Intraligand Charge Transfer Enables Visible‐Light‐Mediated Nickel‐Catalyzed Cross‐Coupling Reactions**

Abstract

We demonstrate that several visible-light-mediated carbonÀ heteroatom cross-coupling reactions can be carried out using a photoactive Ni II precatalyst that forms in situ from a nickel salt and a bipyridine ligand decorated with two carbazole groups (Ni(Czbpy)Cl 2 ). The activation of this precatalyst towards cross-coupling reactions follows a hitherto undisclosed mechanism that is different from previously reported light-responsive nickel complexes that undergo metalto-ligand charge transfer. Theoretical and spectroscopic investigations revealed that irradiation of Ni(Czbpy)Cl 2 with visible light causes an initial intraligand charge transfer event that triggers productive catalysis. Ligand polymerization affords a porous, recyclable organic polymer for heterogeneous nickel catalysis of cross-coupling reactions. The heterogeneous catalyst shows stable performance in a packed-bed flow reactor during a week of continuous operation.

Shining Visible Light on Reductive Elimination: Acridine–Pd-Catalyzed Cross-Coupling of Aryl Halides with Carboxylic Acids

“…Aryl carboxylic acids ( 2h , 2i ) also underwent cross-coupling with 1a smoothly. The observed scope of aryl iodides in this reaction is different from the reported Ni-catalyzed systems, − where aryl bromides and iodides with electron-donating groups showed low reactivity, displaying the advantage of the current protocol. We suppose that the oxidative addition of aryl halides is promoted by the use of more electron-donating phosphinoacridine ligands compared with bipyridine ligands in the previous Ni catalysis.…”

“…The rate of reductive elimination is usually faster at the more electron-deficient metal center, where MLCT excited states are expected to accelerate the reaction . Among numerous cross-coupling reactions, the reaction of aryl halides with carboxylic acids is especially challenging because of the low basicity and nucleophilicity of carboxylate making the reductive elimination sluggish and endergonic. , Since the recent pioneering work on the Ni/photoredox dual-catalyst system by MacMillan and co-workers, several cross-coupling reactions of aryl bromides/iodides with carboxylic acids have been developed using Ni catalysts. − Pd-catalyzed cross-coupling reactions of aryl iodides with carboxylic acids have also recently been reported under thermal conditions. , However, they still suffer from several drawbacks such as limitation to electron-deficient aryl halides (for Ni) − and high temperature with superstoichiometric amounts of Ag salts (for Pd) …”

Photoinduced Nickel‐Catalyzed Carbon–Heteroatom Coupling**

“…[23,24] It is still challenging for the development of visible-light-induced carbon-heteroatom coupling reactions in nickel catalysis. Recently, Thomas, Pieber, and co-workers elegantly installed two carbazole units to bipyridine ligand with the absorption of Ni II species in the range of visible light and succeeded in the coupling of sulfinates, carboxylic acids, and sulfonamides with aryl iodides under mild conditions [25] (Figure 1b). In addition, Miyake and co-workers reported a different mechanism of direct excitation of nickel À amine complexes by UV light to access CÀ N cross-coupling.…”

“…The Ni I /Ni III catalytic cycle, in which Ni III complexes underwent facile reductive elimination to form carbon-heteroatom bonds was widely proposed in nickel/photoredox dual catalysis [3,4,6,7,29] and single nickel catalysis. [21][22][23][24][25][26] Aryl-Ni II -halide (1 b) complexes feature strong absorption in the range of visible light and are likely to generate the active nickel(I) species by homolysis of nickel-aryl bonds in the presence of the light under exogenous photocatalyst-free condition [22] (Figure 1b). Thus, arylÀ Ni II À halide (1 b) complexes should be useful to forge carbon-heteroatom bonds throught the Ni I /Ni III cycle under visible light irradiation.…”

Poly(Heptazine Imide) Ligand Exchange Enables Remarkable Low Catalyst Loadings in Heterogeneous Metallaphotocatalysis

“…30,31 Another attractive strategy was to incorporate Ni(II) and photosensitizing Ir centers into metal/covalent organic frameworks (MOFs/COFs)/flexible polymers containing chelating bpy/phen sites or directly embed Ni(II) in a photosensitive bpy-functionalized COF or poly-Czbpy. [32][33][34][35][36][37][38][39][40] Although appealing, these approaches may require multiple steps to access one solid catalyst with a specific bpy-based ligand, which often deleteriously affects the catalytic activity in comparison with the parent homogeneous Ni catalyst with tunable bpy-based ligands (Fig. 1b).…”