A. I. Poddel’sky

3,6-di-tert-butylcatecholates of triaryl antimony(V): NMR study and redox-transformations

Recently the ability of complexes of non-transition metals to reversibly add molecular oxygen was found by the example of о-amidophenolate and catecholate (Cat) complexes of triphenyl antimony(V) with donor methoxy groups in the 4 and 5 positions of the aromatic ring of the catecholate ligand [1, 2]. Variation in the nature of the substituents in the Cat ligand affects the reactivity of these compounds with respect to oxygen: The introduction of electron-withdrawing groups results in the formation of the air-stable catecholate s, whereas the presence of electron-donor groups makes it possible for these complexes to add molecular oxygen [3–7]. The investigation of electrochemical properties of the complexes has shown that the acceptor groups in the Cat ligand shift the oxidation potential of the catecholate to electropositive region (hamper the anodic process) and decrease the stability of the cationic complexes; the donor substituents, vice versa, facilitate oxidation of the Cat ligand with molecular oxygen (shift the oxidation potential to the cathode field), favoring interaction with oxygen. However, at present there are no data in the literature on the effect of the redox-inert organic fragments at the antimony atom on redox-transformations of the Cat ligand in complexes of the formula (Cat)SbAr3.

Trialkylantimony(V) o-amidophenolates: Electrochemical transformations and antiradical activity

The electrochemical transformations and antiradical activity of trialkylantimony(V) o-amidophenolate derivatives, (AP)SbR3 (AP = 4,6-di-tert-butyl-N-(2,6-diisopropylphenyl)-o-amidophenolate); R = CH3 (I), C2H5 (II), and C6H11 (III), are studied. The electrochemical oxidation of compounds I–III proceeds successively to form mono- and dicationic forms of the complexes. The presence of the donor hydrocarbon groups at the antimony(V) atom shifts the oxidation potentials to the cathodic range and decreases the stability of the monocationic complexes formed in electrochemical oxidation. The second anodic process is irreversible and accompanied by o-iminoquinone decoordination. The antiradical activity of compounds I–III is studied in the reaction with the diphenylpicrylhydrazyl radical and oleic acid autooxidation. The values obtained for indices EC50 and IC50 indicate the antiradical activity of the studied compounds. Complexes I–III were found to be the efficient inhibitors of oleic acid oxidation and act as efficient destructors of hydroperoxides.

The influence of Ph3Sb(V)L complexes with redox-active ligands on in vivo lipid peroxidation

72 Coordination compounds of antimony(III/V) attract researchers’ attention owing to the diverse modes of coordination and pharmacological activity [1, 2]. Study of the biological properties of new anti mony complexes is a topical issue in modern bioinor ganic chemistry [3]. Currently, data on the toxicity of antimony(III,V) organic compounds are rather scarce [4, 5]; meanwhile, inorganic compounds of antimony are known to exhibit genotoxicity in vivo and in vitro caused by the irreversible binding to the thiol contain ing protein groups [6]. The pentavalent antimony derivatives are considered to be less toxic than trivalent ones and are regarded as precursors of pharmacologi cally active forms. Currently, there is no clear understanding of the mechanism of the toxic action of antimony on biolog ical objects. The toxicity of antimony(III) compounds is often attributed to the generation of reactive oxygen species (ROS) and initiation of the oxidative stress [7]. The reactivity of complex compounds varies depend ing on the ligand nature; therefore, the toxic effect can be regularly leveled due to variation of the coordina tion environment of the metal. The combination of redox active ligands with heavy metals gives rise to unusual physicochemical properties. Compounds 1–3 studied here are the first examples of main group metal complexes capable of reversible binding of molecular oxygen under mild conditions [8, 9].

Triaryl- and trialkylantimony(V) Bis(catecholates) based on 1,1′-spirobis[3,3-dimethylindanequinone-5,6]: Spectroscopic and electrochemical studies

A series of new binuclear bis(catecholate) antimony(V) complexes based on 1,1′-spirobis[3,3-dimethylindanequinone-5,6] with various substituents at the central antimony atoms, R3Sb(Cat-Spiro-Cat)SbR3 (I–IV) and R3Sb(CatBr-Spiro-BrCat)SbR3 (V–VIII) (R = p-fluorophenyl, phenyl, p-tolyl, and ethyl), were synthesized. Spirobis(catecholates) I–III exhibit two one-electron oxidation waves on the cyclic voltammograms, whereas bromo-substituted spirobis(catecholates) V–VII undergo two-electron oxidation immediately at the first stage. The two-electron oxidation of the complexes results in the loss of one of the organoantimony fragments and the formation of mononuclear catecholate-quinone complexes (Q-Spiro-Cat)SbR3 or (QBr-Spiro-BrCat)SbR3, respectively. An insignificant delocalization of the charge and spin between two redox centers is observed in the complexes. The nature of substituents at the antimony atom exerts an effect on the values of redox potentials of the complexes: more donating groups decrease the oxidation potentials of the catecholate fragments and more withdrawing groups increases these values.

Synthesis and ESR spectra of [4,6-di-tert-butyl-N-(2,6-diisopropylphenyl)-o-iminobenzosemiquinonato]thallium(i)

Abstract[4,6-Di-tert-butyl-N-(2,6-diisopropylphenyl)-o-iminobenzosemiquinonato]thallium(i) was synthesized and characterized by IR and ESR spectroscopy. The hyperfine coupling constants with 203/205Tl nuclei were found to depend strongly on the nature (solvating ability) of solvents. At 298 K, the HFC constant aTl changes from 3.09 mT in n-hexane up to 19.70 mT in tetramethylethylenediamine. The coordination number of solvation was found to be 1 for DMF—benzene and pyridine—hexane systems. The thermodynamic characteristics of solvation in the pyridine—hexane system were determined.

