Sylvain Nlate

Molecular Batteries: Ferrocenylsilylation of Dendrons, Dendritic Cores, and Dendrimers: New Convergent and Divergent Routes to Ferrocenyl Dendrimers with Stable Redox Activity

The ferrocenylsilylation of the phenol triallyl dendron 2, of the phenol nonaallyl dendron 4, and of the 9-, 27-, 81-, and 243-allyl dendrimers 7-10 (monitored by the disappearance of the signals of the olefinic protons in 1H NMR spectra) has been achieved using ferrocenyldimethylsilane 1 and Karstedts catalyst in diethyl ether at 40 degrees C, yielding the corresponding ferrocenyl dendrons and dendrimers. An alternative convergent synthesis of the nonaferrocenyl dendron 5 was carried out by reaction of the triferrocenyl dendron 2 with a protected triododendron followed by deprotection. Reaction of the nonaferrocenyl dendron 5 with hexakis(bromomethyl)benzene gave the 54-ferrocenyl dendron 6. All the ferrocenyl dendron and dendrimers produce a chemically and electrochemically reversible ferrocenyl oxidation wave at seemingly the same potential. Stable platinum electrodes modified with the high ferrocenyl dendrimers were fabricated. The soluble orange-red ferrocenyl dendrimers can also be oxidized in CH2Cl2 by [NO][PF6] to the insoluble deep blue polyferrocenium dendrimers. For instance, the 243-ferrocenium dendrimer has been characterized by its Mossbauer spectrum, which is of the same type as that of ferrocenium itself. The ferrocenium dendrimers can be reduced without any decomposition back to the ferrocenyl dendrimer, indicating that these multielectronic redoxstable dendrimers behave as molecular batteries.

Dendron-Functionalized Core―Shell Superparamagnetic Nanoparticles: Magnetically Recoverable and Reusable Catalysts for Suzuki C―C Cross-Coupling Reactions

A metallodendron functionalized with dicyclohexyldiphosphino palladium complex was synthesized. The metallodendron was grafted onto core-shell superparamagnetic nanoparticles (gamma-Fe(2)O(3)/polymer, 200-500 nm) to give optimal catalytic reactivity in cross-coupling reactions. The grafted nanoparticles were used as recoverable and reusable catalysts for Suzuki C--C cross-coupling reactions. They showed remarkable reactivity towards iodo- and bromoarenes under mild conditions, and unprecedented reactivity towards chloroarenes. On completion of the catalytic reaction, the catalysts were readily recovered by using a simple magnet to attract the superparamagnetic grafted nanoparticles. Catalysts were recovered more than 25 times with almost no discernable loss of reactivity.

Enantiopure dendritic polyoxometalates: chirality transfer from dendritic wedges to a POM cluster for asymmetric sulfide oxidation.

