Dendritic catalysts and dendrimers in catalysis

Synthesis and hydroformylation evaluation of Fréchet-type organometallic dendrons with N,O-salicylaldimine Rh(i) complexes at the focal point

A series of organometallic dendrons containing N,O-salicylaldimine entities at the focal point were synthesised by reacting the N,O-salicylaldimine-functionalised Fréchet dendrons (G0, G1 and G2) with a [Rh(μ-Cl)(η2:η2-COD)]2 dimer to yield the corresponding Rh(COD) [COD = cyclooctadiene] complexes. These Rh(COD) complexes were exposed to an atmosphere of CO to yield a new class of rhodium carbonyl organometallic dendrons with Rh(CO)2 units at the focal point. All the compounds were characterised using standard spectroscopic and analytical techniques, these include nuclear magnetic resonance, infrared spectroscopy, mass spectrometry and single-crystal X-ray diffraction for compounds 1, 4 and 7. All of the complexes were evaluated in the hydroformylation of 1-octene, with excellent conversion and chemoselectivity towards aldehydes. The G0-(CO)2 catalyst precursor (7) was active in the hydroformylation of 1-octene, styrene, 7-tetradecene, methyl oleate, triolein, d-limonene and R-citronellal. The conversion and chemoselectivity towards aldehydes for 7-tetradecene, methyl oleate, triolein and d-limonene were promising. Across a particular dendron series, an increase in chemoselectivity was observed due to the dendritic effect. Mercury drop tests were performed for the G0-analogues and these confirm that the hydroformylation can be attributed to a combination of homogeneous and heterogeneous catalysis.

Synthesis of Water-Soluble Amino Functionalized Multithiacalix[4]arene via Quaternization of Tertiary Amino Groups

A convenient approach to the synthesis of multithiacalix[4]arene derivatives containing amino groups and phthalimide fragments by the formation of quaternary ammonium salts is presented. As the initial macrocycle for the synthesis of multithiacalix[4]arenes, a differently substituted p-tert-butylthiacalix[4]arene containing bromoacetamide and three phthalimide fragments was used in a 1,3-alternate conformation. The macrocycle in cone conformation containing the tertiary amino groups was found to be a convenient core for the multithiacalix[4]arene systems. Interaction of the core multithiacalix[4]arene with monobromoacetamide derivatives of p-tert-butylthiacalix[4]arene resulted in formation in high yields of pentakisthiacalix[4]arene containing quaternary ammonium and phthalimide fragments. The removal of phthalimide groups led to the formation of amino multithiacalix[4]arene in a good yield. Based on dynamic light scattering, it was shown that the synthesized amino multithiacalix[4]arene, with pronounced hydrophobic and hydrophilic fragments, formed dendrimer-like nanoparticles in water via direct supramolecular self-assembly.

Cylindrical micelles of a POSS amphiphilic dendrimer as nano-reactors for polymerization

A low generation amphiphilic dendrimer, POSS-AD, which has a POSS core and eight amphiphilic arms, was synthesized and used as a nano-reactor to produce well-defined polymer nano-cylinders. Confirmed by small-angle X-ray scattering (SAXS), Raman and NMR spectrometry, monodispersed cylindrical micelles that contain a hydrophilic cavity with a diameter of 2.09 nm and a length of 4.26 nm were produced via co-assembling POSS-AD with hydrophilic liquids, such as H₂O and HEMA in hydrophobic solvents. Taking the HEMA/POSS-AD cylindrical micelles as nano-reactors, polymerization of HEMA within the micelles results in polymer nano-cylinders (POSS-ADNPs) with a diameter of 2.24 nm and a length of 5.02 nm. The study confirmed that despite the inability to maintain specific shape in solution, low generation dendrimers form well-defined nano-containers or nano-reactors, which relies on co-assembling with hydrophilic guest molecules. These nano-reactors are robust enough to maintain their shape during the polymerization of the guest molecules. Polymer nano-cylinders with dimensions less than 10 nm can thus be produced from the HEMA/POSS-AD micelles. Since the chemical structure of low-generation dendrimers and the contents of the co-assembled nano-reactors can be easily adjusted, the concept holds the potential for the further developments of low-generation amphiphilic dendrimers.

