (7.9 kcal·mol –1 for TS3B- S and 8.9 kcal mol –1 for TS3B- R vs 25.1 kcal·mol –1 for TS3A , Scheme c) . The radical addition step is an enantio-determining step, and the relative Gibbs free energy of the transition state TS3B- S is 1.0 kcal mol –1 lower than that of TS3B- R .…”

“…In contrast to the oxidative addition of 31 through the transition state TS3A, the resulting D Ni I is prior to undergo radical addition to radical A via transition states TS3B-S or TS3B-R (7.9 kcal•mol −1 for TS3B-S and 8.9 kcal mol −1 for TS3B-R vs 25.1 kcal•mol −1 for TS3A, Scheme 6c). 66 The radical addition step is an enantiodetermining step, and the relative Gibbs free energy of the transition state TS3B-S is 1.0 kcal mol −1 lower than that of TS3B-R. The origin of enantioselectivity can be further visualized by steric maps around nickel using SambVca 2.1 tool (Scheme 6e).…”

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

“…It means that nickel in different oxidation states may trigger different catalytic cycles, which provides a suitable case for us to answer the above issues by comparing the SET and EnT mechanisms of the same reaction. Considering the complexity of both SET or EnT mechanisms, we adopted a combination of density functional theory (DFT), molecular dynamics (MD), and time-dependent DFT (TD-DFT) computations to investigate the aryl esterification of benzoic acid and aryl bromide catalyzed by Ir III /Ni 0 and Ir III /Ni II dual catalysis. The answers to the above questions would be not only conducive to broadening the application range of Ir/Ni dual catalysis but also provide a robust theoretical basis for seeking low-cost alternatives to the Ir photocatalyst.…”

Heart of gold: enabling ligands for oxidative addition of haloorganics in Au(i)/Au(iii) catalysed cross-coupling reactions

“…One might imagine the photolysis of 5a leading to interesting applications in dual catalysis applications. 56 When considering single electron transfer reactions with photocatalysts or electrodes, ligands adorned to Au( i ) such as MeDalPhos may prove to have unique properties as they are likely non-innocent (oxidation of dimethylaniline portion would be facile) and may lead to metal-to-ligand or ligand-to-metal charge transfer processes; giving light to Au-complexes not accessible through simple chemical reactions. Considering the current state of knowledge and application of logic in ligand enabled Au( i )/Au( iii ) catalysis, the future is rich with opportunities for discovery.…”