PCP- and SCS-pincer palladium complexes immobilized on mesoporous silica: application in C-C bond formation reactions

ECE-pincer palladium(II) complexes {ECE=[C6H3(CH2E)2-2,6]-, E=PPh2 and SPh} thered to a trialkoxysilane moiety through a carbamate linkage were immobilized on ordered mesoporous silicas SBA-15 and MCM-41 using a grafting process. The resulting hybrid materials were characterized by IR spectroscopy, solid-state CP/MAS NMR (13C, 31P, and 29Si), and elemental analyses. These analyses showed the integrity of the pincer-metal complexes on the supports, which highlights their stability under the applied immobilization conditions. An H-bonding interaction between the carbamate carbonyl group of the complex and free silanol groups on the silica surface was also established. The hybrid materials were found to act as Lewis acid catalysts in the aldol reaction between methyl isocyanoacetate and benzaldehyde. SBA-15 modified with the PCP-pincer Pd complex was used in up to five runs without loss of activity. Control experiments showed the true heterogeneous nature of the catalyst in this reaction. Nitrogen physisorption data, XRD, and TEM/EDX analyses of the hybrid materials revealed that the mesoporous structure of these materials was retained during the immobilization process as well as during catalysis.