Molecular Cage Impregnated Palladium Nanoparticles: Efficient, Additive-Free Heterogeneous Catalysts for Cyanation of Aryl Halides

Two shape-persistent covalent cages
(CC1r and CC2r) have
been devised from triphenyl amine-based trialdehydes and cyclohexane
diamine building blocks utilizing the dynamic imine chemistry followed
by imine bond reduction. The cage compounds have been characterized
by several spectroscopic techniques which suggest that CC1r and CC2r are [2+3] and [8+12] self-assembled architectures, respectively.
These state-of-the-art molecules have a porous interior and stable
aromatic backbone with multiple palladium binding sites to engineer
the controlled synthesis and stabilization of ultrafine palladium
nanoparticles (PdNPs). As-synthesized cage-embedded PdNPs have been
characterized by transmission electron microscopy (TEM), scanning
electron microscopy (SEM), and powder X-ray diffraction (PXRD). Inductively
coupled plasma optical emission spectrometry reveals that Pd@CC1r and Pd@CC2r have 40 and 25 wt% palladium loading, respectively. On the
basis of TEM analysis, it has been estimated that as small as ∼1.8
nm PdNPs could be stabilized inside the CC1r, while larger CC2r could stabilize ∼3.7 nm NPs. In contrast, reduction of palladium
salts in the absence of the cages form structure less agglomerates.
The well-dispersed cage-embedded NPs exhibit efficient catalytic performance
in the cyanation of aryl halides under heterogeneous, additive-free
condition. Moreover, these materials have excellent stability and
recyclability without any agglomeration of PdNPs after several cycles.