Dendritic catalysis: Major concepts and recent progress

Dendrimer-Ni-Based Material: Toward an Efficient Ni-Fe Layered Double Hydroxide for Oxygen-Evolution Reaction

Ni/Fe oxides are among the most widely used catalysts for water splitting. This paper outlines a new approach to synthesize Ni-Fe layered double hydroxides (Ni-Fe LDHs) for oxygen-evolution reaction (OER). Herein, we show that a dendrimer with carboxylate surface groups (generation 3.5) could react with Ni(II) ions to form a precatalyst for OER. During electrochemical OER, this precatalyst converted to Ni-Fe LDH, which is an efficient catalyst toward OER in the presence of Fe(III) ions. The catalyst was characterized by a number of methods and applied for OER using fluorine-doped tin oxide (FTO), Au, Pt, Ni foam, and glassy carbon electrodes. The catalyst shows a current density of 100 mA/cm² on the surface of the Ni foam, using only 297 mV overpotential and with the Tafel slope of 60.8 mV/decade. A current density of 50 mA/cm² on the surface of Au or Pt requires 333 and 317 mV overpotentials, respectively. The slopes of the Tafel plots for the catalyst on Au, GC, and Pt are 52.5, 47.1, and 37.4 mV/decade, respectively. The dendrimer resulted in a large dispersibility and an increase in active sites of Ni-Fe LDH, as well as the formation of Ni-Fe LDH.

Design of Organoiron Dendrimers Containing Paracetamol for Enhanced Antibacterial Efficacy

Paracetamol (acetaminophen) is a common painkiller and antipyretic drug used globally. Attachment of paracetamol to a series of organoiron dendrimers was successfully synthesized. The aim of this study is to combine the benefits of the presence of these redox-active organoiron dendrimers, their antimicrobial activities against some human pathogenic Gram-positive, and the therapeutic characteristics of paracetamol. The antimicrobial activity of these dendrimers was investigated and tested with a minimum inhibitory concentration and this has been reported. Some of these newly synthesized dendrimers exhibited the highest inhibitory activity against methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus faecium (VRE), and Staphylococcus warneri compared to reference drugs. The results of this study indicate that the antimicrobial efficacy of the dendrimers is dependent on the size of the redox-active organoiron dendrimer and its terminal functionalities. The best result has been recorded for the fourth-generation dendrimer 11, which attached to 48 paracetamol end groups and has 90 units composed of the η⁶-aryl-η⁵-cyclopentadienyliron (II) complex. This dendrimer presented inhibition of 50% of the growth (IC₅₀) of 0.52 μM for MRSA, 1.02 μM for VRE, and 0.73 μM for Staphylococcus warneri. The structures of the dendrimers were characterized by elemental analysis, Fourier transform infrared (FT-IR), nuclear magnetic resonance (¹H-NMR), and ¹³C-NMR spectroscopic techniques. In addition, all synthesized dendrimers displayed good thermal stability in the range of 300–350 °C following the degradation of the cationic iron moieties which occurred around 200 °C.

Palladium Catalysts Based on Porous Aromatic Frameworks, Modified with Ethanolamino-Groups, for Hydrogenation of Alkynes, Alkenes and Dienes

The current work describes an attempt to synthesize hybrid materials combining porous aromatic frameworks (PAFs) and dendrimers and use them to obtain novel highly active and selective palladium catalysts. PAFs are carbon porous materials with rigid aromatic structure and high stability, and the dendrimers are macromolecules which can effectively stabilize metal nanoparticles and tune their activity in catalytic reactions. Two porous aromatic frameworks, PAF-20 and PAF-30, are modified step-by-step with diethanolamine and hydroxyl groups at the ends of which are replaced by new diethanolamine molecules. Then, palladium nanoparticles are applied to the synthesized materials. Properties of the obtained materials and catalysts are investigated using X-ray photoelectron spectroscopy, transmission electron microscopy, solid state nuclear magnetic resonance spectroscopy, low temperature N2 adsorption and elemental analysis. The resulting catalysts are successfully applied as an efficient and recyclable catalyst for selective hydrogenation of alkynes to alkenes at very high (up to 90,000) substrate/Pd ratios.

