Periodic Reporting for period 1 - PhotoRedOx (Spectroscopic and Computational Elucidation of Transition Metal Photoredox Mechanisms)
Período documentado: 2021-08-01 hasta 2023-07-31
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.
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.