Synthesis and Characterization of Palladium Pincer Bis(carbene) CCC Complexes

Direct Observation of C-F Bond Formation from Isolable Palladium(IV) Systems via an Outer-Sphere Pathway

Despite recent advancements in synthetic methods for palladium-catalyzed C-F couplings, the mechanistic understanding─particularly concerning high-valent palladium species─remains underexplored. In contrast to the commonly proposed inner-sphere mechanism, which involves C-F reductive elimination at a palladium(IV) center, the outer-sphere pathway is poorly understood, and little experimental evidence has been provided using well-defined, isolable high-valent Pd systems. In this study, we present the design and synthesis of a series of isolable and characterizable palladium(IV) alkyl systems. Their reactivity in C-F bond formation has been investigated and systematically compared. Through the isolation of intermediates, reactivity comparisons, kinetic studies, and DFT calculations, this work presents, for the first time, isolable palladium(IV) model systems that enable the direct observation of C-F bond formation via an outer-sphere pathway.

Fluoride-induced redox-switchable behaviour of a palladium(ii)/(iv) couple

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 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 the 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.

An N-heterocyclic carbene-based pincer system of palladium and its versatile reactivity under oxidizing conditions

NHC-based pincers (NHC = N-heterocyclic carbene) have been broadly employed as supporting platforms, and their palladium complexes have found many synthetic applications. However, previous studies mainly focused on the NHC pincers of palladium featuring an oxidation number of +II. In contrast, oxidation of these well-defined Pd(II) species and the study of their fundamental high-valent Pd chemistry remain largely undeveloped. In addition, from a perspective of PdII/PdIV catalysis, the reactivity and degradation of NHC pincers in catalytically relevant reactions have not been well understood. In this work, a series of Pd(II) complexes supported by a well-known NHC^Aryl^NHC pincer platform have been prepared. Their reactivity towards various oxidizing reagents, including halogen surrogates, electrophilic fluorine reagents, and alkyl/aryl halides, has been examined. In some cases, ambient-characterizable high-valent Pd NHCs, which have been scarcely reported, were obtained. The carbenes incorporated into the pincer framework proved to be effective spectator donors. In contrast, the central aryl moiety exhibits versatile reactivity and collapse pathways, allowing it to function either as a spectator or a non-innocent actor.

The Fluoride Method: Access to Silver(III) NHC Complexes

We have synthesized the first silver(III) carbene complexes, (CF3 )3 Ag(NHC), by direct reaction of the silver(III) fluoride precursor complex [PPh4 ][(CF3 )3 AgF] with different imidazolium salts. This novel methodology circumvents the use of free NHC molecules. The silver(III) carbene complexes thus prepared are unprecedented and show remarkable thermal stabilities. They display square-planar or square-pyramidal geometry. Following our calculations, the electronic structure of a model representative complex exhibits Inverse Ligand Field (ILF). The compounds reported herein are synthetic analogues of the elusive difluorocarbene and carbonyl species proposed as intermediates in the acidic decomposition of [Ag(CF3 )4 ]- . The synthetic procedure reported is envisaged to enable access to carbene complexes of other late transition-metals in high oxidation states.