Flexibility Induced Encapsulation of Ultrafine Palladium Nanoparticles into Organic Cages for Tsuji–Trost Allylation

Abstract

A series of three positional isomers of organic cages namely o -OC, m -OC, and p -OC, have been self-assembled using dynamic covalent chemistry. Their room temperature controlled fabrication with palladium gives ultrafine diameter (1–2 nm) of palladium nanoparticles (Pd NPs). We observed that the shape-flexibility of cages have great impact on the formation of Pd NPs. Theoretical calculations reveals that theoretically obtainable size of Pd NPs for each cage which was complementary to the experimental results. Theoretical studies indicate that the driving forces for the specific orientational preference may be ascribed to subtle variations on the level of π–π interactions, which ultimately governs the growth of Pd NPs therein. It is the first example of shape-flexible synthesis of organic cages where flexibility governs the nanoparticle growth. Pd NPs have shown excellent catalysis of Tsuji–Trost allylation at room temperature and pressure in water.

Ultrastable and Highly Catalytically Active N‐Heterocyclic‐Carbene‐Stabilized Gold Nanoparticles in Confined Spaces

“…[19][20][21][22] Of this wide variety of materials,d iscrete three-dimensional (3D) organic molecular cages with guest accessible cavities have recently emerged as an ew type of functional-materials platform to prepare MNPs with precisely controlled sizes and shapes. [23][24][25][26][27][28] Besides the well-defined cage structure, which provides ac onfined cage environment and can effectively prevent particle aggregation, enhancing particle stability and solubility,the anchoring functional groups inside the cavity have been established to be critical. These groups not only accelerate the encapsulation of metal precursors by strongly interacting with metal ions,t hey also promote the controlled nucleation of MNPs with assistance from the stabilizing effect of the aromatic backbones.T hus far, the applied anchoring functional groups have been limited to thioether groups or amine moieties.…”

Encapsulation of Metal Clusters within Porous Organic Materials: From Synthesis to Catalysis Applications

“…For instance, Bharadwaj, Chattaraj and co‐workers successfully governed the mean size of Pd‐MCs with the help of three highly flexible positional isomers of organic cage scaffolds namely o ‐OC, m ‐OC and p ‐OC ( o , m , p stands for ortho, meta and para) (Figure 9). [67] Obviously, the mean sizes of Pd‐MCs (average diameter: 1.22±0.31 nm, 1.62±0.64 nm, 1.68±0.38 nm) corresponded closely to the inner cavity size of the three templates ( o ‐OC: 1.11 nm, m ‐OC: 1.62 nm, p ‐OC: 1.78 nm), demonstrating the effective controlment of cavity size. It is noted that the shape‐flexibility and mobility of cage templates responded to accommodate Pd‐MCs once the nucleation occurred.…”

“…(b) Size distributions of Pd‐MCs supported by o ‐OC, m ‐OC and p ‐OC, respectively. Reproduced with permission [67] . Copyright 2020, American Chemical Society.…”

“…Palladium‐mediated catalysis have gained enormous relevance in promoting coupling reactions in the past decades [70] . Bharadwaj, Chattaraj and co‐workers employed the palladium‐bearing composite Pd@ o ‐OC to catalyze the Tsuji‐Trost allylation reaction of ethyl acetoacetate and acetyl acetone, which could reach ∼100% conversion only within 36 min while the allylations of dimethyl malonate was less selective [67] . The bound of Pd‐MCs inside the cage cavity of o ‐OC prevented the leach to a large extent, which thereby could support at least four cycles of catalytic reactions without substantial change in the size of Pd‐MCs.…”

Transition Metal Nanoparticles‐Catalyzed Organic Reactions within Porous Organic Cages

“…Chattaraj, Bharadwaj, and co‐workers have developed three PdNPs@POCs complexes via Schiff base condensation of o / m / p ‐trialdehydes and (1R, 2R)‐(−)‐1,2‐diaminocyclohexane followed by in situ reduction and Pd encapsulation (Figure 9). [29] Thus formed PdNPs@ o ‐POC‐8, PdNPs@ m ‐POC‐8, and PdNPs@ p ‐POC‐8 have Pd loading of 24, 18, and 16 wt %, respectively. The mean particle size of the three complexes is 1.22±0.31 nm, 1.62±0.46 nm, and 1.68±0.38 nm, respectively, which is corroborated with the inner cavity size of corresponding cages, indicating that the PdNPs were synthesized inside the organic cages.…”