Periodic Reporting for period 2 - PhotoRedOx (Spectroscopic and Computational Elucidation of Transition Metal Photoredox Mechanisms)
Periodo di rendicontazione: 2023-08-01 al 2024-07-31
The merge of thermal catalysis and photochemistry (i.e. photoredox catalysis) has revolutionized organic synthesis through using photons and earth-abundant transition metals instead of precious metal catalysts. Photoredox catalysis has enabled new reactive intermediates and excited states that have unlocked synthetic possibilities for complex molecules, advancing options for drug development and production. However, the mechanisms underlying these processes are not fully understood, and a deeper understanding of the fundamental concepts is necessary for the design of novel photocatalysts and optimization of reactions. In this project, a multi-disciplinary approach was used to provide new mechanistic insights underlying photoredox catalysis by combining spectroscopic and analytical techniques with computational methods. Specifically, the project aimed to understand how to efficiently harvest photons to generate transition metal excited states capable of activating inert ligand-metal bonds for complex reactivity. The focus was on experimentally and computationally quantifying ground and excited state frontier molecular orbitals and potential energy surfaces to define structure-function relationships across excited state photocatalysis.
Conclusions:
The project successfully provided significant mechanistic insights into the photochemistry of Ni(II)–bipyridine complexes. Key findings include the identification of the mechanism for excited-state Ni-C bond homolysis, the impact of ligand structure on photochemical stability and reactivity, and the pathways for oxidative addition and dimerization of Ni(I) intermediates. These discoveries have deepened our understanding of how to control excited-state potential energy surfaces and how ligand design can tune catalytic reactivity. The results offer a foundation for developing more efficient and selective photocatalysts, which have potential applications in green chemistry and sustainable synthesis.
Conclusions:
The project successfully provided significant mechanistic insights into the photochemistry of Ni(II)–bipyridine complexes. Key findings include the identification of the mechanism for excited-state Ni-C bond homolysis, the impact of ligand structure on photochemical stability and reactivity, and the pathways for oxidative addition and dimerization of Ni(I) intermediates. These discoveries have deepened our understanding of how to control excited-state potential energy surfaces and how ligand design can tune catalytic reactivity. The results offer a foundation for developing more efficient and selective photocatalysts, which have potential applications in green chemistry and sustainable synthesis.
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.
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.
1. We have synthesized new Ni(II)–bipyridine aryl halide photoredox catalysts to form a large library of complexes to investigate the structural control over the mechanism of their excited-state bond homolysis.
2. We have provided a new mechanistic insights underlying photoredox catalysis. Specifically, we have found that (i) photochemical homolysis rate constants span 2 orders of magnitude across the Ni(II)–bipyridine aryl halide complexes, (ii) excited-state Ni(II)–C(aryl) bond homolysis is temperature- and wavelength-dependent, (iii) the excited-state homolysis barrier is inconsistent with thermally driven homolysis from a low-energy ligand field excited state, (iv) the homolysis occurs via a repulsive triplet excited-state potential energy surface, featuring an aryl-to-Ni ligand-to-metal charge transfer.
3. We have found that the bipyridine substituents provide a strong influence also on reactivity of Ni(I)-bipyridine halide complexes, determining their reactivity toward oxidative addition or dimerization.
4. We have provided a first direct experimental evidence of these complexes engaging in oxidative addition of strong C(sp2)–Cl bonds.
5. We have demonstrated that structural constraints affect the excited-state dynamics and reactivity of these catalysts.
2. We have provided a new mechanistic insights underlying photoredox catalysis. Specifically, we have found that (i) photochemical homolysis rate constants span 2 orders of magnitude across the Ni(II)–bipyridine aryl halide complexes, (ii) excited-state Ni(II)–C(aryl) bond homolysis is temperature- and wavelength-dependent, (iii) the excited-state homolysis barrier is inconsistent with thermally driven homolysis from a low-energy ligand field excited state, (iv) the homolysis occurs via a repulsive triplet excited-state potential energy surface, featuring an aryl-to-Ni ligand-to-metal charge transfer.
3. We have found that the bipyridine substituents provide a strong influence also on reactivity of Ni(I)-bipyridine halide complexes, determining their reactivity toward oxidative addition or dimerization.
4. We have provided a first direct experimental evidence of these complexes engaging in oxidative addition of strong C(sp2)–Cl bonds.
5. We have demonstrated that structural constraints affect the excited-state dynamics and reactivity of these catalysts.