Schiff Base Derivatives in Zinc(II) and Cadmium(II) Complexation with the closo -Dodecaborate Anion

: A series of Schiff base derivatives, namely N -(4-methoxyphenyl)-1-(1-methylbenzimidazol-2-yl)methanimine (L 1 ), 4-methoxy-N -[(1-methylbenzimidazol-2-yl)methyl]aniline (L 2 ), and 2-[( E )- (1-propylbenzimidazol-2-yl)iminomethyl]phenol (L 3 ), were synthesized. These compounds feature different linker groups, including –CH=N–, –CH 2 –NH–, and –N=CH–, respectively. During the process of zinc(II) and cadmium(II) complexation in the presence of the closo -dodecaborate [B 12 H 12 ] 2– anion, it was observed that ligand L 3 underwent degradation. Consequently, two compounds were isolated, [Zn(Bz-NH 2 ) 2 (CH 3 COO) 2 ] and (HBz-NH 2 ) 2 [B 12 H 12 ] · 2CH 3 CN, both containing 1-propyl-2-aminobenzimidazole (Bz-NH 2 ), which is a degraded fragment of the ligand. Several new zinc(II) and cadmium(II) coordination compounds were synthesized and characterized using various physicochemical analysis methods, including elemental analysis, IR, and UV spectroscopy. Additionally, X-ray diffraction and Hirshfeld surface analysis were performed for compounds [Cd(L 2 ) 2 (CH 3 CN)(H 2 O)][B 12 H 12 ], [Zn(Bz-NH 2 ) 2 (CH 3 COO) 2 ], and (HBz-NH 2 ) 2 [B 12 H 12 ] · 2CH 3 CN, as well as for ligand L 2 .


Elemental Analysis
Carbon, hydrogen, and nitrogen content was analyzed using a Carlo ErbaCHNS-3 FA 1108 automated elemental analyzer (Carlo Erba Instruments, Milan, Italy).Zinc(II), cadmium(II), and boron content was determined on an iCAP 6300 Duo ICP emission spectrometer with inductively coupled plasma (Thermo Scientific, Waltham, MA, USA).

IR Spectroscopy
IR spectra of compounds were recorded on a Lumex Infralum FT-02 Fourier-transform spectrophotometer (Lumex, St. Petersburg, Russia).The measurements were recorded in the range of 4000-600 cm -1 at a resolution of 1 cm -1 .Samples were prepared as Nujol mulls; for measurements, NaCl pellets were used.IR spectra of compounds are shown in Supplementary Materials (Figures S1-S4).

UV Spectroscopy
UV-vis absorption spectra of ligands L 1 -L 3 and compounds 1-3 as glycerol suspensions were recorded using a SF 103 spectrophotometer (Akvilon, Moscow, Russia) in the region of 200-700 nm.UV-vis absorption spectra are shown in Supplementary Materials (Figures S5-S7).

1 H and 13 C NMR Spectroscopy
1 H and 13 C NMR spectra of organic ligands L 1 -L 3 were measured on a Bruker AVANCE-600 spectrometer (Bruker AXC, Inc., Karlsruhe, Germany) in CDCl 3 at 600 and 150 MHz, respectively, with internal deuterium stabilization.The chemical shifts of the 1 H nuclei are given relative to the residual signals of the deuterosolvent.Tetramethylsilane was used as an external standard. 1H and 13 C NMR spectra are present in Supplementary Materials (Figures S8-S13).

X-ray Diffraction
The single-crystal X-ray diffraction data for compounds 2-4•2CH 3 CN and ligand L 2 were collected using a three-circle Bruker D8 Venture diffractometer (Bruker AXS Inc., Karlsruhe, Germany).The studies were performed at the Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences.The data were collected using the ϕ and ω scan modes, indexed, and integrated using the SAINT program [28].Subsequently, the data were scaled and corrected for absorption using the SADABS program [29].Detailed information can be found in Supplementary Materials (Table S1).The structures were determined by direct methods and refined using the full-matrix least squares technique on F 2 with anisotropic displacement parameters applied for non-hydrogen atoms.Hydrogen atoms in all compounds were positioned at calculated positions and refined within a riding model with fixed isotropic displacement parameters [U iso (H) = 1.5U eq (C) for the CH 3 -groups and 1.2U eq (C) for the other groups].All calculations were performed using the SHELXTL program [30] and OLEX2 program package [31].
In the structure of 2, three CH 3 CN molecules were identified to exhibit significant disorders and were subsequently excluded from the analysis using an OLEX2 solvent mask.Errors B in the structure of 2 are associated with the removal of these solvent molecules.

