Novel silica immobilized NCN-pincer palladium(II) and platinum(II) complexes: Application as Lewis acid catalysts

Abstract

2,6-Bis[(dimethylamino)methyl]phenyl (NCN-pincer) palladium(II) and platinum(II) complexes tethered to a trialkoxysilane coupling agent through a carbamate linkage were immobilized on various types of silica using a grafting or a sol–gel process. The resulting hybrid materials were characterized by IR spectroscopy (DRIFT) and solid state CP/MAS NMR ( 13C and 29Si). Based on these analyses, H-bond interactions between the carbamate carbonyl group of the complex and the free silanol groups on silica surface were established. The palladium-based materials were tested for their activity as Lewis acid catalysts in the aldol reaction between methyl isocyanoacetate and benzaldehyde. It was found that these materials can indeed be applied as catalysts in this reaction. Their repetitive reuse showed an inferior catalytic efficacy, which presumably was caused by a reconstitution of the silica support. These studies also revealed that simple silver-based salts are active catalysts in this aldol reaction.