Metal Complexes of Oxadiazole Ligands: An Overview

Antimicrobial, anticancer activities, molecular docking, and DFT/B3LYP/LANL2DZ analysis of heterocyclic cellulose derivative and their Cu-complexes

In this study, novel Cu-complexes of heterocyclic cellulose which were synthesized via the reaction of carboxymethyl cellulose (CMC) from bagasse pulp with NH2NH2 to give hydrazide cellulose which easily reacted with CS2 to form salt and then cyclized in the presence of HCl to afford cellulose oxadiazole, or with hydrazine hydrate to give cellulose triazole. Furthermore, the cellulose oxadiazole and triazole moieties acting as chelating agents with metal ion Cu (II), and all synthesized compounds were examined for their spectral analysis to show the adsorption of Cu (II) on the surface of cellulose through intramolecular hydrogen bonding. Results illustrated that cellulose oxadiazole and Cu- cellulose oxadiazole exhibited antimicrobial activities more than triazole and Cu- cellulose triazole. Furthermore, anticancer results showed that both cellulose oxadiazole and triazole exhibited activity higher than Cu-cellulose oxadiazole and Cu-cellulose triazole, where the cellulose triazole showed the highest activity (IC50 = 58.7 μg/μL). Additionally, the docking simulation of the synthesized cellulose complexes with different proteins such as PDBID:3t88, PDBID:4ynt, PDBID:1tgh, PDBID:2wje, and PDBID:4hdq and shortage bond length to confirm the experimental results. Optimization of metal complexes utilized the DFT/B3LYP/LANL2DZ basis set to confirm the stability of these metals theoretically and their physical descriptors and FMO analysis.

Bioactive Oxadiazoles 3.0

Heterocycles are fundamental moieties for the construction of new compounds with perspective applications ranging from drugs to materials [...].

1,3,4-Oxadiazoles as Anticancer Agents: A Review

Among the deadliest diseases, cancer is characterized by tumors or an increased number of a specific type of cell because of uncontrolled divisions during mitosis. Researchers in the current era concentrated on the development of highly selective anticancer medications due to the substantial toxicities of conventional cytotoxic drugs. Several marketed drug molecules have provided resistance against cancer through interaction with certain targets/growth factors/enzymes, such as Telomerase, Histone Deacetylase (HDAC), Methionine Aminopeptidase (MetAP II), Thymidylate Synthase (TS), Glycogen Synthase Kinase-3 (GSK), Epidermal Growth Factor (EGF), Vascular Endothelial Growth Factor (VEGF), Focal Adhesion Kinase (FAK), STAT3, Thymidine phosphorylase, and Alkaline phosphatase. The molecular structure of these drug molecules contains various heterocyclic moieties that act as pharmacophores. Recently, 1,3,4- oxadiazole (five-membered heterocyclic moiety) and its derivatives attracted researchers as these have been reported with a wide range of pharmacological activities, including anti-cancer. 1,3,4- oxadiazoles have exhibited anti-cancer potential via acting on any of the above targets. The presented study highlights the synthesis of anti-cancer 1,3,4-oxadiazoles, their mechanism of interactions with targets, along with structure-activity relationship concerning anti-cancer potential.

Improved electroluminescence efficiency derived from functionalized decoration of 1,3,4-oxadiazole (OXD)-based Ir(III) complexes