Hirshfeld Surface Analysis
The Crystal Explorer 17.5 program [32] was employed to analyze the interactions occurring within the crystal.The donor-acceptor groups were visualized using a standard (high) surface resolution, and d norm surfaces were mapped across a consistent color scale ranging from −0.640 (red) to 0.986 (blue) a.u.

Results and Discussion
In our previous study [23], we discovered that the closo-dodecaborate anion [B 12 H 12 ] 2-, which has the lowest coordination ability among the polyhedral boron cluster anions [B n H n ] 2-(n = 6-12), may not effectively compete in zinc(II) and cadmium(II) complexation Crystals 2023, 13, 1449 5 of 13 when various organic and inorganic ligands are present in the solution-for instance, when nitrate ions or dimethylformamide molecules are present in excess.To address this challenge, we overcame the issue by utilizing previously synthesized complexes [M(DMF) 6 ][B 12 H 12 ] (where M = Zn(II), Cd(II)) as initial reagents [23,27].Therefore, in this study, we conducted Zn(II) and Cd(II) complexation with benzimidazole derivatives L 1 -L 3 in the presence of the [B 12 H 12 ] 2-anion, utilizing these compounds as our starting materials.
The reaction pathways for zinc(II) and cadmium(II) complexation with ligand L 1 containing the -CH=N-linker group and ligand L 2 with the -CH 2 -NH-linker group in the presence of the closo-dodecaborate anion are quite similar.In the case of cadmium(II) and ligand L 1 , the mixed-ligand complex [Cd(L 1 ) 2 [B 12 H 12 ]] was isolated with a boron cluster anion coordinated by the metal.However, for ligand L 2 , we obtained a mixed-ligand cationic complex [Cd(L 2 ) 2 (CH 3 CN)(H 2 O)] 2+ with the closo-dodecaborate anion serving as a counterion (Scheme 1).In contrast, for zinc(II), the initial components remained unreacted, regardless of U eq the organic ligand used.It's worth noting that upon evaporating a solution containing [Zn(DMF) 6 ][B 12 H 12 ] and ligand L 2 , single crystals of ligand L 2 suitable for X-ray diffraction studies precipitated (Figure S14).

Results and Discussion
In our previous study [23], we discovered that the closo-dodecaborate anion [B12H12] 2- , which has the lowest coordination ability among the polyhedral boron cluster anions [BnHn] 2-(n = 6-12), may not effectively compete in zinc(II) and cadmium(II) complexation when various organic and inorganic ligands are present in the solution-for instance, when nitrate ions or dimethylformamide molecules are present in excess.To address this challenge, we overcame the issue by utilizing previously synthesized complexes [M(DMF)6][B12H12] (where M = Zn(II), Cd(II)) as initial reagents [23,27].Therefore, in this study, we conducted Zn(II) and Cd(II) complexation with benzimidazole derivatives L 1 -L 3 in the presence of the [B12H12] 2-anion, utilizing these compounds as our starting materials.

