A Multifunctional Pincer Ligand Supports Unsaturated Cobalt: Five Functionalities in One Pincer

A pyridyl pincer ligand was developed to incorporate steric bulk, through a PtBu₂ arm, and proton responsivity, through a pyrazole pincer ligand arm, together with reactivity at benzylic hydrogen and redox activity within a 1,4 diazabutadiene moiety. Binding it to CoCl₂ yielded square-pyramidal [(PNNH)CoCl₂ ], which was deprotonated by Li[N(SiMe₃ )₂ ] to form [{Li(THF)₂ PNN}CoCl₂ ]. Reduction of this LiCl adduct with KC₈ under CO atmosphere led to formation of Co^I mono- and dicarbonyl complexes, which can be protonated but also further deprotonated at the benzylic CH group to give a dearomatized pyridyl group. The ligand was characterized in its neutral, monoanionic, and dianionic forms, and the anions were shown to exist as intimate ion pairs with Li⁺ bound to pyrazolate N and chloride bound to Lewis acidic cobalt. X-ray photoelectron spectroscopy was used to assay both Li content and cobalt oxidation states. The general character of binding of LiCl to a metal complex acidic at metal and nucleophilic at ligand (pyrazolate Nβ) is discussed, as are potential catalytic applications of the concept.