3d-d Excited States of Ni(II) Complexes Relevant to Photoredox Catalysis, Spectroscopic Identification and Mechanistic Implications
Published in Journal of the American Chemical Society, 2020
Recommended citation: Ting, Stephen I.; Garakyaraghi, Sofia; Taliaferro, Chelsea M.; Shields, Benjamin J.; Scholes, Gregory D.; Castellano, Felix N.; and Doyle, Abigail G. “3d-d Excited States of Ni(II) Complexes Relevant to Photoredox Catalysis, Spectroscopic Identification and Mechanistic Implications” J. Am. Chem. Soc., 2020, xxx, xxx–xxx. https://pubs.acs.org/doi/10.1021/jacs.0c00781#
Previous work from two of our groups studied the photophysics of Ni(t-Bubpy)(o-Tol)Cl, which is representative of proposed intermediates in many Ni-photoredox reactions. This complex was found to have a long-lived excited state (τ = 4 ns), which was computationally assigned as a metal-to-ligand charge transfer (MLCT) state with an energy of 1.6 eV (38 kcal/mol). This work evaluates the computational assignment experimentally. Ultrafast UV-Vis and mid-IR transient absorption data suggest that a MLCT state is generated initially upon excitation, but decays to a long-lived state that is 3d-d rather than 3MLCT in character. Dynamic cis,trans-isomerization was observed in the dark using 1H NMR techniques, supporting that this 3d-d state is tetrahedral and accessible at ambient temperature. Through a combination of transient absorption and NMR studies, the 3d-d energy was determined to be ~0.5 eV (11 kcal/mol) above the ground state.