The Evaluation of Cellulose Acetate Capsules Functionalized for the Removal of Cd(II)

Cellulose acetate is derived from cellulose and has the characteristics of biodegradability and reusability. So, it has been used for the elimination of toxic compounds capable of producing different diseases, such as cadmium, that result from human and industrial activity. For this reason, capsules functionalized with Cyanex 923 were prepared and characterized by FTIR spectroscopy, Energy Dispersive X-ray Spectroscopy (EDX), and SEM. The functionalized capsules were used for removing and recovering Cd(II) by modifying variables such as HCl concentration in the extraction medium and carrier content in the capsules, among others. The extraction of cadmium from battery leachates and the three isotherm models, Langmuir, Freundlich, and Dubinin Radushkevich, were also tested to model the cadmium removal process. The results showed a favorable physical sorption with a good capacity for extraction and the possibility of reusing the capsules for up to seven cycles without a decrease in the percentage of cadmium recovery.

Synthesis and Solid-State Dynamics of a Crystalline Steroid Molecular Rotor without the Alkyne Axle: Steroid Dimers Based on a 1,4-Di(1,3-dioxan-2-yl)benzene Moiety

Two diastereomeric crystalline steroid dimers were obtained by acid-catalyzed double acetalization of (20S)-5α-pregnan-3β,16β,20-triol 3-monoacetate with terephtalaldehyde. These compounds were characterized by NMR in solution, MS, single-crystal X-ray diffraction, and variable-temperature solid-state NMR by ¹³C cross-polarization magic angle spinning (CPMAS). While the phenylene rotator in the SR diastereomer remains static even at 373 K, the RR isomer shows a slow rotational process of the phenylene ring at temperatures above room temperature and thus may be considered the first crystalline steroid molecular rotor without the alkyne axle.

Kinetic Control in the Synthesis of a Möbius Tris((ethynyl)[5]helicene) Macrocycle Using Alkyne Metathesis

The synthesis of conjugated Möbius molecules remains elusive since twisted and macrocyclic structures are low-entropy species sporting their own synthetic challenges. Here we report the synthesis of a Möbius macrocycle in 84% yield via alkyne metathesis of 2,13-bis(propynyl)[5]helicene. MALDI-MS, NMR spectroscopy, and X-ray diffraction indicated a trimeric product of twofold symmetry with PPM/MMP configurations in the helicene subunits. Alternatively, a threefold-symmetric PPP/MMM structure was determined by DFT calculations to be more thermodynamically stable, illustrating remarkable kinetic selectivity for this alkyne metathesis cyclooligomerization. Computational studies provided insight into the kinetic selectivity, demonstrating a difference of 15.4 kcal/mol between the activation barriers for the PPM/MMP and PPP/MMM diastereodetermining steps. Computational (ACID and EDDB) and experimental (UV-vis and fluorescence spectroscopy and cyclic voltammetry) studies revealed weak conjugation between the alkyne and adjacent helicene groups as well as the lack of significant global aromaticity. Separation of the PPM and MMP enantiomers was achieved via chiral HPLC at the analytical scale.

Origin of the isotropic motion in crystalline molecular rotors with carbazole stators

Herein we report two crystalline molecular rotors 1 and 4 that show extremely narrow signals in deuterium solid-state NMR spectroscopy. Although this line shape is typically associated with fast-moving molecular components, our VT 2H NMR experiments, along with X-ray diffraction analyses and periodic DFT computations show that this spectroscopic feature can also be originated from low-frequency intramolecular rotations of the central phenylene with a cone angle of 54.7° that is attained by the cooperative motion of the entire structure that distorts the molecular axis to rotation. In contrast, two isomeric structures (2 and 3) do not show a noticeable intramolecular rotation, because their crystallographic arrays showed very restricting close contacts. Our findings clearly indicate that the multiple components and phase transitions in crystalline molecular machines can work in concert to achieve the desired motion.

Synthesis, Characterization, and Solid State Dynamic Studies of a Hydrogen Bond-Hindered Steroidal Molecular Rotor with a Flexible Axis

A novel steroid molecular rotor was obtained in four steps from the naturally occurring spirostane sapogenin diosgenin. The structural and dynamic characterization was carried out by solution NMR, VT X-ray diffraction, solid state ¹³C CPMAS, and solid state ²H NMR experiments. They allowed the identification of a fast dynamic process with a frequency of 14 MHz at room temperature, featuring a barrier to rotation Ea = 7.87 kcal mol^-1. The gathered experimental evidence indicated the presence of a hydrogen bond that becomes stronger as the temperature lowers. This interaction was characterized using theoretical calculations, based on topological analyses of the electronic density and energies. In addition, combining theoretical calculations with experimental measurements, it was possible to propose a partition to Ea (∼8 kcal/mol) into three contributions, that are the cost of the intrinsic rotation (∼2 kcal/mol), the hydrogen bond interaction (∼2 kcal/mol), and the packing effects (∼2-3 kcal/mol). The findings from the present work highlight the relevance of the individual components in the function of molecular machines in the solid state.

Isotropic rotation in amphidynamic crystals of stacked carbazole-based rotors featuring halogen-bonded stators

Liquid-like dynamics of a covalent 1,4-phenylene rotator have been unveiled in 1 with a brominated stator showing type-II halogen bonds. This singular rotation is favored by synergistic molecular changes in stacked molecules, according to VT solid state NMR, ¹H T₁ relaxometry and VT X-ray experiments of this highly crystalline compound.

Towards taxane analogues synthesis by dienyne ring closing metathesis

The synthesis of highly functionalized 16,17,18-trinortaxane analogues based on a dienyne cyclization is described.

Thermodynamic evaluation of aromatic CH/π interactions and rotational entropy in a molecular rotor

A molecular rotor built with a stator formed by two rigid 9β-mestranol units having a 90° bent angle linked to a central phenylene rotator has an ideal structure to examine aromatic CH/π interactions. Energies and populations of the multiple solution conformations from quantum-mechanical calculations and molecular dynamics simulations were combined with variable-temperature (VT) (1)H NMR data to establish the enthalpy of this interaction and the entropy associated with rotation about a single bond. Rotational dynamics in the solid state were determined via VT cross-polarization magic-angle spinning (13)C NMR spectroscopy.

Amphidynamic crystals of a steroidal bicyclo[2.2.2]octane rotor: a high symmetry group that rotates faster than smaller methyl and methoxy groups

The synthesis, crystallization, single crystal X-ray structure, and solid state dynamics of molecular rotor 3 provided with a high symmetry order and relatively cylindrical bicyclo[2.2.2]octane (BCO) rotator linked to mestranol fragments were investigated in this work. By use of solid state (13)C NMR, three rotating fragments were identified within the molecule: the BCO, the C19 methoxy and the C18 methyl groups. To determine the dynamics of the BCO group in crystals of 3 by variable temperature (1)H spin-lattice relaxation (VT (1)H T1), we determined the (1)H T1 contributions from the methoxy group C19 by carrying out measurements with the methoxy-deuterated isotopologue rotor 3-d6. The contributions from the quaternary methyl group C18 were estimated by considering the differences between the VT (1)H T1 of mestranol 8 and methoxy-deuterated mestranol 8-d3. From these studies it was determined that the BCO rotator in 3 has an activation energy of only 1.15 kcal mol(-1), with a barrier for site exchange that is smaller than those of methyl (E(a) = 1.35 kcal mol(-1)) and methoxy groups (E(a) = 1.92 kcal mol(-1)), despite their smaller moments of inertia and surface areas.