Tautomeric design of ortho-hydroxyheterocyclic Schiff bases

Fluorescence Sensing of pH and p-Nitrophenol Using an AIEE Active Pyridoxal Derived Schiff Base

An easy-to-prepare aggregation-induced emission enhancement (AIEE) active Schiff base NPY was synthesized by condensing vitamin B6 cofactor pyridoxal with 3-hydroxy-2-naphthoic hydrazide, and employed for the fluorescent sensing of pH and p-nitrophenol (p-NP). The AIEE phenomenon of NPY was investigated in mixed DMSO/H2O medium. The weakly yellow-fluorescent NPY (λem = 535 nm) in pure DMSO turned to a bright cyan-fluorescent NPY (λem = 490 nm) upon addition of poor solvent water. The DLS and SEM analyses supported the self-aggregation of NPY that restricted the intramolecular rotation and activated the excited state intramolecular proton transfer (ESIPT) process. The AIEE luminogen (AIEEgen) NPY containing 90% of water fraction (fwater) was employed for the fluorescent sensing of pH. AIEEgen NPY displays three distinct fluorescent pH windows: non-fluorescent below pH 3.0 and above pH 10.0, cyan fluorescent between pH 3.0 to 8.0, and yellow fluorescent between pH 8.0 to 10.0. AIEEgen NPY was also applied for the detection of nitroaromatics in HEPES buffer (10% DMSO, 10 mM, pH 7.0). The addition of p-NP selectively quenched the fluorescent intensity of AIEEgen NPY with an estimated detection limit of 1.73 µM. The analytical utility of AIEEgen NPY was examined by quantifying p-NP in different real water samples.

Effect of a Substituent on the Properties of Salicylaldehyde Hydrazone Derivatives

The present study investigates the effect of the substitution of salicylaldehyde hydrazones at two selected positions, i.e., the para-position with regard to the proton-donating and proton-accepting centers forming the hydrogen bridge. A detailed analysis of structural data obtained by theoretical approaches and X-ray experiments, together with original resonance Hammett's constants, indicates that the strength of the intramolecular hydrogen bonding present in salicylaldehyde hydrazones can be selectively modulated by substitution of the parent molecular system with the chemical group of known π-electron-donating or -accepting properties. Our findings provide an insight into planning synthesis pathways for salicylaldehyde hydrazone species and predicting their result with regard to their H-bonding and related physical and chemical properties.

Crystal structure, Hirshfeld surface analysis and DFT calculations of (E)-3-[1-(2-hy-droxy-phenyl-anilino)ethyl-idene]-6-methyl-pyran-2,4-dione

The asymmetric unit of the title compound, C₁₄H₁₃NO₄, contains three independent mol-ecules, which differ slightly in conformation. Each contains an intra-molecular N-H⋯O hydrogen bond. In the crystal, O-H⋯O hydrogen bonds form chains of mol-ecules, which are linked into corrugated sheets parallel to (03) plane by C-H⋯O hydrogen bonds together with π inter-actions between the carbonyl groups and the 2-hy-droxy-phenyl rings. The layers are linked by further C-H⋯O hydrogen bonds. The Hirshfeld surface analysis of the crystal structure indicates that the most important contributions for the crystal packing are from H⋯H (49.0%), H⋯O/O⋯H (28.3%) and H⋯C/C⋯H (10.9%) inter-actions. van der Waals inter-actions are the dominant inter-actions in the crystal packing. Moreover, density functional theory (DFT) optimized structures at the B3LYP/ 6-311 G(d,p) level are compared with the experimentally determined mol-ecular structure in the solid state. The HOMO-LUMO behavior was elucidated to determine the energy gap of 4.53 eV.

Ring-Opening Polymerization of ε-Caprolactone and Styrene Oxide-CO2 Coupling Reactions Catalyzed by Chelated Dehydroacetic Acid-Imine Aluminum Complexes

A series of chelated dehydroacetic acid-imine-based ligands L1H~L4H was synthesized by reacting dehydroacetic acid with 2-t-butylaniline, (S)-1-phenyl-ethylamine, 4-methoxylbenzylamine, and 2-(aminoethyl)pyridine, respectively, in moderate yields. Ligands L1H~L4H reacted with AlMe3 in toluene to afford corresponding compounds AlMe2L1 (1), AlMe2L2 (2), AlMe2L3 (3), and AlMe2L4 (4). All the ligands and aluminum compounds were characterized by IR spectra, 1H and 13C NMR spectroscopy. Additionally, the ligands L1H~L4H and corresponding aluminum derivatives 1, 3, and 4 were characterized by single-crystal X-ray diffractometry. The catalytic activities using these aluminum compounds as catalysts for the ε-caprolactone ring-opening polymerization (ROP) and styrene oxide-CO2 coupling reactions were studied. The results show that increases in the reaction temperature and selective solvent intensify the conversions of ε-caprolactone to polycaprolactone. Regarding the coupling reactions of styrene oxide and CO2, the conversion rate is over 90% for a period of 12 h at 90 °C. This strategy dispenses the origination of cyclic styrene carbonates, which is an appealing concern because of the transformation of CO2 into an inexpensive, renewable and easy excess carbon feedstock.

Intramolecular Hydrogen Bond, Hirshfeld Analysis, AIM; DFT Studies of Pyran-2,4-dione Derivatives

Intra and intermolecular interactions found in the developed crystals of the synthesized py-ron-2,4-dione derivatives play crucial rules in the molecular conformations and crystal stabili-ties, respectively. In this regard, Hirshfeld calculations were used to quantitatively analyze the different intermolecular interactions in the crystal structures of some functionalized py-ran-2,4-dione derivatives. The X-ray structure of pyran-2,4-dione derivative namely (3E,3′E)-3,3′-((ethane-1,2-diylbis(azanediyl))bis(phenylmethanylylidene))bis(6-phenyl-2H-pyran-2,4(3H)-dione) was determined. It crystallized in the monoclinic crystal system and C2/c space group with unit cell parameters: a = 14.0869(4) Å, b = 20.9041(5) Å, c = 10.1444(2) Å and β = 99.687(2)°. Generally, the H…H, H…C, O…H and C…C contacts are the most important interactions in the molecular packing of the studied pyran-2,4-diones. The molecular structure of these compounds is stabilized by intramolecular O…H hydrogen bond. The nature and strength of the O…H hy-drogen bonds were analyzed using atoms in molecules calculations. In all compounds, the O…H hydrogen bond belongs to closed-shell interactions where the interaction energies are higher at the optimized geometry than the X-ray one due to the shortening in the A…H distance as a con-sequence of the geometry optimization. These compounds have polar characters with different charged regions which explored using molecular electrostatic potential map. Their natural charges, reactivity descriptors and NMR chemical shifts were computed, discussed and compared.