Long-Lived Charge-Transfer States of Nickel(II) Aryl Halide Complexes Facilitate Bimolecular Photoinduced Electron Transfer

Abstract

Here we investigate the photophysics and photochemistry of Ni(II) aryl halide complexes common to cross-coupling and Ni/photoredox reactions. Computational and ultrafast spectroscopic studies reveal that these complexes feature long-lived ³MLCT excited states, implicating Ni as an underexplored alternative to precious metal photocatalysts. Moreover, we show that ³MLCT Ni(II) engages in bimolecular electron transfer with ground-state Ni(II), which enables access to Ni(III) in the absence of external oxidants or photoredox catalysts. As such, it is possible to facilitate Ni-catalyzed C-O bond formation solely by visible light irradiation, thus representing an alternative strategy for catalyst activation in Ni cross-coupling reactions.

Figure 1.

(A) Electronic structure of d⁶ photocatalysts compared to Ni(II) aryl halide complexes. (B) Structure of Ni complexes and X-ray crystal structure of 1-Cl. (C) Comparison of component absorption spectra. (D) Absorption spectra for aryl halide complexes.

Figure 2.

Summary of computational studies. (A) DFT computed electronic structure of 1-Cl displayed together with Kohn–Sham orbitals. (B) TD-DFT computed absorption spectrum, peak assignments, and population analysis. Difference density isosurfaces demonstrate the loss (red) and build-up (blue) of electron density. Changes in charge are normalized such that Δδ Ni(III)–Ni(II) = 1.

Figure 3.

Initial transient absorption experiments. (A) Full contour plot for TA spectrum of 1-Cl (pump 295 nm). (B) Comparison of TA spectra of free dtbbpy (1–1000 ps, pump 305 nm), Ni(dtbbpy)Cl2 (1–60 ps, pump 295 nm), and 1-Cl (1–1000 ps, pump 295 nm ).(C) Single-wavelength kinetics for Ni(dtbbpy)Cl₂ (367 nm, pump 295 nm) and 1-Cl (476 nm, pump 400 nm). (D) Experimental spectrum versus simulated ³MLCT₁ spectrum of 1-Cl.

Figure 4.

Summary of photophysics Ni(II) aryl halide complexes and evolution of TA spectra for 1-Cl upon visible excitation (λ_pump = 400 nm). (A) Jablonski diagram for all aryl halide complexes. (B) Initial cooling. (C) ISC associated spectral evolution (τ_ISC = 1.1 ps). (D) Bleach broadening. (E) Decay to ground state (τ_nr3 = 4.1 ns). The negative feature at 400 nm is the result of pump scattering.

Figure 5.

Criteria for bimolecular photoinduced electron transfer. (A) Visible light photolysis and thermolysis experiments. (i) Yield determined by ¹H NMR. (ii) Conversion is reported. (B) Experimental concentration dependent kinetics (solid blue) plotted together with 95% confidence interval (blue shading) and simulated saturation kinetics (black dashed). (C) Kinetic hypothesis.

Figure 6.

Ni-catalyzed C–O coupling and proposed mechanism. Yields determined by GC-FID using 1-fluoronaphthalene as an external standard.