Template synthesis and catalytic applications of metal nanoparticles within n-heterocyclic carbene organic cages

This study is led by Dr. Ying-Feng Han (College of Chemistry and Materials Science, Northwest University). Over the past decades, N-heterocyclic carbenes (NHCs) have established themselves as one of the most potent ligands in many catalytic systems, as well as metal-NHC-stabilized MNPs. However, the development of discrete 3D NHC-based architectures to encapsulate MNPs with superior catalytic performance is still in its infancy. Han and his group developed an efficient supramolecular strategy named the metal-carbene template approach (MCTA) to synthesize polyimidazolium organic cages (PICs). Using this strategy, the new type of PIC materials with different shapes and sizes can be readily tailored by the rational design of NHC precursors. “PICs have good solubility and stability in most common solvents and have the potential to support MNPs,” Han says.

Tong Liu and Sha Bai, together with Le Zhang, sought to prepare the PdNPs via a two-step approach. First, a red-orange solution of PIC-T (1 equiv.) and Pd(OAc)₂ (10 equiv.) in CH₃CN was prepared. After stirring, the reaction mixture was reduced by a methanolic solution of NaBH₄, affording Pd@PCC-T along with a sharp color change to deep brown without any precipitation, suggesting the PdNPs formation via reduction and stabilization by in-situ generated PCCs. The particle diameter and size distribution of Pd@PCC-T were analyzed by transmission electron microscopy (TEM) micrographs. The image presented well-dispersed PdNPs with a narrow size distribution of 2.06 ± 0.02 nm, matching well with the optimized cage structure. This indicates that the cage template can offer a protecting shell for the size-controlled synthesis of PdNPs.

In addition, two-dimensional diffusion ordered spectroscopy revealed that the as-synthesized Pd@PCC-T has similar diffusion coefficient to the reported Au@PCC-I (Angew Chem Int Ed 2020; 59: 16683–9), which confirms the similar size and shape of Pd@PCC-T and Au@PCC-I. It provides important evidence to support that the formed PdNPs are entrapped within the cage. To gain further insight into the electronic coupling between PCC-T and PdNPs, N1s XPS analysis was performed. The binding energy assigned to the carbene N shifted from 401.6 eV to 400.7 eV after forming Pd@PCC-T, definitely demonstrating the anchoring of PdNPs by the organic cage.

The team then found that Pd@PCC-T exhibiting excellent catalytic performance and recyclability in the Sonogashira coupling and the tandem reaction to synthesize benzofuran derivatives. The catalytic performance of Pd@PCC-T is superior to the majority of previously reported PdNPs catalysts protected by classic organic supports, presumably due to the small particle size, available surface accessibility, and stability achieved via the well-confined organic cage. Remarkably, a subtle regulation of NHC sites (from imidazolidin-2-ylidene to 1,2,4-triazolin-5-ylidene) binding with PdNPs significantly boosts the catalytic activity toward the tandem reaction to synthesize benzofuran derivatives (8 h, 73% vs. 4 h, 98%). These findings indicate that modulation of the chemical environment (electronic effect, steric effect, etc.) around PdNPs, via ligand modification at the atomic level, can regulate the interaction between metal NPs and substrates for sequential catalysis more or less.

“Their in-situ generated poly-NHC sites are essential in controlling the nucleation and growth of MNPs. The strong σ-donating ability of the NHCs further enhances the interactions between the shells (cages) and the MNPs, consequently leading to encapsulated MNPs within polycarbene cages (PCCs) with unique properties,” Han says.

This work confirmed the crucial role of NHC sites for catalytic efficiency. It is envisioned that MNPs@PCCs, integrating readily tunable NHC sites with a well-confined microenvironment, would be an ideal platform for advancing new outstanding heterogeneous catalysts.

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