Transition metal complexes with “non-innocent” ligands in the activation of hydrogen sulfide

The reactions of hydrogen sulfide with transition metal complexes containing redox-active ligands are studied. A combination of electrochemical and spectral data indicates that the one-electron process affording the hydrogen sulfide radical and monoanionic complexes is an elementary act for the most part of the reactions studied. The accessibility of the metal center in the Co, Ni, Zn, and Pt complexes allows hydrogen sulfide to preliminary coordinate to the metal followed by the inner-sphere electron transfer in the hydrogen sulfide-metal-organic ligand system. Active intermediates (radical cation, thiyl radical, and proton) formed due to oxidation react with aromatic substrates. The substitution reaction in the aromatic ring produces a mixture of isomeric thiols and dimerization products of organylthiyl radicals (disulfides).

Synthesis and antiradical activity of new triphenylantimony(V) catechlolates derived from alkyl gallates

45 Antimony(III/V) compounds are used for a long time in medical practice as efficient antiparasitic drugs, which exhibit a number of side effects. In many cases, the appearance of side effects is caused by the alteration of intracellular redox balance owing to the reaction of the active form of the pharmaceutical with SH groups of protein molecules and production of reactive oxygen species (ROS) and oxidative stress ini tiation [1–3]. One of the methods to reduce the side effect of antimony pharmaceuticals consists in their combination with antioxidants: ascorbic acid and vita min E. This allows one to decrease considerably car dio and hepatotoxicity [2, 4]. Recent studies showed that the combination of redox active ligands with organometallic fragment leads to the emergence of pharmacological activity of such compounds [5, 6]. The introduction of biologi cally active redox center showing antioxidant activity into the structure of nontransition metal compound opens an opportunity to modulate not only the reac tivity on account of an increase in the number of redox states but also the pharmacological activity. We have recently found that triorganylantimony(V) catecholate complexes show antiradical activity: they behave as inhibitors of the lipid peroxidation process in vitro and in vivo [7–9]. The compounds of this class can be obtained in high preparative yield [10], but the preparation of initial o benzoquinones is a rather laborious process. Therefore, we used commercially available esters of gallic (3,4,5 trihydroxybenzoic) acid showing antioxidant activity as ligands containing redox active catecholate fragment. Recent studies showed that triarylantimony(V) dicarboxylates based on substituted benzoic acid derivatives, including gallic acid ethers, exhibit high antiparasitic activity [11, 12].

Reversible binding of molecular oxygen to catecholate and o-amidophenolate complexes of SbV: energy approach

The theoretical study of the reactivity of catecholate and o-amidophenolate complexes of SbV in the reversible binding of molecular oxygen was performed. The evaluation of the strength of intramolecular interactions in pre-reaction complexes provides the assessment of the strength of oxygen binding in the coordination sphere of Sb. The initial complexes, their transition states, and final spiroendoperoxide complexes were modeled. The calculated activation energies of the reaction and ionization can serve as the energy criteria providing the explanation and prediction of interactions of these complexes with molecular oxygen.

Anti- and prooxidant activity of triphenylantimony(V) catecholates derived from alkyl gallates

Antiradical and antiand prooxidant activities of the newly obtained triphenylantimony(V) catecholate complexes derived from gallic acid esters of the general formulas (RO(O)C–Cat)SbPh3 (R = Me (1), C8H17 (2), C12H25 (3)) and (MeO(O)C–(OH•NEt3)Cat)SbPh3 (4) were studied. The comparative study involving gallic acid esters of the general formula RO(O)C–GallH3 (R = Me (5), C8H17 (6), C12H25 (7)) was performed in order to evaluate the effect of organic ligands on the properties of the antimony(V) compounds. The values of EC50, the number of converted molecules of stable radical (nDPPH), as well as the relative content of oleic acid hydroperoxides confirm the antiradical and antioxidant activity of triphenylantimony(V) complexes in reactions with diphenylpicrylhydrazyl radicals and in the antioxidation of oleic acid. The influence of more active compounds 1 and 4 and ester 5 on lipid peroxidation of the rat liver homogenate of male Wistar line was determined. The membraneprotective properties were evaluated for induced hemolysis of erred blood cells as an example, and catalase activity of erythrocyte hemolysate was studied.

Penta- and hexacoordinate antimony(V) compounds with the tridentate O,N,O-donor ligand: Electrochemical transformations and antiradical activity

The electrochemical transformations and antiradical activity of penta- and hexacoordinate antimony(V) complexes I–V containing the tridentate O,N,O-donor ligand, N,N-bis(di-3,5-tert-butyl-2-hydroxyphenyl)amine, are studied. The oxidation of hexacoordinate triarylantimony(V) compounds R3Sb(Cat-NH-Cat) (I–III) leads to the formation of neutral paramagnetic intermediates Ia–IIIa. Two anodic reversible one-electron stages are observed for pentacoordinate complexes R′2Sb(Cat-N-Cat) (IV, V). The possibility of the formation of stable paramagnetic species in electrochemical oxidation is a reason for the antiradical activity of the complexes. The study of the reactions of compounds I–V with the electrogenerated superoxide radical anion, diphenylpicrylhydrazyl radical, peroxy radicals, and hydroperoxides formed by the autooxidation of unsaturated fatty acids (oleic, linoleic) shows that all complexes exhibit a pronounced antiradical activity. The highest effect is observed for compounds I, IV, and V characterized by the prolonged action.