Recent Advances in the Catalytic Applications of Chiral Schiff‐Base Ligands and Metal Complexes in Asymmetric Organic Transformations

Synthesis and characterization of citric acid-modified chitosan Schiff base with enhanced antibacterial properties for the elimination of Bismarck Brown R and Rhodamine B dyes from wastewater

In this study, a new chitosan Schiff base with surface modification using citric acid was synthesized for efficient removal of pernicious dyes, namely Bismarck Brown R (BBR) and Rhodamine B (RhB), from wastewater. The physicochemical properties of the modified chitosan Schiff base were comprehensively investigated. Adsorption studies demonstrated that BBR adsorption occurred through monolayer formation, while RhB adsorption proceeded via multilayer formation on the heterogeneous surface. The synthesized adsorbent exhibited exceptional dye removal efficiency, with a Langmuir saturation capacity of 348 ± 11.0 mg.g-1 for BBR and 145 ± 18.44 mg.g-1 for RhB. Isotherm data fitting revealed consistency with the Langmuir isotherm model for BBR and the Freundlich isotherm model for RhB. Notably, the modified chitosan Schiff base showcased enhanced antibacterial properties, effectively inhibiting both gram-positive and gram-negative bacteria. The study's findings underscore the potential of this novel chitosan-based Schiff base as an efficient adsorbent for the removal of various dyes from wastewater, emphasizing its versatility and practical applicability in water treatment processes.

Hexacoordinated tin complexes catalyse imine hydrogenation with H2

Frustrated Lewis pair (FLP) hydrogenation catalysts predominantly use alkyl- and aryl-substituted Lewis acids (LA) that offer a limited number of combinations of substituents, limiting our ability to tune their properties and, ultimately, their reactivity. Nevertheless, main-group complexes have numerous ligands available for such purposes, which could enable us to broaden the range of FLP catalysis. Supporting this hypothesis, we demonstrate here that hexacoordinated tin complexes with Schiff base ligands catalyse imine hydrogenation via activation of H2(g). As shown by hydrogen-deuterium scrambling, [Sn(tBu2Salen)(OTf)2] activated H2(g) at 25 °C and 10 bar of H2. After tuning the ligands, we found that [Sn(Salen)Cl2] was the most efficient imine hydrogenation catalyst despite having the lowest activity in H2(g) activation. Moreover, various imines were hydrogenated in yields up to 98% thereby opening up opportunities for developing novel FLP hydrogenation catalysts based on hexacoordinated LA of main-group elements.

Harnessing Dpp-Imine as a Powerful Achiral Cocatalyst to Dramatically Increase the Efficiency and Stereoselectivity in a Magnesium-Mediated Oxa-Michael Reaction

Dpp-imines are classic model substrates for synthetic method studies. Here, we disclose their powerful use as achiral coligands in metal-catalyzed reactions. It is highly interesting to find that the Dpp-imine can not only act as powerful ligand to create excellent chiral pockets with magnesium complexes but also, more importantly, this coligand can dramatically enhance the catalytic ability of the metal catalyst. The underlying reaction mechanism was extensively explored by conducting a series of experiments, including 31P NMR studies of the coordination complex between the Dpp-imine coligand and magnesium complexes, ESI capture results, multiple control experiments, studies and comparison of different coligands, 1H NMR studies on the relationship between the substrate and Dpp-imine coligand, as well as the relationship between the substrate and the full complexes. Furthermore, DFT calculation provided valuable insights in the role of the imine additive and demonstrated that adding the Dpp-imine coligand in the magnesium catalyst can switch the deprotonation/nucleophilic addition steps from a stepwise mechanism to a concerted process during the oxa-cyclization reaction. The crucial factors responsible for the excellent enantioselectivity and enhanced reaction efficiency brought by Dpp-imine have been extracted from the calculation model. These mechanistic experiments and DFT calculation data clearly disclose and prove the powerful and interesting functions of the Dpp-imine coligand, which also direct a novel application of this type of active imine as useful ligands in metal-catalyzed asymmetric reactions.

The solvent-regulated excited state reaction mechanism of 2-(2'-hydroxyphenyl)benzothiazole aggregates

The excited state relaxation dynamics of 2-(2'-hydroxyphenyl)benzothiazole (HBT) in the gas phase and the solvents have been explored experimentally and theoretically. However, the fundamental mechanism of its emission in aggregates is still unexplored. In this article, we have presented a detail investigation of solvent-regulated excited state (ES) reactions for HBT aggregates with the aid of several experimental and theoretical research. The careful investigation of solvatochromic and electrochemical behavior elucidates that the emission around 460 nm of HBT in DMSO and DMSO-water fraction correspond to the excited state internal charge transfer (ESICT). The quantum chemical analysis further supports this observation. The concentration-dependent 1H NMR and emission studies of HBT in DMSO revealed the formation of aggregates at higher concentrations that facilitate the charge transfer. The emission pattern of HBT in the AcN-water fraction demonstrates that the sequential internal charge transfer-proton transfer (ESICT-ESIPT) occurs in HBT aggregates. The pH studies show that HBT aggregates are potential ratiometric sensors for near-physiological pH ranges. Moreover, a ground-state zwitterionic conformation of HBT is observed in the basic medium formed by ground-state internal proton transfer (GSIPT). Overall, this study provides a better understanding of solvent-regulated ES reaction mechanism in the case of HBT aggregates and other substituted HBT compound aggregates published previously.

Chitosan-based Schiff bases: Promising materials for biomedical and industrial applications

There is plenty of scope for modifying chitosan, an only polycationic natural polysaccharide, owing to its reactive functional groups, namely hydroxyl and amino groups. Although innumerable numbers of chitosan derivatives have been synthesized by modifying these groups and reported elsewhere, in this review article, an attempt has been exclusively made to demonstrate the syntheses of various chitosan-based Schiff bases (CSBs) simply by allowing the reactions of reactive amino groups of chitosan with different aldehydes/ketones of interest. Due to their very peculiar and unique characteristics, such as biodegradability, biocompatibility, metal-binding capability, etc., they are found to be very useful for diversified applications. Thus, we have also attempted to showcase their very specific biomedical fields, including tissue engineering, drug delivery, and wound healing, to name a few. In addition, we have also discussed the utilization of CSBs for industrial applications such as wastewater treatment, catalysis, corrosion inhibition, sensors, etc.

Antimicrobially Active Zn(II) Complexes of Reduced Schiff Bases Derived from Cyclohexane-1,2-diamine and Fluorinated Benzaldehydes-Synthesis, Crystal Structure and Bioactivity

A series of Schiff base ligands obtained by the condensation of trans-cyclohexane-1,2-diamine and fluorinated benzaldehydes were prepared, followed by their reduction with NaBH₄. The reduced ligands were employed in the synthesis of zinc complexes of the general formula [ZnCl₂(L)]. The structures of both the original and the reduced Schiff bases, as well as of the zinc complexes, were characterized by single-crystal X-ray analysis, along with NMR and IR spectroscopy. The antimicrobial activities of the reduced Schiff bases and their zinc complexes were evaluated in vitro against E. coli, S. aureus, and C. albicans. The compounds containing the 4-(trifluoromethylphenyl) moiety showed marked antibacterial activity. Interestingly, the antimicrobial effect of the zinc complex with this moiety was significantly higher than that of the corresponding free reduced ligand, comparable with ciprofloxacin used as standard. Thus, a synergic effect upon the complexation with zinc can be inferred.