N-heterocyclic carbene-stabilized metal nanoparticles within porous organic cages for catalytic application

Abstract

Tuning the surface-embellishing ligands of metal nanoparticles (NPs) is a powerful strategy to modulate their morphology and surface electronic and functional features, impacting their catalytic activity and selectivity. In this work, we report the design and synthesis of a polytriazolium organic cage PIC-T, capable of stabilizing PdNPs within its discrete cavity. The obtained material (denoted Pd@PCC-T) is highly durable and monodispersed with narrow particle-size distribution of 2.06 ± 0.02 nm, exhibiting excellent catalytic performance and recyclability in the Sonogashira coupling and tandem reaction to synthesize benzofuran derivatives. Further investigation indicates that the modulation of N-heterocyclic carbene sites embedded in the organic cage has an impact on NPs' catalytic efficiency, thus providing a novel methodology to design superior NP catalysts.

Keywords: N-heterocyclic carbene; catalytic reaction; metal nanoparticle; porous organic cage.

Figure 1.

Modulation of the NHC sites embedded in Pd@PCC-I (X = C) and Pd@PCC-T (X = N) for tunable catalytic performance.

Figure 2.

(a) The schematic synthesis of Pd@PCC-T by PIC-T. i: Ag₂O, CH₃CN, 65°C, 24 h; ii: hv, λ = 365 nm, 24 h; iii: NH₄Cl, CH₃OH, rt, 2 h and then anion exchange with NH₄BF₄ in CH₃OH, rt, 2 h; iv: Pd(OAc)₂, CH₃CN, rt, 2 h; v: NaBH₄, CH₃CN and CH₃OH, rt, 3 h. (b) TEM image (scale bar = 10 nm), size distribution and (c) Pd 3d XPS spectrum of Pd@PCC-T. (d) N 1s XPS spectrum of PIC-T and Pd@PCC-T.

Figure 3.

(a) Time course of the formation of 2-phenylbenzofuran by using Pd@PCC-T and Pd@PCC-I respectively. (b) Recyclability of the Pd@PCC-T catalyst in the tandem reaction.

Figure 4.

The substrate scope of the Pd@PCC-T catalyzed tandem reaction for the synthesis of benzofuran derivatives. Reaction conditions: 2-iodophenol (2.5 mmol), phenylacetylene (3.0 mmol), Cs₂CO₃ (7.5 mmol), DMSO (6.0 mL) and Pd@PCC-T (0.5 mol% Pd, 6.0 mg). Yield determined by GC analysis.