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

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


Scheme 2. Preparation of binuclear triphenylantimony(V) catecholates 5-11.
In all cases, subsequent to the experimental manipulations, a slow evaporation of the solvent (toluene, n-hexane, or a mixture of toluene-hexane) allowed the obtaining of light yellow to orange crystalline powders of complexes 1-11.
The syntheses shown in Schemes 1 and 2 should be performed in a dry atmosphere in order to exclude the coordination of water to antimony atom with a formation of complexes of the other types, as it was shown on the example of reaction between triphenylantimony(V) 4,5-dichloro-3,6-di-tert-butylcatecholate with DABCO in an open atmosphere (Scheme 3). The crystallization of this reaction mixture in an open atmosphere leads to the formation of a coordination polymeric complex of the type {[(4,5-Cl2-3,6-DBCat)SbPh3·H 2O]· DABCO}n (12).

Scheme 2. Preparation of binuclear triphenylantimony(V) catecholates 5-11.
In all cases, subsequent to the experimental manipulations, a slow evaporation of the solvent (toluene, n-hexane, or a mixture of toluene-hexane) allowed the obtaining of light yellow to orange crystalline powders of complexes 1-11.
The syntheses shown in Schemes 1 and 2 should be performed in a dry atmosphere in order to exclude the coordination of water to antimony atom with a formation of complexes of the other types, as it was shown on the example of reaction between triphenylantimony(V) 4,5-dichloro-3,6-di-tert-butylcatecholate with DABCO in an open atmosphere (Scheme 3). The crystallization of this reaction mixture in an open atmosphere leads to the formation of a coordination polymeric complex of the type {[(4,5-Cl 2 -3,6-DBCat)SbPh 3 ·H 2 O]·DABCO} n (12). Another interesting type-tetranuclear antimony(V) catecholate-may be prepared starting from Bipy and triphenylantimony(V) spiro-bis-catecholate Ph3Sb(Cat-Spiro-Cat)SbPh3 in an equimolar ratio (Scheme 4). Complex {Ph3Sb(Cat-Spiro-Cat)SbPh3•(Bipy)}2 (13) was isolated as light orange crystals 13•6toluene suitable for single-crystal X-ray analysis.
The structure of 1-13 was confirmed by the IR spectroscopy, 1 Н, 13 С{ 1 H} NMR spectroscopy (Supplementary Materials (ESI): Figures S1-S26), and elemental analysis. The formation of complexes in solution is confirmed by 1 H and 13 C{ 1 H} NMR spectroscopy. For example, the signals from protons of linker ligand in the 1 H NMR spectra of complexes 1 and 2 in CDCl3 solution undergo a shift in comparison with the signals of free ligands: singlet at δ = 8.54 ppm for 1 and δ = 8.59 ppm for uncoordinated pyrazine; multiplets at δ = 8.45 and 7.30 ppm for 2; and at δ = 8.55 and 7.25 ppm for free bis-(pyridine-4-yl)-disulfide, respectively. Another interesting type-tetranuclear antimony(V) catecholate-may be prepared starting from Bipy and triphenylantimony(V) spiro-bis-catecholate Ph 3 Sb(Cat-Spiro-Cat)SbPh 3 in an equimolar ratio (Scheme 4). Complex {Ph 3 Sb(Cat-Spiro-Cat)SbPh 3 ·(Bipy)} 2 (13) was isolated as light orange crystals 13·6toluene suitable for single-crystal X-ray analysis. troscopy (Supplementary Materials (ESI): Figures S1-S26), and elemental analysis. The formation of complexes in solution is confirmed by 1 H and 13 C{ 1 H} NMR spectroscopy. For example, the signals from protons of linker ligand in the 1 H NMR spectra of complexes 1 and 2 in CDCl3 solution undergo a shift in comparison with the signals of free ligands: singlet at δ = 8.54 ppm for 1 and δ = 8.59 ppm for uncoordinated pyrazine; multiplets at δ = 8.45 and 7.30 ppm for 2; and at δ = 8.55 and 7.25 ppm for free bis-(pyridine-4-yl)-disulfide, respectively. According to IR spectroscopy, the presence of the coordinated neutral N-donor ligands in the complexes is determined by the stretching vibrations v(Сarom-N) on the region of 1360-1250 cm −1 and stretching vibrations v(С-N) in the range of 1200-1240 cm −1 .

