Periodic Reporting for period 1 - PhotoRedOx (Spectroscopic and Computational Elucidation of Transition Metal Photoredox Mechanisms)
Période du rapport: 2021-08-01 au 2023-07-31
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 is used to provide new mechanistic insights underlying photoredox catalysis by combining spectroscopic and analytical techniques with computational methods. Specifically, the project aims 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 is 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.
So far, we have reported an extensive experimental and computational analyses of a large library of various Ni(II)–bipyridine aryl halide complexes to illuminate the mechanism of their excited-state Ni-C(aryl) bond homolysis. Photochemical homolysis rate constants were found to vary across the structures and correlate linearly with Hammett parameters of both bipyridine and aryl ligand substitutents, reflecting structural control over key metal-to-ligand charge-transfer and ligand-to-metal charge-transfer excited-state potential energy surfaces. Temperature- and wavelength-dependent investigations revealed moderate excited-state barriers and a minimum energy threshold required for excitation. Correlations to electronic structure calculations further support a mechanism in which repulsive triplet excited-state potential energy surfaces featuring a critical aryl-to-Ni ligand-to-metal charge transfer lead to bond rupture. Structural control over excited-state potential energy surfaces thus provides a rational approach to utilize photonic energy and leverage excited-state bond homolysis processes in synthetic chemistry.
The project has further benchmarked the relative reactivity of photochemically generated Ni(I)–bipyridine halide complexes toward competitive oxidative addition and off-cycle dimerization pathways. Structure-function relationships between ligand set and reactivity were developed, and the origin of substituent influence toward the reactivity of challenging high-energy C(sp2)–Cl bonds activation was investigated. Through a dual Hammett and computational analysis, the mechanism of the formal oxidative addition was found to proceed through an SNAr-type pathway, consisting of a nucleophilic two-electron transfer between the Ni(I) 3d(z2) orbital and the C(aryl)–Cl σ* orbital, which contrasts the mechanism previously observed for activation of weaker C(sp2)–Br/I bonds. The bipyridine substituents were found to provide a strong influence on reactivity, ultimately determining whether oxidative addition or dimerization even occur. Ligand-induced modulation of Ni 3d(z2) orbital energy thus represents a tunable target by which the reactivity of Ni(I) complexes can be altered, providing a direct route to stimulate reactivity with even stronger C–X bonds and potentially unveiling new ways to accomplish Ni-mediated photocatalytic cycles.
The project has further benchmarked the relative reactivity of photochemically generated Ni(I)–bipyridine halide complexes toward competitive oxidative addition and off-cycle dimerization pathways. Structure-function relationships between ligand set and reactivity were developed, and the origin of substituent influence toward the reactivity of challenging high-energy C(sp2)–Cl bonds activation was investigated. Through a dual Hammett and computational analysis, the mechanism of the formal oxidative addition was found to proceed through an SNAr-type pathway, consisting of a nucleophilic two-electron transfer between the Ni(I) 3d(z2) orbital and the C(aryl)–Cl σ* orbital, which contrasts the mechanism previously observed for activation of weaker C(sp2)–Br/I bonds. The bipyridine substituents were found to provide a strong influence on reactivity, ultimately determining whether oxidative addition or dimerization even occur. Ligand-induced modulation of Ni 3d(z2) orbital energy thus represents a tunable target by which the reactivity of Ni(I) complexes can be altered, providing a direct route to stimulate reactivity with even stronger C–X bonds and potentially unveiling new ways to accomplish Ni-mediated photocatalytic cycles.
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.
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.