System [M(DMF
The reaction pathways for zinc(II) and cadmium(II) complexation with ligand L 1 containing the -CH=N-linker group and ligand L 2 with the -CH2-NH-linker group in the presence of the closo-dodecaborate anion are quite similar.In the case of cadmium(II) and ligand L 1 , the mixed-ligand complex [Cd(L 1 )2[B12H12]] was isolated with a boron cluster anion coordinated by the metal.However, for ligand L 2 , we obtained a mixed-ligand cationic complex [Cd(L 2 )2(CH3CN)(H2O)] 2+ with the closo-dodecaborate anion serving as a counterion (Scheme 1).In contrast, for zinc(II), the initial components remained unreacted, regardless of Ueq the organic ligand used.It's worth noting that upon evaporating a solution containing [Zn(DMF)6][B12H12] and ligand L 2 , single crystals of ligand L 2 suitable for X-ray diffraction studies precipitated (Figure S14).Note that a similar reaction of the cadmium(II) complexation with 1-(1-methylbenzimidazol-2-yl)-N-phenylmethanimine (L ) containing the -CH=N-linker group and the [B 12 H 12 ] 2- anion was described [27].The authors isolated the mixed-ligand complex [Cd(L ) 2 [B 12 H 12 ]] with the coordinated closo-dodecaborate anion and determined its structure by X-ray diffraction.
graphically independent part of the triclinic unit cell (space group P-1) of complex 2 contains the cationic complex [CdL2(CH3CN)H2O] 2+ and two crystallographically independent halves of the [B12H12] 2-anion (Figure 1).The cadmium(II) coordination environment is a strongly distorted octahedral and includes two nitrogen atoms of iminium (Nim), amino atoms (Namin) of two azoligands, a nitrogen atom of acetonitrile (NCH3CN), and one oxygen atom of a water molecule.The Cd-Nim bond lengths are 2.339(3) Å and 2.535  Our analysis of the Hirshfeld surface of the closo-dodecaborate anions shows that cationic complexes are associated with the [B12H12] 2-anions by weak OH…HB, NH…HB, CH…HB contacts (Figure 2a), forming 1D polymer chains in the crystal (Figure 2b).IR spectroscopy serves as an excellent tool for the qualitative identification of compound composition and preliminary structure determination.In particular, the presence of an absorption band of medium intensity ν(BH)MHB at 2407 cm -1 in the IR spectrum of complex 1 indicates that the [B12H12] 2-anion is coordinated by the metal.Additionally, an intense absorption band assigned to the stretching vibrations of "free" BH bonds is observed in the region of 2510-2430 cm -1 (Figure S1).
In the IR spectra of ligand L 2 with the -CH2-NH-linker group and complex 2, there is a band of stretching vibrations of the NH bond, ν(NH) with two maxima observed at approximately 3260 and 3257 cm -1 (Figure S2).The coordinated state of the water molecule IR spectroscopy serves as an excellent tool for the qualitative identification of compound composition and preliminary structure determination.In particular, the presence of an absorption band of medium intensity ν(BH) MHB at 2407 cm -1 in the IR spectrum of complex 1 indicates that the [B 12 H 12 ] 2-anion is coordinated by the metal.Additionally, an intense absorption band assigned to the stretching vibrations of "free" BH bonds is observed in the region of 2510-2430 cm -1 (Figure S1).
In the IR spectra of ligand L 2 with the -CH 2 -NH-linker group and complex 2, there is a band of stretching vibrations of the NH bond, ν(NH) with two maxima observed at approximately 3260 and 3257 cm -1 (Figure S2).The coordinated state of the water molecule in complex 2 is evident through the presence of a broadened absorption band in the region of 3585-3510 cm -1 , which corresponds to the stretching vibrations of the OH groups of water molecules (Figure S2).
The coordinated state of organic ligands L 1 and L 2 is reflected in the IR spectra of the synthesized complexes by changes in the number and shift of absorption band maxima, as well as the redistribution of their intensities in the 1600-700 cm -1 region when compared to the IR spectra of uncoordinated ligands (Figures S1 and S2).
The coordinated state of ligands L 1 and L 2 can be inferred from the electronic absorption spectra of the isolated compound.Specifically, a bathochromic shift of the broadened intraligand charge transfer band in the UV-vis absorption spectra of complex 1 (~350 nm in the ligand vs. ~390 nm in the complex) (Figure S5) or a hypsochromic shift in the UV-vis absorption spectra of complex 2 (~310 nm in the ligand vs. ~290 nm in the complex) (Figure S6) reflects the effect of the central metal atom on the π-electron system of coordinated ligand molecules, and indicates the preservation of the ligand electronic structure.

