Transition Metal Complexes with Tridentate Schiff Bases (O N O and O N N) Derived from Salicylaldehyde: An Analysis of Their Potential Anticancer Activity

Although it is known that the first case of cancer was recorded in ancient Egypt around 1600 BC, it was not until 1917 during the First World War and the development of mustard gas that chemotherapy against cancer became relevant; however, its properties were not recognised until 1946 to later be used in patients. In this sense, the use of metallopharmaceuticals in cancer therapy was extensively explored until the 1960s with the discovery of cisplatin and its anticancer activity. From that date to the present, the search for more effective, more selective metallodrugs with fewer side effects has been an area of continuous exploration. Efforts have led to considering a wide variety of metals from the periodic table, mainly from the d-block, as well as a wide variety of organic ligands, preferably with proven biological activity. In this sense, various research groups have found an ideal binder in Schiff bases, since their raw materials are easily accessible, their synthesis conditions are friendly and their denticity can be manipulated. Therefore, in this review, we have explored the anticancer and antitumor activity reported in the literature for coordination complexes of d-block metals coordinated with tridentate Schiff bases (O N O and O N N) derived from salicylaldehyde. For this work, we have used the main scientific databases CCDC® and SciFinder®.

Pincer Complexes Derived from Tridentate Schiff Bases for Their Use as Antimicrobial Metallopharmaceuticals

Within the current challenges in medicinal chemistry, the development of new and better therapeutic agents effective against infectious diseases produced by bacteria, fungi, viruses, and parasites stands out. With chemotherapy as one of the main strategies against these diseases focusing on the administration of organic and inorganic drugs, the latter is generally based on the synergistic effect produced by the formation of metal complexes with biologically active organic compounds. In this sense, Schiff bases (SBs) represent and ideal ligand scaffold since they have demonstrated a broad spectrum of antitumor, antiviral, antimicrobial, and anti-inflammatory activities, among others. In addition, SBs are synthesized in an easy manner from one-step condensation reactions, being thus suitable for facile structural modifications, having the imine group as a coordination point found in most of their metal complexes, and promoting chelation when other donor atoms are three, four, or five bonds apart. However, despite the wide variety of metal complexes found in the literature using this type of ligands, only a handful of them include on their structures tridentate SBs ligands and their biological evaluation has been explored. Hence, this review summarizes the most important antimicrobial activity results reported this far for pincer-type complexes (main group and d-block) derived from SBs tridentate ligands.

Family of Well-Defined Chiral-at-Cobalt(III) Complexes as Metal-Templated Hydrogen-Bond-Donor Catalysts: Effect of Chirality at the Metal Center on the Stereochemical Outcome of the Reaction

A family of well-defined Λ- and Δ-diastereomeric octahedral cationic chiral-at-cobalt complexes were obtained by a simple two-step reaction of (R,R)-1,2-diaminocyclohexane, (R,R)-1,2-diphenylethylenediamine, or (S)-2-(aminomethyl)pyrrolidine and substituted salicylaldehydes with a cobalt(III) salt. It was observed for the first time that the use of an excess of cobalt(III) salt provides both the enantiopure Λ and Δ forms of the corresponding cobalt(III) complexes 1 and 2 in a ratio of diastereomers ranging from 1:1.6 to >20:1 (Λ/Δ) and in 31-95% combined yields. The obtained complexes were robust, air- and bench-stable, soluble in most of organic solvents, and insoluble in water. Through variation of the substituents in the phenyl ring of the salicylaldehyde moiety, it was shown that both steric and electronic effects of substituents have a significant impact on the formation of Λ and Δ isomers. Next, the efficacies of the enantiopure metal-templated complexes 1-3 were investigated in three benchmark asymmetric reactions in order to compare their catalytic activity. The chiral cobalt(III) complexes 1-3 were tested as enantioselective hydrogen-bond-donor catalysts in such important reactions as the Michael addition of the O'Donnell substrate to methyl acrylate, epoxidation of chalcone, and trimethylsilylcyanation of benzaldehyde. It was clearly demonstrated that the chirality at the cobalt center has an impact on the stereochemical outcome of the reactions. In particular, the Λ(R,R)-1 and Δ(R,R)-1 complexes acted as "pseudoenantiomeric" catalysts in the epoxidation and trimethylsilylcyanoation reactions, providing both enantiomers of the products with up to 57% enantiomeric excess.

