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

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

Ivan V. Smolyaninov 1,2 , Andrey I. Poddel’sky 3,* , Susanna A. Smolyaninova 2, Maxim V. Arsenyev 3, Georgy K. Fukin 3 and Nadezhda T. Berberova 2

1 Toxicology Research Group, Federal State Budgetary Institution of Science “Federal Research Centre The Southern Scientiﬁc Centre of the Russian Academy of The Sciences”, Tatischeva str. 16, 414056 Astrakhan, Russia; [email protected] 2 Department of Chemistry, Astrakhan State Technical University, 16 Tatisheva str., Astrakhan 414056, Russia; [email protected] (S.A.S.); [email protected] (N.T.B.) 3 G.A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences, 49 Tropinina str., 603137 Nizhny Novgorod, Russia; [email protected] (M.V.A.); [email protected] (G.K.F.) * Correspondence: [email protected]; Tel./Fax: +7-831-462-7497

Academic Editor: Vito Lippolis Received: 27 March 2020; Accepted: 10 April 2020; Published: 12 April 2020

Abstract: 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.

Keywords: redox-active ligand; catechol; thioether; antimony; X-ray; cyclic voltammetry; electronic paramagnetic resonance

1. Introduction A large amount of data has been accumulated to the present time on the complexes of transition and non-transition elements with redox active ligands: o-benzoquinones, o-iminobenzoquinones and alpha-diimines [1–11]. A feature of these compounds is the participation of not only metal but also redox-active ligands in the processes of electron transfer and, as a result, they are capable of reversible accepting and donating electrons during the diﬀerent chemical transformations, while remaining bound to the metal. This feature can signiﬁcantly expand or change the reactivity of metal complexes. Studies of non-transition metal complexes containing redox-active ligands of the o-quinonato type have allowed to reveal an expansion of the valence potentials of non-transition metals

Molecules 2020, 25, 1770; doi:10.3390/molecules25081770 www.mdpi.com/journal/molecules Molecules 2020, 25, 1770 2 of 23 due to redox-active ligands [12,13], to observe of the phenomenon of redox isomerism in the main group metals’ chemistry [14], and also to observe and investigate the ability of antimony(V) complexes to reversible fixation of molecular oxygen [15–19], to observe the fixation of nitrogen(II) oxide (by zinc(II) and lead(II) catecholates) [20]. There are several approaches that allow to change the properties of ligands and their metal complexes: (1) a modification of the ligands’ structure by the variation of heteroatoms (O, N, S) or the presence of bulky organic substituents; (2) the introduction of different electron donor/acceptor groups or additional redox-active as well as chelating fragments at various positions of the quinoid ring; and (3) a combination of a redox-active ligand and an additional coordination center. Organic and organometallic compounds containing several redox centers are of particular interest in view of a number of their specific features: the possibility of indirect activation through certain functional groups, intramolecular electron transfer, proton-conjugated electron transfer, etc. From the example of ferrocene derivatives (ferrocifenes) as well as quinoid compounds, it was shown that various types of biological activities (cytotoxicity, antibacterial, antiparasitic, antioxidant) depend directly on the presence of a redox-active group and its transformations [21–24]. Redox-asymmetric systems can be built not only on the basis of a ferrocenyl fragment but also on a combination of phenolic, amide, and amino groups, which are also of interest because of the possibility of proton-conjugated charge transfer reactions [25–27]. For a long time, dioxolenes have attracted the attention of researchers because of the possibility of their existence in several redox states. Recently, the focus of research in this area has been aimed at their functionalization by introducing additional redox-active groups [28–31] or coordination sites [32–35]. In the series of functionalized derivatives of catechol/o-quinones, sulfur-containing compounds occupy a special place since sulfide bridge fragments also exhibit redox activity. The introduction of the tetrathiafulvalene linker allows to obtain bis-quinones and their metal complexes which exhibit unusual physicochemical properties [36–39]. The sterically hindered dithiete-annelated o-benzoquinone is a bifunctional ligand, which combines two chalcogen redox-active coordination centers in its structure—dioxolene and dithiolene—allowing it to form complexes in which various coordination sites are involved [40]. S-functionalization of 3,5-di-tert-butyl-o-benzoquinone with cystamine and cysteine has made it possible to obtain Cu(II), Zn(II), and Ni(II) complexes [41,42]. A similar reaction with dithiols makes it possible to synthesize catechols with a free thio group or bis-catechol thioethers [43,44], which have chelating properties and the ability to be absorbed on the surface [45–47]. Variation of the organic groups in thiols allows one to obtain a wide range of thiolated catechols, which exhibit antioxidant, antiradical, and cryoprotective activity [48–50]. Transition metal compounds based on sulfur-containing catecholate/o-semiquinolate ligands demonstrate antifungal and antibacterial activity [51,52]. Thus, the synthesis and study of the properties of thiolated dioxiolene ligands and metal complexes based on them is relevant in view of the diversity of the properties manifested. Antimony(III/V) derivatives are used as antiparasitic agents in the treatment of leishmaniasis [53], and have potential as antitumor, cytotoxic, and antibacterial substances [54,55]. Along with biological activity, antimony complexes are promising compounds for the design of functional materials. The presence of a catecholate cycle at the antimony atom allows the creation of new colorimetric and fluorescent sensors for the fluoride anion [56–59]. The polymer compositions containing catecholate derivatives of triphenylantimony(V) are promising for the creation of polymeric materials capable of fixing oxygen [60–62]. In the present paper, we report on new thioethers combining several redox-active centers: A chelating catechol fragment, a phenolic group, and a thioether linker. The coordinating activity of the catechol fragment was studied in a reaction with triphenylantimony(V) dibromide, and as a result, triphenylantimony(V) catecholates, which contain redox-active centers of a different nature, were synthesized. The molecular structure and electrochemical transformations of the resulting compounds are discussed. Molecules 2020, 25, 1770 3 of 23

2. Results and Discussion

Molecules2.1. Synthesis 2020, 25 and, 1770 Characterization 3 of 22 Molecules 2020, 25, 1770 3 of 22 The sterically hindered catechol (6-(CH2-S-tBu2Phenol)-3,5-DBCat)H2 (L1) was synthesized The sterically hindered catechol (6-(CH2-S-tBu2Phenol)-3,5-DBCat)H2 (L1) was synthesized from fromThe 6-methoxymethyl-3,5-di- sterically hindered catecholtert-butylcatechol (6-(CH2-S-tBu2 (6-CHPhenol)-3,5-DBCat)H2OMe-3,5-Cat)H2 (2L1and) was 2,6-di-synthesizedtert-butyl-4- from 6-methoxymethyl-3,5-di-tert-butylcatechol (6-CH2OMe-3,5-Cat)H2 and 2,6-di-tert-butyl-4- 6-methoxymethyl-3,5-di-mercaptophenol in a solutiontert-butylcatechol of acetic acid at(6-CH 60 C2OMe-3,5-Cat)H (Scheme1(1)).2 Theand 3,5-di- tert2,6-di--butylcatecholstert-butyl-4- mercaptophenol in a solution of acetic acid at 60°C (Scheme◦ 1(1)). The 3,5-di-tert-butylcatechols (6-(S- mercaptophenol(6-(S-tBu2Phenol)-3,5-DBCat)H in a solution of2 (aceticL2) and acid (6-(S-Phenol)-3,5-DBCat)H at 60°C (Scheme 1(1)). The2 (L 3,5-di-3) weretert prepared-butylcatechols by the reaction (6-(S- tBu2Phenol)-3,5-DBCat)H2 (L2) and (6-(S-Phenol)-3,5-DBCat)H2 (L3) were prepared by the reaction of tBuof 3,5-di-2Phenol)-3,5-DBCat)Htert-butyl-o-benzoquinone2 (L2) and (6-(S-Phenol)-3,5-DBCat)H with 2,6-di-tert-butyl-4-mercaptophenol2 (L3) were prepared or 4-mercaptophenol by the reaction [48 of], 3,5-di-tert-butyl-o-benzoquinone with 2,6-di-tert-butyl-4-mercaptophenol or 4-mercaptophenol [48], 3,5-di-respectively,tert-butyl- in ethanolo-benzoquinone solution (Schemewith 2,6-di-1(2)).tert-butyl-4-mercaptophenol or 4-mercaptophenol [48], respectively, in ethanol solution (Scheme 1(2)). respectively, in ethanol solution (Scheme 1(2)).

SchemeScheme 1.1. SynthesisSynthesis ofof catecholscatechols LL11–-LL33. . Scheme 1. Synthesis of catechols L1-L3.

Catechols L1,, LL22 werewere isolated isolated as as pale-yellow pale-yellow microcrystalline microcrystalline powders powders that that were were well-soluble well-soluble in Catechols L1, L2 were isolated as pale-yellow microcrystalline powders that were well-soluble in toluene or chloroform. The The composition composition of these catechols was determined by means of IR-, 11H, and 1 toluene13 1 or chloroform. The composition of these catechols was determined by means of IR-, H, and 13C{C{1H}H} NMR NMR spectroscopy spectroscopy and and elemental elemental anal analysis.ysis. A A prolonged prolonged re recrystallizationcrystallization of L1 fromfrom n-hexane n-hexane 13C{1H} NMR spectroscopy and elemental analysis. A prolonged recrystallization of L1 from n-hexane allowed X-ray-quality white crystals of L1 toto be be grown. grown. allowed X-ray-quality white crystals of L1 to be grown. A substitution substitution reaction reaction between between catechols catechols L1L or1 or L3 Land3 and triphenylantimony(V) triphenylantimony(V) dibromide dibromide in the in A substitution reaction between catechols L1 or L3 and triphenylantimony(V) dibromide in the presencethe presence of two of two equivalents equivalents of oftriethylamine triethylamine in intoluene toluene solution solution leads leads to to the the corresponding presence of two equivalents of triethylamine in toluene solution leads to the corresponding triphenylantimony(V) catecholatescatecholates (Scheme2 2).). triphenylantimony(V) catecholates (Scheme 2).

Scheme 2. Synthesis of complexes 1 and 2. Scheme 2. SynthesisSynthesis of of complexes complexes 11 andand 22.. Complexes 1 and 2 were isolated as yellow solids. These compounds are readily soluble in Complexes 1 and 2 were isolated as yellow solids. These compounds are readily soluble in commonComplexes organic1 solvents,and 2 were such isolated as aliphatic as yellow or aromatic solids. These hydrocarbons, compounds chloroform, are readily ethers, soluble and in common organic solvents, such as aliphatic or aromatic hydrocarbons, chloroform, ethers, and methanol.common organic Complexes solvents, 1 and such 2 aswere aliphatic characterized or aromatic by hydrocarbons, IR-, 1H, and chloroform,13C{1H} NMR ethers, spectroscopy and methanol. and methanol. Complexes 1 and 2 were characterized1 by IR-,13 1H,1 and 13C{1H} NMR spectroscopy and elementalComplexes analysis.1 and 2 Thewere X-ra characterizedy suitable crystals by IR-, ofH, 1 andwere grownC{ H} NMRby the spectroscopy prolonged (during and elemental 5 days) elemental analysis. The X-ray suitable crystals of 1 were grown by the prolonged (during 5 days) crystallizationanalysis. The X-ray of complex suitable in crystals toluene of solution1 were at grown −18°C. by the prolonged (during 5 days) crystallization crystallizationof complex in tolueneof complex solution in toluene at 18 solutionC. at −18°C. − ◦ 2.2. Molecular Structures 2.2. Molecular Structures The molecular structure of catechol L1 in crystal is shown on Figure 1. The general geometrical The molecular structure of catechol L1 in crystal is shown on Figure 1. The general geometrical parameters of catechol fragment are typical for this class of compounds. The carbon–carbon bonds in a parameters of catechol fragment are typical for this class of compounds. The carbon–carbon bonds in a ring C(1-6) have an average distance of 1.401 ± 0.014 Å, which is typical of aromatic systems. The carbon– ring C(1-6) have an average distance of 1.401 ± 0.014 Å, which is typical of aromatic systems. The carbon– oxygen distances are ordinary [63] and close to each other: the bonds O(1)-C(1), O(2)-C(2), and O(3)-C(11) oxygen distances are ordinary [63] and close to each other: the bonds O(1)-C(1), O(2)-C(2), and O(3)-C(11) are 1.3816(14), 1.3804(14), and 1.3806(14) Å, respectively. The molecule is not planar, the angle between are 1.3816(14), 1.3804(14), and 1.3806(14) Å, respectively. The molecule is not planar, the angle between

