2,2′‐Bipyrimidine as Efficient Sensitizer of the Solid‐State Luminescence of Lanthanide and Uranyl Ions from Visible to Near‐Infrared

Treatment of Ln(NO₃)₃?nH₂O with 1 or 2 equiv 2,2′‐bipyrimidine (BPM) in dry THF readily afforded the monometallic complexes [Ln(NO₃)₃(bpm)₂] (Ln=Eu, Gd, Dy, Tm) or [Ln(NO₃)₃(bpm)₂]?THF (Ln=Eu, Tb, Er, Yb) after recrystallization from MeOH or THF, respectively. Reactions with nitrate salts of the larger lanthanide ions (Ln=Ce, Nd, Sm) yielded one of two distinct monometallic complexes, depending on the recrystallization solvent: [Ln(NO₃)₃(bpm)₂]?THF (Ln=Nd, Sm) from THF, or [Ln(NO₃)₃(bpm)(MeOH)₂]?MeOH (Ln=Ce, Nd, Sm) from MeOH. Treatment of UO₂(NO₃)₂?6H₂O with 1 equiv BPM in THF afforded the monoadduct [UO₂(NO₃)₂(bpm)] after recrystallization from MeOH. The complexes were characterized by their crystal structure. Solid‐state luminescence measurements on these monometallic complexes showed that BPM is an efficient sensitizer of the luminescence of both the lanthanide and the uranyl ions emitting visible light, as well as of the Yb^III ion emitting in the near‐IR. For Tb, Dy, Eu, and Yb complexes, energy transfer was quite efficient, resulting in quantum yields of 80.0, 5.1, 70.0, and 0.8 %, respectively. All these complexes in the solid state were stable in air.     

Unraveling gold(I)-specific action towards peptidic disulfide cleavage: a DFT investigation

Ground-state disulfide dissociation is a target of prime importance in structural biochemistry. A main difficulty consists in avoiding competition with carbon–sulfur and backbone scission pathways. In tandem mass spectrometry, such selectivity is afforded using transition elements or coinage-metal ions as catalyst. Yet, the underlying gas-phase mechanistic details remain poorly understood. Gold(I)-assisted disulfide cleavage is investigated by means of DFT calculations, to elucidate the highly selective and specific catalytic action of this transition-metal cation, a most promising one in tandem mass spectrometry. The preferential cleavage of sulfur–sulfur versus carbon–sulfur linkages on dimethyldisulfide, taken as a prototypical aliphatic compound, is rationalized on the basis of molecular orbital arguments. Secondly, it is revealed that the disulfide dissociation profile is dramatically impacted by a peptidic environment. Calculations on L,L-cystine derivatives show two main factors: the topological frustration for an embedded -CH(2)-S-S-CH(2)- motif induces a 5 kcal mol(-1) penalty, whereas electrophilic assistance via complexation of nitrogen and oxygen atoms lowers activation barriers by a factor of 3. S-S weakening is both thermodynamically and kinetically driven by the versatile coordination mode of gold(I). The influence of amine-terminus group protonation is finally sketched: it gives rise to an intermediate reactivity. This study sheds lights on the key action of the peptidic environment in tuning the dissociation profile in the presence of this transition-metal monocation.     

Unravelling the mechanism of glycerol hydrogenolysis over rhodium catalyst through combined experimental-theoretical investigations

We report herein a detailed and accurate study of the mechanism of rhodium-catalysed conversion of glycerol into 1,2-propanediol and lactic acid. The first step of the reaction is particularly debated, as it can be either dehydration or dehydrogenation. It is expected that these elementary reactions can be influenced by pH variations and by the nature of the gas phase. These parameters were consequently investigated experimentally. On the other hand, there was a lack of knowledge about the behaviour of glycerol at the surface of the metallic catalyst. A theoretical approach on a model Rh(111) surface was thus implemented in the framework of density functional theory (DFT) to describe the above-mentioned elementary reactions and to calculate the corresponding transition states. The combination of experiment and theory shows that the dehydrogenation into glyceraldehyde is the first step for the glycerol transformation on the Rh/C catalyst in basic media under He or H(2) atmosphere.     

Surface properties of new virginiamycin M1 derivatives

Three kinds of derivatives of the M₁ factor of virginiamycin have been synthesised: esters with long chain fatty acids, oximes with modified polar amino acids and bis-derivatives with both the ester and oxime function. The study of the surface tension time dependence of M₁ and its derivatives has shown that it is necessary to enhance simultaneously the hydrophobicity and the hydrophilicity of M₁ to render M₁ surface-active. A structure/function relationship study of the surface-active bis-derivatives has shown that enhancing the hydrophobicity of the molecule led to slower adsorption kinetics, higher stability of the monolayers formed and a better capacity to penetrate a membrane model. The repulsive electrostatic forces due to the presence of charges on the amino acids linked to M₁ lead to higher surface tensions, a greater molecular area at the interface and lower penetration into a membrane model.This study has demonstrated that modifying systematically the hydrophobicity and hydrophilicity of a non surface-active molecule allows the production of surface-active derivatives.     

