I. V. Smolyaninov

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.

Antiradical activity of morpholine- and piperazine-functionalized triphenylantimony(V) catecholates

The antiradical activity of the functionalized triphenylantimony(V) catecholates Ph3Sb[4-O(CH2CH2)2N-3,6-DBCat] (I), Ph3Sb[4,5-Piperaz-3,6-DBCat] (II), and Ph3Sb[4-PhN(CH2CH2)2N-3,6-DBCat] (III) (where [4-O(CH2CH2)2N-3,6-DBCat]2−, [4,5-Piperaz-3,6-DBCat]2−, and [4-PhN(CH2CH2)2N-3,6-DBCat]2− are the dianionic ligands 3,6-di-tert-butyl-4-(morpholin-1-yl)-, 3,6-di-tert-butyl-4,5-(piperazine-1,4-diyl)-, and 3,6-di-tert-butyl-4-(4-phenylpiperazin-1-yl)catecholates, respectively) was studied in reactions with the diphenylpicrylhydrazyl radical during autooxidation of unsaturated fatty (oleic and linoleic) acids with lipid peroxidation of Russian sturgeon (Acipenser gueldenstaedti B.) sperm and human blood erythrocytes in vitro as examples. The EC50 and nDPPH values obtained indicate the high antiradical activity of complexes II and III in the reactions with the stable radical. On the whole, complexes I–III inhibit the lipid peroxidation in both model (oxidation of unsaturated fatty acids) and in vitro experiments. The inhibiting effects of the complexes are comparable with and even, in some cases, higher than those of the known antioxidant ionol.

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].

Complexes of the 14th group elements with tridentate redox-active ligand

New methods of synthesis of the 14th group metal (Si, Sn, Pb) complexes containing a redox-active ligand, 3,5-di-tert-butyl-1,2-quinone-1-(2-hydroxy-3,5-di-tert-butylphenyl)imine, in various redox states were developed. New compounds were studied by the ESR spectroscopy in solutions. The redox properties of the paramagnetic tin compounds are characterized by the cyclic voltammetry. The solvent and temperature effect on the ESR spectra parameters of paramagnetic complexes of Sn(IV)and Pb(IV) was revealed.

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.

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].

Complexes of triphenylantimony( v ) catecholates with ammonium salts. Spectroscopic and electrochemical investigations

The reactions of triphenylantimony(V) catecholates (Cat)SbPh3 with different tetraalkyl-ammonium salts (bromides, iodides, hydroxide anions) in acetonitrile were investigated, and their complexes 8–13 of the general formula [R4N]+[(Cat)SbPh3X]− were synthesized. The molecular structure of complex 8 was determined by X-ray diffraction. The electrochemical oxidation of complexes 8–13 proceeds more easily than the oxidation of the starting catecholates (Cat)SbPh3. The coordination of the bromide anion results in the stabilization of the oxidized forms of the complexes [(SQ)SbPh3X]0 and [(Q)SbPh3X]+ (SQ and Q are the o-benzosemiquinone and o-benzoquinone forms of the redox-active ligand, respectively).

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.