Molecules 2020, 25, 1770 4 of 23

2.2. Molecular Structures

The molecular structure of catechol L1 in crystal is shown on Figure1. The general geometrical parameters of catechol fragment are typical for this class of compounds. The carbon–carbon bonds in a ring C(1-6) have an average distance of 1.401 0.014 Å, which is typical of aromatic systems. The ± carbon–oxygen distances are ordinary [63] and close to each other: the bonds O(1)-C(1), O(2)-C(2), and

MoleculesO(3)-C(11) 2020, are25, 1770 1.3816(14), 1.3804(14), and 1.3806(14) Å, respectively. The molecule is not planar,4 of the 22 angle between catechol and phenolic aromatic rings planes is equal to 54.7(1)◦, the torsion angle is catechol47.9(1)◦ and, and phenolic the torsion aromatic angle rings C(1)C(6)C(7)S(1) planes is equal is to 56.7(1) 54.7(1)°,◦. Suchthe torsion distortion angle ofis 47.9(1)°, geometry an isd the supported torsion angleby an C(1)C(6)C(7)S(1) intramolecular hydrogenis 56.7(1)°. bonding Such distortion in the molecule: of geometry the distance is supported O(2)-H(2) by... anO(1) intramolecular is 2.07(1) Å, hydrogenangle O(2)-H(2)-O(1) bonding in the is 126.6(1) molecule:◦, and the the distance distance O(2)-H(2)…O(1) O(1)-H(1) ... S(1)is 2.07(1) is 2.28(1) Å, angle Å, angle O(2)-H(2)-O(1) O(1)-H(1)-S(1) is 126.6(1)°,is 153.0(1) and◦. In the the distance crystal, O(1)-H(1)…S(1) molecules form is pairs 2.28(1) due Å, to angle the intermolecular O(1)-H(1)-S(1) hydrogen is 153.0(1)°. bonding In the betweencrystal, moleculesphenolic hydroxylform pairs groups due to (Figurethe intermolecular S9). The intermolecular hydrogen bonding distances between O(3)-H(3) phenolic... hydroxylO(30) are groups 1.71(1) (FigureÅ, and S9). the correspondingThe intermolecular angles distances O(3)-H(3)-O(3 O(3)-H(3)…O(30) are 113.5(1)′) are 1.71(1)◦. The Å, distance and the betweencorresponding the planes angles of O(3)-H(3)-O(3catecholate rings′) are in 113.5(1)°. neighboring The distance pairs is 3.49(1)between Å, the which planes is closeof catecholate to the sum rings ofthe in neighboring Van der Waals pairs radii is 3.49(1)of carbon Å, which atoms is [ 64close]. to the sum of the Van der Waals radii of carbon atoms [64].

FigureFigure 1. 1. TheThe X-ray X-ray structure of catecholcatechol LL11 (6-(CH(6-(CH22-S-tBu-S-tBu2Phenol)-3,5-DBCat)H22 inin crystal. crystal. The The hydrogenhydrogen atoms atoms except except hydroxyl hydroxyl hydrogens hydrogens are are omitte omittedd for for clarity. clarity. The The ellipsoids ellipsoids of of 50% 50% probability. probability. TheThe selected selected bond bond distances distances (Å): (Å): O(1)-C(1) O(1)-C(1) 1.3816( 1.3816(14),14), O(2)-C(2) O(2)-C(2) 1.3804(14), 1.3804(14), O(3)-C(11) O(3)-C(11) 1.3806(14), 1.3806(14), S(1)- C(7)S(1)-C(7) 1.8527(12), 1.8527(12), S(1)-C(8) S(1)-C(8) 1.7765(11), 1.7765(11), C(1)-C(2) C(1)-C(2) 1.3967(16), 1.3967(16), C(1)-C(6) C(1)-C(6) 1.3987(16), 1.3987(16), C(2)-C(3) C(2)-C(3) 1.3910(16), 1.3910(16), C(3)-C(4)C(3)-C(4) 1.4029(15),1.4029(15), C(4)-C(5) C(4)-C(5) 1.4016(15), 1.4016(15), C(5)-C(6) C(5)-C(6) 1.4150(15), 1.4150(15), C(6)-C(7) C(6)-C(7) 1.5101(15), 1.5101(15), C(8)-C(9) 1.3907(16),C(8)-C(9) 1.3907(16),C(8)-C(13) C(8)-C(13) 1.3893(16), 1.3893(16), C(9)-C(10) C(9)-C(10) 1.3947(16), 1.3947(16), C(10)-C(11) C(10)-C(11) 1.4118(15), 1.4118(15), C(11)-C(12) C(11)-C(12) 1.4139(15), 1.4139(15), C(12)-C(13) C(12)-C(13)1.3949(16). 1.3949(16).

TheThe molecular molecular structure ofof triphenylantimony(V)triphenylantimony(V) catecholate catecholate1 in1 in crystal crystal is shownis shown in Figurein Figure2. The 2. Thecentral central antimony antimony atom atom Sb(1) Sb(1) has ahas distorted a distorted square sq pyramidaluare pyramidal geometry geometry with anwith O,O’-chelating an O,O’-chelating ligand ligandand two and phenyl two phenyl groups groups in the in basal the plane.basal plane. The bond The bond angles angles in the in base the base of pyramid of pyramid O(1)-Sb(1)-C(30) O(1)-Sb(1)- C(30)and O(2)-Sb(1)-C(36) and O(2)-Sb(1)-C(36) are 156.73(6) are 156.73(6) and 142.01(7) and 142.01(7)°.◦. The bond The bond Sb(1)-C(42) Sb(1)-C(42) with thewith apical the apical phenyl phenyl group groupis approximately is approximately 0.028(2)-0.033(2) 0.028(2)-0.033(2) Å shorter Å shorter than thethan equatorial the equatorial bonds bonds Sb(1)-C(30) Sb(1)-C(30) and Sb(1)-C(36).and Sb(1)- C(36).The planes The planes of the of aromatic the aromatic rings C(16-21)rings C(16-21) and C(1-6) and C(1-6) of the phenolicof the phenolic group group and the and catecholate the catecholate ligand, ligand,respectively, respectively, lie at an lie angle at an of angle 57.6(1) ofº .57.6(1)º. The arylthio-fragment The arylthio-fragment in 1 is stronger in 1 is stronger turned away turned from away the from the catecholate ring than in catechol L1, the torsion angle C(2)C(3)C(7)S(1) in 1 is 77.9(1)° (vs. 56.7(1)° in L1), and the torsion angle C(3)C(7)S(1)C(16) is 138.1(1)° (vs. 47.9(1)° in L1). The oxygen-to-carbon bonds O(1)-C(1) and O(2)-C(2) (1.358(2) and 1.366(2) Å, respectively) are ordinary and typical for catecholato complexes of different metals [65], and bond O(3)-C(19) (1.376(2) Å) is the longest O-C bond in 1, which is typical for sterically hindered phenols [66–68]. The six- membered carbon ring C(1-6) is aromatic, with average C-C bond distances of 1.400 ± 0.015 Å, which is very close to the same value for catechol L1. In general, the geometrical characteristics of the redox- active catecholato ligand in 1 are typical for various antimony(V) catecholates [69–79].

Molecules 2020, 25, 1770 5 of 23

catecholate ring than in catechol L1, the torsion angle C(2)C(3)C(7)S(1) in 1 is 77.9(1)◦ (vs. 56.7(1)◦ in LMolecules1), and the2020 torsion, 25, 1770 angle C(3)C(7)S(1)C(16) is 138.1(1)◦ (vs. 47.9(1)◦ in L1). 5 of 22

FigureFigure 2. 2.The The X-ray X-ray structure structure of of1 :(1: a(a)) the the general general view view on on molecule molecule of of1 1;(; b(b)) the the view view on on molecule molecule1 1 alongalong the the dotted dotted line line (view (view b) b) depicted depicted on on Figure Figure2a. 2a. The The hydrogen hydrogen atoms atoms except except hydroxyl hydroxyl hydrogen hydrogen H(1)H(1) are are omittedomitted for clarity. The The ellipsoids ellipsoids of of50% 50% probability. probability. The Theselected selected bond bond distances distances (Å): Sb(1)- (Å): Sb(1)-O(1)O(1) 2.0402(13), 2.0402(13), Sb(1)-O(2) Sb(1)-O(2) 2.0256(13), 2.0256(13), Sb(1)-C(30) Sb(1)-C(30) 2.1328(19), 2.1328(19), Sb(1)-C(36) Sb(1)-C(36) 2.1383(19),2.1383(19), Sb(1)-C(42)Sb(1)-C(42) 2.1054(19),2.1054(19), O(1)-C(1)O(1)-C(1) 1.358(2),1.358(2), O(2)-C(2)O(2)-C(2) 1.366(2),1.366(2), O(3)-C(19)O(3)-C(19) 1.376(2),1.376(2), O(3)-H(1)O(3)-H(1) 0.73(3),0.73(3), S(1)-C(7)S(1)-C(7) 1.8365(18),1.8365(18), S(1)-C(16) S(1)-C(16) 1.7772(19), 1.7772(19), C(1)-C(2) C(1)-C(2) 1.398(3), 1.398(3), C(1)-C(6) C(1)-C(6) 1.393(3), 1.393(3), C(2)-C(3) C(2)-C(3) 1.392(3), 1.392(3), C(3)-C(4) C(3)-C(4) 1.415(3),1.415(3), C(4)-C(5) C(4)-C(5) 1.400(3), 1.400(3), C(5)-C(6) C(5)-C(6) 1.401(3), 1.401(3), C(16)-C(17) C(16)-C(17) 1.385(3), 1.385(3), C(16)-C(21) C(16)-C(21) 1.389(3), 1.389(3), C(17)-C(18) C(17)-C(18) 1.394(3),1.394(3), C(18)-C(19) C(18)-C(19) 1.413(3), 1.413(3), C(19)-C(20) C(19)-C(20) 1.408(3), 1.408(3), C(20)-C(21) C(20)-C(21) 1.390(3). 1.390(3).

TheIn the oxygen-to-carbon crystal, molecules bonds 1 form O(1)-C(1) layers, and where O(2)-C(2) one can (1.358(2) find intermolecular and 1.366(2) Å, hydrogen respectively) bonding are ordinarybetween and the typicalphenol for group catecholato of one complexescomplex molecule of diﬀerent and metals the sulfur [65],and atom bond of a O(3)-C(19) neighboring (1.376(2) complex Å) ismolecule the longest (Figure O-C bondS10) with in 1, whicha distance is typical O(3)-H(1)…S( for sterically1) of hindered2.86(1) Å phenolsand corresponding [66–68]. The angle six-membered O(3)-H(1)- carbon ring C(1-6) is aromatic, with average C-C bond distances of 1.400 0.015 Å, which is very S(1) of 115.5(1)°. ± close to the same value for catechol L1. In general, the geometrical characteristics of the redox-active catecholato2.3. Electrochemistry ligand in 1 are typical for various antimony(V) catecholates [69–79]. In the crystal, molecules 1 form layers, where one can ﬁnd intermolecular hydrogen bonding The electrochemical properties of catechol thioethers L1-L3 and triphenylantimony(V) between the phenol group of one complex molecule and the sulfur atom of a neighboring complex catecholate complexes 1, 2 were investigated by cyclic voltammetry (CV) in CH2Cl2 and MeCN molecule (Figure S10) with a distance O(3)-H(1) ... S(1) of 2.86(1) Å and corresponding angle solutions containing 0.15 M NBu4ClO4 (TBAP) as the supporting electrolyte at a glassy carbon O(3)-H(1)-S(1) of 115.5(1)◦. working electrode. The redox potentials given in Table 1; Table 2 are referenced to Ag/AgCl/KCl(sat.) electrode.

Table 1. The CV data for thioethers L1-L3 and triphenylantimony(V) catecholates 1 and 2 in CH2Cl2

(GC anode, С = 3·10−3 М, Ar, 0.15 M Bu4NClO4, vs Ag/AgCl/KCl(sat.)).