Synthesis and Reactivity of Poly(propyleneimine) Dendrimers Functionalized with Cyclopentadienone N-Heterocyclic-Carbene Ruthenium(0) Complexes

Ligand design in metal chemistry is a fundamental step when pursuing compounds with specific reactivity. In this paper, the functionalization of the OH group in the lateral chain of the N-heterocyclic-carbene (NHC) ligand bound to a bis-carbonyl cyclopentadienone NHC ruthenium(0) complex allowed the decoration of five generations of poly(propyleneimine) (PPIs) dendrimers with up to 64 organometallic moieties. The coupling was achieved by employing carbonyldiimidazole and the formation of carbamate linkages between dendritic peripheral NH2 and lateral OH groups on ruthenium complexes. The synthetic procedure, chemical purification, and spectroscopic characterization of the five generations of dendrimers (3g1–5) are here described. The ruthenium-modified dendrimers were activated as catalysts in the transfer hydrogenation of the model compound 4-fluoroacetophenone in the presence of cerium ammonium nitrate as their mononuclear congeners. The catalytic activity, being similar for the five generations, shows a decrease if compared to mononuclear complexes. This detrimental effect might be ascribed to the –CH2NH– functionalization, largely present in dendrimer skeleton and that can compete with the hydrogen transfer mechanism, but also partially to a dendritic effect caused by steric encumbrance.

The Selective Oxidation of Sulfides to Sulfoxides or Sulfones with Hydrogen Peroxide Catalyzed by a Dendritic Phosphomolybdate Hybrid

The oxidation of sulfides to their corresponding sulfoxides or sulfones has been achieved using a low-cost poly(amidoamine) with a first-generation coupled phosphomolybdate hybrid as the catalyst and aqueous hydrogen peroxide as the oxidant. The reusability of the catalyst was revealed in extensive experiments. The practice of this method in the preparation of a smart drug Modafinil has proved its good applicability.

Dendrimer-Encapsulated Pd Nanoparticles, Immobilized in Silica Pores, as Catalysts for Selective Hydrogenation of Unsaturated Compounds

Heterogeneous Pd-containing nanocatalysts, based on poly (propylene imine) dendrimers immobilized in silica pores and networks, obtained by co-hydrolysis in situ, have been synthesized and examined in the hydrogenation of various unsaturated compounds. The catalyst activity and selectivity were found to strongly depend on the carrier structure as well as on the substrate electron and geometric features. Thus, mesoporous catalyst, synthesized in presence of both polymeric template and tetraethoxysilane, revealed the maximum activity in the hydrogenation of various styrenes, including bulky and rigid stilbene and its isomers, reaching TOF values of about 230000 h-1. Other mesoporous catalyst, synthesized in the presence of polymeric template, but without addition of Si(OEt)4, provided the trans-cyclooctene formation with the selectivity of 90-95 %, appearing as similar to homogeneous dendrimer-based catalysts. Microporous catalyst, obtained only on the presence of Si(OEt)4, while dendrimer molecules acting as both anchored ligands and template, demonstrated the maximum activity in the hydrogenation of terminal linear alkynes and conjugated dienes, reaching TOF values up to 400000 h-1. Herein the total selectivity on alkene in the case of terminal alkynes and conjugated dienes reached 95-99 % even at hydrogen pressure of 30 atm. The catalysts synthesized can be easily isolated from reaction products and recycled without significant loss of activity.

Energy Transfer in Dendritic Systems Having Pyrene Peripheral Groups as Donors and Different Acceptor Groups

In this feature article, a specific overview of resonance energy transfer (FRET) in dendritic molecules was performed. We focused mainly on constructs bearing peripheral pyrene groups as donor moieties using different acceptor groups, such as porphyrin, fullerene C60, ruthenium-bipyridine complexes, and cyclen-core. We have studied the effect of all the different donor-acceptor pairs in the energy transfer efficiency (FRET). In all cases, high FRET efficiency values were observed.

