Amphiphilic palladium NHC-complexes with chelating bis-NHC ligands based on imidazole-4,5-dicarboxylic acid: synthesis and catalysis in water

Abstract

Efficient catalytic systems for various organic transformations in green solvents, especially water, are in great demand. Catalytically active bis-NHC complexes of palladium(II) based on imidazole-4,5-dicarboxylic acid with different lipophilicities were obtained. The synthesis of imidazolium salts was complicated by the formation of side products of nucleophilic substitution by iodide ions in the Menshutkin reaction involving alkyl iodides, which was successfully resolved by using alkyl tosylates. The synthesis of bis-NHC complexes of palladium(II) was carried out in situ using Pd(OAc)₂ and KI from imidazolium tosylate salts. The structures of all compounds were well-characterized by a complex of modern physical methods. Typical for gemini surfactants, imidazolium salts aggregate in water to form submicron 200 nm (in the case of di-butyl salt) or compact 6 nm (in the case of di-tetradecyl salt) particles. The catalytic activity of the complexes and systems in situ with Pd(OAc)₂ in the hydrogenation reaction of nitroaromatics has been studied. The complex with butyl substituents was found to be superior to known catalytic systems in the reduction of p-nitrophenol (k_app = 1.53 min⁻¹). According to microscopy data, after reduction palladium nanoparticles remained uniformly distributed in the butyl complex, while a lipophilic shell in the tetradecyl complex prevented the access of water-soluble regents to Pd centers. However, the lipophilic tetradecyl complex is more active in the reduction of water-insoluble p-ethylnitrobenzene and in cross-coupling reactions using water-insoluble lipophilic aryl halides due to the combination of micellar and metal complex catalysis. Our results provide insight into amphiphilic NHC palladium complexes as promising catalytic systems for aqueous media.