Compound Eox1, V 1 Ic/Ia Eox2, V 1 Ic/Ia Eox3, V L1 1.28 - 1.49 - -

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2.3. Electrochemistry

The electrochemical properties of catechol thioethers L1-L3 and triphenylantimony(V) catecholate complexes 1, 2 were investigated by cyclic voltammetry (CV) in CH2Cl2 and MeCN solutions containing 0.15 M NBu4ClO4 (TBAP) as the supporting electrolyte at a glassy carbon working electrode. The redox potentials given in Table1; Table2 are referenced to Ag /AgCl/KCl(sat.) electrode.

Table 1. The CV data for thioethers L1–L3 and triphenylantimony(V) catecholates 1 and 2 in CH2Cl2 (GC anode, C = 3 10 3 M, Ar, 0.15 M Bu NClO , vs. Ag/AgCl/KCl(sat.)). · − 4 4 ox1 1 ox2 1 ox3 Compound E ,V Ic/Ia E ,V Ic/Ia E ,V

L1 1.28 - 1.49 - - 1.37 1.58 L 0.4 0.6 1.77 2 (1.32) (1.55) L3 1.28 - 1.48 - - 0.96 1.39 1 0.8 0.5 - (0.89) (1.27) 1.04 2 0.7 1.24 - 1.48 (0.96) 1 The half-wave potentials E1/2 for quasi-reversible oxidation processes are given in parentheses.

Table 2. The CV data for thioethers L1–L3 and triphenylantimony(V) catecholates 1 and 2 in CH3CN (GC anode, C = 3 10 3 M, Ar, 0.15 M Bu NClO , vs. Ag/AgCl/KCl(sat.)). · − 4 4 ox1 1 ox2 1 ox3 Compound E ,V Ic/Ia E ,V Ic/Ia E ,V

L1 1.09 - 1.34 - 1.69 L2 * 1.27 - 1.48 0.3 - L3 ** 1.13 - 1.26 - 1.61 0.89 1.23 1 0.4 0.5 1.68 (0.84) (1.12) 0.94 2 0.8 1.36 -- (0.89) 1 The half-wave potentials E1/2 for quasi-reversible oxidation processes are given in parentheses. * Data have been received in a mixture “CH3CN/ CH2Cl2” (2:1). ** Data from ref. [48].

Compounds L1-L3 are characterized by the presence of multiple electroactive fragments: catechol, phenolic group, and thioether linker, which affects their electrochemical behavior. The electrooxidation of the target catechol thioethers occurs in three or two successive stages depending on the applied solvent. Figure3 shows the cyclic voltammograms (CVs) of L1 in acetonitrile solution. The CV of L1 in dichloromethane is shown in Figure S11 of ESI. The first two-electron electrochemical stage is ascribed to the irreversible oxidation of the catechol fragment in the case of compounds L1 and L3. A similar behavior is typical for substituted catechols in aprotic solvents: 3,5-di-tert-butylcatechol has an irreversible oxidation peak at 1.11 (CH3CN) or 1.23 V (CH2Cl2). In the first stage, the mechanism of electooxidation involves electron transfer followed by fast deprotonation, leading to o-quinone formation (Scheme3). Molecules 2020, 25, 1770 6 of 22

1.37 1.58 L2 0.4 0.6 1.77 (1.32) (1.55) L3 1.28 - 1.48 - - 0.96 1.39 1 0.8 0.5 - (0.89) (1.27) 1.04 1.24 1.48 2 0.7 - (0.96)

1 The half-wave potentials E1/2 for quasi-reversible oxidation processes are given in parentheses.

Table 2. The CV data for thioethers L1-L3 and triphenylantimony(V) catecholates 1 and 2 in CH3CN

(GC anode, С = 3·10−3 М, Ar, 0.15 M Bu4NClO4, vs Ag/AgCl/KCl(sat.)).

Compound Eox1, V 1 Ic/Ia Eox2, V 1 Ic/Ia Eox3, V L1 1.09 - 1.34 - 1.69 L2 * 1.27 - 1.48 0.3 - L3 ** 1.13 - 1.26 - 1.61 0.89 1.23 1 0.4 0.5 1.68 (0.84) (1.12) 0.94 1.36 2 0.8 - - (0.89)

1 The half-wave potentials E1/2 for quasi-reversible oxidation processes are given in parentheses. * Data have been received in a mixture “CH3CN/ CH2Cl2” (2:1). ** Data from ref. [48].

Compounds L1-L3 are characterized by the presence of multiple electroactive fragments: catechol, phenolic group, and thioether linker, which affects their electrochemical behavior. The electrooxidation of the target catechol thioethers occurs in three or two successive stages depending on the applied solvent. Figure 3 shows the cyclic voltammograms (CVs) of L1 in acetonitrile solution. The CV of L1 in dichloromethane is shown in Figure S11 of ESI. The first two-electron electrochemical stage is ascribed to the irreversible oxidation of the catechol fragment in the case of compounds L1 and L3. A similar behavior is typical for substituted catechols in aprotic solvents: 3,5-di-tert- butylcatechol has an irreversible oxidation peak at 1.11 (CH3CN) or 1.23 V (CH2Cl2). In the first stage, Moleculesthe mechanism2020, 25, 1770 of electooxidation involves electron transfer followed by fast deprotonation, leading7 of 23 to o-quinone formation (Scheme 3).

FigureFigure 3. 3.Cyclic Cyclic voltammograms voltammograms of of compound compoundL 1L(in1 (in the the potential potential switch switch from from 0.70−0.70 to to 1.5 1.5 V—curve V—curve − 1;1; in in the the potential potential switch switch from from 0.70−0.70 to to 1.85 1.85 V—curve V—curve 2) 2) (MeCN, (MeCN, C C= =3 3 mM, mM, 0.15 0.15 M M TBAP, TBAP, scan scan rate rate 1 − 200 mV s −1). Molecules200 2020 mV·s,· 25−, 1770). 7 of 22 Molecules 2020, 25, 1770 7 of 22 tBu tBu tBu OH tBu O -2e OH O -2e -2H+ tBu OH + tBu O tBu OH -2H tBu O S tBu S tBu S tBu S tBu OH OH tBu OH tBu OH tBu tBu Scheme 3. The first stage of electrochemical oxidation of L1. Scheme 3. The ﬁrst stage of electrochemical oxidation of L1. Scheme 3. The first stage of electrochemical oxidation of L1.

Earlier spectroelectrochemical investigations of of catechol L33 havehave shown shown that that electrolysis at the Earlier spectroelectrochemical investigations of catechol L3 have shown that electrolysis at the controlled potentialpotential leadsleads toto thethe formation formation of of the the corresponding correspondingo-benzoquinone o-benzoquinone [48 [48].]. The The formation formation of controlled potential leads to the formation of the corresponding o-benzoquinone [48]. The formation ofthe theo-quinone o-quinone structure structure is accompanied is accompanied by the by appearancethe appearance of a wide of a wide absorption absorption band atband 400–500 at 400–500 nm. A of the o-quinone structure is accompanied by the appearance of a wide absorption band at 400–500 widenm. A absorption wide absorption band in band the visible in the spectral visible rangespectral is arange characteristic is a characteristic feature of compoundsfeature of compounds containing nm. A wide absorption band in the visible spectral range is a characteristic feature of compounds containinga quinoid fragment a quinoid [80 fragment]. The spectroelectrochemistry [80]. The spectroelectrochemistry of L1 under of the L1 controlled under the potentialcontrolled electrolysis potential containing a quinoid fragment [80]. The spectroelectrochemistry of L1 under the controlled potential electrolysisalso conﬁrms also the confirms formation the of formation the corresponding of the correspondingo-benzoquinone. o-benzoquinone. electrolysis also confirms the formation of the corresponding o-benzoquinone. As a result of electrooxidation, new absorption with a maximum at 410 nm was observed in the As a result of electrooxidation, new absorption with a maximum at 410 nm was observed in the visible regionregion of of the the spectrum spectrum and and the intensity the intensity of this bandof this increased band increased with time (Figurewith time4). Additionaly,(Figure 4). visible region of the spectrum and the intensity of this band increased with time (Figure 4). Additionaly,the second characteristic the second bandcharacteristic of lower band intensity of lowe in ther intensity form of a in shoulder the form to of the a mainshoulder band to appears the main in Additionaly, the second characteristic band of lower intensity in the form of a shoulder to the main bandthe region appears of 590 in the nm, region which of corresponds 590 nm, which to the corresponds n-π * transition to the in n- quinones.π * transition in quinones. band appears in the region of 590 nm, which corresponds to the n-π * transition in quinones.

Figure 4. The changes in the absorption spectrum of thioether L1 during the electrolysis time (90 min) FigureatFigure a controlled 4. 4. TheThe changeschanges potential in in the1.2 the absorptionV absorption (MeCN, Ar,spectrum spectrum C = 0.5 of mM, of thioether thioether 0.15 M L LTBAP).1 1duringduring the the electrolysis electrolysis time time (90 (90 min) min) atat a acontrolled controlled potential potential 1.2 1.2 V V (MeCN, (MeCN, Ar, Ar, C C == 0.50.5 mM, mM, 0.15 0.15 M M TBAP). TBAP). The absorption band in the visible spectral range corresponds to a π-π* transition, which is a characteristicThe absorption for compounds band in the containing visible spectral a quinonoid range fragment corresponds like 3.5-di-to a π-tertπ* -butyl-transition,o-benzoquinone which is a characteristic for compounds containing a quinonoid fragment like 3.5-di-tert-butyl-o-benzoquinone [81]. The CV of L1 after electrolysis in the cathodic region contains a quasi-reversible peak at E1/2 = [81].−0.58 The V (Figure CV of S12L1 after of ESI). electrolysis This peak in is the assigned cathodic to theregion reduction contains of electrogenerateda quasi-reversible o-benzoquinone peak at E1/2 = −to0.58 o-benzosemiquinone. V (Figure S12 of ESI). This peak is assigned to the reduction of electrogenerated o-benzoquinone to o-benzosemiquinone. The second oxidation peak on CVs of catechols L1, L3 may be assigned to the oxidation of the phenolic The second oxidation peak on CVs of catechols L1, L3 may be assigned to the oxidation of the phenolic group. In acetonitrile, for compounds L1 and L3, the shift of the second oxidation potential to the cathode group.region In occurs acetonitrile, as compared for compounds to the oxidation L1 and Lof3, thethe well-knownshift of the second phenolic oxid antioxidantation potential (ionol)—2,6-di- to the cathodetert - region occurs as compared to the oxidation of the well-known phenolic antioxidant (ionol)—2,6-di-tert- butyl-4-methylphenol 1.47 V (CH3CN). In dichloromethane, the oxidation potentials of L1 and L3 are close butyl-4-methylphenolto the value for this 1.47phenolic V (CH compound3CN). In dichloromethane, (1.48 V). The presencethe oxidation of the potentials thioether of Lgroup1 and L causes3 are close the to the value for this phenolic compound (1.48 V). The presence of the thioether group causes the appearance of the third redox wave at high anodic potentials. For L1 and L3 in acetonitrile, the values Еox3 ox3 appearanceare in good of agreement the third redoxwith thewave data at highon the anodic oxidation potentials. of catechol For L 1thioethers: and L3 in acetonitrile, the electrooxidation the values of Е the are in good agreement with the data on the oxidation of catechol thioethers: the electrooxidation of the thioether group occurs in the potential range from 1.59 to 1.67 V. On the CVs of L1, L3 in dichloromethane, thioethertwo oxidation group wavesoccurs onlyin the are potential observ edrange and from they 1.59 are toshifted 1.67 V. to On the the anodic CVs ofregion L1, L 3 byin dichloromethane,0.15 and 0.19 V in twocomparison oxidation with waves the only data are in observMeCN.ed Obviously, and they are the shifted third anodeto the anodicprocess region occurs by at 0.15the andhigher 0.19 anodic V in comparisonpotential restricted with the by data the electrocin MeCN.hemical Obviously, oxidation the of third the solvent. anode process occurs at the higher anodic potential restricted by the electrochemical oxidation of the solvent. The CV of L2 in dichloromethane solution has three oxidation peaks like the CVs of L1 and L3 in acetonitrileThe CV (Figureof L2 in 5).dichloromethane solution has three oxidation peaks like the CVs of L1 and L3 in acetonitrile (Figure 5).

