Dynamic processes in hydrido-carbonyl trirhenium clusters containing bridging nitrogen heterocyclic ligands: An NMR investigation

The dynamic processes occurring in the triangular clusters [Re₃(μ-H)₃(μ-pz-κN¹:κN²)(CO)₁₀]⁻ (pz = pyrazolate, 4), [Re₃(μ-H)₂(μ-pydz-κN¹:κN²)(CO)₁₀] ⁻ (pydz = pyridazine, 5) and [Re₃(μ-H)₃(μ-pydz-κN¹:κN²)(CO)₁₀] (6), have been investigated by ¹H and ¹³C NMR. In the pyrazolate derivative 4 the exchange (k ≈ 1 s⁻¹ at 320 K) between the two carbonyls in the trans-diaxial apical positions has been recognized, and its activation parameters, in C₂D₂Cl₄, have been determined (E_a = 68(3) kJ mol⁻¹). The exchange has been attributed to the rotation of the apical H₂Re(CO)₄ fragment with respect to the Re₂(μ-pz) basal fragment, a process analogous to that previously observed in the unsaturated dianion [Re₃(μ-H)₃(CO)₁₀] ²⁻ (2) and in the monoanion [Re₃(μ-H)₃(μ-NC₅H₄-κN¹:κC⁶)(CO)₁₀]⁻ (1), containing a bridging orthometallated pyridine ligand. The vertex rotation was not observed in the pyridazine derivatives 5 and 6. An explanation for this different behaviour is presented, based on the view of the fluxional clusters 1, 2 and 4 as adducts between the apical and basal moieties (π- or σ-complexes). The ΔG^#_312K value here measured in acetone for the σ-complex 4 (77 kJ mol⁻¹) is very similar to that previously determined for the other σ-complex 1 (ΔG^#_305K = 76 kJ mol^-1) and significantly higher than the values measured for the π-complex 2 (ΔG^#_260K = 60 kJ mol⁻¹). The di-hydrido derivative 5 shows a different much faster dynamic process, namely the hopping of one hydride between the two lateral edges, affording a pseudo C_s symmetry in the molecule. The process has been monitored by both ¹H and ¹³C analysis, affording quite similar activation parameters (E_a = 44(1) and 45(1) kJ mol⁻¹, respectively, in THF-d₈), that did not significantly change in CD₂Cl₂ solution, in agreement with an intramolecular process.     

Heteroscorpionate-based Co2+, Zn2+, and Cu2+ complexes: coordination behavior, aerobic oxidation, and hydrogen sulfide detection

The coordination behavior and reactivity of the phenol-substituted bis(pyrazolyl)methane ligands, (3,5-(t)Bu(2)-2-phenol)bis(3,5-Me(2)-pyrazol-1-yl)methane (L1-H) and 2-phenol-bis(3,5-Me(2)-pyrazol-1-yl)methane (L2-H) have been investigated in the metal complexes (L1-H)CoCl(2) (1), (L1-H)ZnCl(2) (2), (L3)CuCl(2) (3), (L2)(2)Co(2)Cl(2) (4) (L2-H)ZnCl(2) (5), and (L2-H)CuCl(2)·H(2)O (6). The mononuclear tetrahedral cobalt complex 1 was isolated and fully characterized by X-ray single crystal diffraction and (1)H NMR spectroscopy and relaxometry. The neutral L1-H is κ(2)-coordinated to the metal center whereas the not coordinated hydroxy-phenyl group is involved in extended intermolecular hydrogen bonds. Aerobic oxidation of L1-H was observed in the reaction of this ligand with CuCl(2) to yield the para-quinone derivative L3 (L3 = 2-(t)Bu-6-(bis(3,5-Me(2)-pyrazol-1-yl)methyl)cyclohexa-2,5-diene-1,4-dione). Upon oxidation L3 resulted κ(2)-coordinated to the tetrahedral Cu(II) metal center, affording 3. The reaction of L2-H with CoCl(2)·6H(2)O produced the elimination of 1 equiv of hydrochloric acid and the formation of the binuclear complex 4 in which one cobalt is in an octahedral environment featuring two κ(3)-coordinated deprotonated ligands whereas the second cobalt center is detected in tetrahedral coordination geometry, bound to the octahedral cobalt via two phenoxo bridging moieties. Interestingly L2-H, (3-(t)Bu-2-phenol)bis(3,5-Me(2)-pyrazol-1-yl)methane (L4-H), or (5-(t)Bu-2-phenol)bis(3,5-Me(2)-pyrazol-1-yl)methane (L5-H) were not oxidized in the reaction with CuCl(2). The reaction of the ligand L2-H with ZnCl(2) and CuCl(2)·2H(2)O yielded the κ(2)-coordinated tetrahedral complex 5 and the square planar complex 6, respectively. The application of the cobalt complex 1 as molecular dosimeter for H(2)S was explored and compared to that of the zinc analogue 2. Density functional theory (DFT) calculations and NMR experiments to assess the possible mechanisms of H(2)S detection by both 1 and 2 are also described.     