X-ray Structures
The crystal structures of complexes [(3,6-DBCat)SbPh3]2·DABCO (1) (12), and {Ph3Sb(Cat-Spiro-Cat)SbPh3•(Bipy)}2 (13) were determined by means of single-crystal X-ray analysis (Figures 1-8, and Figures S27-S34 of ESI). The experimental and refinement details are given in Table S1 of Supplementary Materials. The selected bond distances and angles are given in Table S2 of Supplementary Materials. The mononuclear moieties (Cat)SbPh3 in molecules of 1, 3, 4, 5, and 10 are crystallographically The structure of 1-13 was confirmed by the IR spectroscopy, 1 H, 13  According to IR spectroscopy, the presence of the coordinated neutral N-donor ligands in the complexes is determined by the stretching vibrations v(C arom -N) on the region of 1360-1250 cm −1 and stretching vibrations v(C-N) in the range of 1200-1240 cm −1 .  Table S1 of Supplementary Materials. The selected bond distances and angles are given in Table S2 [64][65][66][67][68][69][70], the six-membered carbon cycles are aromatic with the average C-C distances of 1  (1)-O(2) bond lengths in complexes are typical for antimony(V) catecholates, the equatorial Sb-CPh bonds are longer than the apical ones, which is also characteristic of such compounds [59,72]. The distances between antimony and nitrogen atoms differ in the range 2.494(2)-2.773(5) Å . This is somewhat larger than the sum of the covalent radii of Sb and N in the octahedral geometry (1. 43 [76]), thus pointing to the donor-acceptor nature of the interaction. These bonds in our complexes lie in the wide range of Sb-N bonds of donor-acceptor nature observed in many different antimony compounds [77]. Thus, all structures have some common features, but they also have certain differences. The molecule of 1 is centrosymmetric, and the mononuclear fragments (Cat)SbPh3 are rotated relative to each other by 180° and the catecholate planes are parallel to each other ( Figure S27 of ESI). The intramolecular distance between two antimony atoms in Figure 1. The X-ray structure of 1 (thermal ellipsoids here and below of 50% probability). The hydrogen atoms are omitted. The selected bond lengths (  (1)-O(2) bond lengths in complexes are typical for antimony(V) catecholates, the equatorial Sb-CPh bonds are longer than the apical ones, which is also characteristic of such compounds [59,72]. The distances between antimony and nitrogen atoms differ in the range 2.494(2)-2.773(5) Å. This is somewhat larger than the sum of the covalent radii of Sb and N in the octahedral geometry (1. 43 [76]), thus pointing to the donor-acceptor nature of the interaction. These bonds in our complexes lie in the wide range of Sb-N bonds of donor-acceptor nature observed in many different antimony compounds [77]. Thus, all structures have some common features, but they also have certain differences. The molecule of 1 is centrosymmetric, and the mononuclear fragments (Cat)SbPh3 are rotated relative to each other by 180° and the catecholate planes are parallel to each other ( Figure S27 of ESI). The intramolecular distance between two antimony atoms in molecule of 1 is 8.236 Å, and between planes of π-systems of two catecholate ligands is 0.273 Å , respectively. The catecholate planes are not parallel, the corresponding angle is 19.13° ( Figure S31 of ESI). In contrast to 1, 3, and 5, the mononuclear fragments (Cat)SbPh3 in 8 are rotated relative to each other by 111°. The angle between pyridine planes in Bipy ligand in 8 is 35.55°, and the angles between pyridine planes and catecholato planes are 61.65° and 74.84° for ligands environment at Sb(a) and Sb(2) atoms, respectively.      [76]), thus pointing to the donor teraction. These bonds in our complexes lie in the wide ran nor-acceptor nature observed in many different antimony c structures have some common features, but they also have cer

