Crystallization mechanisms of acicular crystals

In this contribution, we present an experimental investigation of the growth of four different organic molecules produced at industrial scale with a view to understand the crystallization mechanism of acicular or needle-like crystals. For all organic crystals studied in this article, layer-by-layer growth of the lateral faces is very slow and clear, as soon as the supersaturation is high enough, there is competition between growth and surface-activated secondary nucleation. This gives rise to pseudo-twinned crystals composed of several needle individuals aligned along a crystallographic axis; this is explained by regular over- and inter-growths as in the case of twinning. And when supersaturation is even higher, nucleation is fast and random.In an industrial continuous crystallization, the rapid growth of needle-like crystals is to be avoided as it leads to fragile crystals or needles, which can be partly broken or totally detached from the parent crystals especially along structural anisotropic axis corresponding to weaker chemical bonds, thus leading to slower growing faces. When an activated mechanism is involved such as a secondary surface nucleation, it is no longer possible to obtain a steady state. Therefore, the crystal number, size and habit vary significantly with time, leading to troubles in the downstream processing operations and to modifications of the final solid-specific properties.These results provide valuable information on the unique crystallization mechanisms of acicular crystals, and show that it is important to know these threshold and critical values when running a crystallizer in order to obtain easy-to-handle crystals.     

Investigation of the surface reactivity of a sol-gel derived glass in the ternary system SiO2-CaO-P2O5

A new glass formulation, with the molar composition 60% SiO₂-35% CaO-5% P₂O₅, was synthesized using the sol-gel process, for applications as biomaterial in orthopaedic or maxillo facial surgery. Pellets, made of glass powder, were uniaxially compacted and soaked in simulated body fluid (SBF) for up to 7 days at 37 °C to evaluate glass bioactivity. Ionic exchanges at the interface glass-SBF were evaluated by studying evolutions of calcium, phosphorus and silicon concentrations in SBF using ICP-OES. Changes in glass surface, and the formation of crystalline phases were analyzed using XRD, SEM, EDS and FTIR methods.Results form ICP-OES showed a high reactivity of the glass surface with a very high and continuous release of calcium, a limited glass dissolution and an uptake of phosphorous from SBF. Results from both FTIR and XRD analysis indicated that the glass surface was progressively covered by two different phases: CaCO₃ as calcite and a carbonated apatite layer. The formation of these phases, following two different schemas, was observed after 2 h of immersion and confirmed after 7 days. SEM micrographs and EDS analysis demonstrated that the main phase, a carbonated apatite, was present as micro-spheroids and the secondary phase, calcite, was materialized by agglomerates which have diameters up to 10-15 μm. These results are in accordance with a bioactive feature of the glass studied.     

On the Development of Phenol-Formaldehyde Resins Using a New Type of Lignin Extracted from Pine Wood with a Levulinic-Acid Based Solvent

Resole resins have many applications, especially for foam production. However, the use of phenol, a key ingredient in resoles, has serious environmental and economic disadvantages. In this work, lignin extracted from pine wood using a “green” solvent, levulinic acid, was used to partially replace the non-sustainable phenol. The physicochemical properties of this novel resin were compared with resins composed of different types of commercial lignins. All resins were optimized to keep their free formaldehyde content below 1 wt%, by carefully adjusting the pH of the mixture. Substitution of phenol with lignin generally increases the viscosity of the resins, which is further increased with the lignin mass fraction. The addition of lignin decreases the kinetics of gelification of the resin. The type and amount of lignin also affect the thermal stability of the resins. It was possible to obtain resins with higher thermal stability than the standard phenol-formaldehyde resins without lignin. This work provides new insights regarding the development of lignin-based resoles as a very promising sustainable alternative to petrol-based resins.     

Stabilization of miniemulsion droplets by cerium oxide nanoparticles: a step toward the elaboration of armored composite latexes

Stable methyl methacrylate (MMA) miniemulsions were successfully prepared using for the first time cerium oxide (CeO(2)) nanoparticles as solid stabilizers in the absence of any molecular surfactant. The interaction between MMA droplets and CeO(2) nanoparticles was induced by the use of methacrylic acid (MAA) as a comonomer. Both MAA and CeO(2) contents played a key role on the diameter and the stability of the droplets formed during the emulsification step. Cryo-transmission electron microscopy (TEM) images of the suspensions formed with 35 wt % of CeO(2) showed the presence of polydisperse 50-150 nm spherical droplets. More surprisingly, some nonspherical (likely discoidal) objects that could be the result of the sonication step were also observed. The subsequent polymerization of these Pickering miniemulsion droplets led to the formation of composite PMMA latex particles armored with CeO(2). In all cases, the conversion was limited to ca. 85%, concomitant with a loss of stability of the latex for CeO(2) contents lower than 35 wt %. This stability issues were likely related to the screening of the cationic charges present on CeO(2) nanoparticles upon polymerization. TEM images showed mostly spherical particles with a diameter ranging from 100 to 400 nm and homogeneously covered with CeO(2). Besides, for particles typically larger than 200 nm, a buckled morphology was observed supporting the presence of residual monomer at the end of the polymerization and consistent with the limited conversion. The versatility of these systems was further demonstrated using 35 wt % of CeO(2) and replacing MMA by n-butyl acrylate (BA) either alone or in combination with MMA. Stable monomer emulsions were always obtained, with the droplet size increasing with the hydrophobicity of the oil phase, pointing out the key influence of the wettability of the solid stabilizer. The polymerization of Pickering miniemulsion stabilized by CeO(2) nanoparticles proved to be an efficient strategy to form armored composite latex particles which may find applications in coating technology.     

