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

Abstract

We demonstrate that several visible-light-mediated carbon-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)Cl₂ ). 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 metal-to-ligand charge transfer. Theoretical and spectroscopic investigations revealed that irradiation of Ni(Czbpy)Cl₂ 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.

Keywords: Flow Chemistry; Heterogeneous Catalysis; Homogeneous Catalysis; Nickel Catalysis; Photocatalysis.

Figure 1

Strategies for visible‐light‐mediated nickel‐catalyzed carbon−heteroatom cross‐couplings. a) Dual photo/nickel‐catalysis uses in situ‐formed nickel precatalyst and an exogenous photocatalyst. b) Ni^II(dtbbpy) aryl halide complexes can be activated via MLCT. c) In situ‐formed nickel precatalysts can be activated via ILCT (this work).

Figure 2

Synthesis and characterization of Czbpy and Ni(Czbpy)Cl₂. a) The ligand was synthesized via an Ullmann C−N coupling and forms the desired complex upon treatment with NiCl₂⋅glyme. b) Cyclic voltammetry studies of Ni(Czbpy)Cl₂ and Ni(dtbbpy)Cl₂. c) Experimental and calculated UV/Visible spectra of Czbpy and Ni(Czbpy)Cl₂. The band indicated by a vertical line is assigned to an intraligand charge transfer (ILCT) transition. The theoretical spectra have been shifted by −0.25 eV. d) Natural transition orbitals (NTOs) of Ni(Czbpy)Cl₂. e) A simplified excited state relaxation diagram of Ni(Czbpy)Cl₂. The states are separately labeled for the metal center and Czbpy ligand in parenthesis, respectively. f) Optical transient absorption data of Ni(Czbpy)Cl₂ and Czbpy at 10 ps after photoexcitation at 415 nm (arrow). For comparison, the inverted static absorption spectra are shown as well. The solid black curve is the simulated transient spectrum from TD‐DFT calculated by subtracting the ground state spectrum from the spectrum of the lowest‐triplet T1 state. The spectra have been arbitrarily scaled to approximately match the amplitude of the transient czpbpy spectrum (blue). g) Spin trapping experiment of Ni(Czbpy)Cl₂ in DMAc in the presence of phenyl N‐t‐butylnitrone (PBN) with and without illumination. Radical species are formed upon irradiation.

Figure 3

Optimized reaction conditions and control experiments for light‐mediated carbon−heteroatom cross‐coupling reactions catalyzed by Ni(Czbpy)Cl₂. a) C−S arylation of sodium p‐toluensulfinate with 4‐iodobenzotrifluoride. b) C−O arylation of N‐Boc‐proline with 4‐iodobenzotrifluoride. c) C−N arylation of p‐toluensulfonamide with 4‐iodobenzotrifluoride. [a] NMR yields determined by ¹H NMR spectroscopy using 1,3,5‐trimethoxybenzene as an internal standard. n.d.=not detected.

Figure 4

Polymerization of Czbpy enables heterogeneous visible‐light‐mediated metallaphotocatalyzed cross‐coupling reactions. a) Preparation of the porous organic polymer poly‐Czbpy and complexation with NiCl₂. b) Characterization of poly‐Czbpy and Ni@poly‐Czbpy by UV/Visible spectroscopy. c) XPS analysis of Ni@Czbpy: N 1s and Ni 2p core‐level spectra. d) Visible‐light‐mediated carbon−heteroatom cross‐coupling reactions using Ni@poly‐Czbpy as a heterogeneous metallaphotocatalyst. e) Catalyst recycling study (orange: 5 mol % of NiCl₂⋅glyme added at each reaction cycle; green: 5 mol % of NiCl₂⋅glyme added only for the first cycle; blue: 2.5 mol % of NiCl₂⋅glyme added only for the first cycle). f) Catalyst lifetime study using a continuous long‐run C−S cross‐coupling experiment over seven days flow in a packed‐bed reactor.