X-ray Structures
The molecule of 1 is centrosymmetric, and the mononuc are rotated relative to each other by 180° and the catecholate other ( Figure S27 of ESI). The intramolecular distance betwee molecule of 1 is 8.236 Å, and between planes of π-systems of 7.372 Å. The angle between catecholate and pyrazine planes is planes in Bipy ligand in 8 is 35.55°, and the angles between pyridine planes and catecholato planes are 61.65° and 74.84° for ligands environment at Sb(a) and Sb(2) atoms, respectively.   In all structures, the redox active ligand is in catecholato form. The O-C bonds in molecules of 1, 3, 4, 5, 8, and 10 lie in the range of 1.357-1.366 Ǻ typical for ordinary O-C bonds in antimony catecholates [64][65][66][67][68][69][70], the six-membered carbon cycles are aromatic with the average C-C distances of 1.400, 1.401, 1.402, 1.402, 1.405, and 1.407 Ǻ, respectively. The metrical oxidation state (MOS) for complexes was calculated according to S.N. Brown [71] as −1.932, −1.959, −2.008, −1.971, −1.953, −1.900, and −1.904 (for 1, 3, 4, 5, 8, and 10 respectively), confirming the dianionic nature of O,O'-chelating ligands. The Sb(1)-O(1) and Sb(1)-O(2) bond lengths in complexes are typical for antimony(V) catecholates, the equatorial Sb-CPh bonds are longer than the apical ones, which is also characteristic of such compounds [59,72]. The distances between antimony and nitrogen atoms differ in the range 2.494(2)-2.773(5) Å. This is somewhat larger than the sum of the covalent radii of Sb and N in the octahedral geometry (1. 43 [76]), thus pointing to the donor-acceptor nature of the interaction. These bonds in our complexes lie in the wide range of Sb-N bonds of donor-acceptor nature observed in many different antimony compounds [77]. Thus, all structures have some common features, but they also have certain differences. The molecule of 1 is centrosymmetric, and the mononuclear fragments (Cat)SbPh3 are rotated relative to each other by 180° and the catecholate planes are parallel to each other ( Figure S27 of ESI). The intramolecular distance between two antimony atoms in molecule of 1 is 8.236 Å, and between planes of π-systems of two catecholate ligands is 7.372 Å. The angle between catecholate and pyrazine planes is 54.2°. The antimony atom Sb(1)     bonds are longer than the apica acteristic of such compounds [59,72]. The distances between oms differ in the range 2.494(2)-2.773(5) Å. This is somewhat covalent radii of Sb and N in the octahedral geometry (1.43 + 0 = 2.10 Å [74]), but significantly less than the sum of their van 3.8 Å [75]; 2.06 + 1.55 = 3.61 Å [76]), thus pointing to the donor teraction. These bonds in our complexes lie in the wide ran nor-acceptor nature observed in many different antimony c structures have some common features, but they also have cer The molecule of 1 is centrosymmetric, and the mononuc are rotated relative to each other by 180° and the catecholate other ( Figure S27 of ESI). The intramolecular distance betwee molecule of 1 is 8.236 Å, and between planes of π-systems of 7.372 Å. The angle between catecholate and pyrazine planes is Sb (1) [76]), thus pointing to the donor teraction. These bonds in our complexes lie in the wide ran nor-acceptor nature observed in many different antimony c structures have some common features, but they also have cer Complex 12 has rather different structure. In solid state, molecules of (4,5-Cl2-3,6-Cat)SbPh3•H2O and DABCO form 1D polymeric chains via the intermolecular hydrogen bonding of water and DABCO molecules (Figure 7, Figure S33 of ESI). bonds