Published April 13, 2022 | Accepted Version + Supplemental Material
Journal Article Open

Elucidating the Mechanism of Excited-State Bond Homolysis in Nickel–Bipyridine Photoredox Catalysts

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Abstract

Ni 2,2′–bipyridine (bpy) complexes are commonly employed photoredox catalysts of bond-forming reactions in organic chemistry. However, the mechanisms by which they operate are still under investigation. One potential mode of catalysis is via entry into Ni(I)/Ni(III) cycles, which can be made possible by light-induced, excited-state Ni(II)–C bond homolysis. Here, we report experimental and computational analyses of a library of Ni(II)–bpy aryl halide complexes, Ni(^Rbpy)(^(R′)Ph)Cl (R = MeO, t-Bu, H, MeOOC; R′ = CH₃, H, OMe, F, CF₃), to illuminate the mechanism of excited-state bond homolysis. At given excitation wavelengths, photochemical homolysis rate constants span 2 orders of magnitude across these structures and correlate linearly with Hammett parameters of both bpy and aryl ligands, reflecting structural control over key metal-to-ligand charge-transfer (MLCT) and ligand-to-metal charge-transfer (LMCT) excited-state potential energy surfaces (PESs). Temperature- and wavelength-dependent investigations reveal moderate excited-state barriers (ΔH^‡ ∼ 4 kcal mol⁻¹) and a minimum energy excitation threshold (∼55 kcal mol⁻¹, 525 nm), respectively. Correlations to electronic structure calculations further support a mechanism in which repulsive triplet excited-state PESs featuring a critical aryl-to-Ni LMCT lead to bond rupture. Structural control over excited-state PESs provides a rational approach to utilize photonic energy and leverage excited-state bond homolysis processes in synthetic chemistry.

Additional Information

© 2022 American Chemical Society. Received: February 3, 2022; Published: March 30, 2022. D.A.C. is a National Science Foundation Graduate Research Fellow (DGE-1745301) and is supported by a National Academies of Science, Engineering, and Medicine Ford Foundation Predoctoral Fellowship. B.S. acknowledges funding through a Southern California Edison WAVE fellowship at Caltech. N.P.K. acknowledges support from the Hertz Fellowship and from the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1745301. This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 883987 (D.B.). Support has been provided by the National Institutes of Health (National Institute of General Medical Sciences, R35-GM142595). The authors also acknowledge M. K. Takase in the Beckman Institute X-ray crystallography facility. The computations presented here were conducted in the Resnick High Performance Computing Center, a facility supported by Resnick Sustainability Institute at the California Institute of Technology. The authors declare no competing financial interest.

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Accepted Version - nihms-1870778.pdf

Supplemental Material - ja2c01356_si_001.pdf

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Additional details

Created:
August 22, 2023
Modified:
October 23, 2023