3d-d Excited States of Ni(II) Complexes Relevant to Photoredox Catalysis: Spectroscopic Identification and Mechanistic Implications

Stephen I. Ting, Sofia Garakyaraghi, Chelsea M. Taliaferro, Benjamin J. Shields, Gregory D. Scholes, Felix N. Castellano, Abigail G. Doyle

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173 Scopus citations


Synthetic organic chemistry has seen major advances due to the merger of nickel and photoredox catalysis. A growing number of Ni-photoredox reactions are proposed to involve generation of excited nickel species, sometimes even in the absence of a photoredox catalyst. To gain insights about these excited states, two of our groups previously 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 using a series of related complexes. 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 of the square planar complexes 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 state was determined to lie ∼0.5 eV (12 kcal/mol) above the ground state. Because the 3d-d state features a weak Ni-aryl bond, the excited Ni(II) complexes can undergo Ni homolysis to generate aryl radicals and Ni(I), both of which are supported experimentally. Thus, photoinduced Ni-aryl homolysis offers a novel mechanism of initiating catalysis by Ni(I).

Original languageEnglish (US)
Pages (from-to)5800-5810
Number of pages11
JournalJournal of the American Chemical Society
Issue number12
StatePublished - Mar 25 2020

All Science Journal Classification (ASJC) codes

  • General Chemistry
  • Biochemistry
  • Catalysis
  • Colloid and Surface Chemistry


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