Hydrophobic Periphery Tails of Polyphenylenepyridyl Dendrons Control Nanoparticle Formation and Catalytic Properties

Abstract

Here we report control of iron oxide and palladium nanoparticle (NP) formation via stabilization with polyphenylenepyridyl dendrons of the second and third generations with dodecyl periphery. These nanomaterials are developed as magnetically recoverable catalysts. To accurately assess the influence of the dodecyl exterior for the same dendron generation, we also designed a second generation dendron with partial dodecyl periphery. For all dendrons studied, the multicore iron oxide mesocrystals were formed, the sizes and morphology of which were controlled by the dendron generation. Analysis of the static and dynamic magnetic properties, in combination with transmission electron microscopy observations, demonstrate that magnetism is sensitive on the structure-directing capabilities of the type of the dendron which was employed for the mesocrystal stabilization. Close proximity of single cores in such multicore mesocrystals promotes the coupling of the neighboring magnetic moments, thus boosting their magnetization and allowing easy crossover between superparamagnetic and ferrimagnetic behaviors at room temperature. The particularly dramatic role of the dendron structure was also witnessed via the Pd NP formation, which was found to depend on both the dendron generation and its dodecyl periphery. In the case of the catalyst based on the second generation dendron with full dodecyl periphery, no Pd NPs were observed by TEM indicating that these species are of a subnanometer size and are not visible on or near the iron oxide NPs. For the catalyst based on the second generation dendron with partial dodecyl periphery, hydrogen reduction leads to much larger Pd NPs (2.7 nm) due to an unimpeded exchange of Pd species between dendrons and nondense dendron coating with asymmetrical dendrons. The third generation dendron with full dodecyl periphery allows nearly monodisperse 1.2 nm Pd NPs in the shells of iron oxide mesocrystals and the best catalytic properties in selective hydrogenation of dimethylethynylcarbinol. This study suggests a robust approach to control NP formation in magnetically recoverable catalysts for a wide variety of catalytic reactions using dendrons combining rigidity and flexibility in one molecule.

Colloidal magnetic nanocrystal clusters: variable length-scale interaction mechanisms, synergetic functionalities and technological advantages

“…Similar features in the imaginary part of the ac susceptibility, χ″(T), have been found in magnetite nanoclusters capped with dendrons ( Figure 16C) [90]. In this system though, the maximum χ″(T) at low temperature is attributed to superparamagnetism [90] and not to surface spin disorder like that established in polyacrylic acid-capped maghemite nanoclusters [21].…”

“…In this system though, the maximum χ″(T) at low temperature is attributed to superparamagnetism [90] and not to surface spin disorder like that established in polyacrylic acid-capped maghemite nanoclusters [21]. In such dendron-capped magnetite nanoclusters, the Vogel-Fulcher law has been utilized to fit the ac susceptibility data, and the resultant value of the attempt time was found to be comparable to that reported for nanoparticulate systems with intermediate-strength dipolar interactions [123].…”

“…A metallic precursor, such as metal chlorides [14-16, 25, 26, 28, 31, 33, 35, 77-87] or acetylacetonates [36,[88][89][90] are dissolved in a solvent like ether [36,[88][89][90], or glycol [14,15,25,26,31,33,35,77,[79][80][81][82][83][84][85][86][87], or water [16], or THF-ethanol [28] in a glass flask equipped with a heating mantle or in an autoclave (Figure 2A-E). A capping agent is also added in the reaction mixture at room temperature (Table 1) to control the size and the shape of the system.…”

“…Polycrystalline nanoclusters are obtained when the solvothermal reaction occurs in water, and the nanoclusters are stabilized with polyacrylamide [16]. However, efficient crystallographic alignment is observed by utilizing large poly(phenylenepyridyl) dendron/dendrimer as capping molecules and ether as a solvent ( Figure 2E) [36,89,90]. The dendron/dendrimer surfactants self-assemble in the reaction mixture and "embrace" the nanocrystal surface by means of both A B Figure 4: Fast Fourier transform (FFT) patterns of magnetite nanoclusters prepared in a single-step synthesis protocol, with 1,2-propylene glycol (A) and ethylene glycol (B) as solvents.…”

“…In the dendron-based system, the superparamagnetic behavior likely originates from nanoclusters incorporating a larger average distance among their constituent nanocrystals. The extra potential of such nanoclusters arises from our ability to tune the interparticle separation by changing the generation of the dendron capping ligand [90].…”