Effect of Dendrigraft Generation on the Interaction between Anionic Polyelectrolytes and Dendrigraft Poly(l-Lysine)

In this present work, three generations of dendrigraft poly(l-Lysine) (DGL) were studied regarding their ability to interact with linear poly (acrylamide-co-2-acrylamido-2-methyl-1-propanesulfonate) (PAMAMPS) of different chemical charge densities (30% and 100%). Frontal analysis continuous capillary electrophoresis (FACCE) was successfully applied to determine binding constants and binding stoichiometries. The effect of DGL generation on the interaction was evaluated for the first three generations (G2, G3, and G4) at different ionic strengths, and the effect of ligand topology (linear PLL vs. dendrigraft DGL) on binding parameters was evaluated. An increase of the biding site constants accompanied with a decrease of the DGL-PAMAMPS (n:1) stoichiometry was observed for increasing DGL generation. The logarithm of the global binding constants decreased linearly with the logarithm of the ionic strength. This double logarithmic representation allowed determining the extent of counter-ions released from the association of DGL molecules onto one PAMAMPS chain that was compared to the total entropic reservoir constituted by the total number of condensed counter-ions before the association.

Synthesis, photophysical and electrochemical properties and DSSC application of triphenylaminochalcone cored dendrimers with anthracene and pyrene as surface groups and triazolylcholanoate as the bridging unit

Triphenylaminochalcone-cored dendrimers decorated with anthracene and pyrene at the periphery have been synthesized by a convergent methodology using a click chemistry approach and characterized from spectral data and elemental analyses.

Protoenzymes: the case of hyperbranched polyesters

Enzymes are biopolymeric complexes that catalyse biochemical reactions and shape metabolic pathways. Enzymes usually work with small molecule cofactors that actively participate in reaction mechanisms and complex, usually globular, polymeric structures capable of specific substrate binding, encapsulation and orientation. Moreover, the globular structures of enzymes possess cavities with modulated microenvironments, facilitating the progression of reaction(s). The globular structure is ensured by long folded protein or RNA strands. Synthesis of such elaborate complexes has proven difficult under prebiotically plausible conditions. We explore here that catalysis may have been performed by alternative polymeric structures, namely hyperbranched polymers. Hyperbranched polymers are relatively complex structures that can be synthesized under prebiotically plausible conditions; their globular structure is ensured by virtue of their architecture rather than folding. In this study, we probe the ability of tertiary amine-bearing hyperbranched polyesters to form hydrophobic pockets as a reaction-promoting medium for the Kemp elimination reaction. Our results show that polyesters formed upon reaction between glycerol, triethanolamine and organic acid containing hydrophobic groups, i.e. adipic and methylsuccinic acid, are capable of increasing the rate of Kemp elimination by a factor of up to 3 over monomeric triethanolamine.This article is part of the themed issue 'Reconceptualizing the origins of life'.

Dendrimer-functionalized electrospun nanofibres as dual-action water treatment membranes

This work reports the preparation of composite electrospun membranes combining antimicrobial action with the capacity of retaining low-molecular weight non-polar pollutants. The membranes were electrospun blends of polyvinyl alcohol (PVA) and poly(acrylic acid) (PAA) stabilized using heat curing. The membranes were functionalized by grafting amino-terminated poly(amidoamine) (PAMAM) G3 dendrimers. The antimicrobial effect was assessed using strains of Escherichia coli and Staphylococcus aureus by tracking their capacity to form new colonies and their metabolic impairment upon contact with membranes. The antimicrobial activity was particularly high to the gram-positive bacterium S. aureus with a 3-log reduction in their capacity to colonize dendrimer-functionalized membranes with respect to neat PVA/PAA fibers. The effect to gram-positive bacteria was attributed to the interaction of dendrimers with the negatively charged bacterial membranes and resulted in membranes essentially free of bacterial colonization after 20h in contact with cultures at 36°C. The adsorption of toluene on PAA/PVA fibers and on dendrimer-functionalized membranes was assayed using toluene over a broad concentration range. The host-guest encapsulation of toluene inside dendrimer molecules was computed through docking studies, which allowed calculating a maximum capacity of 14 molecules of toluene per molecule of PAMAM G3. The theoretical prediction was in good agreement with the experimental capacity at the higher concentrations assayed.

Synthesis of structurally controlled hyperbranched polymers using a monomer having hierarchical reactivity

Hyperbranched polymers (HBPs) have attracted significant attention because of their characteristic topological structure associated with their unique physical properties compared with those of the corresponding linear polymers. Dendrimers are the most structurally controlled HBPs, but the necessity of a stepwise synthesis significantly limits their applications in materials science. Several methods have been developed to synthesize HBPs by a one-step procedure, as exemplified by the use of AB2 monomers and AB' inimers under condensation and self-condensing vinyl polymerization conditions. However, none of these methods provides structurally controlled HBPs over the three-dimensional (3D) structure, i.e., molecular weight, dispersity, number of branching points, branching density, and chain-end functionalities, except under special conditions. Here, we introduce a monomer design concept involving two functional groups with hierarchical reactivity and demonstrate the controlled synthesis of dendritic HBPs over the 3D structure by the copolymerization of the designed monomer and acrylates under living radical polymerization conditions.