Structure and properties of clay nano‐biocomposites based on poly(lactic acid) plasticized with polyadipates

The use of nano‐biocomposites based on plasticized poly(lactic acid) (PLA) has been proposed as a way to improve the polymer ductility and to expand PLA applications window. Novative nano‐biocomposites were elaborated with PLA plasticized by polyadipates (15 wt%) with different molar masses (from 1500 to 2500 Da), with 2.1 wt% of an organo‐modified montmorillonite (O‐MMT). These materials showed enhanced ductility and barrier properties. The clay was swelled in liquid polyadipates prior to their blending with PLA to facilitate chains intercalation and nanofiller exfoliation during melt‐blending. In certain processing conditions, quite homogenous and exfoliated structures were obtained, as shown by X‐ray diffraction (XRD) and transmission electronic microscopy (TEM) results. Irrespective of the average molar mass of the polyadipate, the clay addition induced a reduction in around 25% in oxygen transmission rate (OTR) without an important detriment in tensile properties. Nano‐biocomposites prepared with higher molar masses polyadipates showed the highest thermal stability as well as the lowest OTR, resulting in very promising and novative materials for different applications such as soft packaging. Copyright © 2010 John Wiley & Sons, Ltd.     

First-principles studies of H2S adsorption and dissociation on metal surfaces

Density functional theory calculations were employed to investigate the molecular and dissociative adsorption of H₂S on the closed packed surfaces of a number of important noble metals (Ag(1 1 1), Au(1 1 1) and Cu(1 1 1)) and transition metals (Ir(1 1 1), Ni(1 1 1), Pd(1 1 1) and Pt(1 1 1)). Energy minima corresponding to adsorbed states were identified with H₂S binding preferentially at the top sites. The adsorption of other S moieties (SH and S) was also examined. SH and S were found to prefer bridge sites and hollow sites, respectively. The binding of H₂S and its S-containing dissociated species is stronger on the transition metals. The elementary reactions of abstraction of H from H₂S to form a surface SH intermediate and abstraction of H from SH to form a surface S intermediate as model pathways for the dissociation of H₂S were examined. Our results suggest that H₂S decomposition on the aforementioned transition metal surfaces is more facile, both thermodynamically and kinetically.     

Production and Characterization of Nanostructured Powders of Nd2Fe14B and Fe90Al10 by Mechanical Alloying

The objective of this work is to evaluate the applicability of exchange coupling between nanoparticles of Nd₂Fe₁₄B (hard magnetic material) and Fe₉₀Al₁₀ (soft magnetic material), as permanent magnets produced by surfactant-assisted mechanical alloying. The obtained powders were then mixed with 85% of the Nd₂Fe₁₄B system and 15% of the Fe₉₀Al₁₀ system and subsequently sintered at 300 °C, 400 °C and 500 °C for one hour. The results obtained by Mössbauer spectrometry (MS) show a ferromagnetic behavior with six magnetic sites represented by sextets (16k1, 16k2, 8j1, 8j2, 4c and 4e), characteristic of the Nd₂Fe₁₄B system. X-ray diffraction (XRD) results show a tetragonal and BCC structure for the Nd₂Fe₁₄B and FeAl systems, respectively. The results obtained by vibrating sample magnetometry (VSM), for mixtures of the Nd₂Fe₁₄B and Fe₉₀Al₁₀ sy stems sintered at 300 °C, 400 °C and 500 °C, allow for the conclusion that the coercive field (Hc) decreases drastically with temperature and the percentage of soft phase at values of Hc = 132 Oe compared to the coercive field values reported for Nd₂Fe₁₄B Hc = 6883 Oe, respectively. Images obtained by transmission electron microscopy (TEM), for the Fe₉₀Al₁₀ system, show a tendency for the nanoparticles to agglomerate.     

Carbon Quantum Dot Optical Properties for potential infiltration into Hollow Core Photonic Crystal Fibers

Carbon quantum dots (CQD) have received significant attention in recent years due to their potential applications in optics and sensing. In this study, the authors report on the first characterization of the optical activity and broad absorption spectrum covering from short-wave ultraviolet, at 200 nm, to mid-infrared, at 1600 nm, of CQD synthesized using the “low-molecular-weight alcohols electrochemical carbonization” method. The CQD are analyzed using spectroscopic techniques, optical activity in the infrared, and high-resolution transmission electron microscopy. Results show a CQD size distribution of 5±3 nm and spherical morphology. The absorption spectra show increased absorption at both, high and low frequency. Additionally, the specific rotation of the CQD solution is significantly higher than that of pure ethanol, by three orders of magnitude. These findings suggest that CQD may have potential applications in polarized infrared filters and/or sensors due to their ability to rotate the polarization state of light at 1550 nm. The results of this study provide valuable insights into the optical properties of CQD and their potential for infiltration into hollow core photonic crystal fibers, making them a promising material for future research and development in the field of optics and sensing.     

A simple model of flow-induced crystallization memory

Flow induced crystallization of polymer systems exhibits strong memory effects. Crystalline structures gradually change when the flow is switched off and the polymer is relaxed prior to crystallization. A simple model based on the multidimensional theory of crystal nucleation^[1] is proposed. Steady, potential flow applied to a polymer fluid above melting temperature (T_p > T_m) results in molecular orientation of crystallizing units. The flow controls formation of molecular clusters which convert into athermal nuclei when the system is cooled down to crystallization temperature, T_c < T_m. Orientation effects gradually disappear when the melt is relaxed above T_m in the absence of flow or stress.