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

Abstract

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.