Multivacant polyoxometalate-stabilizing palladium nanoparticles catalyze the N-formylation of amines with CO2 and H2

Abstract

In this work, the Keggin-type tri- and mono-palladium-substituted silicotungstates (POMs) were constructed by the reaction of tri- and monovacant silicotungstates ([SiW₉O₃₄]¹⁰⁻ or [SiW₁₁O₃₉]⁸⁻) with palladium chloride. The as-obtained potassium salts of POMs demonstrated that Pd²⁺ ions were incorporated into POM frameworks. Notably, Pd nanoparticles were formed when the tri-palladium-substituted Keggin-type POM salts were reduced by H₂. It was found that [SiW₉O₃₄]¹⁰⁻ anions could act as multivacant coordination ligands to effectively stabilize palladium nanoparticles. The Pd nanoparticles were catalytically active for the reductive amination of carbon dioxide, and the formylamides were achieved in high yields under mild reaction conditions with methanol as the solvent. Notably, the Pd nanocatalyst exhibited outstanding recyclability and was recycled at least eight times without any obvious loss of the catalytic activity. The characterization by HAADF-STEM revealed that Pd nanoparticles formed a stable structure with POMs. Further studies demonstrated that the N-formylation reaction proceeds by the “methyl formate” pathway, involving CO₂ activation by an in situ reaction with methanol and K₂CO₃ to generate potassium methyl carbonate. H₂ underwent heterolytic dissociation with the assistance of a base over a Pd⁰–SiW₉ catalyst, leading to one proton bound to carbonates and the hydride on a Pd atom (Pd–H). Moreover, the coupling of potassium methyl carbonate and Pd–H species afforded methyl formate intermediates in methanol, and then the reaction of methyl formate with amines gave formamides. This catalytic system demonstrated the benefits of excellent reactivity, stability, and recyclability for the N-formylation reaction.