Laurent Plasseraud

CATALYTIC HYDROGENATION OF BENZENE DERIVATIVES UNDER BIPHASIC CONDITIONS

The catalytic hydrogenation of various benzene derivatives was studied, using the salt [(η6-C6H6)4Ru4H4]Cl2 in aqueous solution as the catalyst precursor. Under these biphasic conditions, the corresponding cyclohexane derivatives are obtained with catalytic turnover rates of more than 100 cycles per hour. In the case of the hydrogenation of the parent benzene, the NMR analysis of the aqueous phase revealed the presence of the hexahydrido species [(η6-C6H6)4Ru4H6]2+ together with the tetrahydrido species [(η6-C6H6)4Ru4H4]2+ under hydrogen pressure. An exchange study of the reaction of the para-cymene analogue [(η6-C6H4MePri-p)4Ru4H4]2+ with benzene showed the four arene ligands in the tetrahydrido cluster to be successively replaced by other aromatic ligands under catalytic conditions.

Organo-catalyzed synthesis of aliphatic polycarbonates in solvent-free conditions

A new efficient and expeditious route to the synthesis of aliphatic polycarbonates, in solvent-free conditions and using 1-n-butyl-3-methylimidazolium-2-carboxylate (BMIM-2-CO2) as a catalyst precursor, is described. The protocol consists of a two-step polymerization process involving the transesterification of dimethyl carbonate (DMC) with linear alkane diols and leading to high molecular weight homopolymers. The reaction went to completion quantitatively with the liberation of methanol as the only by-product. The in situ formation of N-heterocyclic carbene species resulting from BMIM-2-CO2 decarboxylation is suggested to be a key feature of the condensation process. The protocol was then applied to the copolymerization of diverse diols leading to the synthesis of polycarbonates with average segment lengths and random sequences.

Insertion reaction of carbon dioxide into Sn–OR bond. Synthesis, structure and DFT calculations of di- and tetranuclear isopropylcarbonato tin(IV) complexes

The reaction of carbon dioxide with the stannane nBu2Sn(OiPr)2 and distannoxane [nBu2(iPrO)Sn]2O leads to the selective insertion into one Sn-OiPr bond generating the corresponding nBu2Sn(OiPr)(OCO2(i)Pr) and nBu2(iPrO)SnOSn(OCO2(i)Pr)nBu2 species. Both compounds are characterised by multinuclear NMR, FT-IR and single-crystal X-ray crystallography. In the solid state, they adopt a dimeric arrangement with bridging isopropoxy and terminal isopropylcarbonato ligands. The X-ray crystal structure of the dinuclear stannane shows that the Sn2O2 ring and the two Sn-OCO2C fragments are nearby coplanar. The same holds for the ladder-type tetranuclear distannoxane. The dimeric structures are also evidenced by solution NMR in non-coordinating solvents. Interestingly, the assignment of the exo and endo tin resonances of the dimeric distannoxane is unambiguous using a labeled 13CO2 experiment. The stability of the dimeric association has been probed in the stannane series on the basis of DFT calculations.

Catalysis in aqueous solution: Hydrogenation of benzene derivatives catalysed by (η6-C6H6)2Ru2Cl4

Abstract The catalytic hydrogenation of various benzene derivatives was studied, using (η6-C6H6)2Ru2Cl4 in aqueous solution as the catalyst precursor. Under biphasic conditions, the corresponding cyclohexane derivatives are obtained with catalytic turnover rates which vary, depending on the substrate, from 20 to 2000 cycles per h. After a catalytic run, the aqueous solution contains the two tetranuclear cations [(η6-C6H6)4Ru4H4]2+ and [(η6-C6H6)4Ru4H6]2+ which are known to catalyse the hydrogenation of aromatic compounds, but the activity of which is considerably lower than that of the (η6-C6H6)2Ru2Cl4 precursor. An intermediate, presumably the more active species, was detected by 1 H -NMR spectroscopy under catalytic conditions and identified as the trinuclear cluster cation [Ru3(η6-C6H6)3(μ2-Cl)(μ3-O)(μ2-H)2]+.

Tin‐Based Mesoporous Silica for the Conversion of CO2 into Dimethyl Carbonate

Sn-based SBA-15 was prepared by reacting di-n-butyldimethoxystannane with SBA-15 pretreated with trimethylchlorosilane (TMCS) to cap the external hydroxyl groups. Small-angle X-ray diffraction (SXRD), infrared spectroscopy (IR), nitrogen adsorption/desorption, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and inductively coupled plasma atomic emission (ICP-AES) measurements allow us to propose that the organotin species are located within the pore channels of the mesoporous host. This novel material catalyzes selectively the coupling of CO(2) with methanol to dimethyl carbonate (DMC). The reaction time-conversion dependence shows that a turnover number (TON) of 16 can be reached at 423 K under 20 MPa, which is among the highest reported so far in the absence of water traps. Moreover, as the catalytic activity is retained after recycling, even higher values can be obtained on a cumulative basis. A further TON increase is observed with the reaction temperature. Interestingly, the tin-based SBA-15 mesoporous material exhibits lower TONs if the TMCS pretreatment is left out. Therefore, the organotin species located outside the channels are far less active than those located within.