The quest for new catalytic and highly enantioselective processes is currently one of the most intensively studied areas in chemical synthesis. In this context, chiral polyoxometalates (POMs) have become a topic of recent interest owing to their potential application in medicine and asymmetric catalysis. Among numerous applications of POMs, catalysis is by far the most studied, owing to the enormous versatility that POMs offer in the clean synthesis of fine chemicals. However, few chiral POM compounds for asymmetric catalysis are known. Of those reported, none are based on dendritic structures, although dendritic counter cations are known to play a critical role in determining the properties of anionic POMs. Two main approaches have been developed for the synthesis of chiral POM-based frameworks. The first involves the use of a chiral species (organic moieties or metal complexes) as a chirality transfer agent. The second strategy is based on spontaneous resolution upon crystallization in the absence of any chiral auxiliary, which yields conglomerates. For example, enantiopure hafnium-substituted POMs, which are obtained by spontaneous resolution upon crystallization in the absence of any chiral source have been reported. Recently, two enantiomerically pure 3D chiral POM-based architectures were developed from achiral moieties without any chiral auxiliaACHTUNGTRENNUNGry.[6b–c] However, despite their potential applications in catalysis and separation, enantiopure POM frameworks are still under represented. The search for suitable enantiomerically pure POM-based hybrid materials remains a challenging issue in synthetic chemistry and materials science. To date, chiral dendritic POM systems, as well as the study of chiroptical activity of enantiopure POM-based frameworks in asymmetric catalysis have, to the best of our knowledge, never been reported. Herein, we report the synthesis and characterization of four enantiopure polyoxometalate-cored dendrimers, which contain n-propyl [(R)-(+)-6 and (S)-( )-6] and epoxy [(R)(+)-7 and (S)-( )-7] groups, based on electrostatic interactions between enantiopure dendritic quaternary ammonium ions and an achiral trianionic POM. To the best of our knowledge, these compounds represent the first examples of optically active dendritic POM systems. Importantly, the POM cluster displays chiroptical effects, indicating chirality transfer from the enantiopure dendritic ammonium ions. The optical and chiroptical properties of the n-propyl-terminated POM-cored dendrimers (R)-(+)-6 and (S)-( )-6 have been demonstrated in solution by UV/Vis spectroscopy, circular dichroism (CD) and vibrational circular dichroism (VCD) spectroscopy, as well as fluorimetry. In addition, the use of 6 as a catalyst in the asymmetric oxidation of thioanisole with aqueous H2O2 provides the corresponding optically active sulfoxide with up to 14% enantiomeric excess (ee) as a result of a chirality transfer process from the dendritic wedge to the catalytically active POM unit. Despite the modest enantioselectivity, this proof-of-principle catalytic experiment demonstrates and confirms the transfer of chiroptical properties from organic moieties to a catalytically active POM unit. Interestingly, the catalyst was recovered and reused in three successive cycles without discernable [a] C. Jahier, Dr. M. Cantuel, Dr. N. D. McClenaghan, Dr. T. Buffeteau, Dr. D. Cavagnat, Dr. S. Nlate ISM, UMR CNRS No. 5255, Universit Bordeaux I 351 Cours de la Lib ration, 33405 Talence Cedex (France) Fax: (+33)5-4000-6994 E-mail : s.nlate@ism.u-bordeaux1.fr [b] Dr. F. Agbossou UCCS UMR CNRS N8 8181, Universit de Lille 59652 Villeneuve d’Asq Cedex (France) [c] Dr. M. Carraro, Prof. M. Bonchio Department of Chemical Sciences and ITM-CNR University of Padova, Via Marzolo, 1 35131 Padova (Italy) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200901512.

Metallodendritic Grafted Core–Shell γ‐Fe2O3 Nanoparticles Used as Recoverable Catalysts in Suzuki CC Coupling Reactions

The use of dendritic structures for the grafting of core-shell γ-Fe(2)O(3)/polymer 300 nm superparamagnetic nanoparticles (MNPs) has been performed with four metallodendrons that were functionalized with diphosphinopalladium complexes. The catalytic performance of these nanocatalysts was optimized for the Suzuki C-C cross-coupling reaction. These results demonstrated the importance of optimizing the catalytic efficiency of grafted MNPs by optimizing the dendritic structures and the nature of the peripheral phosphine ligands. All of these nanocatalysts showed remarkable reactivity towards bromoarenes and they were recovered and efficiently reused by magnetic separation with almost no loss of reactivity, even after 25 cycles.

Gold-nanoparticle-cored Polyferrocenyl Dendrimers: Modes of Synthesis and Functions as Exoreceptors of Biologically Important Anions and Re-usable Redox Sensors

Polyamidoferrocenylalkylthiolate-Au nanoparticles (AuNP) and AuNP-cored polyferrocenyl dendrimers have been assembled either by the ligand-substitution method from dodecanethiolate-AuNP (AB3 units) or Brust-type direct synthesis from a 1-1 mixture of dodecanethiol and dendronized thiol (AB9 units). The AuNP-cored polyferrocenyl dendrimers are a new type of dendrimers with a AuNP core. AuNPs containing a nonasilylferrocenyl dendron have been synthesized; they bear respectively 180 and 360 ferrocenyl units at the periphery. These AuNPs selectively recognize the anions H2PO4− and adenosine-5′-triphosphate (ATP2−) with a positive dendritic effect. Recognition is monitored by the appearance of a new wave at a less positive potential in cyclic voltammetry (CV). These AuNP-cored polyferrocenyl dendrimers can form robust modified electrodes that serve as re-usable sensors of ATP2− by simple washing with CH2Cl2.

