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

Two shape-persistent covalent cages (<b>CC1</b>^<b>r</b> and <b>CC2</b>^<b>r</b>) 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 <b>CC1</b>^<b>r</b> and <b>CC2</b>^<b>r</b> 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 <b>Pd@CC1</b>^<b>r</b> and <b>Pd@CC2</b>^<b>r</b> 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 <b>CC1</b>^<b>r</b>, while larger <b>CC2</b>^<b>r</b> 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

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

Two shape-persistent covalent cages (<b>CC1</b>^<b>r</b> and <b>CC2</b>^<b>r</b>) 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 <b>CC1</b>^<b>r</b> and <b>CC2</b>^<b>r</b> 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 <b>Pd@CC1</b>^<b>r</b> and <b>Pd@CC2</b>^<b>r</b> 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 <b>CC1</b>^<b>r</b>, while larger <b>CC2</b>^<b>r</b> 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

Urea-Functionalized Self-Assembled Molecular Prism for Heterogeneous Catalysis in Water

Reaction of a ditopic urea “strut” (<b>L</b>_<b>1</b>) with <i>cis</i>-(tmen)­Pd­(NO₃)₂ yielded a [3+3] self-assembled molecular triangle (<b>T</b>) [<b>L</b>_<b>1</b> = 1,4-di­(4-pyridylureido)­benzene; tmen = <i>N,N,N′,N′</i>-tetramethylethane-1,2-diamine]. Replacing <i>cis</i>-(tmen)­Pd­(NO₃)₂ in the above reaction with an equimolar mixture of Pd­(NO₃)₂ and a clip-type donor (<b>L</b>_<b>2</b>) yielded a template-free multicomponent 3D trigonal prism (<b>P</b>) decorated with multiple urea moieties [<b>L</b>_<b>2</b> = 3,3′-(1<i>H</i>-1,2,4-triazole-3,5-diyl)­dipyridine]. This prism (<b>P</b>) was characterized by NMR spectroscopy, and the structure was confirmed by X-ray crystallography. The <b>P</b> was employed as an effective hydrogen-bond-donor catalyst for Michael reactions of a series of water-insoluble nitro-olefins in an aqueous medium. The <b>P</b> showed better catalytic activity compared to the urea-based ligand <b>L</b>_<b>1</b> and the triangle <b>T</b>. Moreover, the confined nanospace of <b>P</b> in addition to large product outlet windows makes this 3D architecture a perfect molecular vessel to catalyze Diels–Alder reactions of 9-hydroxymethylanthracene with N-substituted maleimide in the aqueous medium. The present results demonstrate new observations on catalytic aqueous Diels–Alder and Michael reactions in heterogeneous fashion employing a discrete 3D architecture of Pd­(II). The prism was recycled by simple filtration and reused several times without significant loss of activity