No abstract

For the first time we report syntheses of a family of functional polyphenylenepyridyl dendrons with different generations and structures such as focal groups, periphery, and a combination of phenylene and pyridyl moieties in the dendron interior using a Diels−Alder approach and a divergent method. The dendron structure and composition were confirmed using NMR spectroscopy, MALDI-TOF mass spectrometry, FTIR, and elemental analysis. As a proof of concept that these dendrons can be successfully used for the development of nanocomposites, synthesis of iron oxide nanoparticles was carried out in the presence of thermally stable dendrons as capping molecules followed by formation of Pd NPs in the dendron shells. This resulted in magnetically recoverable catalysts exhibiting exceptional performance in selective hydrogenation of dimethylethynylcarbinol (DMEC) to dimethylvinylcarbinol (DMVC).

We report the formation of multicore iron oxide mesocrystals using the thermal decomposition of iron acetyl acetonate in the presence of the multifunctional and rigid poly(phenylenepyridyl) dendron and dendrimer. We thoroughly analyze the influence of capping molecules of two different architectures and demonstrate for the first time that dendron/dendrimer self-assembly leads to multicore morphologies. Single-crystalline ordering in multicore NPs leads to cooperative magnetic behavior: mesocrystals exhibit ambient blocking temperatures, allowing subtle control over magnetic properties using a minor temperature change.

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.