REVIEW: Biomedical applications of Schiff base metal complexes

Manganese Schiff Base Complexes, Crystallographic Studies, Anticancer Activities, and Molecular Docking

Choice of ligands is significant to successful synthesis of metal complexes (coordination compounds). This study reports the use of Schiff base as the right ligand to control the poor bioavailability and neurodegenerative toxicity challenges of manganese ion. In line with this study, document analysis was used as the methodological approach to evaluate the significance of Schiff base ligands in easing these manganese’s challenges and aligning the resultant coordination compounds (manganese Schiff base complexes) as therapeutic agents in anticancer studies. Report also involves crystallographic studies where single crystal X-ray crystallography was used as a chemical characterization technique. In addition, molecular docking studies, MOE2008, and AutoDock software were used to reveal the mode of interaction between the Schiff base and the manganese(II) and (III) ions, as well as scrutinizing the biological efficacy of the manganese(II) and manganese(III) Schiff bases coordination compounds as anticancer agents against some anticancer cell lines. Conclusion drawn was that manganese(II) and manganese(III) Schiff bases coordination compounds gave more active and potent activities than the corresponding Schiff bases. As a result, challenges of neurodegenerative toxicity and poor bioavailability of manganese ion were overcome, and the chelation therapy was fulfilled. Results from single crystal X-ray crystallography confirmed the successful synthesis of manganese(II) and manganese(III) Schiff bases coordination compounds and revealed the mechanism of reaction, while the molecular docking buttressed the biological activities of the Schiff base ligand and manganese Schiff base coordination compounds by portraying the structure activity relationship (SAR) between either Schiff base or the manganese Schiff base coordination compounds and the virtual cancer cell line (receptor protein), where hits were obtained for lead optimizations.

Probing biothiols using a red-emitting pyridoxal derivative by adopting copper(II) displacement approach and cell imaging

An aggregation-induced emission (AIE) active Schiff base L was synthesized by condensing pyridoxal and 2-hydroxy-1-naphthaldehyde with p -phenylenediamine in two simple steps in ethanol. The colorimetric, UV-Vis and fluorescence studies of L revealed that the weak yellow emissive L (λ em =540 nm, λ ex = 450 nm) in pure DMSO turned to a red-emissive L upon increasing the poor solvent fraction (HEPES buffer, 10 mM, pH 7.4) above 50% in DMSO. The DLS and SEM analysis supported the formation of self-aggregates of L that restricted the intramolecular motion and activated the ESIPT. The cations sensing ability of the AIEgen L was explored in HEPES buffer (5% DMSO, 10 mM, pH 7.4), where Cu 2+ selectively quenched the fluorescence at 608 nm due to the chelation-enhanced fluorescence quenching (CHEQ) effect with an estimated sensitivity limit of 0.9 µM. Subsequently, the in situ formed AIEgen L-Cu 2+ complex was applied for the cascade detection of cysteine (Cys), homocysteine (Hcy) and glutathione (GSH). The decomplexation of Cu 2+ from the AIEgen L-Cu 2+ upon addition of Cys, Hcy and GSH restored the quenched fluorescence emission of AIEgen L at 608 nm. With this Cu 2+ displacement approach, the concentration of Cys, Hcy and GSH can be detected down to 2.8 µM, 3.12 µM and 2.0 µM, respectively. The practical utility of AIEgen L and AIEgen L-Cu 2+ was validated by quantifying the selective analytes in various environmental and biological samples, and also applied successfully for the cell imaging applications.

Design, synthesis, and biological evaluation of dinuclear bismuth(III) complexes with Isoniazid-derived Schiff bases

Four dinuclear bismuth(III) Schiff-base complexes bearing Schiff-base ligands have been synthesized and structurally characterized by single-crystal X-ray diffraction, elemental analysis, and spectral techniques (FT-IR, NMR and MS). The analytical data reveal the bismuth(III) complexes possess 1:1 metal-ligand ratios. In vitro biological studies have revealed that bismuth(III) complexes displayed much higher antibacterial and antitumor activities than their parent ligands, which involves two gram-negative (S. aureus, B. subtili) and two gram-positive (E. coli, P. aeruginosa) bacteria, and human gastric cancer SNU-16 cells. The power-time curves of S. pombe exposed to tested compounds were detected by bio-microcalorimetry. Some thermokinetic parameters (k, P_max,t_G and Q_total) were derived based on the metabolic power-time curves, and their quantitative relationships with the concentrations (c) were further discussed.