Peripheral functionalisation of dendrimers with polyoxotungstate complexes assembled by ionic bonding and their use as oxidation catalysts : Influence of the tether length

Two series of dendritic compounds functionalized at the periphery with polyoxometalate (POM) units were synthesized using two synthetic strategies. The first series involved the [CpFe]+-induced functionalization of polymethylarenes, leading to dendrimers in which the dendronic tripod is directly bonded to the arene core. In the second procedure, dendrimers with a spacer group of six atoms between the arene core and the tripod units were synthesized through a coupling reaction between the phenol dendron and bromobenzyl derivatives. These polyallyl dendrimers were functionalized at the periphery to give quaternary poly-ammonium salts. Reactions of the latter with H3PW12O40 in the presence of hydrogen peroxide led to dendrimers containing {PO4[WO(O2)2]4}3− species at the periphery. These compounds are efficient catalysts for the selective oxidation of alkenes, sulfides and alcohols in an aqueous–CDCl3 biphasic system, using hydrogen peroxide as the primary oxidant. The complexes with dendritic wedges directly bonded to the arene core were found to be efficient, epoxidation catalysts that are stable if they are stored under an inert atmosphere, but they do not efficiently catalyze oxidation of alcohols. The dendritic POMs containing spacers, on the other hand, are shown to be more stable in air, they could be recycled and were efficient for the catalytic oxidation of alcohols. Recycling was carried out for the epoxidation reaction using both series of POM catalysts and yielded 50% to 85% of epoxides using the recovered catalysts, the best yields after recycling being obtained with the tris-POM catalyst built from mesitylene and containing spacers (100% conversion; yields in epoxide: first run: 90%; recycled catalyst: 85%). In the case of alcohol oxidation, the tris-POM catalyst was recycled with moderate success (92% conversion; ketone yields: first run: 75%; recycled catalyst: 50%). Among these POM catalysts, dendritic effects on the oxidation yields were found concerning the number of POM units.

Ferrocenylsilylation of dendrons: a fast convergent route to redox-stable ferrocene dendrimers

A 54-ferrocene dendrimer is synthesized by a convergent route and can be used to modify a Pt electrode in CH2Cl2; it can be reversibly oxidized in DMF in a single 54-electron wave (and with NO+).

Efficient strategy to increase the surface functionalization of core–shell superparamagnetic nanoparticles using dendron grafting

Core–shell γ-Fe2O3/polymer 300 nm superparamagnetic nanoparticles, grafted by fluorescent dendrons using a convergent approach, showed an increase in their surface functionalization compared to grafting using a linear analogue.

Encapsulation of Polyoxotungstate into Dendrimers by Ionic Bonding and Their Use As Oxidation Catalyst

A family of 36-armed dendrimers containing six internal amino groups was synthesized and used to incorporate polyoxometalates (POMs) into their structures by ionic bonding. Allyl-terminated dendrimer 17 (with oxidizable end groups) and methylphenyl-terminated dendrimer 18 (with non-oxidizable end groups) were used for these studies. It was found that the electrostatic incorporation of the tri-anionic POM [PO4{WO(O2)2}4]3− into the methylphenyl-terminated dendrimer 18, in an acidic medium, leads to the dendritic POM hybrid 19, bearing two POM units in its structure. In contrast, attempts to encapsulate POMs into allyl-terminated dendrimer 17 gave unsatisfactory results. Indeed, the epoxidation kinetics of the 36-olefinic terminated dendrimer 17 was too slow, and the expected 36-epoxy-dendritic POM framework 20 was not obtained. Lengthening the reaction up to six hours led to the decomposition of POM species. The solubility in organic solvents of the dendritic POM hybrid 19, combined with its NMR and infrared data, indicate that POM units are clearly connected to the dendritic structure. The catalytic performance of this hybrid material in the oxidation of cyclooctene shows that the properties of POM are retained. The POM-encapsulated dendrimer 19 was found to be an effective catalyst in the oxidation of cyclooctene.

2,3,5,6-Tetrakis[3,5-bis(trifluoromethyl)phenoxy]-2,5-bis(dimethylamino)2,3,5,6-tetrabora-1,4-dioxane diethyl ether 0.667-solvate.

The title compound, C(36)H(26)B(4)F(24)N(2)O(6).0.667C(4)H(10)O, has centrosymmetric tetraboradioxane molecules, half each of three of these comprising the asymmetric unit together with a molecule of diethyl ether. Disorder affects most of the CF(3) groups and one ethyl group of the solvent molecule. The B(4)O(2) rings are approximately planar and contain two B atoms with trigonal geometry and two with distorted tetrahedral geometry, the B-O bonds for the four-coordinate B atoms being longer than those for the three-coordinate B atoms. N-H...O hydrogen bonds link two of the crystallographically independent molecules together in chains, while the third molecule forms discrete trimolecular clusters with two solvent molecules via N-H...O hydrogen bonds. This is the first crystallographically characterized example of a tetrabora-dioxane molecule containing both four- and three-coordinate B atoms.