Pd immobilized on dendrimer decorated halloysite clay: Computational and experimental study on the effect of dendrimer generation, Pd valance and incorporation of terminal functionality on the catalytic activity.

Dendrimer (PAMAM) decorated halloysite nanoclay (Hal) was prepared and applied for immobilization of Pd catalytic species to develop an efficient catalyst for copper and ligand-free CC coupling reactions. The effect of dendrimer generation, Pd valance and incorporation of anthranilamide as terminal functionality of dendrimer on the catalytic activity of the hybrid catalyst was studied experimentally and theoretically. The results of DFT computational studies on the effect of Pd valence showed that Pd(0) has much higher binding energy (-44.3 kcal/mol) on the modified halloysite surface, exposing PAMAM group, than Pd(OAc)2 with the binding energy of only -14.3 kcal/mol. Hence, Pd(0) was selected for experimental investigations. Again, computational studies on the two dendrimer generations, I and II, revealed higher stability of Pd(0) impregnated on Hal-PAMAM in generation I (G1) than it in G2. These results were experimentally confirmed by the synthesis of both Pd@Hal-PAMAM-G1 and Pd@Hal-PAMAM-G2 and comparing their catalytic activities for promoting the model coupling reactions. Additionally, the effect of terminal groups of dendrimer periphery was studied by comparing the catalytic activity of the catalysts with amine and anthranilamide terminated dendrimer, Pd@Hal-PAMAM-G1-ISA and Pd@Hal-PAMAM-G1. Interestingly, the experimental results were in good agreement with the theoretical findings and established Pd@Hal-PAMAM-G1-ISA as the catalyst of the choice. It was found that Pd@Hal-PAMAM-G1-ISA could promote both Sonogashira and Heck C-C coupling reactions efficiently in aqueous media (1:1 mixture of H2O and EtOH) and could be successfully recovered and reused for 10 reaction times with slight loss of the catalytic activity and Pd leaching.