are longer than the apica acteristic of such compounds [59,72]. The distances between oms differ in the range 2.494(2)-2.773(5) Å. This is somewhat covalent radii of Sb and N in the octahedral geometry (1.43 + 0 = 2.10 Å [74]), but significantly less than the sum of their van 3.8 Å [75]; 2.06 + 1.55 = 3.61 Å [76]), thus pointing to the donor teraction. These bonds in our complexes lie in the wide ran nor-acceptor nature observed in many different antimony c structures have some common features, but they also have cer The molecule of 1 is centrosymmetric, and the mononuc are rotated relative to each other by 180° and the catecholate other ( Figure S27 of ESI). The intramolecular distance betwee molecule of 1 is 8.236 Å, and between planes of π-systems of 7.372 Å. The angle between catecholate and pyrazine planes is Sb (1)  bonds are longer than the apical ones, which is also cha acteristic of such compounds [59,72]. The distances between antimony and nitrogen a oms differ in the range 2.494(2)-2.773(5) Å. This is somewhat larger than the sum of th covalent radii of Sb  ), thus pointing to the donor-acceptor nature of the i teraction. These bonds in our complexes lie in the wide range of Sb-N bonds of d nor-acceptor nature observed in many different antimony compounds [77]. Thus, a structures have some common features, but they also have certain differences.  [77]. structures have some common features, but they also have certain differences.  (1)-O(2) bond lengths in complexes are typical for antimony(V) catecholates, the equatorial Sb-C Ph bonds are longer than the apical ones, which is also characteristic of such compounds [59,72]. The distances between antimony and nitrogen atoms differ in the range 2.494(2)-2.773(5) Å. This is somewhat larger than the sum of the covalent radii of Sb  These bonds in our complexes lie in the wide range of Sb-N bonds of donor-acceptor nature observed in many different antimony compounds [77]. Thus, all structures have some common features, but they also have certain differences. Complex 12 has rather different structure. In solid state, molecules of (4,5-Cl2-3,6-Cat)SbPh3•H2O and DABCO form 1D polymeric chains via the intermolecular hydrogen bonding of water and DABCO molecules (Figure 7, Figure S33 of ESI).   ), thus pointing to the donor-acceptor nature of the inse bonds in our complexes lie in the wide range of Sb-N bonds of donature observed in many different antimony compounds [77]. Thus, all e some common features, but they also have certain differences.   Figure S34 of ESI) and −2.064 (Cat2, Figure S34 of ESI), whic the biggest values in the series of complexes in this work.

Charge Density in 5ED
Due to the fact that crystals of complex 5 (hereinafter referred to as 5ED) had a reflectivity, we carried out a high resolution X-ray diffraction experiment to invest The molecule of 1 is centrosymmetric, and the mononuclear fragments (Cat)SbPh 3 are rotated relative to each other by 180 • and the catecholate planes are parallel to each other ( Figure S27 of ESI). The intramolecular distance between two antimony atoms in molecule of 1 is 8.236 Å, and between planes of π-systems of two catecholate ligands is 7.372 Å. The angle between catecholate and pyrazine planes is 54.  The molecule of 1 is centrosymmetric, and the mononuclear are rotated relative to each other by 180° and the catecholate plane other ( Figure S27 of ESI). The intramolecular distance between tw molecule of 1 is 8.236 Å, and between planes of π-systems of two 7.372 Å. The angle between catecholate and pyrazine planes is 54.  Figure S34 of ESI) and −2.064 (Cat2, Figure S34 of ESI), which are the biggest values in the series of complexes in this work.

