Probing the support effect at the molecular level in the polyaniline-supported palladium nanoparticle-catalyzed Ullmann reaction of aryl iodides

A series of polyaniline (PANI)-supported palladium nanoparticles (Pd@PANIs) were fabricated from electron-deficient and enriched substituted anilines and palladium chloride. PANIs' support effect in the Pd@PANIs-catalyzed Ullmann reaction of aryl iodides for generation of biaryls was then studied. The results revealed that, though having similar Pd content and morphology, these Pd@PANI catalysts behaved quite differently in catalytic activities in the Ullmann reaction of aryl iodides, with the electron-enriched PANI supports having obviously enhanced activities for the reaction. Mechanistic studies revealed this is due to the stronger coordination of the nitrogen ligands on the electron-enriched PANI support with the Pd centers, higher adsorption rates of the reactant on the electron-enriched Pd@PANI catalysts, especially those promoted by the Pd loading, and easier generation and higher ratio of the Pd(0) species in the catalyst, which is the active catalysts in the Ullmann reaction responsible for the products' yields and the TONs of the reactions. In contrast, the specific surface area of the Pd@PANI catalyst is found to have little influence on the catalytic activity and adsorption capacity of the catalysts, and thus its influence may be omitted in the present Pd@PANIs-catalyzed Ullmann reaction of aryl iodides. Therefore, a simple fine tuning of the PANI support's electronic property by introducing electron donating groups can be an effective protocol for enhancing the catalyst's activity. This work may be the first example of a support effect study in PANI-supported metal nanoparticle-catalyzed reactions. (C) 2018 Elsevier Inc. All rights reserved.