System [M(DMF) 6 ][B 12 H 12 ] (M = Zn(II), Cd(II))/L 3
The zinc(II) and cadmium(II) complexation with ligand L 3 , which contains the -N=CH-linker group, follows a fundamentally different pathway in the presence of the closo-dodecaborate anion [B 12 H 12 ] 2-compared to the processes described above.The study revealed the degradation of ligand L 3 , which was visually evident.The initially bright yellow solution gradually changed to a pale yellow color after 4 h, eventually becoming almost completely colorless after 24 h.Consequently, for zinc(II), complex [Zn(Bz-NH 2 ) 2 (CH 3 COO) 2 ] (3) was isolated.Notably, Bz-NH 2 (1-propyl-2-aminobenzimidazole) represents a fragment of the degraded ligand, and this complex does not contain the boron cluster anion.In the case of cadmium(II), a precipitate was obtained, consisting of a metalfree salt (HBz-NH 2 ) 2 [B 12 H 12 ]•2CH 3 CN (4•2CH 3 CN), along with a cadmium(II) complex involving salicylaldehyde (Scheme 2).It appears that the formation of compound 3 is influenced by the specific characteristics of the complexing metal, as well as the presence of both organic and inorganic bases.In this context, it's important to note that zinc(II) is considered a "harder" acid according to the Pearson's concept compared to cadmium(II), and it exhibits a high affinity for oxygen-containing species.As a result, the preferential formation of a bond with the oxygencontaining "hard" component of the system becomes more favorable compared to forming a bond with the [B12H12] 2-anion, which acts as a "soft" inorganic base.An analysis of the results presented above supports this hypothesis.When conducting complexation with stable "soft" organic ligands L 1 and L 2 , which maintain their structures without degradation, the reaction with zinc(II) does not occur because of the absence of oxygen-containing degradation fragments.It appears that the formation of compound 3 is influenced by the specific characteristics of the complexing metal, as well as the presence of both organic and inorganic bases.In this context, it's important to note that zinc(II) is considered a "harder" acid according to the Pearson's concept compared to cadmium(II), and it exhibits a high affinity for oxygen-containing species.As a result, the preferential formation of a bond with the oxygen-containing "hard" component of the system becomes more favorable compared to forming a bond with the [B 12 H 12 ] 2-anion, which acts as a "soft" inorganic base.An analysis of the results presented above supports this hypothesis.When conducting complexation with stable "soft" organic ligands L 1 and L 2 , which maintain their structures without degradation, the reaction with zinc(II) does not occur because of the absence of oxygencontaining degradation fragments.
According to the X-ray diffraction data, complex 3 crystallizes in the monoclinic system (P2 1 /n).The coordination environment of the zinc(II) atom is distorted and tetrahedral; it includes two iminium nitrogen atoms from two ligands Bz-NH 2 , and two oxygen atoms from two acetates (Figure 3).The Zn-N im distances are 2.0059( 14 It appears that the formation of compound 3 is influenced by the specific characteristics of the complexing metal, as well as the presence of both organic and inorganic bases.In this context, it's important to note that zinc(II) is considered a "harder" acid according to the Pearson's concept compared to cadmium(II), and it exhibits a high affinity for oxygen-containing species.As a result, the preferential formation of a bond with the oxygencontaining "hard" component of the system becomes more favorable compared to forming a bond with the [B12H12] 2-anion, which acts as a "soft" inorganic base.An analysis of the results presented above supports this hypothesis.When conducting complexation with stable "soft" organic ligands L 1 and L 2 , which maintain their structures without degradation, the reaction with zinc(II) does not occur because of the absence of oxygen-containing degradation fragments.
Table 1.Hydrogen bonds in the structure of complex 3.

D-H-A d(H-A)/Å d(D-A)/Å D-H-A/
The data of the IR spectrum of complex 3 align with the findings obtained by X-ray diffraction.Firstly, the appearance of absorption bands ν as (NH) NH2 at 3333 cm -1 and ν s (NH) NH2 at 3189 cm -1 , compared to the IR spectrum of ligand L 3 , indicates the formation of a compound containing an NH 2 group, specifically 1-propyl-2-aminobenzimidazole (Figure S3).Secondly, the presence of a strong absorption band of stretching vibrations ν(CO) COO at 1663 cm -1 in a lower frequency region compared to the position of this absorption band in uncoordinated acetate (ν(COO) COOH ~1700 cm -1 ) suggests the coordination of the acetate group in complex 3.Both ligands Bz-NH2 participate in intra-and intermolecular NH…O hydrogen bonds with acetate ions in complex 3 (Table 1), forming 2D polymer planes (Figure 4).These planes are connected to each other due to π-π stacking interactions (centroid-centroid distance 3.718 Å, shift distance 1.548 Å, angle 0.00°).148.4 N6-H6B-O2 1  1.95 2.8131(18) 166.6