Charge Density in 5 ED
Due to the fact that crystals of complex 5 (hereinafter referred to as 5 ED ) had a high reflectivity, we carried out a high resolution X-ray diffraction experiment to investigate the electron density distribution in this complex. Details of multipole refinement are given in Supplementary Materials (Figures S35-S37 of ESI). The distribution of the deformation electron density (DED) around the Sb atom in 5 ED has a clearly pronounced polar (probably ionic) character (Figure 9a,b). The DED maxima on the Sb-O and Sb-C bonds are markedly shifted toward the oxygen and carbon atoms, respectively. The DED maxima on the O-C bonds are located at the middle of the corresponding distances in 5ED, whereas in the related catecholate Sb(V) complexes [78], the maxima are shifted toward the oxygen atoms. The angle between the lone electron pairs of oxygen atoms and the Sb-O and O-C bonds is close to 120º. The lone electron pair of the nitrogen atom of the 4,4′-dipyridyl molecule is directed away from the antimony atom ( Figure 10) towards approximately the middle of the Sb(1)-C(15) bond ( Figure S38 of ESI). A similar situation is observed in complexes 1 and 3. The DED maxima on the Sb-O and Sb-C bonds are markedly shifted toward the oxygen and carbon atoms, respectively. The DED maxima on the O-C bonds are located at the middle of the corresponding distances in 5 ED , whereas in the related catecholate Sb(V) complexes [78], the maxima are shifted toward the oxygen atoms. The angle between the lone electron pairs of oxygen atoms and the Sb-O and O-C bonds is close to 120 • . The lone electron pair of the nitrogen atom of the 4,4 -dipyridyl molecule is directed away from the antimony atom ( Figure 10) towards approximately the middle of the Sb(1)-C(15) bond ( Figure S38 of ESI). A similar situation is observed in complexes 1 and 3.
The analysis of charges obtained by electron density integration inside the atomic basin has revealed that the charge on antimony atom is 2.07 e. The charges on phenyls and catecholate substituents and also 4,4 -dipyridyl molecule are −0.4 ÷ −0.53 e, −0.41 and −0.32 e, respectively. Thus, the bridging 4,4 -dipyridyl molecule is negatively charged and apparently can coordinate on itself in complex 5 ED small electrophilic molecules, which potentially can undergo unusual transformations in the coordination sphere of antimony atom.
The DED maxima on the Sb-O and Sb-C bonds are markedly shifted toward the oxygen and carbon atoms, respectively. The DED maxima on the O-C bonds are located at the middle of the corresponding distances in 5ED, whereas in the related catecholate Sb(V) complexes [78], the maxima are shifted toward the oxygen atoms. The angle between the lone electron pairs of oxygen atoms and the Sb-O and O-C bonds is close to 120º. The lone electron pair of the nitrogen atom of the 4,4′-dipyridyl molecule is directed away from the antimony atom ( Figure 10) towards approximately the middle of the Sb(1)-C(15) bond ( Figure S38 of ESI). A similar situation is observed in complexes 1 and 3. To understand the nature of the chemical bonds in 5ED (Table 1), we used Bader's theory [79]. The corresponding calculations have shown that the Sb-O, Sb-N, and Sb-C whereas the O-C, N-C, and C-C bonds are shared (2(r) < 0, he(r) < 0; covalent bonds). A similar situation is observed in related catecholate complexes (Ph3(4,5-OMe-3,6-tBu-Cat)Sb•MeCN and Ph3(4,5-N2C4H6-3,6-tBu-Cat)Sb•MeOH) [78]. Using the Espinosa-Molins-Lecomte correlation [80], we estimated the energy of the donor-acceptor interaction between the antimony(V) and nitrogen atom of the 4,4′-dipyridyl molecule. The energy of this interaction is 7.5 kcal/mol.   Figure 11. Experimental molecular graph of the complex 5 ED . The hydrogen atoms, except those participating in intramolecular interaction, are omitted for clarity. Only critical points (3,-1) (blue color) are presented for clarity.

Cyclic Voltammetry
The cyclic voltammetry can provide an additional information about the electronic structure and the nature of interactions between different ligands in a complex molecule. The binuclear antimony(V) complexes with bidentate ligands 1-7, 9, and tetranuclear antimony(V) complex 13 were investigated by means of CV. The electrochemical potentials of these catecholates are given in Table 2. 05 -E 1 pa -the peak potential for the first oxidation process; E 2 pa -the peak potential for the second oxidation process; E 3 pa -the peak potential for the third oxidation process. *-the half-wave potential value.