Design of a Peripheral Building Block for H-Bonded Dendritic Frameworks and Analysis of the Void Space in the Bulk Dendrimers

Three dendrimers, (t-Bu-G 2 N) 2 , CC(t-Bu-G 1 N) 3 and (t-Bu-G 1 N) 2 , with 3,5-di-tert-butyl amidobenzene as a common peripheral moiety were prepared in 64-83% yields and characterized. The bulk solids had high BET surface areas of 136-138 m2/g, which were similar for the three dendrimers in spite of their different molecular weight (ranging from 1791 to 2890). It was concluded that the peripheral amide groups do not imbed in the interstitial space of neighbouring dendrimer molecules but rather build a supramolecular architecture through strong intermolecular H-bonds. This mode of assembly generates voids in the bulk dendrimers responsible for sorption properties. The X-ray crystal structure analysis of a compound representing the peripheral moiety of the dendrimers and the FT-IR and powder-XRD data for (t-Bu-G 1 N) 2 suggest the proposed supramolecular structure. The isosteric heats of CO2 sorption (Q st) for (t-Bu-G 2 N) 2 were significantly higher than those for the other two dendrimers, which is consistent with the formation of a different type of voids within the interstitial space of the molecule. It is suggested that the interstitial void space can be designed and tuned to adjust its properties to a particular task, such as the separation of gases or a catalytic reaction facilitated by the dendrimer.

Electron and Phonon Dynamics in Hexagonal Pd Nanosheets and Ag/Pd/Ag Sandwich Nanoplates

Pd and its hybrid nanostructures have attracted considerable attention over the past decade, with both catalytic and plasmonic properties. The electron and phonon properties directly govern conversion efficiencies in applications such as energy collectors and photocatalysts. We report the dynamic processes of electron-phonon coupling and coherent acoustic phonon vibration in hexagonal Pd nanosheets and Ag/Pd/Ag sandwich nanoplates using transient absorption spectroscopy. The electron-phonon coupling constant of Pd nanosheets, G_Pd-nanosheet (8.7 × 10¹⁷ W/(m³·K)) is larger than that of the bulk G_Pd (5.0 × 10¹⁷ W/(m³·K)). The effective coupling constant G_eff of Ag/Pd/Ag nanoplates decreases with increasing Ag shell thickness, finally approaching the bulk G_Ag. The variation of G_eff is explained in terms of reduced density of states near Fermi level of Pd nanosheets with 1.8 nm ultrathin thickness. Coherent acoustic phonon vibration in Pd nanosheets is assigned to a fundamental breathing mode, similar to the vibration of benzene. The period increases with increasing Ag shell thickness. For Ag/Pd/Ag nanoplates with 20 nm thick Ag shells, the vibrational mode is ascribed to a quasi-extensional mode. The results show that the modes of the coherent acoustic phonon vibration transform with the geometric variation of Pd nanosheets and Ag/Pd/Ag nanoplates. Our results represent an understanding of quantum-confinement related electron dynamics and bulk-like phonon kinetics in the ultrathin Pd nanosheets and their hybrid nanostructures.

Multi-scale characterization of thermoresponsive dendritic elastin-like peptides

Elastin like peptides (ELPs)-polypeptides based on the protein elastin-are used widely as thermoresponsive components in biomaterials due to the presence of a sharp soluble-to-insoluble phase change at a characteristic transition temperature (T_t). While linear ELPs have been thoroughly studied, few investigations into branched ELPs have been carried out. Using lysine amino acids as branching and terminal units with 1-3 pentameric repeats between each branch, ELP dendrimers were prepared by solid-phase peptide synthesis with molecular weights as high as 14kDa. A conformation change from random coil to β-turn upon heating through the T_t, typical of ELPs, was observed by circular dichroism spectroscopy for all peptides. The high molecular weights of these peptides enabled the use of characterization techniques typically reserved for polymers. Variable-temperature small-angle X-ray scattering measurements in dilute solution revealed an increase in size and fractal dimension upon heating, even well below the T_t. These results were corroborated by cryogenic transmission electron microscopy, which confirmed the presence of aggregates below the T_t, and micro differential scanning calorimetry, which showed a broad endothermic peak below the T_t. These results collectively indicate the presence of a pre-coacervation step in the phase transition of ELP dendrimers.