The performance of organic light emitting devices (OLEDs) fabricated using Ir(III) complexes bearing 1,3,4-oxadiazole (OXD)-based cyclometallic ligands still needs to be improved. In this work, Ir3+ was coordinated with a 2-(9,9-diethyl-9H-fluoren-2-yl)-1,3,4-oxadiazole (F-OXD) fragment, which was modified with various functionalized substituents, including fluorenyl, OXD and carbazolyl groups. Three complexes, named Ir-Flu, Ir-OXD and Ir-Cz, were synthesized successfully and their photophysical, electrochemical and electroluminescence properties were investigated in detail. All these complexes exhibited yellow-orange emission in solution and a distinct aggregation-induced phosphorescent emission (AIPE) phenomenon was observed. Monochrome OLEDs were fabricated using these phosphorescent dopants, and the turn-on voltage (V), luminance (L) and current efficiency (CE) showed significant improvement compared to analogous OXD-based Ir(III) complexes reported before. In particular, the device with Ir-OXD as the dopant achieved the highest maximum brightness of 25 014 cd m-2 and the lowest efficiency roll-off (42.6%) at the maximum luminance among all the devices. These results provided a proven strategy of functionalized decoration of OXD-based complexes to achieve superior luminous efficiency devices.

Expanding the Library of 1,2,4-Oxadiazole Derivatives: Discovery of New Farnesoid X Receptor (FXR) Antagonists/Pregnane X Receptor (PXR) Agonists

Compounds featuring a 1,2,4-oxadiazole core have been recently identified as a new chemotype of farnesoid X receptor (FXR) antagonists. With the aim to expand this class of compounds and to understand the building blocks necessary to maintain the antagonistic activity, we describe herein the synthesis, the pharmacological evaluation, and the in vitro pharmacokinetic properties of a novel series of 1,2,4-oxadiazole derivatives decorated on the nitrogen of the piperidine ring with different N-alkyl and N-aryl side chains. In vitro pharmacological evaluation showed compounds 5 and 11 as the first examples of nonsteroidal dual FXR/Pregnane X receptor (PXR) modulators. In HepG2 cells, these compounds modulated PXR- and FXR-regulated genes, resulting in interesting leads in the treatment of inflammatory disorders. Moreover, molecular docking studies supported the experimental results, disclosing the ligand binding mode and allowing rationalization of the activities of compounds 5 and 11.

Design, synthesis, DFT calculations, molecular docking and antimicrobial activities of novel cobalt, chromium metal complexes of heterocyclic moiety-based 1,3,4-oxadiazole derivatives

A Schiff base, 5-(4-methylphenyl)-4-[(pyridin-2-ylmethylidene)amino]-4H-1,2,4-oxadiazole as a bidentate ligand has been synthesized by the reaction between the 4-amino-5-(4-methylphenyl)-4H-1,3,4-oxadiazole and aromatic aldehyde. The Schiff base reacted with CoCl₃·6H₂O and CrCl₃·6H₂O in ethanol to yield 1,3,4-oxadiazole complexes. The structures of synthesized ligand and their complexes have been established on the basis of their IR, Mass and ¹H-NMR spectra. Electronic and geometric structures of both cobalt and chromium complexes were investigated by density functional theory (DFT) and quantum theory of atoms in molecules (QTAIM) calculations. DFT-based reactivity calculations estimated the studied system as strong electrophile and/or strong nucleophile in polar organic reactions. Moreover, most reactive sites were predicted theoretically based on the delocalized and localized indexes. The nature of Ligand-Metal chemical bonding is discussed in terms of the natural bond orbital (NBO) and QTAIM analysis. Accordingly, the metal ions such as cobalt and chromium are bidentate coordinated with the Schiff base by nitrogen atoms of imine function and pyridine, to form stable complexes. Furthermore, the chromium ions have an affinity superior to the cobalt ions towards Schiff base ligand. In addition, the results of the antibacterial activity in-vitro show that the metal complexation confers an increase in the antibacterial activity of the complexed ligand compared to the free ligand against both Gram-positive and Gram-negative bacteria with broad spectrum activity. In silico molecular docking studies of the ligands and their complexes were applied to describe the probable binding modes into the active site of Escherichia coli (E. coli) FabH and Salmonella typhimurium LT2 neuraminidase (STNA) receptors. The increase in biological activity could be attributed to the high stability of the complexes and strong affinities to bacterial enzyme receptors.Communicated by Ramaswamy H. Sarma.