The electrochemical behavior of the complexes depends on the coordinated N-donor ligand. Conventionally, these complexes can be divided into two groups: the first one contains Pyr, Bipy, PySSPy (complexes 1-3, 5-7, 13), and the second one-complexes 4 and 9 with DABCO. The coordination of pyrazine, Bipy, and PySSPy at the antimony atom changes their mechanism of electrooxidation: instead of two successive redox stages Cat/SQ and SQ/Cat, one multielectron stage is observed ( Figure 12). Previously, a similar effect was observed for mononuclear triphenylantimony(V) catecholates, containing coordinated pyridine molecules [62]. In the case of pyrazine, the potential value coincides with that for the complex with pyridine. For complexes with bridging Bipy and PySSPy (complexes 2, 3, 5, 7), there is an insignificant shift of the peaks to the anodic region (0.95-1.05 V). The cyclic voltammetry can provide an additional information about the electronic structure and the nature of interactions between different ligands in a complex molecule. The binuclear antimony(V) complexes with bidentate ligands 1-7, 9, and tetranuclear antimony(V) complex 13 were investigated by means of CV. The electrochemical potentials of these catecholates are given in Table 2. 0.83 1.05 -Е 1 ра-the peak potential for the first oxidation process; Е 2 pа-the peak potential for the second oxidation process; Е 3 ра-the peak potential for the third oxidation process. *-the half-wave potential value.
The electrochemical behavior of the complexes depends on the coordinated N-donor ligand. Conventionally, these complexes can be divided into two groups: the first one contains Pyr, Bipy, PySSPy (complexes 1-3, 5-7, 13), and the second one-complexes 4 and 9 with DABCO. The coordination of pyrazine, Bipy, and PySSPy at the antimony atom changes their mechanism of electrooxidation: instead of two successive redox stages Cat/SQ and SQ/Cat, one multielectron stage is observed ( Figure 12). Previously, a similar effect was observed for mononuclear triphenylantimony(V) catecholates, containing coordinated pyridine molecules [62]. In the case of pyrazine, the potential value coincides with that for the complex with pyridine. For complexes with bridging Bipy and PySSPy (complexes 2, 3, 5, 7), there is an insignificant shift of the peaks to the anodic region (0.95-1.05 V). The two-electron oxidation of the catecholate group results in the formation of o-benzoquinone and its subsequent de-coordination. A quasi-reversible reduction peak The two-electron oxidation of the catecholate group results in the formation of obenzoquinone and its subsequent de-coordination. A quasi-reversible reduction peak of the de-coordinated quinone is fixed on the reverse branch of the CV during a pulsed potential sweep.
We have carried out the model CV experiments with free PySSPy and an antimony complex (3,6-DBCat)SbPh 3 to confirm the coordination of pyridine-containing ligands in a solution. The addition of triphenylantimony(V) 3,6-di-tert-butylcatecholate to a solution of PySSPy in a molar ratio of 1:1 leads to a significant decrease in the intensity of the free ligand oxidation wave (E p = 1.98 V), while in a ratio of 2:1-to its disappearance. In this case, an oxidation wave of complex 2 appears (Figure 13). Oxidation waves of (3,6-DBCat)SbPh 3 (at E 1 1/2 = 0.89 V, E 2 p = 1.40 V vs. Ag/AgCl/KCl(sat.) [81]) are not observed on the CV curve.