The skeleton of the staghorn coral Acropora millepora: molecular and structural characterization

The scleractinian coral Acropora millepora is one of the most studied species from the Great Barrier Reef. This species has been used to understand evolutionary, immune and developmental processes in cnidarians. It has also been subject of several ecological studies in order to elucidate reef responses to environmental changes such as temperature rise and ocean acidification (OA). In these contexts, several nucleic acid resources were made available. When combined to a recent proteomic analysis of the coral skeletal organic matrix (SOM), they enabled the identification of several skeletal matrix proteins, making A. millepora into an emerging model for biomineralization studies. Here we describe the skeletal microstructure of A. millepora skeleton, together with a functional and biochemical characterization of its occluded SOM that focuses on the protein and saccharidic moieties. The skeletal matrix proteins show a large range of isoelectric points, compositional patterns and signatures. Besides secreted proteins, there are a significant number of proteins with membrane attachment sites such as transmembrane domains and GPI anchors as well as proteins with integrin binding sites. These features show that the skeletal proteins must have strong adhesion properties in order to function in the calcifying space. Moreover this data suggest a molecular connection between the calcifying epithelium and the skeletal tissue during biocalcification. In terms of sugar moieties, the enrichment of the SOM in arabinose is striking, and the monosaccharide composition exhibits the same signature as that of mucus of acroporid corals. Finally, we observe that the interaction of the acetic acid soluble SOM on the morphology of in vitro grown CaCO3 crystals is very pronounced when compared with the calcifying matrices of some mollusks. In light of these results, we wish to commend Acropora millepora as a model for biocalcification studies in scleractinians, from molecular and structural viewpoints.

The cluster dication [H6Ru4(C6H6)4]2+ revisited: the first cluster complex containing an intact dihydrogen ligand?

Abstract A low-temperature 1H-NMR study suggests the tetranuclear cluster dication [H6Ru4(C6H6)4]2+ (1) to contain an H2 ligand that undergoes, upon warming of the solution, an intramolecular exchange with the four hydride ligands at the Ru4 framework. Whereas two of the three NMR signals at −120°C in the hydride region show T1 values in the range 200–300 ms, the least deshielded resonance at δ=−17.33 ppm exhibits a T1 value of only 34 ms, characteristic of an H2 ligand. A re-examination of the single-crystal X-ray structure analysis of the chloride salt of 1 supports this interpretation by a short distance of 1.14(0.15) A between two hydrogen atoms coordinated as a HH ligand in a side-on fashion to one of the triangular faces of the Ru4 tetrahedron. The distance between one of the two hydrogen atoms of the H2 ligand and one of the four hydride ligands is also very short [1.33(0.15) A], suggesting an additional H2⋯H interaction. The presence of this H3 unit over one of the three Ru3 faces in 1 may explain the deformation of the Ru4 skeleton from the expected tetrahedral symmetry. Density functional theory (DFT) calculations on 1 indicate a very soft potential energy surface associated with the respective displacement of the three interacting cofacial hydrogen atoms. In accordance with these results, the cluster dication 1 tends to loose molecular hydrogen to form the cluster dication [H4Ru4(C6H6)4]2+ (2). The equilibrium between 1 and 2 can be used for catalytic hydrogenation reactions.

Hexacatenar liquid-crystalline complexes of palladium(II) and platinum(II) based on trialkoxystilbazole esters

The synthesis and characterisation of 4-(3′,4′,5′-trialkoxybenzoyloxy)pyridines (1a–e), and of their corresponding palladium(II), (2a–e), and platinum(II), (3a–e), complexes are described. The pyridine-based ligands are not mesomorphic, but upon complexation to PdCl2 or PtCl2, new hexacatenar mesogens are formed which show exclusively the hexagonal columnar mesophase. The mesomorphic behaviour of the complexes was characterised by polarised optical microscopy, differential scanning calorimetry and X-ray diffraction. The metal seems to influence the crystal phase and mesophase stability as well as the mesomorphic temperature range.

[(η6-p-PriC6H4Me)2Ru2Mo2O6(OMe)4]: a new tetranuclear mixed-metal oxo cluster presenting a cube-based chair structure

Abstract The title complex was obtained by reaction of [(η6-p-PriC6H4Me)4Ru4Mo4O16] with methanol in the presence of p-hydroquinone. It contains an Mo2Ru2O2(OMe)4 core consisting of two Mo2RuO(OMe)3 half-cubes fused together to form a chair-like structure.

Nautilin‐63, a novel acidic glycoprotein from the shell nacre of Nautilus macromphalus

In molluscs, and more generally in metazoan organisms, the production of a calcified skeleton is a complex molecular process that is regulated by the secretion of an extracellular organic matrix. This matrix constitutes a cohesive and functional macromolecular assemblage, containing mainly proteins, glycoproteins and polysaccharides that, together, control the biomineral formation. These macromolecules interact with the extruded precursor mineral ions, mainly calcium and bicarbonate, to form complex organo‐mineral composites of well‐defined microstructures. For several reasons related to its remarkable mechanical properties and to its high value in jewelry, nacre is by far the most studied molluscan shell microstructure and constitutes a key model in biomineralization research. To understand the molecular mechanism that controls the formation of the shell nacreous layer, we have investigated the biochemistry of Nautilin‐63, one of the main nacre matrix proteins of the cephalopod Nautilus macromphalus. After purification of Nautilin‐63 by preparative electrophoresis, we demonstrate that this soluble protein is glycine‐aspartate‐rich, that it is highly glycosylated, that its sugar moieties are acidic, and that it is able to bind chitin in vitro. Interestingly, Nautilin‐63 strongly interacts with the morphology of CaCO3 crystals precipitated in vitro but, unexpectedly, it exhibits an extremely weak ability to inhibit in vitro the precipitation of CaCO3. The partial resolution of its amino acid sequence by de novo sequencing of its tryptic peptides indicates that Nautilin‐63 exhibits short collagenous‐like domains. Owing to specific polyclonal antibodies raised against the purified protein, Nautilin‐63 was immunolocalized mainly in the intertabular nacre matrix. In conclusion, Nautilin‐63 exhibits ‘hybrid’ biochemical properties that are found both in the soluble and insoluble proteins, rendering it difficult to classify according to the standard view on nacre proteins.