Palladium(II) and Platinum(II) Deprotonated Diaminocarbene Complexes Based on N-(2-Pyridyl)ureas with Oxadiazole Periphery

Metal mediated coupling of isocyanides with substituted N-(pyridine-2-yl) ureas was first used to incorporate privileged biological motifs into platinum metal complexes. We synthesized two palladium(II) and two platinum(II) cyclometallated species with oxadiazole cores. The compounds were isolated in good yields (61–73%) and characterized by high-resolution mass spectrometry and 1H, 13C, and 195Pt NMR spectroscopies. The structures of three complexes were additionally elucidated by X-ray diffraction analysis. These complexes indeed showed cytotoxic activity. The species bearing the 1,3,4-oxadiazole moiety exhibit more potency than the ones with the 1,2,4-oxadiazole ring. Particularly, the cytotoxic effect of both 1,3,4-oxadiazole-based complexes towards T98G cells significantly exceeds the common antitumor metal-drug cisplatin.

Synthesis, crystal structure and Hirshfeld surface analysis of a zinc(II) coordination polymer of 5-phenyl-1,3,4-oxadiazole-2-thiolate

The mol­ecular and crystal structure of a zinc coordination polymer with 5-phenyl-1,3,4-oxa­diazole-2-thiol­ate were studied and Hirshfeld surfaces and fingerprint plots were generated to investigate various inter­molecular inter­actions.

A new zinc coordination polymer with 5-phenyl-1,3,4-oxa­diazole-2-thiol­ate, namely, catena-poly[zinc(II)-bis­(μ₂-5-phenyl-1,3,4-oxa­diazole-2-thiol­ato)-κ²N³:S;κ²S:N³], [Zn(C₈H₅N₂OS)₂]_n, was synthesized. The single-crystal X-ray diffraction analysis shows that the polymeric structure crystallizes in the centrosymmetric monoclinic C2/c space group. The Zn^II atom is coordinated to two S and two N atoms from four crystallographically independent (L) ligands, forming zigzag chains along the [001] direction. This polymer complex forms an eight-membered [Zn–S–C–N–Zn–S–C–N] chair-like ring with two Zn^II atoms and two ligand mol­ecules. On the Hirshfeld surface, the largest contributions come from the short contacts such as van der Waals forces, including H⋯H, C⋯H and S⋯H. Inter­actions including N⋯H, O⋯H and C⋯C contacts were also observed; however, their contribution to the overall stability of the crystal lattice is minor.

Potential Synthetic Routes and Metal-Ion Sensing Applications of 1,3,4-Oxadiazoles: An Integrative Review

Oxadiazoles, especially 1,3,4-oxadiazole scaffolds, stand among the foremost heterocyclic fragments with a broad spectrum of applications in diverse fields, including pharmacology, polymers, material science, and organic electronics, among others. In this comprehensive review, we summarize the pivotal synthetic strategies for 1,3,4-oxadiazole derivatives including dehydrogenative cyclization of 1,2-diacylhydrazines, oxidative cyclization of acylhydrazones, condensation cyclization, C-H activation of oxadiazole ring, decarboxylative cyclization and oxidative annulation along with plausible mechanisms. The set of 1,3,4-oxadiazoles selected from the literature and discussed herein epitomize the ease of synthesis as well as the possibility of linking π-conjugated groups; thereby encouraging the use of these molecules as important starting building blocks for a wide variety of fluorescent frameworks, particularly in the development of potential chemosensors. High photoluminescent quantum yield, excellent thermal and chemical stability, and the presence of potential coordination (N and O donor atoms) sites make these molecules a prominent choice for metal-ions sensors. An overview of selective metal-ion sensing, the detection limit along with the sensing mechanisms (photo-induced electron transfer, excited-state intramolecular proton transfer, and complex formation) is also included.