Evaluation of the anticancer activities with various ligand substituents in Co(II/III)-picolyl phenolate derivatives: synthesis, characterization, DFT, DNA cleavage, and molecular docking studies

The reactions between 2-(pyridine-2-ylmethoxy)-benzaldehyde (L) and various primary amines furnish tridentate (L1 to L3) and tetradentate (L4) chelating ligands. The choice of different primary amines in the condensation reaction incorporates the chiral carbon atom in L2 and L3. A series of mononuclear cobalt(II) complexes, [Co^II(L1)(Cl)₂] (1), [Co^II(L2)(Cl)₂]·CH₃CN (2), [Co^II(L3)(Cl)₂] (3), and [Co^III(L4)(N₃)₂] (4) are synthesized in the pure crystalline state from the resulting solution of cobalt(II) chloride and/or azide and respective ligand. The new ligands and cobalt complexes are characterized using spectral (UV-Vis, ¹H-NMR, IR, and HRMS), cyclovoltammetric, and DFT studies. The structure of L1, L2, and all four cobalt complexes are determined by single X-ray crystallography. Cytotoxic activity of the compounds is evaluated using three different tissues of origin e.g., U-937 (histiocytic lymphoma), HEK293T (embryonic kidney), and A549 (lung carcinoma). The cobalt complexes are more active than the corresponding ligands against U-937 and HEK293T. The MTT assay demonstrates that the cobalt compounds are more effective anticancer agents against U-937 cancer cells than HEK293T and A549. The toxicity order, 1 (7.2 ± 0.3 μM) > 3 (11.4 ± 0.6 μM) > 2 (12 ± 0.1 μM) > 4 (29 ± 1 μM) is observed against U-937 cancer cells. All the compounds induce cell death through an apoptosis mechanism and all are ineffective against PBMCs. The mechanism of activity against U937 cancer cells involves caspase-3 activation and two different mitogen-activated protein kinases attesting the programmed cell death. Among the compounds, complexes 1, 2, and 3 show DNA cleavage activity both in oxidizing (H₂O₂) and reducing (GSH) environments. The mechanistic study reveals that singlet oxygen (¹O₂) is the major species involved in DNA cleavage. The absolute chemical hardness values of the ligands and 4 are relatively higher than 1, 2, and 3, which tacitly support the DNA cleavage experiment. The docking result indicates that the compounds under investigation strongly interact with DNA base pairs through a variety of interactions which attests successfully to the experimental results. A structure-activity relationship has been drawn to correlate the variation of antitumor activity with ligand conformations.

Enantioselective "organocatalysis in disguise" by the ligand sphere of chiral metal-templated complexes

Asymmetric catalysis holds a prominent position among the important developments in chemistry during the 20th century. This was acknowledged by the 2001 Nobel Prize in chemistry awarded to Knowles, Noyori, and Sharpless for their development of chiral metal catalysts for organic transformations. The key feature of the catalysts was the crucial role of the chiral ligand and the nature of the metal ions, which promoted the catalytic conversions of the substrates via direct coordination. Subsequently the development of asymmetric organic catalysis opened new avenues to the synthesis of enantiopure compounds, avoiding any use of metal ions. Recently, an alternative approach to asymmetric catalysis emerged that relied on the catalytic functions of the ligands themselves boosted by coordination to metal ions. In other words, in these hybrid chiral catalysts the substrates are activated not by the metal ions but by the ligands. The activation and enantioselective control occurred via well-orchestrated and custom-tailored non-covalent interactions of the substrates with the ligand sphere of chiral metal complexes. In these metal-templated catalysts, the metal served either as a template (a purely structural role), or it constituted the exclusive source of chirality (metal-centred chirality due to the spatial arrangement of achiral or chiral bi-/tridentate ligands around an octahedral metal centre), and/or it increased the Brønsted acidity of the ligands. Although the field is still in its infancy, it represents an inspiring combination of both metal and organic catalysis and holds major unexplored potential to push the frontiers of asymmetric catalysis. Here we present an overview of this emerging field discussing the principles, applications and perspectives on the catalytic use of chiral metal complexes that operate as "organocatalysts in disguise". It has been demonstrated that these chiral metal complexes are efficient and provide high stereoselective control in asymmetric hydrogen bonding catalysis, phase-transfer catalysis, Brønsted acid/base catalysis, enamine catalysis, nucleophilic catalysis, and photocatalysis as well as bifunctional catalysis. Also, many of the catalysts have been identified as highly effective catalysts at remarkably low catalyst loadings. These hybrid systems offer many opportunities in the synthesis of chiral compounds and represent promising alternatives to metal-based and organocatalytic asymmetric transformations.