Molecules 2020, 25, 1770 8 of 23

The absorption band in the visible spectral range corresponds to a π-π* transition, which is a characteristic for compounds containing a quinonoid fragment like 3.5-di-tert-butyl-o- benzoquinone [81]. The CV of L1 after electrolysis in the cathodic region contains a quasi-reversible peak at E = 0.58 V (Figure S12 of ESI). This peak is assigned to the reduction of electrogenerated 1/2 − o-benzoquinone to o-benzosemiquinone. The second oxidation peak on CVs of catechols L1, L3 may be assigned to the oxidation of the phenolic group. In acetonitrile, for compounds L1 and L3, the shift of the second oxidation potential to the cathode region occurs as compared to the oxidation of the well-known phenolic antioxidant (ionol)—2,6-di-tert-butyl-4-methylphenol 1.47 V (CH3CN). In dichloromethane, the oxidation potentials of L1 and L3 are close to the value for this phenolic compound (1.48 V). The presence of the thioether group causes the appearance of the third redox wave at high anodic potentials. For L1 and L3 in acetonitrile, the values Eox3 are in good agreement with the data on the oxidation of catechol thioethers: the electrooxidation of the thioether group occurs in the potential range from 1.59 to 1.67 V. On the CVs of L1, L3 in dichloromethane, two oxidation waves only are observed and they are shifted to the anodic region by 0.15 and 0.19 V in comparison with the data in MeCN. Obviously, the third anode process occurs at the higher anodic potential restricted by the electrochemical oxidation of the solvent. The CV of L2 in dichloromethane solution has three oxidation peaks like the CVs of L1 and L3 in acetonitrile (Figure5). Molecules 2020, 25, 1770 8 of 22

Figure 5. The CV curves of L2 (in the potential switch from 0.60 to 1.50 V—curve 1; in the potential Figure 5. The CV curves of L2 (in the potential switch from− −0.60 to 1.50 V—curve 1; in the potential switchswitch from from 0.60−0.60 to to 1.70 1.70 V—curve V—curve 2; 2; in in the the potential potential switch switch from from 0.60−0.60 to to 1.85 1.85 V—curve V—curve 3) 3) (CH (CH2Cl2Cl2,2, − 1 − C = 1.5 mM, 0.15 M TBAP, scan rate 200 mV s −).1 C = 1.5 mM, 0.15 M TBAP, scan rate 200 mV·s· − ).

In contrast to the above described catechol-thioesters L1 and L3, the ﬁrst and second oxidation In contrast to the above described catechol-thioesters L1 and L3, the first and second oxidation stages for compound L2 are quasi-reversible. The obtained values of the current ratio (Ic/Ia) indicate a stages for compound L2 are quasi-reversible. The obtained values of the current ratio (Ic/Ia) indicate a low stability of the electrically generated intermediates. The introduction of tert-butyl groups helps to low stability of the electrically generated intermediates. The introduction of tert-butyl groups helps stabilize the dication formed in the ﬁrst stage (like in the case of L1); however, nevertheless, a rapid to stabilize the dication formed in the first stage (like in the case of L1); however, nevertheless, a rapid deprotonation reaction occurs in the solution, and, as a result, a small reduction peak is observed in deprotonation reaction occurs in the solution, and, as a result, a small reduction peak is observed in the reverse scan of CV. At the second stage, one-electron oxidation of a sterically hindered phenolic the reverse scan of CV. At the second stage, one-electron oxidation of a sterically hindered phenolic fragment leads to the formation of a cation-radical intermediate, which is more stable over the time of fragment leads to the formation of a cation-radical intermediate, which is more stable over the time the CV experiment (Scheme4). of the CV experiment (Scheme 4).

Scheme 4. The electrochemical oxidation of L2.

The third anode peak at the potential of 1.77 V suggests the participation of the thioether fragment in further redox transformations. It should be noted that the presence of an electron-withdrawing sulfur atom in the catecholate ring or the presence of a methylene bridge between the aromatic ring and the heteroatom does not practically affect the oxidation potentials of compounds L1 and L3 in dichloromethane (Table 1). At the same time, the first oxidation potential (0.05 V) for L3 is shifted to the anode region as compared with 3,5-di-tert-butylcatechol in accordance with the electro-acceptor effect of the sulfur atom. In acetonitrile, such changes are less pronounced. In dichloromethane, for compounds L2 and L3, which differ by the absence or presence of tert-butyl groups in the phenolic fragment, a shift of the oxidation

Molecules 2020, 25, 1770 8 of 22

Figure 5. The CV curves of L2 (in the potential switch from −0.60 to 1.50 V—curve 1; in the potential

switch from −0.60 to 1.70 V—curve 2; in the potential switch from −0.60 to 1.85 V—curve 3) (CH2Cl2, C = 1.5 mM, 0.15 M TBAP, scan rate 200 mV·s−1).

In contrast to the above described catechol-thioesters L1 and L3, the first and second oxidation stages for compound L2 are quasi-reversible. The obtained values of the current ratio (Ic/Ia) indicate a low stability of the electrically generated intermediates. The introduction of tert-butyl groups helps to stabilize the dication formed in the first stage (like in the case of L1); however, nevertheless, a rapid deprotonation reaction occurs in the solution, and, as a result, a small reduction peak is observed in the reverse scan of CV. At the second stage, one-electron oxidation of a sterically hindered phenolic fragmentMolecules 2020 leads, 25, 1770to the formation of a cation-radical intermediate, which is more stable over the 9time of 23 of the CV experiment (Scheme 4).

Scheme 4. The electrochemical oxidation of L2. Scheme 4. The electrochemical oxidation of L2.

TheThe thirdthird anodeanode peak peak at at the the potential potential of 1.77 of V1.77 suggests V suggests the participation the participation of the thioether of the thioether fragment fragmentin further in redox further transformations. redox transformations. ItIt should should be be noted noted that that the presence of an electron-withdrawingelectron-withdrawing sulfur sulfur atom atom in in the catecholate ringring or or the the presence presence of of a methylene bridge bridge between between the the aromatic aromatic ring ring and and the the heteroatom heteroatom does does not not practically affect affect the oxidation potentials of compounds L1 and L3 inin dichloromethane dichloromethane (Table (Table 11).). AtAt thethe same same time, time, the the first first oxidation oxidation potential potential (0.05 (0.05 V) V) for for L3 isis shifted shifted to to the the anode anode region region as as compared compared with 3,5-di- terttert-butylcatechol-butylcatechol in in accordance accordance with with the the electro-acceptor effect effect of the sulfur atom.atom. In In acetonitrile,acetonitrile, such changes are less pronounced. In dichloromethane, for compounds LL22 and L3,, which which differdiffer by the absence absence or or presence presence of terttert-butyl-butyl groups groups in in the the phenolic phenolic fragment, fragment, a a shift shift of of the the oxidation oxidation potentials (0.11 and 0.10 V) for L2 to the anode region is observed. This behavior can be rationalized by the formation of stronger intramolecular hydrogen bonds between the catechol hydroxyl and the sulfur atom than in the case of compound L1. Using S-functionalized phenols as an example, such an interaction led to an increase in the bond-breaking energy, BDE (O-H) [82]. The 4-hydroxyphetylthio group introduced into the catecholate cycle exhibits an electron-withdrawing effect, which is expressed for L2 in a significant shift (0.14 V) of the first oxidation potential to the anode region as compared with 3,5-di-tert-butylcatechol. As in the case of free ligands, the solvent has a significant effect on the electrochemical behavior of complexes 1 and 2. In dichloromethane, for both complexes, the first stage of oxidation has a quasi-reversible one-electron character and leads to the formation of relatively stable (in the CV time + ox1 scale) monocationic complexes of the type [(SQ)SbPh3] (Scheme5)[ 83,84]. The value of the E 1/2 potential for 1 is similar to the data obtained for triphenylantimony(V) 3,6-di-tert-butylcatecholate [85]. For the complex 2, this value shifts by 0.06 V to the anode region, which is associated with the electron-withdrawing effect of the sulfur atom, which is not separated from the catecholate ring by the methylene group as in case of 1. Such changes are in good agreement with previously obtained results for triarylantimony(V) 6-chloro(bromo)-3,5-di-tert-butylcatecholates [86,87]. The introduction of a methylene group between the catecholate fragment and the 3,5-di-tert-butyl-4-hydroxyphenylthio group ox1 has no practically effect on the value of the E 1/2 potential, as in the case of 6-alkoxymethyl-substituted 3,5-di-tert-butylcatecholates of triphenylantimony(V) [88]. MoleculesMolecules 20202020,, 2525,, 17701770 99 ofof 2222

potentials (0.11 and 0.10 V) for L2 to the anode region is observed. This behavior can be rationalized potentials (0.11 and 0.10 V) for L2 to the anode region is observed. This behavior can be rationalized byby thethe formationformation ofof strongerstronger intramolecularintramolecular hydrhydrogenogen bondsbonds betweenbetween thethe catecholcatechol hydroxylhydroxyl andand thethe sulfur atom than in the case of compound L1. Using S-functionalized phenols as an example, such an sulfur atom than in the case of compound L1. Using S-functionalized phenols as an example, such an interactioninteraction ledled toto anan increaseincrease inin thethe bond-breakingbond-breaking energy,energy, BDEBDE ((ОО--НН)) [82].[82]. TheThe 4-hydroxyphetylthio4-hydroxyphetylthio groupgroup introducedintroduced intointo thethe catecholatecatecholate cyclecycle exhiexhibitsbits anan electron-withdrawingelectron-withdrawing effect,effect, whichwhich isis expressed for L2 in a significant shift (0.14 V) of the first oxidation potential to the anode region as expressed for L2 in a significant shift (0.14 V) of the first oxidation potential to the anode region as comparedcompared withwith 3,5-di-3,5-di-terttert-butylcatechol.-butylcatechol. AsAs inin thethe casecase ofof freefree ligands,ligands, thethe solventsolvent hashas aa significantsignificant effecteffect onon thethe electrochemicalelectrochemical behaviorbehavior ofof complexescomplexes 11 andand 22.. InIn dichloromethane,dichloromethane, forfor bothboth complexes,complexes, thethe firstfirst stagestage ofof oxidationoxidation hashas aa quasi-quasi- reversiblereversible one-electronone-electron charactercharacter andand leadsleads toto thethe formformationation ofof relativelyrelatively stablestable (in(in thethe CVCV timetime scale)scale) monocationic complexes of the type [(SQ)SbPh3]+ (Scheme 5) [83,84]. The value of the Eox11/2 potential monocationic complexes of the type [(SQ)SbPh3]+ (Scheme 5) [83,84]. The value of the Eox11/2 potential forfor 11 isis similarsimilar toto thethe datadata obtainedobtained forfor triphenylantimony(V)triphenylantimony(V) 3,6-di-3,6-di-terttert-butylcatecholate-butylcatecholate [85].[85]. ForFor thethe complexcomplex 22,, thisthis valuevalue shiftsshifts byby 0.060.06 VV toto thethe anodeanode reregion,gion, whichwhich isis associatedassociated withwith thethe electron-electron- withdrawingwithdrawing effecteffect ofof thethe sulfursulfur atom,atom, whichwhich isis notnot separatedseparated fromfrom thethe cacatecholatetecholate ringring byby thethe methylenemethylene groupgroup asas inin casecase ofof 11.. SuchSuch changeschanges areare inin goodgood agreementagreement withwith previouslypreviously obtainedobtained resultsresults forfor triarylantimony(V)triarylantimony(V) 6-chloro(bromo)-3,5-di-6-chloro(bromo)-3,5-di-terttert-butylcatecholates-butylcatecholates [86,87].[86,87]. TheThe introductionintroduction ofof aa methylenemethylene groupgroup betweenbetween thethe catecholatecatecholate fragmentfragment andand thethe 3,5-di-3,5-di-terttert-butyl-4-hydroxyphenylthio-butyl-4-hydroxyphenylthio Molecules 2020, 25, 1770 10 of 23 group has no practically effect on the value of the Eox11/2 potential, as in the case of 6-alkoxymethyl- group has no practically effect on the value of the Eox11/2 potential, as in the case of 6-alkoxymethyl- substitutedsubstituted 3,5-di-3,5-di-terttert-butylcatecholates-butylcatecholates ofof trtriphenylantimony(V)iphenylantimony(V) [88].[88].

SchemeScheme 5.5. TheTheThe electrochemicalelectrochemical electrochemical oxidationoxidation ofof complexcomplex 11.. .

