Description
The goal of this research is to synthesize pincer ligands and complexes with a central pyridine ligand substituted at positions 2 and 6 by two N-heterocyclic carbenes, each bearing a pendant heteroaryl substituent. Related pincers without heteroaryl groups have been reported in the past 15 years as catalysts, and our ultimate goal was to enable the study of the heteroaryl moieties as pendant bases, creating improved bifunctional catalysts. Two classes of pincers were made: one with methylene spacers between the central pyridine and the imidazol-2-ylidene groups, and those whose imidazol-2-ylidene groups were attached directly to the pyridine without any spacers. Four bis(imidazolium) salt pincer precursors with methylene spacers were made, and reaction with Ag₂O produced silver-carbene complexes which act as transmetallation agents for further complexation. Two of the analogs were able to be metallated with (COD)PdCl₂ to form palladium-carbene complexes, NMR data for which are in accord with a puckered structure induced by the methylene spacers. One of these complexes showed catalytic activity in the Heck coupling reaction. Synthesis of analogs lacking spacers under literature conditions proved to be impossible, presumably because of reduced nucleophilicity of 1-heteroarylimidazoles. Thus, a systematic study of reactivity of 1-substituted imidazoles with five 2,6-disubstituted pyridines conclusively showed that 2,6-bis(trifluoromethanesulfonyloxy)pyridine (16) was the best precursor. A well-described synthesis of 16 was developed, paving the way to making seven analogs of pincer precursors without methylene spacers. Reaction of two of these pincer precursors with Ag₂O and NaOH produced silver-carbene bridged-dimer complexes which were characterized by X-ray diffraction and show promise as sources of other metal complexes.