D-H-A d(H-A)/Å d(D-A)/Å D-H-A/°
The data of the IR spectrum of complex 3 align with the findings obtained by X-ray diffraction.Firstly, the appearance of absorption bands νas(NH)NH2 at 3333 cm -1 and νs(NH)NH2 at 3189 cm -1 , compared to the IR spectrum of ligand L 3 , indicates the formation of a compound containing an NH2 group, specifically 1-propyl-2-aminobenzimidazole (Figure S3).Secondly, the presence of a strong absorption band of stretching vibrations ν(CO)COO at 1663 cm -1 in a lower frequency region compared to the position of this absorption band in uncoordinated acetate (ν(COO)COOH~1700 cm -1 ) suggests the coordination of the acetate group in complex 3.
The composition of the products resulting from the reaction of cadmium(II) with ligand L 3 in the presence of the closo-dodecaborate anion was identified based on IR spectroscopy and X-ray diffraction data.
According to the X-ray diffraction data, one of the products is a salt-containing protonated 1-propyl-2-aminobenzimidazole (HBz-NH2) as a cation, whereas the closo-dodecaborate anion acts as a counterion.It's worth noting that the protonation of nitrogen- The composition of the products resulting from the reaction of cadmium(II) with ligand L 3 in the presence of the closo-dodecaborate anion was identified based on IR spectroscopy and X-ray diffraction data.
According to the X-ray diffraction data, one of the products is a salt-containing protonated 1-propyl-2-aminobenzimidazole (HBz-NH 2 ) as a cation, whereas the closo-dodecaborate anion acts as a counterion.It's worth noting that the protonation of nitrogen-containing ligands during complexation in the presence of closo-borohydride anions is not without precedent.In [33,34], the authors describe the isolation of salts (H 2 phen)[B 10 H 10 ] and (Hbpa) 2 [B 12 H 11 OH] during the cobalt(II) complexation in the CoCl 2 /phen/[B 10 H 10 ] 2-system and lead(II) complexation in the Cs 2 [B 12 H 11 OH]/Pb(NO 3 ) 2 /bpa system, respectively.
The crystallographically independent part of the triclinic unit cell (P-1) of salt 4•2CH 3 CN contains one HBz-NH 2 cation, half of the [B 12 H 12 ] 2-anion, and an acetonitrile molecule (Figure 5a).One of the hydrogen atoms of the amino group of the cation and the iminium proton form short NH...HB contacts with the anion, linking cations and anions into a 1D polymer chain along axis c (Figure 5b).The second hydrogen atom of the amino group of the cation is hydrogen bonded to the solvate molecule of acetonitrile.The cations are connected by π-π stacking interactions (centroid-centroid distance 3.761 Å, shift distance 1.576 Å, angle 0.00 • ).In addition, the chains are connected by the CH...HB contacts between cations and anions (white spots on the Hirshfeld surface of the anion).
However, in the IR spectrum of the resulting precipitate, in addition to the absorption bands characteristic of a compound containing NH and NH 2 groups (broadened absorption band with maxima at 3340, 3334, 3298, 3225 cm -1 ) and the [B 12 H 12 ] 2-anion (intense absorption band with a maximum at 2470 cm -1 ), there is a band of stretching vibrations of the free OH group ν(OH) at 3605, 3566 cm -1 , as well as an intense absorption band at 1663 cm -1 , related to stretching vibrations ν(C=O) of the coordinated aldehyde group (Figure S4).Considering that during complexation, the ligand L 3 is degraded and one of the products is 1-propyl-2-aminobenzimidazole, the second product is probably salicylaldehyde.Therefore, based on the IR spectroscopy data, the second product is likely to be a cadmium(II) complex with salicylaldehyde.molecule (Figure 5a).One of the hydrogen atoms of the amino group of the cation and the iminium proton form short NH...HB contacts with the anion, linking cations and anions into a 1D polymer chain along axis c (Figure 5b).The second hydrogen atom of the amino group of the cation is hydrogen bonded to the solvate molecule of acetonitrile.The cations are connected by π-π stacking interactions (centroid-centroid distance 3.761 Å, shift distance 1.576 Å, angle 0.00°).In addition, the chains are connected by the CH...HB contacts between cations and anions (white spots on the Hirshfeld surface of the anion).However, in the IR spectrum of the resulting precipitate, in addition to the absorption bands characteristic of a compound containing NH and NH2 groups (broadened absorption band with maxima at 3340, 3334, 3298, 3225 cm -1 ) and the [B12H12] 2-anion (intense absorption band with a maximum at 2470 cm -1 ), there is a band of stretching vibrations of the free OH group ν(OH) at 3605, 3566 cm -1 , as well as an intense absorption band at 1663 cm -1 , related to stretching vibrations ν(C=O) of the coordinated aldehyde group (Figure S4).Considering that during complexation, the ligand L 3 is degraded and one of the products is 1-propyl-2-aminobenzimidazole, the second product is probably salicylaldehyde.Therefore, based on the IR spectroscopy data, the second product is likely to be a cadmium(II) complex with salicylaldehyde.
Note that several complexes of transition metals, such as Cd(II), Ni(II), Co(II), Mg(II) [35], Fe(III) [36], Re(I) [37], and N-(benzimidazol-2-yl)salicylaldimine containing the linker -N=CH-group-like ligand L 3 used in this work-are known.In the case of metals M(II), this ligand was formed during complexation in the M + /salicylaldehyde/2-aminobenzimidazole system, while for iron(III) and rhenium(I), this ligand was used as an individual compound.Moreover, this ligand retains its structure in the Fe(III) and Re(I) complexes.According to the methodology presented by the authors [36], we tried to realize the abovedescribed cadmium(II) complexation in methanol.However, the initially bright yellow solution also became almost colorless after 24 h.Evaporation of the reaction solution for 48 h led to the formation of a precipitate, the IR spectrum of which completely coincided with that shown in Figure S4.Note that several complexes of transition metals, such as Cd(II), Ni(II), Co(II), Mg(II) [35], Fe(III) [36], Re(I) [37], and N-(benzimidazol-2-yl)salicylaldimine containing the linker -N=CHgroup-like ligand L 3 used in this work-are known.In the case of metals M(II), this ligand was formed during complexation in the M + /salicylaldehyde/2-aminobenzimidazole system, while for iron(III) and rhenium(I), this ligand was used as an individual compound.Moreover, this ligand retains its structure in the Fe(III) and Re(I) complexes.According to the methodology presented by the authors [36], we tried to realize the above-described cadmium(II) complexation in methanol.However, the initially bright yellow solution also became almost colorless after 24 h.Evaporation of the reaction solution for 48 h led to the formation of a precipitate, the IR spectrum of which completely coincided with that shown in Figure S4.
For compound 3, the UV spectrum of a suspension in glycerol was recorded and analyzed.The disappearance of the absorption band related to intraligand charge transfer in the electronic absorption spectrum of 3 indicates the disappearance of the π-conjugated double bond system as a result of ligand degradation.