For complex 1, the second anode peak has a quasi-reversible character (Figure6). A double ForFor complexcomplex 11,, thethe secondsecond anodeanode peakpeak hashas aa quasi-reversiblequasi-reversible charactercharacter (Figure(Figure 6).6). AA doubledouble increase in the current of the second oxidation peak as compared to the ﬁrst one indicates an increase increaseincrease inin thethe currentcurrent ofof thethe secondsecond oxidationoxidation peakpeak asas comparedcompared toto thethe firstfirst oneone indicatesindicates anan increaseincrease in the number of electrons involved in this electrode process to two. inin thethe numbernumber ofof electronselectrons involvedinvolved inin thisthis electrodeelectrode processprocess toto two.two.

FigureFigure 6.6. 6. CyclicCyclicCyclic voltammogramsvoltammograms voltammograms ofof compoundofcompound compound 11 (in(in1 the(inthe potential thepotential potential switchswitch switch fromfrom from −−0.50.5 toto 1.10.51.1 V—curve toV—curve 1.1 V—curve 1;1; inin thethe 1; − −1 potentialin the potential switch from switch −0.5 from to 1.6 0.5V—curve to 1.6 V—curve2) (CH2Cl2 2), C (CH = 3 mM,Cl ,C 0.15= M3 mM,TBAP, 0.15 scan M rate TBAP, 200 scanmV·s− rate1 ). 200 potential switch from −0.5 to 1.6− V—curve 2) (CH2Cl2, C = 3 mM,2 2 0.15 M TBAP, scan rate 200 mV·s ). mV s 1). · − The value of the peak potential Eox2p = 1.39 V on the CV of 1 is fixed in the potential range of 1.37– The value of the peak potential Eox2p = 1.39 V on the CV of 1 is fixed in the potential range of 1.37– The value of the peak potential Eox2 = 1.39 V on the CV of 1 is ﬁxed in the potential range of 1.601.60 V,V, whichwhich isis characteristiccharacteristic ofof thethe secondsecondp redoxredox processprocess ““oo-semiquinone-semiquinone // oo-benzoquinone”-benzoquinone” inin suchsuch 1.37–1.60 V, which is characteristic of the second redox process “o-semiquinone/o-benzoquinone” in such triarylantimony(V) catecholates [89–91]. This electrochemical picture in the case of 1 indicates the convergence of the boundary redox orbitals of o-semiquinone and phenolic fragments in 1+, which leads to their simultaneous electrooxidation. A decrease in the ratio of currents to 0.5 for the second oxidation process is caused by the subsequent chemical stages in the solution—deprotonation of the phenoxyl radical cation and decoordination of o-benzoquinone moiety from the tricationic intermediate. In complex 2, these redox centers behave separately: The second oxidation stage at 1.24 V is irreversible and assigned to the further oxidation of coordinated o-benzosemiquinone in the intermediate + + 2+ [(SQ)SbPh3] (2 ) to o-benzoquinone, with the subsequent decoordination of o-benzoquinone from 2 leading to the complex decomposition (Scheme6). The third redox process is a two-electronic peak, which is an oxidation of the phenolic group in free o-benzoquinone at 1.48 V (Figure7). MoleculesMolecules 2020 2020, ,25 25, ,1770 1770 1010 of of 22 22 triarylantimony(V)triarylantimony(V) catecholatescatecholates [89–91].[89–91]. ThisThis electrochemicalelectrochemical picturepicture inin thethe casecase ofof 11 indicatesindicates thethe convergenceconvergence ofof thethe boundaryboundary redoxredox orbitalsorbitals ofof oo-semiquinone-semiquinone andand phenolicphenolic fragmentsfragments inin 11++, , whichwhich leadsleads to to their their simultaneous simultaneous electrooxidation. electrooxidation. A A decre decreasease in in the the ratio ratio of of currents currents to to 0.5 0.5 for for the the second second oxidationoxidation process process is is caused caused by by the the subsequent subsequent chemical chemical stages stages in in the the solution—deprotonation solution—deprotonation of of the the phenoxylphenoxyl radicalradical cationcation andand decoordinationdecoordination ofof oo-benzoquinone-benzoquinone moietymoiety fromfrom thethe tricationictricationic intermediate.intermediate. InIn complexcomplex 22, , thesethese redoxredox centerscenters behavebehave separately:separately: TheThe secondsecond oxidationoxidation stagestage atat 1.241.24 VV isis irreversibleirreversible and and assigned assigned to to the the further further oxidation oxidation of of coordinated coordinated o o-benzosemiquinone-benzosemiquinone in in the the intermediate intermediate + + 2+ [(SQ)SbPh[(SQ)SbPh3]3]+ ((22+)) toto oo-benzoquinone,-benzoquinone, withwith thethe subsequentsubsequent decoordinationdecoordination ofof oo-benzoquinone-benzoquinone fromfrom 222+ Molecules 2020, 25, 1770 11 of 23 leadingleading to to the the complex complex decomposition decomposition (Scheme (Scheme 6). 6). The The third third redox redox process process is is a a two-electronic two-electronic peak, peak, which which isis an an oxidation oxidation of of the the phenolic phenolic group group in in free free o o-benzoquinone-benzoquinone at at 1.48 1.48 V V (Figure (Figure 7). 7).

tButBu tButBu tButBu OO OO OO -e-e ++ -e-e 2+2+ SbPhSbPh3 SbPhSbPh3 SbPhSbPh3 3 +e 3 3 2+2+ +e -[SbPh-[SbPh]3] tButBu OO tButBu OO tButBu OO 3 SS SS SS

OHOH OHOH OHOH

2222++ 222+2+

tButBu tButBu OO OO -2e-2e -H-H++ tButBu OO tButBu OO SS SS ++ OHOH OO SchemeScheme 6. 6. The TheThe electrochemical electrochemicalelectrochemical oxidation oxidation of of complex complex 2 2.. .

TheThe potentialpotential value value of of thisthis thirdthird redox-stageredox-stage isis identicalidentical withwith thatthat oneone forfor thethe oxidationoxidation ofof thethe phenolicphenolic group group inin freefree L L33 and and 2,6-di- 2,6-di-terttert-butyl-4-methylphenol-butyl-4-methylphenol (1.48 (1.48 V V vs.vs. Ag/AgCl) AgAg/AgCl)/AgCl) [92]. [[92].92].

Figure 7. Cyclic voltammograms of compound 2 (in the potential switch from 0.50 to 1.15 V—curve 1; FigureFigure 7. 7. Cyclic Cyclic voltammograms voltammograms of of compound compound 2 2 (in (in the the potential potential switch switch from from− − −0.500.50 to to 1.15 1.15 V—curve V—curve 1;in1; in thein the the potential potential potential switch switch switch from from from0.50 − −0.500.50 to 1.30to to 1.30 1.30 V—curve V—curve V—curve 2; in 2; 2; the in in potentialthe the potential potential switch switch switch from from from0.50 − − to0.500.50 1.70 to to V—curve1.70 1.70 V— V— − 1 − 3) (CH Cl ,C = 3 mM, 0.15 M TBAP, scan rate 200 mV s ). −1 curvecurve 3) 23) (CH (CH2 2Cl2Cl2,2 ,C C = = 3 3 mM, mM, 0.15 0.15 M M TBAP, TBAP, scan scan rate rate 200 200· −mV·s mV·s−1).). In a coordinating solvent acetonitrile (Table2), the oxidation potentials for complexes 1 and 2, as InIn aa coordinatingcoordinating solventsolvent acetonitrileacetonitrile (Table(Table 2),2), thethe oxidationoxidation potentialspotentials forfor complexescomplexes 11 andand 22, , it was expected, shift slightly to the cathode region. In the whole, the electrochemical behavior of the asas it it was was expected, expected, shift shift slightly slightly to to the the cathode cathode re region.gion. In In the the whole, whole, the the electrochemical electrochemical behavior behavior of of complexes changes. The CV of 1 in acetonitrile has three oxidation waves (Figure S13). thethe complexes complexes changes. changes. The The CV CV of of 1 1 in in acetonitrile acetonitrile has has three three oxidation oxidation waves waves (Figure (Figure S13). S13). In acetonitrile, the current of the first quasi-reversible oxidation peak for 1 increases twice to a two-electron level, and the ratio of currents Ic/Ia decreases in comparison with the data obtained for 1 in dichloromethane. These changes indicate the generation of an unstable dicationic intermediate 2+ [(Q)SbPh3] in the near-electrode region. Earlier, we observed a similar electrochemical activity for the triphenylantimony(V) catecholates containing electron-withdrawing groups in the catecholate cycle, but in most cases, the dicationic derivatives formed in the electrooxidation were unstable [90,91]. The second wide quasi-reversible oxidation peak at 1.24 V is single electron in nature. The low value of the current ratio, as in the first case, indicates the instability of the resulting intermediate and the course of the subsequent chemical stage. The proximity of the redox orbitals of the catecholate ligand and the sterically hindered phenol fragment does not allow us to unambiguously describe the oxidative transformations at the first and second stages of oxidation. Based on the value of the Molecules 2020, 25, 1770 11 of 22 Molecules 2020, 25, 1770 11 of 22

InIn acetonitrile,acetonitrile, thethe currentcurrent ofof thethe firsfirstt quasi-reversiblequasi-reversible oxidationoxidation peakpeak forfor 1 increasesincreases twicetwice toto aa two-electron level, and the ratio of currents Ic/Ia decreases in comparison with the data obtained for two-electron level, and the ratio of currents Ic/Ia decreases in comparison with the data obtained for 1 inin dichloromethane.dichloromethane. TheseThese changeschanges indicateindicate thethe generationgeneration ofof anan unstableunstable dicationicdicationic intermediateintermediate [(Q)SbPh3]2+ in the near-electrode region. Earlier, we observed a similar electrochemical activity for [(Q)SbPh3]2+ in the near-electrode region. Earlier, we observed a similar electrochemical activity for thethe triphenylantimony(V)triphenylantimony(V) catecholcatecholates containing electron-withdrawing groups in the catecholate cycle, but in most cases, the dicationic derivatives formed in the electrooxidation were unstable [90,91].[90,91]. TheThe secondsecond widewide quasi-reversiblequasi-reversible oxidationoxidation peakpeak atat 1.241.24 VV isis singlesingle electronelectron inin nature.nature. TheThe lowlow valuevalue ofof thethe currentcurrent ratio,ratio, asas inin thethe firstfirst case,case, indicatesindicates thethe instabilityinstability ofof thethe resultingresulting intermediateintermediate Moleculesand the 2020course, 25, 1770of the subsequent chemical stage. The proximity of the redox orbitals of the catecholate12 of 23 ligandligand andand thethe stericallysterically hinderedhindered phenolphenol fragmentfragment does not allow us to unambiguously describe the oxidative transformations at the first and second stages of oxidation. Based on the value of the first ﬁrstoxidative oxidation transformations potential, we at may the assumefirst and that second the dianionicstages of catecholateoxidation. Based fragment on the is oxidizedvalue of initiallythe first oxidation potential, we may assume that the dianionic catecholate fragment is oxidized initially to o- tooxidationo-benzoquinone, potential, accompaniedwe may assume by that an electronthe dianionic transfer catecholate from the fragment phenolic is groupoxidized to theinitially dication to o- benzoquinone, accompanied by an electron transfer from the phenolic group to the dication (Scheme (Schemebenzoquinone,7). The accompanied second anode by process an electron may transfer equally befrom attributed the phenolic to the group oxidation to the ofdication a coordinated (Scheme 7). The second anode process may equally be attributed to the oxidation of a coordinated o- o7).-semiquinone The second radical anode anion process to o-benzoquinone may equally orbe toattributed the oxidation to the of aoxidation di-tert-butylphenol of a coordinated moiety to o a- semiquinone radical anion to o-benzoquinone or to the oxidation of a di-tert-butylphenol moiety to a radicalsemiquinone cation. radical anion to o-benzoquinone or to the oxidation of a di-tert-butylphenol moiety to a radical cation.

SchemeScheme 7.7. TheThe electrochemicalelectrochemical oxidationoxidation ofof complexcomplex 11 ininin acetonitrile.acetonitrile.acetonitrile.

