Redox-active metal(II) complexes of sterically hindered phenolic ligands: Antibacterial activity and reduction of cytochrome c. Part II. Metal(II) complexes of o-diphenol derivatives of thioglycolic acid

Synthesis and Antioxidant Activity of New Catechol Thioethers with the Methylene Linker

Novel catechol thio-ethers with different heterocyclic substituents at sulfur atom were prepared by reacting 3,5-di-tert-butyl-6-methoxymethylcatechol with functionalized thiols under acidic conditions. A common feature of compounds is a methylene bridge between the catechol ring and thioether group. Two catechols with the thio-ether group, bound directly to the catechol ring, were also considered to assess the effect of the methylene linker on the antioxidant properties. The crystal structures of thio-ethers with benzo-thiazole moieties were established by single-crystal X-ray analysis. The radical scavenging and antioxidant activities were determined using 2,2′-diphenyl-1-picrylhydrazyl radical test, ABTS^∙+, CUPRAC (TEAC) assays, the reaction with superoxide radical anion generated by xanthine oxidase (NBT assay), the oxidative damage of the DNA, and the process of lipid peroxidation of rat liver (Wistar) homogenates in vitro. Most catechol-thioethers exhibit the antioxidant effect, which varies from mild to moderate depending on the model system. The dual anti/prooxidant activity characterizes compounds with adamantyl or thio-phenol substituent at the sulfur atom. Catechol thio-ethers containing heterocyclic groups (thiazole, thiazoline, benzo-thiazole, benzo-xazole) can be considered effective antioxidants with cytoprotective properties. These compounds can protect molecules of DNA and lipids from the different radical species.

Triphenylantimony(V) Catecholates of the Type (3-RS-4,6-DBCat)SbPh3-Catechol Thioether Derivatives: Structure, Electrochemical Properties, and Antiradical Activity

A new series of triphenylantimony(V) 3-alkylthio/arylthio-substituted 4,6-di-tert-butylcatecholates of the type (3-RS-4,6-DBCat)SbPh3, where R = n-butyl (1), n-hexyl (2), n-octyl (3), cyclopentyl (4), cyclohexyl (5), benzyl (6), phenyl (7), and naphthyl-2 (8), were synthesized from the corresponding catechol thioethers and Ph3SbBr2 in the presence of a base. The crystal structures of 1, 2, 3, and 5 were determined by single-crystal X-ray analysis. The coordination polyhedron of 1-3 is better described as a tetragonal pyramid with a different degree of distortion, while that for 5- was a distorted trigonal bipyramid (τ = 0.014, 0.177, 0.26, 0.56, respectively). Complexes demonstrated different crystal packing of molecules. The electrochemical oxidation of the complexes involved the catecholate group as well as the thioether linker. The introduction of a thioether fragment into the aromatic ring of catechol ligand led to a shift in the potential of the "catechol/o-semiquinone" redox transition to the anodic region, which indicated the electron-withdrawing nature of the RS group. The radical scavenging activity of the complexes was determined in the reaction with DPPH radical.

Metal Complexes as Promising Agents for Biomedical Applications

Background::

In this review article, a brief overview of novel metallotherapeutic agents (with an emphasis on the complexes of essential biometals) promising for medical application is presented. We have also focused on the recent work carried out by our research team, specifically the development of redox-active antimicrobial complexes of sterically hindered diphenols with some essential biometals (copper, zinc, nickel).

Results::

The complexes of essential metals (manganese, iron, cobalt, nickel, copper, zinc) described in the review show diverse in vitro biological activities, ranging from antimicrobial and antiinflammatory to antiproliferative and enzyme inhibitory. It is necessary to emphasize that the type of organic ligands in these metal complexes seems to be responsible for their pharmacological activities. In the last decades, there has been a significant interest in synthesis and biological evaluation of metal complexes with redox-active ligands. A substantial step in the development of these redox-active agents is the study of their physicochemical and biological properties, including investigations in vitro of model enzyme systems, which can provide evidence on a plausible mechanism underlying the pharmacological activity. When considering the peculiarities of the pharmacological activity of the sterically hindered diphenol derivatives and their nickel(II), copper(II) and zinc(II) complexes synthesized, we took into account the following: (i) all these compounds are potential antioxidants and (ii) their antimicrobial activity possibly results from their ability to affect the electron-transport chain.

Conclusion::

We obtained novel data demonstrating that the level of antibacterial and antifungal activity in the series of the above-mentioned metal-based antimicrobials depends not only on the nature of the phenolic ligands and complexing metal ions, but also on the lipophilicity and reducing ability of the ligands and metal complexes, specifically regarding the potential biotargets of their antimicrobial action – ferricytochrome c and the superoxide anion radical. The combination of antibacterial, antifungal and antioxidant activity allows one to consider these compounds as promising substances for developing therapeutic agents with a broad spectrum of activities.

Polyfunctional Sterically Hindered Catechols with Additional Phenolic Group and Their Triphenylantimony(V) Catecholates: Synthesis, Structure, and Redox Properties

New polyfunctional sterically hindered 3,5-di-tert-butylcatechols with an additional phenolic group in the sixth position connected by a bridging sulfur atom—(6-(CH₂-S-tBu₂Phenol)-3,5-DBCat)H₂ (L₁), (6-(S-tBu₂Phenol)-3,5-DBCat)H₂ (L₂), and (6-(S-Phenol)-3,5-DBCat)H₂ (L₃) (3,5-DBCat is dianion 3,5-di-tert-butylcatecolate)—were synthesized and characterized in detail. The exchange reaction between catechols L₁ and L₃ with triphenylantimony(V) dibromide in the presence of triethylamine leads to the corresponding triphenylantimony(V) catecholates (6-(CH₂-S-tBu₂Phenol)-3,5-DBCat)SbPh₃ (1) and (6-(S-Phenol)-3,5-DBCat)SbPh₃ (2). The electrochemical properties of catechols L₁–L₃ and catecholates 1 and 2 were investigated using cyclic voltammetry. The electrochemical oxidation of L₁–L₃ at the first stage proceeds with the formation of the corresponding o-benzoquinones. The second process is the oxidation of the phenolic moiety. Complexes 1 and 2 significantly expand their redox capabilities, owing to the fact that they can act as the electron donors due to the catecholate metallocycle capable of sequential oxidations, and as donors of the hydrogen atoms, thus forming a stable phenoxyl radical. The molecular structures of the free ligand L₁ and complex 1 in the crystal state were determined by single-crystal X-ray analysis.