Crystal structure of (E)-amino(2-(thiazol-2-ylmethylene)hydrazineyl)methaniminium nitrate, C10H16N12O6S2

C10H16N12O6S2, triclinic, P 1 ‾ $P\overline{1}$ (no. 2), a = 7.6977(2) Å, b = 11.5353(3) Å, c = 11.7533(3) Å, α = 67.973(2)°, β = 87.916(2)°, γ = 88.347(2)°, V = 966.69(5) Å3, Z = 2, R gt (F) = 0.0290, wR ref (F 2) = 0.0818, T = 170K.

Shedding Light on the Synthesis, Crystal Structure, Characterization, and Computational Study of Optoelectronic Properties and Bioactivity of Imine derivatives

Two imine compounds named as (E)-2-(((3,4-dichlorophenyl)imino)methyl)phenol (DC2H) and (E)-4-(((2,4-dimethylphenyl)imino)methyl)phenol (DM4H) are synthesized, and their crystal structures are verified using the single-crystal X-ray diffraction (XRD) technique. The crystal structures of the compounds are compared with the closely related crystal structures using the Cambridge Structural Database (CSD). The crystal packing in terms of intermolecular interactions is fully explored by Hirshfeld surface analysis. Void analysis is carried out for both compounds to check the strength of the crystal packing. Furthermore, a state-of-the-art dual computational technique consisting of quantum chemical and molecular docking methods is used to shed light on the molecular structure, optoelectronic properties, and bioactivity of indigenously synthesized compounds. The optimized molecular geometries are compared with their counterpart experimental values. Based on previous reports of biofunctions of the indigenously synthesized imine derivatives, they are explored for their potential inhibition properties against two very crucial proteins (main protease (M^pro) and nonstructural protein 9 (NSP9)) of SARS-CoV-2. The calculated interaction energy values of DC2H and DM4H with M^pro are found to be -6.3 and -6.6 kcal/mol, respectively, and for NSP9, the calculated interaction energy value is found to be -6.5 kcal/mol. We believe that the current combined study through experiments and computational techniques will not only pique the interest of the broad scientific community but also evoke interest in their further in vitro and in vivo investigations.

Crystal Structure of a Chiral Sec-Amine, 4-Chloro-N-(1-(pyridin-2-yl)ethyl)aniline

In this communication, we present the crystal structure of a secondary amine: 4-chloro-N-(1-(pyridin-2-yl)ethyl)aniline (Lb) obtained from a stepwise reduction of an imine, (E)-N-(4-chlorophenyl)-1-(pyridin-2-yl)ethan-1-imine (La) with sodium borohydride. The structure was characterized by FT-IR, 1H and 13C NMR, Mass Spectroscopy and X-ray diffraction.

White light-induced AIEgen polyurethane films containing Schiff base copper(ii) complexes for synergistic chemo/photodynamic antibacterial therapy

Polyurethane films containing AIEgens and copper complexes can act as a potential antibacterial agent for multi-mode combined antibacterial therapy.

A study of the photochemical behaviour and relaxation mechanisms of anti–syn isomerisation around quinazolinone –N–N bonds

High-resolution NMR experiments revealed that differently substituted quinazolinone-based Schiff bases undergo anti to syn isomerisation on exposure to ultraviolet light in DMSO solution.

Cu(II), Mn(II) and Zn(II) complexes of hydrazones with a quaternary ammonium moiety: synthesis, experimental and theoretical characterization and cytotoxic activity

In this paper, Cu(II), Mn(II) and Zn(II) complexes with N,N,N-trimethyl-2-oxo-2-(2-(1-(thiazol-2-yl)ethylidene)hydrazinyl)ethan-1-aminium chloride (HL1Cl) were synthesized and characterized by single-crystal X-ray diffraction, IR spectroscopy, elemental analysis and DFT calculations. In all three complexes, a ligand (L1) is coordinated in a deprotonated formally neutral zwitterionic form via NNO donor set atoms. Cu(II) and Zn(II) form mononuclear penta-coordinated complexes [CuL1(N₃)(CH₃OH)]BF₄ and [ZnL1(N₃)₂], respectively, while Mn(II) forms a binuclear [Mn₂L1₂(μ-_1,1-N₃)₂(N₃)₂]·2CH₃OH complex, with unusual distorted trigonal-prismatic geometry around the metal centers. The antimicrobial activity of these complexes was tested against a panel of Gram-negative and Gram-positive bacteria, two yeasts and one fungal strain. The binuclear Mn(II) complex showed antifungal activity of similar intensity to amphotericin B. Based on the results of the brine shrimp test and DPPH radical scavenging activity, the most active Cu(II) and Mn(II) complexes were selected for evaluation of cytotoxic activity against five malignant cancer cell lines (HeLa, A375, MCF7, PC-3 and A549) and one normal cell line HaCaT. Both complexes showed significant activity. It should be pointed out that the activity of the Mn(II) complex against the MCF7 breast cancer cell line is only slightly weaker than that of cisplatin, but with selectivity to the tumor cell line in comparison to normal HaCaT cells, which is non-existent in the case of cisplatin.