TheThe electrooxidationelectrooxidation ofof 22 ininin acetonitrileacetonitrileacetonitrile (Figure(Figure(Figure8 8)8)) has hashas a aa quasi-reversible quasi-reversiblequasi-reversible one-electron one-electronone-electron character charactercharacter atat thethe ﬁrstfirst stagestage (as(as inin dichloromethane),dichloromethane), whichwhich indicatesindicates thethe generationgeneration ofof aa monocationmonocation complexcomplex containingcontaining aa coordinatedcoordinated oo-semiquinone-semiquinone-semiquinone ligand. ligand.ligand.

FigureFigure 8.8. Cyclic voltammogramsvoltammograms ofof compoundcompound2 2(in (in the the potential potential switch switch from from 0.50−0.50 to to 1.05 1.05 V—curve V—curve 1; Figure 8. Cyclic voltammograms of compound 2 (in the potential switch from− −0.50 to 1.05 V—curve in1; in the the potential potential switch switch from from 0.50−0.50 to to 1.50 1.50 V—curve V—curve 2) 2) (CH (CH3CN,3CN, C C= = 44 mM,mM, 0.150.15 MM TBAP,TBAP, scanscan raterate 1; in the potential switch from− −0.50 to 1.50 V—curve 2) (CH3CN, C = 4 mM, 0.15 M TBAP, scan rate 200200 mVmV·ss −1).). 200 mV·s· −−1). A signiﬁcant diﬀerence of the electrochemical behavior of 2 in acetonitrile from the electrochemical A significant difference of the electrochemical behavior of 2 inin acetonitrileacetonitrile fromfrom thethe picture of 2 in dichloromethane is the absence of a separation of the second and third stages of oxidation: electrochemical picture of 2 inin dichloromethanedichloromethane isis thethe absenceabsence ofof aa separationseparation ofof thethe secondsecond andand thirdthird There is one two-electron oxidation peak. Complex 2 in acetonitrile exhibits a behavior similar to 1 stages of oxidation: There is one two-electrontwo-electron oxidationoxidation peak.peak. ComplexComplex 2 inin acetonitrileacetonitrile exhibitsexhibits aa in dichloromethane: Both redox active fragments of the ligand are involved in the second stage of behavior similar to 1 inin dichloromethane:dichloromethane: BothBoth redoxredox activeactive fragmentsfragments ofof thethe ligandligand areare involvedinvolved inin oxidation, namely the o-semiquinone ligand is converted to o-benzoquinone, and the phenolic group is oxidized to the radical cation.

The synthesized ligands (as well as the complexes based on them) contain several electroactive centers. Complexes 1 and 2 significantly expand their redox capabilities owing to the fact that they can act as: (1) electron donors due to the catecholate metallocycle capable of sequential oxidations, and (2) donors of the hydrogen atoms forming a stable phenoxyl radical. This ability of ligands can be used in catalysis in coordination with transition metal ions. In contrast to the previously studied triphenylantimony(V) catecholato complexes with additional redox centers (ferrocenyl, morpholine, piperazine groups [18,24,30]), whose electrochemical activation Molecules 2020, 25, 1770 12 of 22 the second stage of oxidation, namely the o-semiquinone ligand is converted to o-benzoquinone, and the phenolic group is oxidized to the radical cation. The synthesized ligands (as well as the complexes based on them) contain several electroactive centers. Complexes 1 and 2 significantly expand their redox capabilities owing to the fact that they can act as: (1) electron donors due to the catecholate metallocycle capable of sequential oxidations, and (2) donors of the hydrogen atoms forming a stable phenoxyl radical. This ability of ligands can Moleculesbe used2020 in catalysis, 25, 1770 in coordination with transition metal ions. 13 of 23 In contrast to the previously studied triphenylantimony(V) catecholato complexes with additional redox centers (ferrocenyl, morpholine, piperazine groups [18,24,30]), whose iselectrochemical observed at potentials activation shifted is observ to theed cathodeat potentials region shifted as compared to the cathode to the region “catechol as /comparedo-semiquinone” to the transition,“catechol/o compounds-semiquinone”1 and transition,2 are characterized compounds 1 byand electrooxidation 2 are characterized of theby electrooxidation phenolic fragment of the at potentialsphenolic fragment that are close at potentials to the second that redoxare close transition to the “secondo-semiquinone redox transition/o-benzoquinone”. “o-semiquinone/ This facto- indicatesbenzoquinone”. the convergence This fact ofindicates the boundary the convergence redox orbitals of ofthe the boundary phenolic redox and o -semiquinoneorbitals of the fragments phenolic inand the o-semiquinone electro-generated fragments monocationic in the electr complexes.o-generated monocationic complexes. 2.4. EPR Experiments 2.4. EPR Experiments The oxidation of the catechol thioether L1 with lead dioxide in toluene proceeds slowly. In this case, The oxidation of the catechol thioether L1 with lead dioxide in toluene proceeds slowly. In this immediately after mixing the reagents, a superposition of two doublets was observed (Figure S14(1)). case, immediately after mixing the reagents, a superposition of 1two doublets was observed (Figure One of the doublets has the following parameters: gi = 2.0020, ai( H) = 3.61 G, the second: gi = 2.0009, S14(1)).1 One of the doublets has the following parameters: gi = 2.0020, ai( H) = 3.6 G, the second: gi = ai( H) = 3.85 G. The intensive stirring and heating of the reaction mixture leads to an increase in 2.0009, ai(1H) = 3.85 G. The intensive stirring and heating of the reaction mixture leads to an increase the intensity of the EPR spectrum, and the intensity of the second doublet grows significantly more in the intensity of the EPR spectrum, and the intensity of the second doublet grows significantly more noticeably in comparison with the intensity of the first doublet (Figure S14(2)). An increase in the noticeably in comparison with the intensity of the first doublet (Figure S14(2)). An increase in the signal intensity allows one to see the satellite splitting of the second signal on magnetic isotopes of lead signal207 intensity allows one to see the satellite splitting of the second signal207 on magnetic isotopes of ( Pb, I = 1/2, 22.1% [93]) with the hyper-fine structure (HFS) constant ai( Pb) = 70.5 G. The doublet lead (207Pb, I = 1/2, 22.1% [93]) with the hyper-fine structure (HFS) constant ai(207Pb) = 70.5 G. The with satellite splitting was simulated using the WinEPR SimFonia 1.25 program, and the simulation doublet with satellite splitting was simulated using the WinEPR SimFonia 1.25 program, and the spectrum is shown in Figure S14(3). simulation spectrum is shown in Figure S14(3). It can be concluded that the interaction of catechol L1 with lead dioxide slowly leads to the It can be concluded that the interaction of catechol L1 with lead dioxide slowly leads to the deprotonation of catechol with the formation of the o-semiquinone radical anion at the first stage deprotonation of catechol with the formation of the o-semiquinone1 radical anion at the first stage (Scheme8), which has a doublet with parameters g i = 2.0020, ai( H) = 3.6 G without any satellite (Scheme 8), which has a doublet with parameters gi = 2.0020, ai(1H) = 3.6 G without any satellite splitting on the lead isotopes. splitting on the lead isotopes.

SchemeScheme 8.8. TheThe oxidationoxidation ofof catecholcatechol LL11 with lead(IV) oxide.

TheThe processprocess acceleratesaccelerates whenwhen thethe mixturemixture isis heated;heated; however,however, thethe formationformation ofof oo-semiquinone-semiquinone complexcomplex withwith lead(II)lead(II) occurs, occurs, the the EPR EPR spectrum spectrum of of which which increases increases in intensityin intensity with with the the heating heating of the of samplethe sample (the (the second second EPR EPR spectrum). spectrum). TheThe oxidationoxidation ofof catecholcatechol LL22 with leadlead dioxidedioxide inin aa toluenetoluene solutionsolution proceedsproceeds moremore easilyeasily thanthan thethe oxidationoxidation ofof LL11 andand doesdoes notnot requirerequire heating.heating. In thisthis case,case, initiallyinitially (Figure(Figure9 9(1)),(1)), a a wide wide doublet doublet 1 appearsappears withwith thethe parametersparameters ggi i == 1.9967,1.9967, aaii((1H) = 2.3 G, which disappearsdisappears graduallygradually withwith furtherfurther stirringstirring ofof thethe reactionreaction mixturemixture (Figure(Figure9 9(2))(2)) with with the the simultaneous simultaneous appearance appearance of of a a triplet triplet with with some some 1 otherother parameters (g ii == 2.0050,2.0050, ai a(2i(21H)H) = 1.1= 1.1 G). G). Twenty Twenty minutes minutes after after the thebeginning beginning reaction, reaction, only only one onetriplet triplet signal signal is observed is observed practically practically (Figure (Figure 9(93)).(3)). Both Both spectra spectra were simulatedsimulated usingusing WinEPRWinEPR SimFoniaSimFonia 1.25.1.25. Simulations Simulations of theof tripletthe triplet and doubletand doublet spectra arespectra shown are in Figureshown9 (4),(5),in Figure respectively. 9(4),(5), respectively.

Molecules 2020, 25, 1770 14 of 23 MoleculesMolecules 2020 2020, 25, ,25 1770, 1770 13 13of of22 22

FigureFigureFigure 9. 9. 9.TheThe The X-band X-band X-band EPR EPR spectrum spectrum ofof theofthe the mixture mixture mixture “ L“2L “+2 L+PbO2 PbO+ PbO2”2” in 2in” toluene intoluene toluene immediately immediately immediately after after after mixing mixing mixing the thereagentsthe reagents reagents (spectrum (spectrum (spectrum 1), 1), after 1), after after stirring sti stirringrring for for 10 for 10 min 10 min (spectrummin (spectrum (spectrum 2), 2), and 2), and afterand af teraf stirringter stirring stirring for for 20 for 20 min 20 min min (spectrum (spectrum (spectrum 3); 1 Simulated EPR spectra (WinEPR SimFonia 1.25): triplet with parameters g = 2.0050, a (21 H)1 = 1.1 G 3);3); Simulated Simulated EPR EPR spectra spectra (WinEPR (WinEPR SimFo SimFoniania 1.25): 1.25): triplet triplet with with parameters parameters gi i g= i 2.0050,= 2.0050, aii(2 ai(2H)H) = 1.1= 1.1 G G 1 (spectrum 4), doublet with parameters g = 1.9967, a1( H) = 2.3 G (spectrum 5). (spectrum(spectrum 4), 4), doublet doublet with with parameters parameters gi ig= i 1.9967,= 1.9967, ai( ai H)i(1H) = 2.3= 2.3 G G(spectrum (spectrum 5). 5).

The oxidation of L2 with lead dioxide proceeds with the deprotonation of catechol with the TheThe oxidation oxidation of of L 2L with2 with lead lead dioxide dioxide proceeds proceeds with with the the deprotonation deprotonation of of catechol catechol with with the the formation of the o-semiquinone radical (Scheme9), and the doublet signal from this radical is formationformation of of the the o -semiquinoneo-semiquinone radical radical (Scheme (Scheme 9), 9), and and the the doublet doublet signal signal from from this this radical radical is is recorded at the initial time. However, a decrease in the intensity of this signal and an increase in recordedrecorded at atthe the initial initial time. time. However, However, a decreasea decrease in in the the intensity intensity of of this this signal signal and and an an increase increase in in the the the intensity of the triplet with the HFS constant on two equivalent protons of 1.1 G characteristic intensityintensity of of the the triplet triplet with with the the HFS HFS constant constant on on two two equivalent equivalent protons protons of of 1.1 1.1 G Gcharacteristic characteristic of of 4,6- 4,6- of 4,6-di-tert-butyl-phenoxyl radicals gradually occurs. Thus, we can conclude that either the di-di-terttert-butyl-phenoxyl-butyl-phenoxyl radicals radicals grad graduallyually occurs. occurs. Thus, Thus, we we can can conclude conclude that that either either the the o -o- o-semiquinone formed at the initial stage oxidizes and deprotonates the phenolic group to form semiquinonesemiquinone formed formed at atthe the initial initial stage stage oxidizes oxidizes and and deprotonates deprotonates the the phenolic phenolic group group to to form form a a a phenoxyl radical during the reaction, or (since the reaction with lead dioxide is heterogeneous, phenoxylphenoxyl radical radical during during the the reacti reaction,on, or or (since (since the the reaction reaction with with lead lead dioxide dioxide is isheterogeneous, heterogeneous, requiring an excess of oxidizing agent) during the reaction, oxidation of the 4,6-di-tert-butylphenol requiringrequiring an an excess excess of of oxidizing oxidizing agent) agent) during during the the reaction, reaction, oxidation oxidation of of the the 4,6-di- 4,6-di-terttert-butylphenol-butylphenol group in the catechol proceeds more slowly than the oxidation of the catechol fragment, and at a groupgroup in in the the catechol catechol proceeds proceeds more more slowly slowly than than the the oxidation oxidation of of the the catechol catechol fragment, fragment, and and at ata a certain moment, the concentration of the o-semiquinone radical begins to decrease with a simultaneous certaincertain moment, moment, the the concentration concentration of of the the o -semiquinoneo-semiquinone radical radical begi beginsns to to decrease decrease with with a a increase in the concentration of the phenoxyl radical. simultaneoussimultaneous increase increase in in the the concen concentrationtration of of the the phenoxyl phenoxyl radical. radical.

