Elucidating the Mechanism of Excited-State Bond Homolysis in Nickel–Bipyridine Photoredox Catalysts
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.Attached Files
Accepted Version - nihms-1870778.pdf
Supplemental Material - ja2c01356_si_001.pdf
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Additional details
- PMCID
- PMC9979631
- Eprint ID
- 114168
- DOI
- 10.1021/jacs.2c01356
- Resolver ID
- CaltechAUTHORS:20220406-18315469
- NSF Graduate Research Fellowship
- DGE-1745301
- Ford Foundation
- Southern California Edison
- Fannie and John Hertz Foundation
- Marie Curie Fellowship
- 883987
- NIH
- R35GM142595
- Resnick Sustainability Institute
- Created
-
2022-04-06Created from EPrint's datestamp field
- Updated
-
2023-07-06Created from EPrint's last_modified field
- Caltech groups
- Resnick Sustainability Institute