Figure 1 .
Figure 1.Crystallographically independent part of the unit cell of complex 2.

Figure 2 .
Figure 2. (a) OH…HB, NH…HB, and CH…HB contacts on the Hirshfeld surfaces of the [B12H12] 2- anions, and (b) 1D polymer chains formed by these contacts.Dashed green lines and dashed red lines show H…B and H…H contacts shorter than the sum of the van der Waals radii of the corresponding atoms.

Figure 2 .
Figure 2. (a) OH. ..HB, NH. ..HB, and CH. ..HB contacts on the Hirshfeld surfaces of the [B 12 H 12 ] 2- anions, and (b) 1D polymer chains formed by these contacts.Dashed green lines and dashed red lines show H. ..B and H. ..H contacts shorter than the sum of the van der Waals radii of the corresponding atoms.

Figure 4 .
Figure 4. 2D polymer planes in crystal 3 formed by NH…O hydrogen bonds.

Figure 5 .
Figure 5. (a) Structures of the cation and anion in salt 4•2CH3CN, (b) Hirshfeld surface of the [B12H12] 2-anion.The dotted green, red, and yellow lines show H…B, H…H, and H…N contacts shorter than the sum of the Van der Waals radii of the corresponding atoms.

Figure 5 .
Figure 5. (a) Structures of the cation and anion in salt 4•2CH 3 CN, (b) Hirshfeld surface of the [B 12 H 12 ] 2-anion.The dotted green, red, and yellow lines show H. ..B, H. ..H, and H. ..N contacts shorter than the sum of the Van der Waals radii of the corresponding atoms.

Table 1 .
Hydrogen bonds in the structure of complex 3.