A novel Schiff base cobalt(III) complex induces a synergistic effect on cervical cancer cells by arresting early apoptosis stage

A new schiff base cobalt(III) complex [N,N'-bis(2'-hydroxyphenylacetone)-o-ethanediamine] cobalt(III) (M3) has been synthesized and characterized by single X-ray crystallography. The cytotoxicity of complex M3 was evaluated against HeLa, LoVo, A549, A549/cis cancer cell lines, and the normal cell lines LO2 by MTT assays. The IC₅₀ is in the range of 6.27-22.68 μM, which is somewhat lower than cisplatin on the basis of platinum molar concentration. Furthermore, anticancer mechanistic studies showed that the complex M3 inhibited cell proliferation by blocking DNA synthesis and then acted on nuclear division of HeLa cells over time. Moreover, western blot analysis indicated M3 dramatically decreased the target protein c-Myc and KLF5 expression levels, and activated many signaling pathways including ER stress, apoptosis, cell cycle and DNA damage in HeLa. M3 did not affect proteasomal activity.

Physical properties, ligand substitution reactions, and biological activity of Co(iii)-Schiff base complexes

Four cobalt(iii) complexes of the general formula [Co(Schiff base)(L)2]+, where L is ammonia (NH3) or 3-fluorobenzylamine (3F-BnNH2), were synthesized. The complexes were characterized by NMR spectroscopy, mass spectrometry, and X-ray crystallography. Their electrochemical properties, ligand substitution mechanisms, and ligand exchange rates in aqueous buffer were investigated. These physical properties were correlated to the cellular uptake and anticancer activities of the complexes. The complexes undergo sequential, dissociative ligand substitution, with the exchange rates depending heavily on the axial ligands. Eyring analyses revealed that the relative ligand exchange rates were largely impacted by differences in the entropy, rather than enthalpy, of activation for the complexes. Performing the substitution reactions in the presence of ascorbate led to a change in the reaction profile and kinetics, but no change in the final product. The cytotoxic activity of the complexes correlates with both the ligand exchange rate and reduction potential, with the more easily reduced and rapidly substituted complexes showing higher toxicity. These relationships may be valuable for the rational design of Co(iii) complexes as anticancer or antiviral prodrugs.

Advances in cobalt complexes as anticancer agents

The evolution of resistance to traditional platinum-based anticancer drugs has compelled researchers to investigate the cytostatic properties of alternative transition metal-based compounds. The anticancer potential of cobalt complexes has been extensively studied over the last three decades, and much time has been devoted to understanding their mechanisms of action. This perspective catalogues the development of antiproliferative cobalt complexes, and provides an in depth analysis of their mode of action. Early studies on simple cobalt coordination complexes, Schiff base complexes, and cobalt-carbonyl clusters will be documented. The physiologically relevant redox properties of cobalt will be highlighted and the role this plays in the preparation of hypoxia selective prodrugs and imaging agents will be discussed. The use of cobalt-containing cobalamin as a cancer specific delivery agent for cytotoxins will also be described. The work summarised in this perspective shows that the biochemical and biophysical properties of cobalt-containing compounds can be fine-tuned to produce new generations of anticancer agents with clinically relevant efficacies.