New dendritic ionic liquids (DILs) for the extraction of metallic species from water

Four dendritic ionic liquids (DILs) have been easily synthesized from 3rd generation PAMAM and PPI: PAMAM G3 NH₃⁺ Tf₂N⁻ (1), PPI G3 NH₃⁺ Tf₂N⁻ (2), PPI G3 NH₂ Me⁺ BF₄⁻ (3) and PAMAM G3 NH₂ Me⁺ BF₄⁻ (4).

Windmill-shaped octanuclear Zn/Ln (LnIII = Dy, Tb, Ho) heterometallic ensembles supported by a tetraferrocene scaffold

Utilizing a new ferrocene-based compartmental ligand, H₄L (1), a series of novel heterometallic complexes [{LZn(μ-OAc)Dy}₄(μ₄-H₂O)] (2), [{LZn(μ-OAc)Tb}₄(μ₄-H₂O)] (3), [{LZn(μ-OAc)Ho}₄(μ₄-H₂O)] (4), [L = Fe[(C₅H₄){-C(Me)[double bond, length as m-dash]N-N[double bond, length as m-dash]C₆H₃-(o-O)(m-O)}]₂] were synthesized and characterized. 2 and 3 crystallize in the monoclinic crystal system in the I2/m space group, whereas 4 crystallizes in the tetragonal crystal system in the I4/m space group. The tetra deprotonated ligand L^4- has two distinct coordination compartments: one pocket (2N, 2O) suitable for the transition metal (3d) ions and another pocket (4O) suitable for lanthanide (4f) metal ions. Additionally, the terminal phenoxo group can be utilized for cluster expansion. In all the complexes, the Zn^II ion is in a perfect square pyramidal (2N, 3O) geometry whereas the lanthanide ion has a coordination number of eight (8O) in a distorted biaugmented trigonal-prism geometry. The electrochemical properties of 2 and 3 along with ligand H₄L (1) were studied by cyclic voltammetry (CV). All the complexes display a similar type of electrochemical behavior viz., one quasi-reversible oxidation typical of a ferrocene/ferrocenium motif. The magnetic properties of all the complexes have also been investigated.

Recent Advances in Recoverable Systems for the Copper-Catalyzed Azide-Alkyne Cycloaddition Reaction (CuAAC)

The explosively-growing applications of the Cu-catalyzed Huisgen 1,3-dipolar cycloaddition reaction between organic azides and alkynes (CuAAC) have stimulated an impressive number of reports, in the last years, focusing on recoverable variants of the homogeneous or quasi-homogeneous catalysts. Recent advances in the field are reviewed, with particular emphasis on systems immobilized onto polymeric organic or inorganic supports.

When CuAAC 'Click Chemistry' goes heterogeneous

Within the green chemistry context, heterogeneous catalysis is more and more applied to organic synthesis. The well known ‘click chemistry’ and especially its flagship, the copper-catalyzed azide–alkyne cycloaddition reaction (CuAAC), is now catch up by such heterogenisation process and copper ions or metals have been grafted or deposited on or into various solids, such as (bio)polymers, charcoal, silica, zeolites, POM or MOF.

Phase transition behaviours of a single dendritic polymer

Dendritic polymers with highly branching structures exhibit many unique properties. In this paper, a computational study using the Wang-Landau sampling technique is carried out to reveal the phase transition behaviours of dendritic homopolymers with various branching structures. Two types of dendritic homopolymers, dendrimers/dendrigrafts (D/D) and hyperbranched (HB) polymers are studied. It is found that with increasing degree of branching in the dendritic polymer, the liquid-solid (LS) transition temperature increases and the coil-globule (CG) transition becomes weak. Additionally, under similar degrees of branching and polymerization, D/D polymers have a higher LS transition temperature than HB polymers. The reason is that the D/D polymers have greater regularity in the radial distribution of the branching units, which facilitates monomer packing during the LS transition. The distinctive internal unit distribution at various temperatures is quantitatively analysed. Our results show the importance of dendritic polymer structure regularity in phase transition behaviours and are valuable in guiding the structural design of dendritic macromolecules for functionalization applications.

