Hydrogen bond is the most abundant noncovalent interaction of water molecules. Previous studies have shown that the strength of the water–water hydrogen bond (-5.0 kcal/mol) increases if one of the water molecules coordinates to transition metal (-9.7 kcal/mol). One of the indicators of this strengthening is the shortening of H∙∙∙O hydrogen bond distances in the crystal structures deposited in the Cambridge Structural Database (CSD).

To study how the coordination of both water molecules influences their hydrogen bonds, we used the ConQuest program to screen high-quality crystal structures deposited in the CSD. The contacts between two aqua ligands in these crystal structures were accepted as hydrogen bonds if the O∙∙∙O distance was shorter than 4.0 Å and the O-H∙∙∙O angle was bigger than 110°. The energies of hydrogen bonds were calculated using the B97D/def2-TZVP level of theory.

The majority of the obtained hydrogen bonds have H∙∙∙O distances between 1.8 Å and 2.0 Å, which is longer than hydrogen bonds between coordinated and free water (1.6 – 1.8 Å). Although this might point towards the weakening of hydrogen bonds by the coordination of both water molecules, the DFT calculations show that the energy of a single hydrogen bond between aqua ligands reaches -11.0 kcal/mol, most likely due to increased dispersion effects. We found that the observed increase in the lengths of hydrogen bonds between aqua ligands is induced by the size of aqua complexes and their tendency to form multiple simultaneous hydrogen bonds. Our DFT calculations show that the supramolecular structures with multiple hydrogen bonds between aqua ligands reach an interaction energy of ‑70.0 kcal/mol.

This study implies that the coordination of both water molecules further increases the strength of their hydrogen bonds and shows that hydrogen bonds between aqua ligands are significant contributors to the stability of supramolecular systems of metal complexes.

C-H/O interactions are among the most abundant weak hydrogen bonds due to the omnipresence of C-H groups and oxygen atoms. Previous studies have shown that coordination with metals can cause the strengthening of noncovalent interactions, such as hydrogen bonds, stacking interactions, cation–π, and anion–π interactions. In this work, we studied the influence of transition metal coordination on C-H/O interactions of aromatic ligands in organometallic complexes.

Crystal structures of high quality, deposited in the Cambridge Structural Database (CSD, version 5.43, November 2022), were studied using the ConQuest program (version 2022.3.0) to find C-H/O interactions between the η⁶-coordinated benzene and the oxygen atom of a Z₁-O-Z₂ fragment, where Z₁ or Z₂ could be any atom. The contact was accepted as a C-H/O interaction if the O∙∙∙H distance was shorter than 2.9 Å and the C-H∙∙∙O angle was bigger than 110°. The density functional theory was employed to calculate the energies of C-H/O interactions.

Our CSD search resulted in 152 C-H/O interactions of coordinated benzene, mostly in organometallic half-sandwich compounds. The analysis of geometrical parameters shows that preferred O∙∙∙H distances in these interactions are between 2.3 Å and 2.5 Å. These O∙∙∙H distances are shorter for coordinated than uncoordinated aromatic rings, indicating that coordinated aromatic rings form stronger C-H/O hydrogen bonds than uncoordinated ones. These findings are confirmed by our preliminary B3LYP-D3/aug-cc-pVDZ calculations, which showed that in the organometallic half-sandwich model system Cr(C₆H₆)(CO)₃∙∙∙H₂O the C-H/O interaction reaches the energy of -3.88 kcal/mol, which is considerably stronger than the C-H/O interaction between (uncoordinated) benzene and water (-1.64 kcal/mol).

Our joint crystallographic and computational study points towards the enhanced ability of coordinated aromatic rings to form substantial C-H/O interactions. This study provides further insights into the strengthening of noncovalent interactions via transition metal coordination.

In a study of the chiral enantiomer structure of cyclic trilactams possessing C3 symmetry, the dimer structure was established through the formation of three NH···O type complementary H-bonds at three amides which were applied as agents for volatile compound profile extraction in a sample via the liquid phase extraction technique.

The extraction efficiency of cyclic trilactam materials for analytes found in perfume samples was assessed using HS-SPME-GC-MS. The experimental arrangement of perfume solution was combined with the materials in EtOH solvent in an open vial, while the perfume in EtOH (without the materials) served as the control. Following overnight evaporation at room temperature, the remaining liquid was collected for HS-SPME-GC-MS analysis. Extraction took place overnight to explore the materials' extraction efficiency and facilitate re-crystallization. The materials underwent re-crystallization, with the collected crystals revealing a similar shape to the observed dimers. This suggests a robust hydrogen bonding interaction within the dimer and the volatile compounds interacting. A list of identified compounds during extraction was compiled for each material and the control. The enrichment peak areas in the chromatogram results were determined as the analyte peak area in the sample containing the materials to the area of the same analyte in the control. This indicated the extraction performance of volatile analytes in the sample containing the materials compared with the control sample, such as Linalylformate (5.1±0.015)×10⁷, Citronellol (5.1±4.2)×10⁶, Carvone (4.2±3.5)×10⁵, Linalylacetate (2.5±2.2)×10⁷ and α-Terpinylacetate (2.3±1.7)×10⁵ of the main potential compounds. Higher enrichment signifies stronger interactions between the analytes and these materials.

