3d-d Excited States of Ni(II) ComplexesRelevant to Photoredox Catalysis: Spectroscopic Identification andMechanistic Implications
2020
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).
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