Spectroscopic Recognition of Metal Ions and Non-Linear Optical (NLO) Properties of Some Fluorinated Poly(1,3,4-Oxadiazole-Ether)s

In this paper, we examined the sensing ability of some fluorinated 1,3,4-oxadiazole-containing assemblies toward various metal ions and their nonlinear optical (NLO) properties. The changes in the spectral characteristics of these compounds in the existence of Mg2+, Mn2+, Ni2+, Cd2+, Zn2+, Co2+, Cu2+, Hg2+, Sn2+, and Ag+ metal ions were performed, and they were found to be selective and more sensitive toward the addition of Ag+, Co2+, and Cu2+ ions (new bands appeared). Instead, spectral changes in the presence of Mg2+, Mn2+, Ni2+, Cd2+, Zn2+, Hg2+, and Sn2+ were not significant, so we did not evaluate the corresponding binding parameters. Therefore, all of these compounds were found to be selective and sensitive to Ag+, Co2+, and Cu2+ ions. Furthermore, the first-order polarizability (αCT), the first-order hyperpolarizability (βCT), and the second-order hyperpolarizability (γCT) were evaluated using the solvatochromic approach, and the intramolecular charge transfer (ICT) characteristics were investigated using a generalized Mulliken–Hush (GMH) analysis.

Facile strategy to incorporate amidoxime groups into elastomers toward self-crosslinking and self-reinforcement

Amidoxime modification of NBR and the formation of a multi-crosslinking network structure by self-crosslinking of AO-NBR.

Multi-activity tetracoordinated pallado-oxadiazole thiones as anti-inflammatory, anti-Alzheimer, and anti-microbial agents: Structure, stability and bioactivity comparison with pallado-hydrazides

Herein, we report a comparative study based on structure, thermal and solution stability, and biopotency against lipoxygenase (LOX), butyrylcholinesterase (BChE) and microbes for Pd(II) compounds of N,O,S bearing 5-(C₅H₄XR)-1,3,4-oxadiazole-2-thiones (L') of type [PdL'Cl₂] (P'n) and N,O bearing respective hydrazides (L) of type trans-[PdL₂Cl₂] (Pn) {X = C, R = 4-I, 2-Br, 4-NO₂, 3-NO₂, 2-Cl, 3-Cl (n = 1-6, serially); X = N (n = 7)}. Spectral techniques (IR, EI-MS, NMR) and physicochemical evaluations successfully characterized the new compounds. The L' behaved as bidentate S-N donors bonded through exocyclic sulfur and N-3' nitrogen, while L acted as amino N donors. UV-vis (solution speciation) and thermal degradation profiles consistently confirmed the greater stability for P'n than Pn compounds. These compounds manifested varying degree in vitro potential to inhibit LOX, BChE and several bacteria and fungi, affected mainly by Pd(II) presence, M-L binding mode, nature and position of R, or halo groups electronegativity. Molecular docking with human 5-LOX and BChE further validated the respective experimental inhibition findings and explored several putative mechanistic interactions (H-bonding, π-stacking, π-alkyl, π-S, etc.) at the enzyme active sites. Pn generally offered superior antimicrobial and anti-LOX (anti-inflammatory) potential than respective P'n compounds, with P3/P'5, P(2,3,7)/P'3, and P6 being comparable, better and equivalent to ampicillin, nystatin and baicalein, the reference antibacterial, antifungal and anti-LOX drugs, respectively. Contrarily, the anti-BChE activity of P'n was found better than Pn compounds, showing P'2/P1 as the most promising anti-Alzheimer drug candidates. This study bares important structural and mechanistic aspects in optimizing antimicrobial, anti-inflammatory and anti-Alzheimer activities, highlighting some potential future pallado-drug candidates.

