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

Cagan, David A. and Bím, Daniel and Silva, Breno and Kazmierczak, Nathanael P. and McNicholas, Brendon and Hadt, Ryan G. (2021) Elucidating the Mechanism of Excited State Bond Homolysis in Nickel–Bipyridine Photoredox Catalysts. . (Unpublished)

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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 rates span two 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.

Item Type:Report or Paper (Discussion Paper)
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URLURL TypeDescription Paper ItemJournal Article
Cagan, David A.0000-0002-4719-2789
Bím, Daniel0000-0003-3100-4293
Kazmierczak, Nathanael P.0000-0002-7822-6769
McNicholas, Brendon0000-0002-3654-681X
Hadt, Ryan G.0000-0001-6026-1358
Additional Information:The content is available under CC BY NC ND 4.0 License. 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). We 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 author(s) have declared they have no conflict of interest with regard to this content. The author(s) have declared ethics committee/IRB approval is not relevant to this content.
Group:Resnick Sustainability Institute
Funding AgencyGrant Number
NSF Graduate Research FellowshipDGE-1745301
Ford FoundationUNSPECIFIED
Caltech WAVE FellowshipUNSPECIFIED
Marie Curie Fellowship883987
Resnick Sustainability InstituteUNSPECIFIED
Southern California Edison (SCE)UNSPECIFIED
Record Number:CaltechAUTHORS:20211209-231174000
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Official Citation:Cagan D, Bím D, Silva B, Kazmierczak N, McNicholas B, Hadt R. Elucidating the Mechanism of Excited State Bond Homolysis in Nickel–Bipyridine Photoredox Catalysts. ChemRxiv. 2021. doi:10.26434/chemrxiv-2021-g0dc2
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:112351
Deposited By: George Porter
Deposited On:10 Dec 2021 20:29
Last Modified:24 Mar 2023 19:30

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