Multi-Stimuli-Responsive Amphiphilic Assemblies through Simple Postpolymerization Modifications

A strategy to construct different stimuli responsive polymers from post polymerization modifications of a single polymer scaffold via thiol-disulfide exchange has been developed. Here, we report on a random copolymer that enables the design and syntheses of a series of dual or multi-stimuli responsive nanoassemblies using a simple post-polymerization modification step. The reactive functional group involves a side chain monopyridyl disulfide unit, which rapidly and quantitatively reacts with various thiols under mild conditions. Independent and concurrent incorporation of physical, chemical or biologically responsive properties have been demonstrated. We envision that this strategy may open up opportunities to simplify the synthesis of multi-functional polymers with broad implications in a variety of biological applications.

Synthesis of Dendrimers with a Bidentate Phosphine Core Ligand Having Carboxy Groups at the Peripheral Layer and Their Application to Aqueous Media Cross-Coupling Reactions

We prepared a series of dendrimers with a bidentate phosphine core ligand having carboxy groups at the peripheral layer. By employing the corresponding water-soluble dendritic phosphine-palladium complex as a catalyst, the aqueous media Suzuki-Miyaura reaction and Tsuji-Trost reaction proceeded to provide the corresponding cross-coupling product.

Partially oxidized iridium clusters within dendrimers: size-controlled synthesis and selective hydrogenation of 2-nitrobenzaldehyde

Iridium clusters nominally composed of 15, 30 or 60 atoms were size-selectively synthesized within OH-terminated poly(amidoamine) dendrimers of generation 6. Spectroscopic characterization revealed that the Ir clusters were partially oxidized. All the Ir clusters efficiently converted 2-nitrobenzaldehyde to anthranil and 2-aminobenzaldehyde under atmospheric hydrogen at room temperature in toluene via selective hydrogenation of the NO2 group. The selectivity toward 2-aminobenzaldehyde over anthranil was improved with the reduction of the cluster size. The improved selectivity is ascribed to more efficient reduction than intramolecular heterocyclization of a hydroxylamine intermediate on smaller clusters that have a higher Ir(0)-phase population on the surface.

Role of Aromatic Interactions in Temperature-Sensitive Amphiphilic Supramolecular Assemblies

Aromatic interactions were found to greatly influence the temperature-dependent dynamic behavior within supramolecular assemblies. Using an amphiphilic dendron, we systematically changed the hydrophobic groups introducing increasing levels of aromaticity while keeping the hydrophilic part constant. We show that the supramolecular assemblies become less sensitive to temperature changes when aromatic interactions in the aggregate are increased. Conversely, the absence of aromaticity in the hydrophobic moieties produces temperature-sensitive aggregates. These results show that subtle molecular-level interactions can be utilized to control temperature-sensitive behavior in the nanoscale. These findings open up new design strategies to rationally tune the behavior of stimuli-responsive supramolecular assemblies on multiple spatiotemporal scales.

Manganese(iii)salophen supported on a silica containing triazine dendrimer: an efficient catalyst for epoxidation of alkenes with sodium periodate

Mn(iii) salophen supported on a nanosilica triazine dendrimer was synthesized and used for epoxidation of alkenes with sodium periodate.

Syntheses and catalytic activities of new metallodendritic catalysts

Metallodendritic catalysts showed high selectivity in the epoxidation of olefins even after being used for at least five cycles.

Functional dendritic compounds: potential prospective candidates for dental restorative materials and in situ re-mineralization of human tooth enamel

This review provides a snapshot of recent progress in the synthesis and application of dendritic compounds as potential prospective candidates for dental restorative materials andin siture-mineralization of human tooth enamel.

Polyamino amphiphile mediated support of platinum nanoparticles on polyHIPE as an over 1500-time recyclable catalyst

For supported metal nanoparticles, the ligand/support is crucial to their catalytic activity, stability and recyclability.

Synthesis, photophysical properties and anticancer activity of micro-environment sensitive amphiphilic bile acid dendrimers

Porphyrin-cored bile acid dendrimers containing deoxymethyl cholate and methyl cholate units at the periphery, have been synthesized by convergent methodology using a click chemistry approach and characterized by ¹H and ¹³C NMR and MALDI-TOF MS data.

Dendrimer-encapsulated Pd(0) nanoparticles immobilized on nanosilica as a highly active and recyclable catalyst for the copper- and phosphine-free Sonogashira–Hagihara coupling reactions in water

Recyclable dendrimer-encapsulated Pd(0) nanoparticles immobilized on nanosilica in the Sonogashira–Hagihara reaction under copper(i) and phosphine ligand-free conditions in water.