Ammonium Formate-Pd/C as a New Reducing System for 1,2,4-Oxadiazoles. Synthesis of Guanidine Derivatives and Reductive Rearrangement to Quinazolin-4-Ones with Potential Anti-Diabetic Activity

1,2,4-Oxadiazole is a heterocycle with wide reactivity and many useful applications. The reactive O-N bond is usually reduced using molecular hydrogen to obtain amidine derivatives. NH4CO2H-Pd/C is here demonstrated as a new system for the O-N reduction, allowing us to obtain differently substituted acylamidine, acylguanidine and diacylguanidine derivatives. The proposed system is also effective for the achievement of a reductive rearrangement of 5-(2'-aminophenyl)-1,2,4-oxadiazoles into 1-alkylquinazolin-4(1H)-ones. The alkaloid glycosine was also obtained with this method. The obtained compounds were preliminarily tested for their biological activity in terms of their cytotoxicity, induced oxidative stress, α-glucosidase and DPP4 inhibition, showing potential application as anti-diabetics.

Beyond direct Nrf2 activation; reinvestigating 1,2,4-oxadiazole scaffold as a master key unlocking the antioxidant cellular machinery for cancer therapy

Harnessing the antioxidant cellular machinery has sparked considerable interest as an efficient anticancer strategy. Activating Nrf2, the master switch of the cellular redox system, suppresses ROS, alleviates oxidative stress, and halts cancer progression. 1,2,4-oxadiazoles are iconic direct Nrf2 activators that disrupt Nrf2 interaction with its endogenous repressor Keap1. This study introduces rationally designed 1,2,4-oxadiazole derivatives that inhibit other Nrf2 suppressors (TrxR1, IKKα, and NF-kB) thus enhancing Nrf2 activation for preventing oxidative stress and carcinogenesis. Preliminary screening showed that the phenolic oxadiazoles 11, 15, and 19 were comparable to ascorbic acid (ROS scavenging) and EDTA (iron chelation), and superior to doxorubicin against HepG-2, MDA-MB231, and Caco-2 cells. They suppressed ROS by 3 folds and activated Nrf2 by 2 folds in HepG-2 cells. Mechanistically, they inhibited TrxR1 (IC₅₀; 13.19, 17.89, and 9.21 nM) and IKKα (IC₅₀; 11.0, 15.94, and 19.58 nM), and downregulated NF-κB (7.6, 1.4 and 1.9 folds in HepG-2), respectively. They inhibited NADPH oxidase (IC₅₀; 16.4, 21.94, and 10.71 nM, respectively) that potentiates their antioxidant activities. Docking studies predicted their important structural features. Finally, they recorded drug-like in silico physicochemical properties, ADMET, and ligand efficiency metrics.

Groundbreaking Anticancer Activity of Highly Diversified Oxadiazole Scaffolds

Nowadays, an increasing number of heterocyclic-based drugs found application in medicinal chemistry and, in particular, as anticancer agents. In this context, oxadiazoles—five-membered aromatic rings—emerged for their interesting biological properties. Modification of oxadiazole scaffolds represents a valid strategy to increase their anticancer activity, especially on 1,2,4 and 1,3,4 regioisomers. In the last years, an increasing number of oxadiazole derivatives, with remarkable cytotoxicity for several tumor lines, were identified. Structural modifications, that ensure higher cytotoxicity towards malignant cells, represent a solid starting point in the development of novel oxadiazole-based drugs. To increase the specificity of this strategy, outstanding oxadiazole scaffolds have been designed to selectively interact with biological targets, including enzymes, globular proteins, and nucleic acids, showing more promising antitumor effects. In the present work, we aim to provide a comprehensive overview of the anticancer activity of these heterocycles, describing their effect on different targets and highlighting how their structural versatility has been exploited to modulate their biological properties.