Molecules 2022, 27, x FOR PEER REVIEW 13 of 23 of the de-coordinated quinone is fixed on the reverse branch of the CV during a pulsed potential sweep. We have carried out the model CV experiments with free PySSPy and an antimony complex (3,6-DBCat)SbPh3 to confirm the coordination of pyridine-containing ligands in a solution. The addition of triphenylantimony(V) 3,6-di-tert-butylcatecholate to a solution of PySSPy in a molar ratio of 1:1 leads to a significant decrease in the intensity of the free ligand oxidation wave (Ep = 1.98 V), while in a ratio of 2:1-to its disappearance. In this case, an oxidation wave of complex 2 appears (Figure 13). Oxidation waves of (3,6-DBCat)SbPh3 (at E 1 1/2 = 0.89 V, E 2 p = 1.40 V vs. Ag/AgCl/KCl(sat.) [81]) are not observed on the CV curve. The coordination of the DABCO ligand is accompanied by a significant shift in the oxidation potentials of the catecholate ligand to the cathodic region (by 0.4 V), compared to the initial complex. Such a significant shift of the oxidation potentials to the cathodic region, in comparison with the initial catecholate is explained by a significant increase in the total electron density of the six-coordination metal site upon coordination of the donor diazabicyclooctane. The observed effect on the values of the oxidation potentials of the complexes is similar to the effect of coordination over the antimony atom of charged nucleophilic agents (bromide anion, hydroxy group) [61]. The CVs of DABCO containing complexes 4 and 9 ( Figure 14) contain three oxidation waves at the potential range to 1.30 V. The first oxidation is quasi-reversible and may be assigned to the oxidation process Cat/SQ. As compared with the initial complexes, the current ratio Ic/Ia decreases to 0.7, pointing out the lower stability of the electrogenerated particles. Moreover, the reverse branch of CV shows the appearance of the reduction peak of the product of the chemical stage following the electron transfer. The second irreversible redox transition we assign to the stage of the oxidation of a coordinated o-benzosemiquinone to o-benzoquinone. The coordination of the DABCO ligand is accompanied by a significant shift in the oxidation potentials of the catecholate ligand to the cathodic region (by 0.4 V), compared to the initial complex. Such a significant shift of the oxidation potentials to the cathodic region, in comparison with the initial catecholate is explained by a significant increase in the total electron density of the six-coordination metal site upon coordination of the donor diazabicyclooctane. The observed effect on the values of the oxidation potentials of the complexes is similar to the effect of coordination over the antimony atom of charged nucleophilic agents (bromide anion, hydroxy group) [61]. The CVs of DABCO containing complexes 4 and 9 ( Figure 14) contain three oxidation waves at the potential range to 1.30 V. The first oxidation is quasi-reversible and may be assigned to the oxidation process Cat/SQ. As compared with the initial complexes, the current ratio I c /I a decreases to 0.7, pointing out the lower stability of the electrogenerated particles. Moreover, the reverse branch of CV shows the appearance of the reduction peak of the product of the chemical stage following the electron transfer. The second irreversible redox transition we assign to the stage of the oxidation of a coordinated o-benzosemiquinone to o-benzoquinone.
However, the additional peaks observed in CV indicate a more complex mechanism of electrotransformations of the complexes. Suppose the complex was destroyed after the first one-electron oxidation. In that case, a second oxidation peak could be observed at a potential characteristic of (3,6-DBCat)SbPh 3 , but this value is also shifted to the cathodic region. Based on potential values of 0.88 and 0.91, these peaks can correspond to free antimony catecholate complexes not bound to DABCO or de-coordinated nitrogencontaining ligand (0.90 V). With an increase in the potential sweep to 1.7 V, it is possible to fix a peak at 1.40 V, which is characteristic of the second redox transition [Ph 3 Sb(3.6-DBSQ)] + /[Ph 3 Sb(3.6-DBBQ)] 2+ of mononuclear complex without N-donor ligand [81]. One can assume that during the CV experiment, the binuclear complex decomposes as a result of the initial oxidation of the complex with DABCO. This decomposition accompanied by the de-coordination of one of the catecholate fragments of (3,6-DBCat)SbPh 3 and the release of DABCO. However, the additional peaks observed in CV indicate a more complex mechanism of electrotransformations of the complexes. Suppose the complex was destroyed after the first one-electron oxidation. In that case, a second oxidation peak could be observed at a potential characteristic of (3,6-DBCat)SbPh3, but this value is also shifted to the cathodic region. Based on potential values of 0.88 and 0.91, these peaks can correspond to free antimony catecholate complexes not bound to DABCO or de-coordinated nitrogen-containing ligand (0.90 V). With an increase in the potential sweep to 1.7 V, it is possible to fix a peak at 1.40 V, which is characteristic of the second redox transition [Ph3Sb(3.6-DBSQ)] + / [Ph3Sb(3.6-DBBQ)] 2+ of mononuclear complex without N-donor ligand [81]. One can assume that during the CV experiment, the binuclear complex decomposes as a result of the initial oxidation of the complex with DABCO. This decomposition accompanied by the de-coordination of one of the catecholate fragments of (3,6-DBCat)SbPh3 and the release of DABCO.