A review: Pharmacological aspects of metal based 1,2,4-triazole derived Schiff bases

Clinical reports have highlighted the radical increase of antibiotic resistance. As a result, multidrug resistance has emerged as a serious threat to human health. Many organic compounds commonly used as drugs in the past, no longer have pure organic mode of action rather need bio-transformation or more activation. Bulk of research has shown that they need trace amount of metal ions incorporated within the chemistry of bioactive molecules for enhancement of their potentiality to fight aggressively against resistance. The deficiency of some metal ions can also be responsible for many diseases like growth retardation, pernicious anemia and heart diseases in infants. To overcome these problems, there is a need to introduce novel strategies which have new mechanism of action along with significant spectrum of biological activity, enhanced safety and efficacy. Bioinorganic compounds have played imperative role in developing the new strategy in the form of "Metal Based Drugs". In current years there have been momentous rise of interest in the application of metal based Schiff base compounds to treat various diseases which are difficult to be treated with conventional methodologies. The unique properties of metal chelates acting as an intermediate between conventional organic and inorganic compounds provided innovative opportunities in the field of pharmaceutical chemistry. In this review, we have exclusively focused on the search of metal based 1,2,4-triazole derived Schiff base compounds (synthesized, reported and reviewed in the past ten years) that possess various biological activities such as antifungal, antibacterial, antioxidant, antidiabetic, anthelmintic, anticancer, antiproliferative, cytotoxic and DNA-intercalation activity.

When the Metal Makes the Difference: Template Syntheses of Tridentate and Tetradentate Salen-Type Schiff Base Ligands and Related Complexes

The reaction of organic molecules mediated by a metal center (template synthesis) can result in a final connectivity that may differ from the one obtained in the absence of the metal. The condensation of carbonyl fragments with primary amines form C=N iminic bonds, the so-called Schiff bases, which can act as ligands for the templating metal center by means of the lone pair on the nitrogen atom. This review focuses on the template methods for the reaction between a carbonyl compound (mainly salicylaldehyde) and a primary aliphatic diamine able to prevent the double condensation on both amine groups and obtain tridentate N2O ligands. These adducts, still having one free amino group, can further react, yielding tetradentate salen-type Schiff base ligands. A screening over the transition metals able to show such a template effect will be presented, with particular attention to copper(II), together with their peculiar reactivity and the available crystal structure of the metal complexes and related coordination geometries.

Substitution reactivity and structural variability induced by tryptamine on the biomimetic rhenium tricarbonyl complex

A series of seven fac-[Re(CO)₃(5Me-Sal-Trypt)(L)] complexes containing tryptamine on the N,O 5-methyl-salicylidene bidentate ligand backbone and where L is MeOH, Py, Imi, DMAP, PPh₃ coordinated to the 6th position have been studied, including the formation of a dinuclear Re₂ cluster. The crystallographic solid state structures show marked similarity in structural tendency, in particular the rigidity of the Re core and the hydrogen bond interactions similar to those found in protein structures. The rates of formation and stability of the complexes were evaluated by rapid time-resolved stopped-flow techniques and the methanol substitution reaction indicates the significant activation induced by the use of the N,O salicylidene bidentate ligand as manifested by the second-order rate constants for the entering nucleophiles. Both linear and limiting kinetics were observed and a systematic evaluation of the kinetics is reported clearly indicating an interchange type of intimate mechanism for the methanol substitution. The anticancer activity of compounds 1–7 was tested on HeLa cells and it was found that all compounds showed similar cytotoxicity where solubility allowed. IC₅₀-values between ca. 11 and 22 μM indicate that some cytotoxicity resides most likely on the salicylidene–tryptamine ligand. The photoluminescence of the seven complexes is similar in maximum emission wavelength with little variation despite the broad range of ligands coordinated to the 6th position on the metal centre.

The biomimetic tryptamine rhenium tricarbonyl complex shows rapid substitution reactivity on the 6th position as well as cytotoxicity and photoluminescence capability induced by the salicylidene bidentate ligand.