Fluoride-induced redox-switchable behaviour of a palladium (II)/(IV) couple

Abstract

We report the preparation of two Pd(II) complexes based on (CCC)-pincer-NHC (NHC = N-heterocyclic carbene) ligands. One of these complexes features an NDI unit (NDI = naphthalene-diimide) attached to the CCC-pincer ligand. Reacting these complexes with bromine results in the formation of the corresponding palladium(IV) complexes, [PdBr3(NDI-CCC)] and [PdBr3(CCC)]. The NDI-containing pincer complex, (CCC)-pincer-NDI, exhibits a strong sensitivity to fluoride ions, which can induce a one-electron reduction of the naphthalene-diimide moiety. We demonstrate that the addition of fluoride induces the reduction of the NDI moiety via de formation of hydroxide anions, which are the effective reducing agents of the process. The addition of fluoride significantly affects the reactivity of the NDI-containing palladium complexes. For example, the palladium(IV) complex [PdBr3(NDI-CCC)] can transfer bromide to styrene in a stoichiometric manner, but this reaction is inhibited in the presence of fluoride. Similarly, the palladium(II) complex [PdI(NDI-CCC)] catalyzes the oxidative homocoupling of arylpyridines, but its catalytic activity is quenched when excess fluoride is added. Notably, we demonstrate that this process can be deactivated and reactivated by sequentially introducing an excess of fluoride and NOBF4, revealing a rare instance of a redox-switchable process within a Pd(II)/Pd(IV) catalytic cycle. In contrast, the (CCC)-pincer palladium(II) and (IV) complexes lacking the NDI unit show no sensitivity to fluoride. Our study demonstrates that a simple reagent, such as the fluoride anion, can effectively modulate the reactivity of the Pd(II)/Pd(IV) pair. More broadly, it shows that fluoride serves as a simpler alternative to the metal-based reducing agents commonly used in redox-switchable catalysis.