The CV of complex 13 shows two stages of oxidation. The coordination of the bipyridine ligand leads to an insignificant shift of the oxidation potential to the cathodic region (0.83 V) relative to the initial binuclear derivative (0.85 V) [58]. In this case, we do not fix the separation of two oxidation stages; just as in the case of pyridine coordination, a multielectron process occurs, which leads to the oxidation of catecholate groups and subsequent possible elimination of the Ph3Sb 2+ dication and the formation of a quinone-catecholate complex. The absence of separation of the second stage into two peaks indirectly testifies to the preservation of the coordination of the bridging nitrogen-containing ligand.
The IR spectra were recorded on an FSM-1201 FT-IR spectrometer in KBr pellets. The CV of complex 13 shows two stages of oxidation. The coordination of the bipyridine ligand leads to an insignificant shift of the oxidation potential to the cathodic region (0.83 V) relative to the initial binuclear derivative (0.85 V) [58]. In this case, we do not fix the separation of two oxidation stages; just as in the case of pyridine coordination, a multielectron process occurs, which leads to the oxidation of catecholate groups and subsequent possible elimination of the Ph 3 Sb 2+ dication and the formation of a quinone-catecholate complex. The absence of separation of the second stage into two peaks indirectly testifies to the preservation of the coordination of the bridging nitrogen-containing ligand.
The IR spectra were recorded on an FSM-1201 FT-IR spectrometer in KBr pellets. The NMR spectra were registered using Bruker Avance DPX-200 (200 MHz) and Bruker AVANCE New (300 MHz) spectrometers with TMS as an internal standard and CDCl 3 as a solvent. The chemical shift values are given in ppm with the reference to solvent and the coupling constants (J) are given in Hz. The elemental analysis (C, H) was carried out on a Euro EA 3000 elemental analyzer, as well as (Sb) by a pyrolytic decomposition in an oxygen flow.
Electrochemical studies were carried out using VERSASTAT-3 potentiostate (PAR) in three-electrode mode. The stationary glassy carbon (d = 2 mm) disk was used as working electrode; the auxiliary electrode was a platinum-flag electrode. The reference electrode was Ag/AgCl/KCl (sat.) with watertight diaphragm. All measurements were carried out under argon. The samples were dissolved in the pre-deaerated solvent. The scan rate was 0.2 V·s −1 . The supporting electrolyte 0.1 M Bu 4 NClO 4 (99%, electrochemical grade, Fluka) was dried in vacuum (48 h) at 50 • C.

The General Synthetic Method of Binuclear Triphenylantimony(V) Complexes with Linker N-Donor Ligands
The toluene solution of the corresponding starting triphenylantimony(V) catecholate (0.5 mmol, 40 mL) was slowly added with a stirring to the toluene solution of N-donor ligand (0.25 mmol, 30 mL). The stirring was continued for 1 h at temperatures of 40-50 • C. The toluene was completely removed under reduced pressure, and the formed residue was recrystallized from different solvents depending on complex: a toluene/n-hexane mixture (1:2) for complexes 1, 2 and 12; toluene/n-hexane mixture (1:1) for complexes 6, 7, 8, 11; toluene for complexes 3, 5, 10 and 13; and n-hexane for complexes 4 and 9.