The solubility between the compounds in less polar solvents can be attributed to their structural features and the nature of the solvent. Specifically, the solubility of each compound is influenced by factors such as polarity, hydrogen bonding capability, and the molecular size and shape of the compounds can influence their interactions with the solvent molecules, further impacting solubility differences between compounds in less polar solvents.

In recent years, concerns about heavy metal pollution and rising CO₂ levels contributing to global warming have intensified. This study focuses on synthesizing and functionalizing mesostructured silicas (MCM-41, MCM-48) to develop amine–silica sorbents for potential applications in removing Cd²⁺ from aqueous solutions and in Carbon Capture and Utilization (CCU) technologies. Due to their ordered mesoporous structure, nitrogen's lone pairs donating to Cd²⁺, and the high affinity of amines for CO₂, amine–silica sorbents are suitable for these applications. Additionally, mesostructured silica-based composites were developed by impregnating MCM-41 with copper, zinc, and zirconium oxidic phases (CZZ) for potential use in the catalytic hydrogenation of CO₂ to methanol.
Mesostructured silica was synthesized using the sol–gel technique with a templating agent, a siliceous alkoxide precursor, and two distinct solvent systems—water (MCM-41) and water/ethanol (MCM-48). Amine–silica sorbents were obtained through post-synthesis grafting with aminopropyltriethoxysilane, and were tested for Cd²⁺ removal and CO₂ capture. To reduce the environmental impact of the grafting process, an eco-friendly solvent (butanol instead of toluene) was used. The CZZ@MCM-41 composites were prepared through auto-combustion. For developing Cd²⁺-removal sorbents and CZZ catalysts, mesostructured silica was also synthesized from industrial waste (hexafluorosilicic acid, FSA). All sorbents were characterized using XRD, TEM-EDX, TEM, N₂-physisorption, thermogravimetric analysis, and ATR-FTIR.
MCM-48 was obtained by adding ethanol as a co-solvent, maintaining other experimental conditions equal to MCM-41 synthesis. MCM-41 from both FSA and TEOS exhibited similar textural properties. The amino groups' concentration was similar (~2 mmol/g) for MCM-41 functionalized with either toluene or butanol; however, the use butanol instead of toluene resulted in a lower CO₂ adsorption performance (968 μmol CO₂/g sorbent vs. 480 μmol CO₂/g sorbent). Amine–silica sorbents showed a Cd²⁺ removal efficiency of 98-99% and an adsorbed cadmium amount (q_e) of ~40-45 mgCd(II)/g sorbent for MCM-41 from both TEOS and FSA.

Recently, metal oxide nanostructures have seen significant advancements in synthesis and device integration. Considering the sustainability of materials and processes, as well as multifunctionality, zinc-tin oxide nanostructures are among the most promising oxide nanostructures being researched. Their ternary oxide nature allows for impressive multifunctionality, with applications including catalysis, electronics, sensors, and energy harvesting. To synthesize these materials, the use of seed layers can be beneficial since it can influence the growth of nanostructures and is advantageous for applications where nanostructures on film are desired, such as photocatalysis and sensors.

In this work, several seed layers (namely Cu, stainless steel, Cr, Ni, etc.) were tested for the synthesis of ZTO nanostructures. It was generally observed that the structures grown on the seed layers differed from those obtained with the seed-layer-free hydrothermal method (under similar synthesis conditions [1-3]), demonstrating the effectiveness of seed layers in influencing growth. Various types of structures were obtained, such as ZnSnO₃ nanowires and Zn₂SnO₄ nanoparticles, octahedrons, and nanowires. The results suggest a correlation between the phase of the employed seed layer and the resultant phase of the nanostructures. Furthermore, it was generally seen that while shorter times favored the production of nanostructures, longer times resulted in thin films with a nanostructured surface when employing the seed layers [4]. This emphasizes the influence of seed layers on nanostructure growth, not only by inducing the phase but also by accelerating the growth process.

References:

1. Rovisco, A. et al., ACS Appl. Nano Mater. 2018, 1, 3986–3997.
2. Rovisco, A. et al., Nanomaterials 2019, 9, 1002.
3. Rovisco, A. et al., In Novel Nanomaterials; IntechOpen, 2021.
4. Rovisco, A., PhD Thesis, Universidade NOVA de Lisboa, 2019.