Ditopic Bis(Pyridinium Amidate) Ligands with N , N - and O , O -Bidentate Bonding Sites: Oxygen-Coordinated Alkali Metals Systematically Impact the Reactivity of the N-Bound Palladium Center

Alkali metal (AM) salts are widely used as bases or additives in homogeneous catalysis, yet the influence of the AM cation itself is often overlooked. Here, we demonstrate that AM binding to the backbone of a bis(pyridinium amidate) (bisPYA) ligand affects the reactivity of the coordinated palladium center. Specifically the ortho-pyridinium bisPYA ligand adopts an N,N′-bidentate bonding mode to PdX2 fragments with X = MeCN(BF)4 (3), Cl (4), or OAc (5). When exposed to AM carbonates, the dicarbonyl backbone of the bis(amidate) ligand coordinates in an O,O′-bidentate fashion to the AM to form stable and crystalline complexes of the type [AM(bisPYA-PdOAc2)2]+ with AM = Li (6a), Na (6b), K (6c), Rb (6d), and Cs (6e). The stability of these AM adducts decreases according to competitive binding and dilution experiments along the series Li+ > Na+ > K+ > Rb+ > Cs+. In agreement with this trend, rates of electrophilic cyclometalation involving C–H bond activation of the pyridinium N–CH3 group correlate with the stability of the adducts, thus providing valuable insights relevant to understanding specific AM effects in transition metal catalysis and offering opportunities to remotely tune the electron density of catalytic centers in this bisPYA and related ligand classes.