2 SchemeSchemeScheme 9. 9. 9.TheThe The oxidation oxidation oxidation of of ofLL 2Lwith2with with lead(IV) lead(IV) lead(IV) oxide. oxide. oxide.

Earlier,Earlier,Earlier, it it itwas was was shown shown shown that that that thioether thioether thioether LL 33L is3is isoxidized oxidized oxidized by by by lead(IV) lead(IV) lead(IV) oxid oxide oxide ine in intoluene toluene toluene solution solution solution with with with the the the formationformationformation of of oflead leadlead o-semiquinonato oo-semiquinonato-semiquinonato derivatives derivatives derivatives [14a]. [14a]. [14a]. In In the In the theEPR EPR EPR spectrum, spectrum, spectrum, a doubleta doublet a doublet at atgi g= at i 2.0004= g 2.0004i = 2.0004 with with satellitewithsatellite satellite splitting, splitting, splitting, typical typical typical for for lead(II) lead(II) for lead(II) o-semiquinolates, o-semiquinolates,o-semiquinolates, was was ob observed. wasserved. observed. This This fact fact This is isalso fact also explained is explained also explained by by the the initialbyinitial the formation initialformation formation of ofthe the o-semiquinone ofo-semiquinone the o-semiquinone radical radical anion radical anion further further anion coordi furthercoordinatednated coordinated to tometal. metal. The to The metal. hyperfine hyperfine The hyperfinestructure structure ofstructure ofthe the EPR EPR ofspectrum thespectrum EPR of spectrum ofa systema system of “L a“3 systemL+PbO3+PbO2” “2is”L is3caused+ causedPbO2 by” by is hyperfine caused hyperfine by splitting hyperfinesplitting of ofa splitting signala signal on ofon the a the signal one one proton on proton the nucleusonenucleus proton in inthe nucleusthe fifth fifth position inposition the fifth of ofthe position the o-semiquinone o-semiquinone of the o-semiquinone six-membered six-membered six-membered carbon carbon ri ngring with carbon with a satellitea ring satellite with splitting splitting a satellite on on 207207 207 1 1 2071207 207 thesplittingthe magnetic magnetic on thelead lead magnetic isotope isotope ( lead (Pb,Pb, isotope22.1%, 22.1%, I (= I 1/2):= Pb,1/2): a 22.1%,i (aH)i( H) = 2.65 I= =2.651 G,/2): G, ai (aaii(Pb)H)Pb) == 22.75=2.65 22.75 G,G G[48]. a i[48].( Pb) = 22.75 G [48]. TheTheThe oxidation oxidation oxidation of of triphenylantimony(V)triphenylantimony(V) triphenylantimony(V) catecholate catecholate catecholate1 with1 1with with lead(IV) lead(IV) lead(IV) oxide oxide oxide in toluenein in toluene toluene proceeds proceeds proceeds with withweakwith weak heatingweak heating heating (to 50 ◦(toC) (to and50°C) 50°C) leads and and to leads theleads formation to to the the formation offormation the corresponding of of the the corresponding corresponding phenoxyl radicalphenoxyl phenoxyl (Scheme radical radical 10 ) (Schemewith(Scheme an isotropic10) 10) with with an multiplet an isotropic isotropic EPR multiplet multiplet spectrum EPR EPR (Figure spectrum spectrum 10) (Figure with (Figure gi =10) 10)2.0050. with with g Thei g=i 2.0050.= spectrum2.0050. The The wasspectrum spectrum simulated was was simulatedwithsimulated Easyspin with with 5.2.25 Easyspin Easyspin [94], 5.2.25 and 5.2.25 the [94], following[94], and and the the HFS following following constants HFS HFS were constants constants revealed: were were the revealed: hyperfine revealed: the coupling the hyperfine hyperfine with couplingcoupling with with two two aromatic aromatic protons protons in in me meta-positionsta-positions to to the the oxy oxy group group with with ai (2ai (21H) 1H) = 0.89= 0.89 G, G, and and couplingcoupling with with two two non-equivalent non-equivalent protons protons of of the the S-CH S-CH2 group2 group in in the the para-position para-position of of the the phenoxyl phenoxyl

Molecules 2020, 25, 1770 15 of 23

1 two aromatic protons in meta-positions to the oxy group with ai(2 H) = 0.89 G, and coupling with two MoleculesMolecules 2020 2020, 25, 25, 1770, 1770 1414 of 1of 22 22 non-equivalent protons of the S-CH2 group in the para-position of the phenoxyl ring with ai(1 H) 1 = 1.97 G and a1 i(1 H) = 3.30 G. The1 nonequivalence of these methylene protons is caused by their ringring with with a ai(1i(1 H)1H) = = 1.97 1.97 G G and and a ai(1i(1 H)1H) = = 3.30 3.30 G. G. The The nonequivalence nonequivalence of of these these methylene methylene protons protons is is diﬀerent positions towards the pi-system of the phenoxyl radical. causedcaused by by their their different different positions positions towards towards the the pi-system pi-system of of the the phenoxyl phenoxyl radical. radical.

SchemeScheme 10. 10. TheThe The oxidation oxidation oxidation of of complex complex 1 1with with PbO PbO2 2withwith with formationformation formation ofof of thethe the phenoxylphenoxyl phenoxyl radical. radical. radical.

Figure 10. The X-band EPR spectrum of a mixture ”1+PbO2”: exp.—the experimental spectrum FigureFigure 10.10. TheThe X-bandX-band EPREPR spectrumspectrum of of a a mixture mixture ” 1”+1PbO+PbO2”:2”: exp.—theexp.—the experimentalexperimental spectrumspectrum (toluene,(toluene, 298 298 K); K); sim.—its sim.—its simu simulation simulationlation (WinEPR (WinEPR SimFoniaSimFonia SimFonia 1.25).1.25). 1.25).

TheThe EPR EPR data data ofof of experimentsexperiments experiments onon on the the the oxidation oxidation oxidation of of of free free free ligands ligands ligands indicate indicate indicate the the the tendency tendency tendency of Lof of1 Land L1 1and andL2 Lto L2 2 toformto form formo-semiquinone o -semiquinoneo-semiquinone radical radical radical anions. anions. anions. The The diTheﬀ erencedifference difference is the is is great the the great propensitygreat propensity propensity of L1 toof of form L L1 1to to stable form form chelatestable stable chelaterings,chelate while rings, rings, for while whileL2 the for for subsequentL L2 2the the subsequent subsequent generation generation generation of a stable of of a a stable phenoxylstable phenoxyl phenoxyl radical radical radical is possible. is is possible. possible. Complex Complex Complex1 in 1reaction1 in in reaction reaction with lead(IV)with with lead(IV) lead(IV) oxide tendsoxide oxide totends tends form to ato phenoxylform form a a phenoxyl phenoxyl radical, whichradical, radical, is consistentwhichwhich is is consistent withconsistent the possibility with with the the possibilityofpossibility simultaneous ofof simultaneoussimultaneous activation of theactivationactivation catecholate ofof the cyclethe caca andtecholatetecholate the phenolic cyclecycle groupandand thethe during phenolicphenolic electrooxidation groupgroup duringduring in a electrooxidationpolarelectrooxidation solvent (MeCN). in in a a polar polar solvent solvent (MeCN). (MeCN).

3.3. Materials MaterialsMaterials and andand Methods MethodsMethods

3.1. General Remarks 3.1.3.1. General General Remarks Remarks All the experiments on the synthesis and study of properties of complexes were carried out in AllAll the the experiments experiments on on the the synthesis synthesis and and study study of of properties properties of of complexes complexes were were carried carried out out in in evacuatedevacuated ampoules ampoules in in the the absence absence of of oxygen oxygen and and water. water. The The The solvents solvents used used used were were were purified puriﬁed purified and and dried dried 1 by standard methods [95]. The infrared spectra of the complexes in the 4000-400 cm −1 range were byby standard standard methods methods [95]. [95]. The The infrared infrared spectra spectra of of the the complexes complexes in in the the 4000-400 4000-400 cm cm− − 1range range were were recorded on an FSM 1201 Fourier-IR spectrometer in nujol mull. The NMR spectra of L1 and L2 were recordedrecorded on on an an FSM FSM 1201 1201 Fourier-IR Fourier-IR spectr spectrometerometer in in nujol nujol mull. mull. The The NMR NMR spectra spectra of of L L1 1and and L L2 2were were 1 recorded in CDCl3 solution using a “Bruker DPX 200” instrument (200 MHz for1 H, and ~50 MHz recordedrecorded in in CDCl CDCl3 3solution solution using using a a “Bruker “Bruker DPX DPX 200” 200” instrument instrument (200 (200 MHz MHz for for H,1H, and and ~50 ~50 MHz MHz for for 13 13for C), and the NMR spectra of 1 and 2 were recorded in CDCl3 solution using a “Bruker ARX 400” 13C),C), and and the the NMR NMR spectra spectra of of 1 1 and and 2 2 were were recorded recorded in in CDCl CDCl3 3solution solution using using a a “Bruker “Bruker ARX ARX 400” 400” 1 13 instrument (400 MHz for1 H, and ~100 MHz for13 C), with Me4Si as the internal standard. The C, instrumentinstrument (400 (400 MHz MHz for for H,1H, and and ~100 ~100 MHz MHz for for 13C),C), with with Me Me4Si4Si as as the the internal internal standard. standard. The The C, C, H, H, S S elementalH,elemental S elemental analysisanalysis analysis waswas was performedperformed performed onon onanan an ElementalElemental Elemental AnalyzerAnalyzer Analyzer “Elementar“Elementar “Elementar variovario vario EL EL Cube”; Cube”;Cube”; the the antimonyantimony content content was was accomplis accomplished accomplishedhed by by combustion combustion analysis. analysis.

3.2.3.2. Cyclic Cyclic Voltammetry Voltammetry TheThe electrochemical electrochemical studies studies were were carried carried out out using using IPC-Pro IPC-Pro potentiostate potentiostate in in three-electrode three-electrode mode.mode. The The stationary stationary glassy glassy carbon carbon (d (d = = 2 2 mm) mm) disk disk was was used used as as the the working working electrode; electrode; the the auxiliary auxiliary electrodeelectrode waswas aa platinum-flagplatinum-flag electrode.electrode. TheThe rereferenceference electrodeelectrode waswas Ag/AgCl/KCl(sat.)Ag/AgCl/KCl(sat.) withwith aa watertightwatertight diaphragm. diaphragm. All All measurements measurements were were carried carried out out under under argon. argon. The The samples samples were were dissolved dissolved

Molecules 2020, 25, 1770 16 of 23

3.2. Cyclic Voltammetry The electrochemical studies were carried out using IPC-Pro potentiostate in three-electrode mode. The stationary glassy carbon (d = 2 mm) disk was used as the working electrode; the auxiliary electrode was a platinum-ﬂag electrode. The reference electrode was Ag/AgCl/KCl(sat.) with a watertight diaphragm. All measurements were carried out under argon. The samples were dissolved in the 1 pre-deaerated solvent. The rate scan was 0.2 Vs− . The supporting electrolyte 0.15 M [Bu4N]ClO4 (99%, «Acros») underwent recrystallization twice from aqueous EtOH and then it was dried in vacuum (48 h) under 50◦C. The concentration of the compounds was 1.5–4.0 mM.