Synthesis and reactivity of an isolable cobalt(I) complex containing a β-diketiminate-based acyclic tetradentate ligand

A model for cobalamin was synthesized using a new monoanionic tetradentate nitrogen donor ligand; 2-(4-tolyl)-1,3-bis(2-isopropylpyridyl)propenediimine (Tol-BDI((2-pp)2)H) (1), which utilizes isopropylpyridines as pendant arms on a β-diketiminate (BDI) backbone. During the synthesis of 1, the rearrangement product, Tol-BDI((2-pp)(4-pp))H (2) was observed. Metalation of 1 with zinc iodide and cobalt chloride yielded the corresponding Tol-BDI((2-pp)2)ZnI (3) and Tol-BDI((2-pp)2)CoCl (4) complexes. The redox properties of 4 in comparison to cobalamin were examined through electrochemical studies. Electrochemical and bulk reduction of complex 4 gave a diamagnetic cobalt(I) complex, Tol-BDI((2-pp)2)Co (5). Reactivity of 5 toward C-X bonds was investigated using methyl iodide and 1-iodo-2-(trimethylsilyl)acetylene, yielding Tol-BDI((2-pp)2)Co(CH(3))I and Tol-BDI((2-pp)2)Co(C(2)Si(CH(3))(3))I respectively. Synthesis and characterization details for these complexes, including the crystal structure of 3, are reported.     

Solvent effect on the singlet excited-state dynamics of 5-fluorouracil in acetonitrile as compared with water

The excited-state dynamics of 5-fluorouracil in acetonitrile has been investigated by femtosecond fluorescence upconversion spectroscopy in combination with quantum chemistry TD-DFT calculations ((PCM/TD-PBE0). Experimentally, it was found that when going from water to acetonitrile solution the fluorescence decay of 5FU becomes much faster. The calculations show that this is related to the opening of an additional decay channel in acetonitrile solution since the dark n/pi* excited state becomes near degenerate with the bright pi/pi* state, forming a conical intersection close to the Franck-Condon region. In both solvents, a S1-S0 conical intersection, governed by the out-of-plane motion of the fluorine atom, is active, allowing an ultrafast internal conversion to the ground state.     

Mapping the cooperativity pathways in spin crossover complexes

Crystal packing energy calculations are applied to the [Fe(PM-L)₂(NCS)₂] family of spin crossover (SCO) complexes (PM-L = 4-substituted derivatives of the N-(2-pyridylmethylene)-4-aminobiphenyl ligand) with the aim of relating quantitatively the cooperativity of observed SCO transitions to intermolecular interactions in the crystal structures. This approach reveals a linear variation of the transition abruptness with the sum of the magnitudes of the interaction energy changes within the first molecular coordination sphere in the crystal structure. Abrupt transitions are associated with the presence of significant stabilising and destabilising changes in intermolecular interaction energies. While the numerical trend established for the PM-L family does not directly extend to other classes of SCO complex in which the intermolecular interactions may be very different, a plot of transition abruptness against the range of interaction energy changes normalised by the largest change shows a clustering of complexes with similar transition abruptness. The changes in intermolecular interactions are conveniently visualised using energy difference frameworks, which illustrate the cooperativity pathways of an SCO transition.

The abruptness of spin crossover (SCO) is related to intermolecular energy changes occurring over the course of an SCO transition. Crossover is abrupt when SCO-induced strain is accommodated synergistically in a few key interactions.     

Synthesis of potentially photoactivatable coumarin derivatives via a 1,3‐dipolar cycloaddition

A copper (1)‐catalyzed 1,3‐dipolar cycloaddition reaction was used to prepare a series of mono and disubstituted 1,2,3‐triazolyl‐coumarins using a 1,3‐cycloaddition (“Click Chemistry”). Starting coumarins were synthesized using classical or modified Pechmann's reaction. The propargyl group was introduced as either propargylether or as a propargylamide. Azides were prepared in a three steps procedure. Cycloaddition products, containing a coumarin and a photoactivatable moiety, were obtained in good yields.     

Spirocyclic Amide Acetal Synthesis by [CpRu]-Catalyzed Condensations of α-Diazo-β-Ketoesters with γ-Lactams

The synthesis of spirocyclic amide acetals (33–93 %) has been achieved through Ru(II)-catalyzed condensations of N-carbamate protected pyrrolidinones with metal carbenes derived from α-diazo-β-ketoesters. Thanks to the mildness of the diazo decomposition conditions induced by a 1 : 1 combination of [CpRu(MeCN)₃][BAr_F] and 1,10-phenanthroline, the formation of the sensitive products is possible. Full characterization of this carbonyl-ylide mediated process is provided by DFT calculations.