Coordination Polymers Based on Highly Emissive Ligands: Synthesis and Functional Properties

Coordination polymers are constructed from metal ions and bridging ligands, linking them into solid-state structures extending in one (1D), two (2D) or three dimensions (3D). Two- and three-dimensional coordination polymers with potential voids are often referred to as metal-organic frameworks (MOFs) or porous coordination polymers. Luminescence is an important property of coordination polymers, often playing a key role in their applications. Photophysical properties of the coordination polymers can be associated with intraligand, metal-centered, guest-centered, metal-to-ligand and ligand-to-metal electron transitions. In recent years, a rapid growth of publications devoted to luminescent or fluorescent coordination polymers can be observed. In this review the use of fluorescent ligands, namely, 4,4'-stilbenedicarboxylic acid, 1,3,4-oxadiazole, thiazole, 2,1,3-benzothiadiazole, terpyridine and carbazole derivatives, naphthalene diimides, 4,4',4''-nitrilotribenzoic acid, ruthenium(II) and iridium(III) complexes, boron-dipyrromethene (BODIPY) derivatives, porphyrins, for the construction of coordination polymers are surveyed. Applications of such coordination polymers based on their photophysical properties will be discussed. The review covers the literature published before April 2020.

Novel 3'-Substituted-1',2',4'-Oxadiazole Derivatives of 18βH-Glycyrrhetinic Acid and Their O-Acylated Amidoximes: Synthesis and Evaluation of Antitumor and Anti-Inflammatory Potential In Vitro and In Vivo

A series of novel 18βH-glycyrrhetinic acid (GA) derivatives containing 3'-(alkyl/phenyl/pyridin(-2″, -3″, and -4″)-yl)-1',2',4'-oxadiazole moieties at the C-30 position were synthesized by condensation of triterpenoid's carboxyl group with corresponding amidoximes and further cyclization. Screening of the cytotoxicity of novel GA derivatives on a panel of tumor cell lines showed that the 3-acetoxy triterpenoid intermediates-O-acylated amidoxime 3a-h-display better solubility under bioassay conditions and more pronounced cytotoxicity compared to their 1',2',4'-oxadiazole analogs 4f-h (median IC50 = 7.0 and 49.7 µM, respectively). Subsequent replacement of the 3-acetoxy group by the hydroxyl group of pyridin(-2″, 3″, and -4″)-yl-1',2',4'-oxadiazole-bearing GA derivatives produced compounds 5f-h, showing the most pronounced selective toxicity toward tumor cells (median selectivity index (SI) > 12.1). Further detailed analysis of the antitumor activity of hit derivative 5f revealed its marked proapoptotic activity and inhibitory effects on clonogenicity and motility of HeLa cervical carcinoma cells in vitro, and the metastatic growth of B16 melanoma in vivo. Additionally, the comprehensive in silico study revealed intermediate 3d, bearing the tert-butyl moiety in O-acylated amidoxime, as a potent anti-inflammatory candidate, which was able to effectively inhibit inflammatory response induced by IFNγ in macrophages in vitro and carrageenan in murine models in vivo, probably by primary interactions with active sites of MMP9, neutrophil elastase, and thrombin. Taken together, our findings provide a basis for a better understanding of the structure-activity relationship of 1',2',4'-oxadiazole-containing triterpenoids and reveal two hit molecules with pronounced antitumor (5f) and anti-inflammatory (3d) activities.

Crystal structure of poly[(μ3-4-amino-1,2,5-oxa-diazole-3-hydroxamato)thallium(I)]

The title compound represents the thallium(I) salt of a substituted 1,2,5-oxa-diazole, [Tl(C3H3N4O3)] n , with amino- and hydroxamate groups in the 4- and 3- positions of the oxa-diazole ring, respectively. In the crystal, the deprotonated hydroxamate group represents an inter-mediate between the keto/enol tautomers and forms a five-membered chelate ring with the thallium(I) cation. The coordination sphere of the cation is augmented to a distorted disphenoid by two monodentately binding O atoms from two adjacent anions, leading to the formation of zigzag chains extending parallel to the b axis. The cohesion within the chains is supported by π-π stacking [centroid-centroid distance = 3.746 (3) Å] and inter-molecular N-H⋯N hydrogen bonds.