Unlocking Distinct Excited States of a Nickel(II) Complex Without Modulating the Primary Coordination Sphere.

Traditional efforts in modulating the reactivity of metal centers focus on tuning parameters in the primary coordination sphere. The ability to embed stimuli-responsive functionality in the secondary coordination sphere to modulate metal center reactivity represents a distinct strategy for expanding transition metal reactivity while leaving the primary coordination sphere unmodified. Herein, we describe the incorporation of a photoresponsive unit, fluorenone, into the secondary coordination sphere of a Ni-(II) complex resulting in a significant perturbation of the complex electronic structure, manifesting in distinct excited states that cannot be predicted by the metal complex or fluorenone unit alone. Using the activation of strong nickel-(II) trifluoromethyl bonds as a readout of this strategy, the incorporation of latent ligand-centered radicals enabled near-quantitative generation of trifluoromethyl radical. Comparisons with a control complex lacking the appended fluorenone unit demonstrated the unique role of the appended fluorenone, showcasing the ability to endow photoreactivity to an otherwise unreactive complex. Mechanistic studies including TD-DFT calculations, cyclic voltammetry, and transient absorption spectroscopy support that irradiation with visible light leads to metal-to-ligand charge transfer followed by relaxation to a long-lived 3d-d state. The studies presented herein showcase a ligand design principle focused on secondary coordination sphere edits through the introduction of latent ligand-centered radicals, allowing for the direct tuning of the metal center reactivity.