Redox transformations and antiradical activity of triarylantimony(V) 3,6-di-tert-butyl-4,5-dimethoxycatecholates

Abstract

Triarylantimony(V) catecholate complexes were synthesized by the oxidative addition of 3,6-ditert-butyl-4,5-dimethoxy-o-benzoquinone to triarylstibines. The electrochemical properties and antiradical activity of the synthesized compounds were studied. According to cyclic viltammetry data, the complexes are oxidized via two consecutive quasi-reversible stages. Introduction of halogen atoms in para-position of phenyl groups at Sb(V) causes anodic shifts of the oxidation potentials and enhances stability of the mono-and dicationic forms of the compounds, which form in the course of electrochemical transformations. Triarylantimony(V) catecholate complexes exhibit appreciable antiradical activity in the auto-oxidation of oleic acid. In was found that the inhibitory activity of the complexes depends on their redox potential.Previously we showed that the redox characteristics, antiradical activity of organo-antimony(V) complexes with redox-active ligands, as well as their ability to bind molecular oxygen depends on a variety of factors [1-6]. Electron-donating substituent group in the redox-active fragment facilitates O 2 binding, shifts anodic oxidation potentials to the cathodic region, and enhances the antiradical activity of Sb(V) compounds [7,8]. Electron withdrawing groups in the catecholate ligand increase the potentials of the catecholate-о-semiquinolate-о-benzoquinone redox transitions and sometimes alter the mechanism of electrochemical oxidation of triphenylantimony(V) catecholate complexes from a two-stage one-electron transfer to one-stage two-electron transfer process [9, 10]. The stability of the cationic complexes resulting from electrochemical oxidation is also affected by the nature of the heteroatoms forming five-membered N, O,Oor S,. The properties of Sb(V) catecholate complexes can also be varied not only by varying substituents in the redox-active ligand, but also by functionalizing groups linked to the central antimony atom [15]. In the present work we continued to study the effect of aryl substituents (phenyl, pchlorophenyl, p-fluorophenyl) in the organometallic fragment SbR 3 of triarylantimony(V) 3,6-di-tert-butyl-4,5-dimethoxycatecholate complexes on the electrochemical transformations and antiradical activity in the auto-oxidation of oleic acid.We have synthesized triphenylantimony, tri-pchlorophenyl-, and tri-p-chlorophenylantimony catecholate complexes I-III by the oxidative addition of the corresponding triarylstibines to 3,6-di-tert-butyl-4,5-dimethoxy-о-benzoquinone (Scheme 1).Complex I was described in [5]. Complexes II and III were characterized by 1 H and 13 C NMR spectroscopy and mass spectrometry; their structure was found to be consistent with the suggested formulas. The oxidation potential relates to the reducing ability of a compound and is a factor which allows one to predict the antiradical activity. The electrochemical

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

“…As in the case of free catechol ligands [ 28 , 29 ], the presence of an electron-withdrawing sulfur atom at position 3 of Cat ligand in 1 – 8 leads to a shift of the first oxidation potential of the complexes, which characterizes the oxidation stage “catecholate/o-semiquinone”, to the anodic region by 0.06–0.08 V as compared with the related triphenylantimony(V) 3,6-di-tert-butylcatecholate [ 66 , 72 ]. In contrast to the pronounced donor effect of RO-substituents in the redox-active catecholate ligand [ 75 , 80 ], the introduction of the RS-group complicates the oxidation process of the complexes and indicates its electron-withdrawing effect. So, these catecholates 1 – 8 do not tend to bind molecular oxygen due to high anodic potential of the oxidation potential “catecholate/o-semiquinone” (e.g., in contrast to that for triphenylantimony(V) 4-methoxy- or 4,5-dimethoxy-3,6-di-tert-butylcatecholates [ 75 , 80 ]).…”

“…In contrast to the pronounced donor effect of RO-substituents in the redox-active catecholate ligand [ 75 , 80 ], the introduction of the RS-group complicates the oxidation process of the complexes and indicates its electron-withdrawing effect. So, these catecholates 1 – 8 do not tend to bind molecular oxygen due to high anodic potential of the oxidation potential “catecholate/o-semiquinone” (e.g., in contrast to that for triphenylantimony(V) 4-methoxy- or 4,5-dimethoxy-3,6-di-tert-butylcatecholates [ 75 , 80 ]).…”

“…We have previously shown that triarylantimony (V) complexes with catecholate ligands act as effective traps and can not only intercept molecular oxygen, but also neutralize alkylperoxy radicals and DPPH, promote the destruction of hydroperoxides, and inhibit lipid peroxidation processes [ 75 , 76 , 77 , 78 , 79 , 80 ]. In the continuation of our research, it was interesting to evaluate the effect of the RS group on the radical scavenging activity in the reaction with the stable DPPH radical.…”

“…In contrast to the transition metals, the O 2 binding does not lead to a change in the antimony oxidation state, while the redox active ligand is involved in the redox reaction. The nature of functional groups in redox active ligands [ 65 , 66 , 67 , 68 ], as well as in substituents at a central antimony atom [ 69 , 70 , 71 , 72 , 73 , 74 ], affects the redox properties of antimony catecholates and their antioxidant activity [ 75 , 76 , 77 , 78 , 79 , 80 ]. Electron-donating groups in catecholate or at the antimony atom in complexes of the type CatSbR 3 shift the oxidation potential to the anodic region.…”