Throughout the course of the project, we have investigated various Ni(II)–bipyridine aryl halide complexes, which are prominent catalysts in Ni photoredox catalysis. We have carried out extensive experimental and computational analyses to investigate the excited-state Ni-C(aryl) bond homolysis mechanism in a variety of these complexes. Photochemical homolysis rates were found to differ across structures, correlating with Hammett parameters of both bipyridine and aryl ligand substituents. This provides insight into how structural variations affect key metal-to-ligand and ligand-to-metal charge-transfer excited-state potential energy surfaces, ultimately leading to bond rupture. Our findings suggest that controlling the structure of the complexes offers a rational strategy for utilizing photonic energy in synthetic chemistry.
Additionally, we benchmarked the reactivity of photochemically generated Ni(I)–bipyridine halide complexes and explored how their structure influences pathways like oxidative addition and dimerization. Notably, we uncovered that the formal oxidative addition of high-energy C(sp2)–Cl bonds proceeds through an SNAr-type pathway, contrasting with the behavior of weaker C(sp2)–Br/I bonds. This is a significant result, as ligand-induced modulation of the Ni(I) 3d(z2) orbital energy can be used to tune reactivity, opening possibilities for more efficient activation of strong C–X bonds and advancing Ni-mediated photocatalytic cycles.
Moreover, our study of two synthesized Ni(II)–bipyridine aryl halide complexes—a novel structurally constrained complex not evaluated in photoredox catalysis and a traditional untethered analogue—revealed that the constrained complex shows greater photochemical stability under prolonged light exposure. This suggests that structural rigidity can prevent degradation during photoexcitation. The tethered complex’s reactivity also points to its potential use as a stable and selective catalyst in photoredox cross-coupling reactions.
Key results achieved include:
1. Identification of the mechanism driving excited-state Ni-C bond homolysis in Ni(II)–bipyridine aryl halide complexes.
2. Development of structure-function correlations that influence homolysis rates.
3. Discovery of a straightforward method for photochemical generation of Ni(I)–bipyridine halide complexes.
4. Insights into competitive pathways involving oxidative addition and dimerization of Ni(I)–bipyridine halide complexes.
5. A detailed spectroscopic, electrochemical, and computational study of Ni(II)-based chiral reductive cross-coupling catalysts.
6. Connection between ligand field strength and catalytically relevant Ni-based reduction potentials.
7. Identification of variable speciation for Ni complexes dependent on solvent and temperature.
8. Findings that structural constraints can significantly alter excited-state relaxation pathways.
Dissemination of these findings has been achieved through original peer-reviewed journal publications and a comprehensive review article on metallaphotoredox catalysis, with a specific focus on nickel(II)–bipyridine complex mechanisms in cross-coupling reactions.
