Elucidating the Mechanism of Excited-State Bond Homolysis in Nickel–Bipyridine Photoredox Catalysts
Citations Over TimeTop 1% of 2022 papers
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.
Related Papers
- → Hydroperoxysterols as a probe for the mechanism of cytochrome P-450scc-mediated hydroxylation. Homolytic versus heterolytic oxygen-oxygen bond scission.(1986)29 cited
- → Accelerated mechanochemical bond scission and stabilization against heat and light in carbamoyloxime mechanophores(2024)1 cited
- → Pressure effects as mechanistic probes for homolytic thermal cobaltcarbon bond cleavage of cobalamins(1992)9 cited
- → Ab initio calculations on P?C bond cleavage in phosphoranyl radicals: implications for the biodegradation of organophosphonate derivatives(1998)8 cited
- → High pressure studies. XVIII. One-bond and two-bond homolytic scission of tert-butyl p-nitrophenylperacetate(1975)5 cited