Dendritic phosphotungstate hybrids efficiently catalyze the selective oxidation of alcohols with H2O2

Dendritic phosphotungstate hybrids are highly efficient and recyclable catalysts for the selective oxidation of alcohols with H₂O₂.

Guanosine and isoguanosine derivatives for supramolecular devices

Guanosine (G) and isoguanosine (isoG) derivatives can self-assemble, yielding supramolecules that have found broad applications in diverse fields.

Dendrimers - from organic synthesis to pharmaceutical applications: an update

Dendrimers are a relatively new class of monodisperse polymers, which have tree-like spherical structures with well-defined sizes and shapes. Their unique structure has a significant impact on their physical and chemical properties. Research on dendrimers is of significant interest to scientists from all areas and their utility in various scientific fields, including pharmaceuticals, is expanding. The present review is comprehensive and covers different aspects of dendrimers viz. (1) synthesis, (2) properties and (3) pharmaceutical applications. The emphasis is on their applications as well as the current ongoing research status for drug targeting.

Nanostructured materials for applications in heterogeneous catalysis

In this review, a brief survey is offered on the main nanotechnology synthetic approaches available to heterogeneous catalysis, and a few examples are provided of their usefulness for such applications. We start by discussing the use of colloidal, reverse micelle, and dendrimer chemistry in the production of active metal and metal oxide nanoparticles with well-defined sizes, shapes, and compositions, as a way to control the surface atomic ensembles available for selective catalysis. Next we introduce the use of sol-gel and atomic layer deposition chemistry for the production and modification of high-surface-area supports and active phases. Reference is then made to the more complex active sites that can be created or carved on such supports by using organic structure-directing agents. We follow with an examination of the ability to achieve multiple functionality in catalysis via the design of dumbbells, core@shell, and other complex nanostructures. Finally, we consider the mixed molecular-nanostructure approach that can be used to develop more demanding catalytic sites, by derivatizing the surface of solids or tethering or immobilizing homogeneous catalysts or other chemical functionalities. We conclude with a personal and critical perspective on the importance of fully exploiting the synergies between nanotechnology and surface science to optimize the search for new catalysts and catalytic processes.

Acyclic diene metathesis polymerization and heck polymer-polymer conjugation for the synthesis of star-shaped block copolymers

Three- and four-arm star shaped polymers, as well as diblock copolymers, are synthesized via acyclic diene metathesis (ADMET) polymerization. This is accomplished by using an asymmetric α,ω-diene containing a terminal double bond and an acrylate, which is polymerized in the presence of multifunctional acrylates as selective and irreversible chain transfer agents using Hoveyda-Grubbs second generation catalyst. High cross-metathesis selectivities are achieved at low temperatures enabling good control over molecular weights. Furthermore, additional polyethyleneglycol (PEG) blocks are attached to these polymers via Heck coupling of the acrylate end-groups of these polymers with aryl iodide functionalized PEG, obtaining three- and four-arm star shaped di- and triblock copolymers with molecular weights up to 31 kDa.

Aqueous reductive amination using a dendritic metal catalyst in a dialysis bag

Water-soluble dendritic iridium catalysts were synthesized by attaching a reactive metal complex to DAB-Am dendrimers via an adapted asymmetric bipyridine ligand. These dendritic catalysts were applied in the aqueous reductive amination of valine while contained in a dialysis bag. Comparable conversions were observed as for the noncompartmentalized counterparts, albeit with somewhat longer reaction times. These results clearly show that the encapsulated catalyst system is suitable to successfully drive a complex reaction mixture with various equilibrium reactions to completion.