Radical scavenging activity of sterically hindered catecholate and o-amidophenolate complexes of LSbVPh3 type

Synthesis, Structure, Electrochemical Properties, and Antioxidant Activity of Organogermanium(IV) Catecholate Complexes

A series of novel organogermanium(IV) catecholates 1-9 of the general formula R'2Ge(Cat), where R' = Ph, Et, have been synthesized. Compounds were characterized by 1H, 13C NMR, IR spectroscopy, and elemental analysis. The molecular structures of 1-3, 6, and 8 in crystal state were established using single-crystal X-ray analysis. The complexes are tetracoordinate germanium(IV) compounds containing a dioxolene ligand in a dianion (catecholato) form. Electrochemical transformations of target germanium(IV) complexes have been studied by cyclic voltammetry. The electro-oxidation mechanism of complexes 1-5, 7, and 10 (the related complex Ph2Ge(3,5-Cat) where 3,5-Cat is 3,5-di-tert-butylcatecholate) involves the consecutive formation of mono- and dicationic derivatives containing the oxidized forms of redox-active ligands. The stability of the generated monocations depends both on the hydrocarbon groups at the germanium atom and on the substituents in the catecholate ring. Compounds 6, 8, and 9 are oxidized irreversibly under the electrochemical conditions with the formation of unstable complexes. The radical scavenging activity and antioxidant properties of new complexes were estimated in the reaction with DPPH radical, ABTS radical cation, and CUPRACTEAC assay. It has been found that compounds 8 and 9 with benzothiazole or phenol fragments are more active in DPPH test. The presence of electron-rich moieties in the catecholate ligand makes complexes 5 and 7-9 more reactive to ABTS radical cation. The value of CUPRACTEAC for organogermanium(IV) catecholates varies from 0.23 to 1.45. The effect of compounds 1-9 in the process of lipid peroxidation of rat liver (Wistar) homogenate was determined in vitro. It was found that most compounds are characterized by pronounced antioxidant activity. A feature of complexes 1, 3, and 5-9 is the intensification of the antioxidant action with the incubation time. In the presence of additives of complexes 3, 5, 6, and 8, an induction period was observed during the process of lipid peroxidation.

The Synthesis and Biological Activity of Organotin Complexes with Thio-Schiff Bases Bearing Phenol Fragments

A number of novel di- and triorganotin(IV) complexes 1-5 (Ph₂SnL¹, Ph₂SnL², Et₂SnL², Ph₃SnL³, Ph₃SnL⁴) with mono- or dianionic forms of thio-Schiff bases containing antioxidant sterically hindered phenol or catechol fragments were synthesized. Compounds 1-5 were characterized by ¹H, ¹³C NMR, IR spectroscopy, and elemental analysis. The molecular structures of complexes 1 and 2 in the crystal state were established by single-crystal X-ray analysis. The antioxidant activity of new complexes as radical scavengers was estimated in DPPH and ABTS assays. It was found that compounds 4 and 5 with free phenol or catechol fragments are more active in these tests than complexes 1-3 with tridentate O,N,S-coordinated ligands. The effect of compounds 1-5 on the promoted oxidative damage of the DNA by 2,2'-azobis(2-amidinopropane) dihydrochloride and in the process of rat liver (Wistar) homogenate lipid peroxidation in vitro was determined. Complexes 4 and 5 were characterized by more pronounced antioxidant activity in the reaction of lipid peroxidation in vitro than compounds 1-3. The antiproliferative activity of compounds 1-5 was investigated against MCF-7, HTC-116, and A-549 cell lines by an MTT test. The values of IC₅₀ are significantly affected by the presence of free antioxidant fragments and the coordination site for binding.

Binuclear Triphenylantimony(V) Catecholates through N-Donor Linkers: Structural Features and Redox Properties

A series of binuclear triphenylantimony(V) bis-catecholato complexes 1–11 of the type (Cat)Ph₃Sb-linker-SbPh₃(Cat) was prepared by a reaction of the corresponding mononuclear catecholates (Cat)SbPh₃ with a neutral bidentate donor linker ligands pyrazine (Pyr), 4,4′-dipyridyl (Bipy), bis-(pyridine-4-yl)-disulfide (PySSPy), and diazobicyclo[2,2,2]octane (DABCO) in a dry toluene: Cat = 3,6-di-tert-butyl-catecholate (3,6-DBCat), linker = Pyr (1); PySSPy (2); Bipy (3); DABCO (4); Cat = 3,5-di-tert-butyl-catecholate (3,5-DBCat), linker = Bipy (5); DABCO (9); Cat = 4,5-(piperazine-1,4-diyl)-3,6-di-tert-butylcatecholate (pip-3,6-DBCat), linker = Bipy (6); DABCO (10); Cat = 4,5-dichloro-3,6-di-tert-butylcatecholate (4,5-Cl₂-3,6-DBCat), linker = Bipy (7); DABCO (11); and Cat = 4,5-dimethoxy-3,6-di-tert-butylcatecholate (4,5-(MeO)₂-3,6-DBCat), linker = Bipy (8). The same reaction of (4,5-Cl₂-3,6-DBCat)SbPh₃ with DABCO in an open atmosphere results in a formation of 1D coordination polymer {[(4,5-Cl₂-3,6-DBCat)SbPh₃·H₂O]·DABCO}_n (12). Bis-catecholate complex Ph₃Sb(Cat-Spiro-Cat)SbPh₃ reacts with Bipy as 1:1 yielding a rare macrocyclic tetranuclear compound {Ph₃Sb(Cat-Spiro-Cat)SbPh₃∙(Bipy)}₂ (13). The molecular structures of 1, 3, 4, 5, 8, 10, 12, and 13 in crystal state were established by single-crystal X-ray analysis. Complexes demonstrate different types of relative spatial positions of mononuclear moieties. The nature of chemical bonds, charges distribution, and the energy of Sb...N interaction were investigated in the example of complex 5. The electrochemical behavior of the complexes depends on the coordinated N-donor ligand. The coordination of pyrazine, Bipy, and PySSPy at the antimony atom changes their mechanism of electrooxidation: instead of two successive redox stages Cat/SQ and SQ/Cat, one multielectron stage was observed. The coordination of the DABCO ligand is accompanied by a significant shift in the oxidation potentials of the catecholate ligand to the cathodic region (by 0.4 V), compared to the initial complex.

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.

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.

Temperature Dependence of Electron Spin Relaxation of 2,2-diphenyl-1-picrylhydrazyl in Polystyrene

The electron spin relaxation rates for the stable radical DPPH (2,2-diphenyl-1-picrylhydrazyl) doped into polystyrene were studied by inversion recovery and electron spin echo at X-band and Q-band between 20 and 295 K. At low concentration (340 μM, 0.01%) spin-lattice relaxation was dominated by the Raman process and a local mode. At high concentration (140 mM, 5%) relaxation is orders of magnitude faster than at the lower concentration, and 1/T₁ is approximately linearly dependent on temperature. Spin lattice relaxation rates are similar at X-band and Q-band. The temperature dependence of spin echo dephasing was faster at about 140 K than at higher or lower temperatures, which is attributed to a wagging motion of the phenyl groups.