3.3. X-Ray Diﬀraction Studies

The X-ray diffraction data were collected on a SMART APEX I (L1, 1) diffractometer (graphite monochromated, MoKα-radiation, ω-scan technique, λ = 0.71073 Å) at 100 K. The intensity data were integrated by the SAINT program [96]. SADABS [97] was used to perform area-detector scaling and absorption corrections. The structures were solved by direct methods and were refined on F2 using the SHELXTL package [98]. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed in geometrically idealized positions and treated as riding with Uiso(H) = 1.2 Ueq (Uiso(H) = 1.5 Ueq for the hydrogen atoms in CH3 groups) of their parent atoms. Crystal data and details of the data collection and structure refinement for L1 and 1 are given in Table S1. The selected bond lengths for L1 are listed in Table S2, and the selected bond angles for 1 are in Table S3 of ESI. The structure parameters were deposited with the Cambridge Structural Database (CCDC 1978538 (L1) and 1978539 (1); [email protected] or http://www.ccdc.cam.ac.uk/data_request/cif).

3.4. Synthesis of Catechol Thioethers 3,5-Di-tert-butyl-6-methoxymethylcatechol and 4,6-di-tert-butyl-3-(4-hydroxyphenylthio)catechol (L3) were synthesized according to previous reports [8c,14a].

3.4.1. 4,6-Di-tert-butyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl-thiomethyl)catechol L1 Ligand was prepared according the next procedure: 3,5-di-tert-butyl-6-methoxymethylcatechol (1.5 mmol, 0.400 g) and 2,6-di-tert-butyl-4-mercaptophenol (1.5 mmol, 0.357 g) were dissolved in acetic acid (15 mL) under argon atmosphere, then the mixture was heated for 12 h at 60 ◦C. Further water (25 mL) was added to the reaction mixture and a precipitate was ﬁltrated. The product was dried under vacuum, then it was recrystallized from n-hexane to form white crystals. The yield was 0.37 g (52%). M.p. 174–176 ◦C. Anal. calc. for C29H44O3S (%): C, 73.68; H, 9.38; S, 6.78; found: C, 73.54; H, 9.42; S, 1 6.70. H NMR (200 MHz, CDCl3, δ, ppm): 1.22 (s., 9 H, tBu, Cat), 1.37 (s., 18 H, tBu, Phenol), 1.42 (s., 9 H, tBu, Cat), 4.35 (s., 2 H, CH2-S), 5.30 (s., 1 H, OH), 6.12 (s., 1 H, OH), 6.89 (s., 1 H, arom. C6H1), 7.18 13 1 (s., 2 H, arom. C6H2), 7.26 (s., 1 H, OH). C{ H} NMR (50 MHz, CDCl3, δ, ppm): 29.48, 30.07, 31.99, 34.27, 34.93, 35.43, 36.40, 116.73, 119.28, 121.59, 130.95, 133.64, 136.78, 138.71, 142.91, 143.34, 154.59. 1 FT-IR (KBr): ν = 3629, 3510, 3198, 3057, 2955, 2910, 2870, 1481, 1425, 1370, 1290, 1235, 1144 cm− .

3.4.2. 4,6-Di-tert-butyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl-thio)catechol L2 This ligand was prepared according to the followed protocol. 2,6-Di-tert-butyl-4-mercaptophenol (1 mmol, 0.238 g) in 10 mL ethanol was added dropwise to a solution of 3,5-di-tert-butyl-o-benzoquione in 20 mL of ethanol (0.5 mmol, 0.110 g) over a period of 2–3 h and the mixture was stirred under argon till decoloration of the reaction media at room temperature. The volume was concentrated under a reduced pressure to yield a crude solid. The product was recrystallized from dry ethanol, dried under vacuum, and isolated as yellow powder. The yield was 0.08 g (35%). M.p. 165–167 ◦C. Anal. calc. for 1 C28H42O3S (%): C, 73.32; H, 9.23; S, 6.99; found: C, 73.48; H, 9.26; S, 6.87. H NMR (200 MHz, CDCl3, δ, ppm): 1.25 (s., 18H, tBu), 1.31 (s., 9H, tBu), 1.41 (s., 9H, tBu), 4.93 (s., 1H, OH), 6.87 (s., 1H, C6H1), 6.91 13 1 (s, 2H, C6H2). C{ H} NMR (50 MHz, CDCl3, δ, ppm): 29.35, 29.63, 34.90, 34.98, 35.02, 118.05, 119.13, Molecules 2020, 25, 1770 17 of 23

127.73, 134.06, 136.00, 136.29, 138.40, 143.23, 146.14. FT-IR (KBr): ν = 3629, 3485, 2958, 2908, 2868, 1644, 1 1663, 1485, 1458, 1392, 1363, 1218, 1164 cm− .

3.5. Synthesis of Complexes

3.5.1. Complex (6-(CH2-S-tBu2Phenol)-3,5-DBCat)SbPh3 (1)

A solution of 4,6-di-tert-butyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl-thiomethyl)catechol L1 (0.236 g, 0.5 mmol) in 25 mL of toluene was added with stirring to a toluene solution of triphenylantimony dibromide (0.256 g, 0.5 mmol, 10–15 mL of toluene) under argon atmosphere. Then two equivalents of the triethylamine (0.14 mL, 1 mmol) were added to the toluene solution. After the addition of reagents and changing the color of the solution by yellow to orange, the reaction mixture was stirred for 2 h. The formed white precipitate of triethylammonium bromide was filtered off. The volume of filtrate was concentrated under a reduced pressure to a half volume and stored at 18 C for five days. The − ◦ X-ray-suitable yellow crystals of 1 were collected by decantation, and dried under vacuum. The yield was 0.300 g (73%). Anal. calc. for C47H57O3SSb (%): C, 68.53; H, 6.97; S, 3.89; Sb, 14.78. Found (%): C, 68.62; H, 7.05; S, 4.01; Sb, 14.53. 1H NMR (400 MHz, δ, ppm): 1.40 (s, 18H, tBu), 1.41 (s, 9H, tBu), 1.42 (s, 9H, tBu), 4.55 (s, 2H, CH2S), 5.18 (s, 1H, OH), 6.70 (s, 1H, arom. C6H1), 7.36 (s, 2H, arom. C6H2), 7.40–7.50 (m, 9H, Ph), 7.87–7.91 (m, 6H, Ph). 13C{1H} NMR (100 MHz, δ, ppm): 29.63, 30.19, 32.43, 34.36, 34.65, 35.85, 36.48, 112.61, 117.58, 127.81, 128.75, 129.06, 131.02, 131.66, 135.44, 136.27, 136.75, 137.81, 142.56, 147.52, 152.86. FT-IR (KBr): ν = 3541, 3068, 3045, 2957, 2910, 2873, 1580, 1480, 1431, 1394, 1 1360, 1258, 1240, 1168 cm− .

3.5.2. Complex (6-(S-Phenol)-3,5-DBCat)SbPh3 (2) A solution of the 4,6-di-tert-butyl-3-(4-hydroxyphenylthio)catechol (0.173 g, 0.5 mmol) in 20 mL of toluene was added dropwise with stirring to a toluene solution of triphenylantimony dibromide (0.256 g, 0.5 mmol, 10–15 mL of toluene) under argon atmosphere. Then two equivalents of the triethylamine (0.14 mL, 1 mmol) were added to toluene solution. After the addition of reagents and changing the color of the solution to pale yellow, the reaction mixture was stirred for 2 h. The formed white precipitate of ammonium salt was ﬁltered oﬀ. The volume was concentrated under a reduced pressure to yield a yellow crude solid. The product was recrystallized from a mixture of the solvents pentane and hexane (v/v = 1/1), dried under vacuum, and isolated as a yellow powder of 2. The yield was 0.175 g (50%). Anal. calc. for C38H39O3SSb (%): C, 65.43; H, 5.64; S, 4.60; Sb, 17.46. Found (%): C, 65.30; H, 5.72; S, 4.55; Sb, 17.60. 1H NMR (400 MHz, δ, ppm): 1.45 (s, 9H, tBu), 1.51 (s, 9H, tBu), 4.51 (br.s, 1H, OH), 6.51 (d.m, J = 8.7 Hz, 2H,C6H4), 6.82 (s, 1H, Ar), 6.94 (d.m, J = 8.7 Hz, 2H, C6H4), 7.35–7.41 (m, 6H, Ar), 7.43–7.48 (m, 3H, Ar), 7.51–7.56 (m, 6H, Ar). 13C{1H} NMR (100 MHz, δ, ppm): 29.53, 31.46, 34.80, 36.69, 112.63, 113.01, 115.48, 127.64, 129.03, 131.00, 131.34, 133.45, 135.10, 137.32, 141.25, 143.19, 149.86, 152.48. FT-IR (KBr): ν = 3411, 3056, 2958, 2910, 2870, 1490, 1431, 1397, 1257, 1238, 1 1167 cm− .

4. Conclusions Thus, in this work, new polyfunctional sterically hindered catechols with an additional phenolic group connected by a bridging sulfur atom, as well as triphenylantimony(V) catecholates based on these ligands, were synthesized. The molecular structures of free ligand L1 and complex 1 in the crystal state were determined by single-crystal X-ray analysis. The electrochemical investigations have shown that the electrooxidation of ligands in the first stage leads to the formation of the corresponding o-benzoquinones. The second anode stage involves the oxidation of the phenolic moiety. The results of the EPR studies confirmed the primary oxidation of the catechol group with the formation of the o-semiquinone radical anion. The electrooxidation of complexes 1 and 2 in dichloromethane is somewhat different from each other. For compound 2, the three oxidation stages are clearly fixed, the first two stages correspond to the conversion of the catecholate form of the ligand to o-benzosemiquinone Molecules 2020, 25, 1770 18 of 23 and then to o-benzoquinone, and the third peak characterizes the oxidation of the phenolic fragment. For complex 1, the first redox stage also affects the catecholate group, and the second redox process can involve both o-semiquinone and phenolic fragments in this electrode process. In the coordinating solvent (acetonitrile), the electrochemical behavior of compound 2 becomes identical to that one for complex 1 in dichloromethane. At the same time, in acetonitrile for complex 1, the current of the first anode peak increases significantly, which implies two-electron oxidation leading to a dication containing the one-electron oxidized form of the catecholate ligand and phenoxyl radical. The oxidation of complex 1 in toluene with lead dioxide leads to the fixation of the phenoxyl radical. This indicates the possibility of activation of the phenolic group during electrooxidation. The combination of electrochemical and spectral data indicates the convergence of the boundary redox orbitals of the phenolic and o-semiquinone fragments in the electro-generated monocationic triphenylantimony(V) complexes.

Supplementary Materials: The following are available online at http://www.mdpi.com/1420-3049/25/8/1770/s1, Figures S1–S8: The NMR spectra of compounds, Table S1: Crystal data and structure refinement for L1 and 1, Table S2: The selected bond lengths for L1, Table S3: The selected bond lengths for 1, Figure S9: The intermolecular hydrogen bonds in crystals of L1, Figure S10: The order of complex 1 molecules in crystal cell with the indication of intermolecular hydrogen bonding, Figure S11: The CVs of compound L1 (CH2Cl2), Figure S12: the CV of the electrolysis products of compound L1, Figure S13: The CVs of compound 1 (CH3CN), Figure S14: The X-band EPR spectrum of the mixture “L1 + PbO2”. CCDC 1978538 (L1) and 1978539 (1) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Author Contributions: Methodology, I.V.S.; resources and synthesis of precursors, M.V.A. and S.A.S.; synthesis of ligands, S.A.S.; synthesis of complexes, I.V.S. and A.I.P.; NMR and EPR investigations, A.I.P.; cyclic voltammetry, S.A.S. and I.V.S.; X-ray analysis, G.K.F.; supervision and project administration, N.T.B.; writing—original draft preparation, review and editing, I.V.S., M.V.A. and A.I.P.; funding acquisition, I.V.S., A.I.P. and N.T.B. All authors have read and agreed to the published version of the manuscript. Funding: The reported study was funded by Russian Foundation for Basic Researches, project number 19-29-08003 mk. The spectroscopic and X-ray structural investigations of compounds were performed in the accordance with state assignment of IOMC RAS. Acknowledgments: The spectroscopic and X-ray structural investigations of compounds were performed using the equipment of the analytical center of IOMC RAS. Conflicts of Interest: The authors declare no conflict of interest.

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Sample Availability: Samples of the compounds are available from the authors.

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