Ionic Core−Shell Dendrimers with an Octacationic Core as Noncovalent Supports for Homogeneous Catalysts

Abstract

Ionic core-shell dendrimers with an octacationic core have been applied as noncovalent supports for homogeneous catalysts. Catalytically active arylpalladium complexes, which bear a tethered sulfato group, were noncovalently attached to the ionic core-shell dendritic supports via a straightforward ionexchange reaction under mild conditions. Diagnostic shifts in 1 H NMR and Overhauser contacts show that the sulfato groups of the catalysts are located close to the octacationic core of the dendritic support in the resulting assemblies. The location of the catalytic Pd(II) sites has been varied via two strategies: by increasing the dendrimer generation and/or by shortening of the sulfato tether. In addition, a metallodendritic assembly was prepared, which bears an alternative shell of apolar dodecyl groups. Both the dendrimer size and the nature of the dendritic shell have no influence on the binding properties of the dendritic supports, i.e., the octacationic dendrimers of generations 1-3 form discrete 1:8 assemblies with the arylpalladium complexes. The structural aspects and the nature of the metallodendritic assemblies have been studied by means of pulse gradient spin-echo NMR diffusion methods, Overhauser spectroscopy, and electron microscopy (TEM). These techniques showed that the dendritic supports and arylpalladium complexes are strongly associated in solution to give unimolecular assemblies of nanoscopic dimensions. Membrane dialysis can recover these metallodendritic assemblies due to their nanoscopic size. The catalytic performances of the metallodendritic assemblies are comparable, but slightly lower than the performance of the unsupported catalyst.

Where Organometallics and Dendrimers Merge: The Incorporation of Organometallic Species into Dendritic Molecules

“…Reaction of the Pd-Cl dendrimer 30 with eight equivalents of AgBF 4 generated 31. The structures of the dendrimers 30 and 31 are shown in Scheme 8 and have been characterized by multinuclear NMR spectroscopic studies, including DOSY experiments, and by ESI mass spectrometry [53]. Although halide scrambling is observed, the MS data clearly indicate that an octamer was formed and was stable in solution.…”

“…21. All of the protected dendrimers (52)(53)(54)(55) were synthesized via reaction of a pendent amine on the dendrons with a mono-or bis(chlorocarbonyl)ferrocene in the presence of Et 3 N and isolated in good to excellent yields (54-92%). The deprotection reactions to generate the free carbohydrate dendrimers 56-59 were quantitative.…”

Highlights of 45 years of research: A personal account

“…Non-covalent anchoring of catalysts to molecular supports; "tulips in a vase" approach [46,55,63] Next to the covalent attachment of catalytic groupings to nanosized supports, we also studied an approach in which the catalytic unit(s) and support could be realized by self-assembling rather than by divergent synthesis. To this end, we synthesize core-shell material with a distinct rigid, octa-cationic core and a neutral shell comprising dendritic wedges.…”

PCP‐ and SCS‐Pincer Palladium Complexes Immobilized on Mesoporous Silica: Application in CC Bond Formation Reactions

“…[6][7][8][9][10] The robustness of the ECE-pincer metal complexes make them attractive candidates for processes for which recycling and reuse is a prerequisite. Various soluble supports have already been used for the immobilization and recycling of pincer-based metal catalysts, among which are hyperbranched polymers, [9,11] oligo(ethylene glycol), [12] dendronized polymers, [13] carbosilane dendrimers, [14] cartwheel molecules, [8,15] polycationic dendrimers, [16] and soluble polymers. [10,17] Recently, we as well as others have reported on the grafting of ECE-pincer metal complexes on silica and on their use in aldol-type [18,19] and Heck reactions.…”