Stereoselective synthesis of 13Z retinoic acids via β-methylenealdehydes as synthons

A stereoselective synthesis of 13Z retinoic acids via β-methylenealdehydes is described. In methylene-de-oxo-bisubstitution reactions (Knoevenagel, Stobbe, etc.), these new synthons produce stereoselectively E olefins. Hence, a synthesis of 13Z retinoic acids is described, via a stereospecific monodecarboxylation of carboxy-14-retinoic acids.     

Synthesis of 2,3-anti-3,4-anti and 2,3-anti-3,4-syn propionate motifs: a diastereoselective tandem sequence of Mukaiyama and free-radical-based hydrogen transfer reactions

[reaction: see text] Reported herein is a strategy employing a Mukaiyama reaction in tandem with a hydrogen transfer reaction for the elaboration of 2,3-anti-3,4-anti and 2,3-anti-3,4-syn propionate motifs. The mode of complexation is controlled through monodentate or chelate pathways for the Mukaiyama reaction to give access to either syn or anti aldol products, precursors of the free-radical reduction reaction. Boron Lewis acid is used to control the free-radical reaction through the exocyclic pathway.     

A dinuclear uranium(IV) complex of the chelating ligand 1,2,3,4‐tetra­methyl‐5‐(2‐pyridyl)­cyclo­penta­diene

The ligand 1,2,3,4‐tetra­methyl‐5‐(2‐pyridyl)­cyclo­penta­diene (cp*py) forms a dinuclear complex with U^IV, i.e. di‐μ‐oxo‐bis­{chloro­(diethyl ether‐κO)[(η⁵,κN)‐1,2,3,4‐tetra­methyl‐5‐(2‐pyridyl)­cyclo­penta­dienyl]uranium(IV)}, [U₂Cl₂O₂(C₁₄H₁₆N)₂(C₄H₁₀O)₂], in which cp*py acts as a chelating ligand, being bound to the metal atom by the cyclo­penta­dienyl unit and also by the N atom of the pyridyl ring.     

High‐Pressure Raman and Calorimetry Studies of Vanadium(III) Alkyl Hydrides for Kubas‐Type Hydrogen Storage

Reversible hydrogen storage under ambient conditions has been identified as a major bottleneck in enabling a future hydrogen economy. Herein, we report an amorphous vanadium(III) alkyl hydride gel that binds hydrogen through the Kubas interaction. The material possesses a gravimetric adsorption capacity of 5.42 wt % H₂ at 120 bar and 298 K reversibly at saturation with no loss of capacity after ten cycles. This corresponds to a volumetric capacity of 75.4 kgH₂ m^?3. Raman experiments at 100 bar confirm that Kubas binding is involved in the adsorption mechanism. The material possesses an enthalpy of H₂ adsorption of +0.52 kJ mol^?1 H₂, as measured directly by calorimetry, and this is practical for use in a vehicles without a complex heat management system.     

Synthesis of 3-methoxyazetidines via an aziridine to azetidine rearrangement and theoretical rationalization of the reaction mechanism

The synthetic utility of N-alkylidene-(2,3-dibromo-2-methylpropyl)amines and N-(2,3-dibromo-2-methylpropylidene)benzylamines was demonstrated by the unexpected synthesis of 3-methoxy-3-methylazetidines upon treatment with sodium borohydride in methanol under reflux through a rare aziridine to azetidine rearrangement. These findings stand in contrast to the known reactivity of the closely related N-alkylidene-(2,3-dibromopropyl)amines, which are easily converted into 2-(bromomethyl)aziridines under the same reaction conditions. A thorough insight into the reaction mechanism was provided by both experimental study and theoretical rationalization.     

Synthesis of dodecavalent fullerene-based glycoclusters and evaluation of their binding properties towards a bacterial lectin

Multivalency is playing a major role in biological processes and particularly in lectin-carbohydrate interactions. The design of high-affinity ligands of lectins should provide molecules capable of interfering with these biological processes and potentially inhibit bacterial or viral infections. Azide-alkyne "click" chemistry was applied to the synthesis of dodecavalent fullerene-based glycoclusters. The conjugation could be efficiently performed from alkyne or azide functions on either partners (i.e. hexakis-fullerene adduct or glycoside). PA-IL is a bacterial lectin from the opportunistic pathogen Pseudomonas aeruginosa and is involved in the recognition of glycoconjugates on human tissues. The glycoclusters obtained were evaluated as ligands of PA-IL and for their potential for competing with its binding to glycosylated surfaces. The affinities measured by hemagglutination inhibition assay (HIA), enzyme-linked lectin assay (ELLA), and surface plasmon resonance (SPR) displayed a significant "glycoside cluster effect" with up to a 12,000-fold increase in binding when comparing a monovalent carbohydrate reference probe with a dodecavalent fullerene-based glycocluster, albeit with some differences depending on the analytical technique.