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

5 years ago

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

Kenneth G. Caulton, Chun-Hsing Chen, Yaroslav Losovyj, Alexander V. Polezhaev

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)2PNN}CoCl2]. Reduction of this LiCl adduct with KC8 under CO atmosphere led to formation of CoI 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. Five in one: A pyridyl pincer ligand was designed to incorporate five different functionalities through PtBu2 and pyrazole pincer ligand arms (see figure). Ligand properties in neutral, monoanionic, and dianionic form were investigated in CoI and CoII complexes. The concept of a single-molecule frustrated Lewis pair (FLP) is discussed, and an alkali metal chloride was used as a model substrate to show FLP behavior.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201700859