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

Abstract

A series of Schiff base derivatives, namely N-(4-methoxyphenyl)-1-(1-methylbenzimidazol-2-yl)methanimine (L¹), 4-methoxy-N-[(1-methylbenzimidazol-2-yl)methyl]aniline (L²), and 2-[(E)-(1-propylbenzimidazol-2-yl)iminomethyl]phenol (L³), were synthesized. These compounds feature different linker groups, including –CH=N–, –CH₂–NH–, and –N=CH–, respectively. During the process of zinc(II) and cadmium(II) complexation in the presence of the closo-dodecaborate [B₁₂H₁₂]^2– anion, it was observed that ligand L³ underwent degradation. Consequently, two compounds were isolated, [Zn(Bz-NH₂)₂(CH₃COO)₂] and (HBz-NH₂)₂[B₁₂H₁₂]∙2CH₃CN, both containing 1-propyl-2-aminobenzimidazole (Bz-NH₂), 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(L²)₂(CH₃CN)(H₂O)][B₁₂H₁₂], [Zn(Bz-NH₂)₂(CH₃COO)₂], and (HBz-NH₂)₂[B₁₂H₁₂]∙2CH₃CN, as well as for ligand L².

1. Introduction

The remarkable properties of compounds that incorporate both benzimidazole derivatives [1,2,3,4,5,6,7,8,9,10] and closo-dodecaborate anions [11,12,13,14,15,16,17,18,19] have sparked significant interest among chemists. This interest arises from not only their potential practical applications, but also from a fundamental perspective. A judicious selection of synthetic conditions, including initial reagents, solvents, and other factors, plays a crucial role in obtaining stable compounds with the desired composition.

As of today, zinc(II) and cadmium(II) coordination compounds incorporating the closo-dodecaborate anion are relatively scarce, primarily consisting of various cationic complexes stabilized by the closo-borate anion or its derivatives. Among these complexes, compounds with coordinated solvent molecules [M(H₂O)₆][B₁₂X₁₂] (X is H and M is Zn(II) [20] or Cd(II) [21]; X is F and M is Zn(II) [22]), [M(DMF)₆][B₁₂H₁₂] [23], [Zn(CH₃CN)₆][B₁₂I₁₂] [24], as well as various salts [ZnL₂][B₁₂I₁₂] (L is tris(2-methyl-6-pyridylmethyl)phosphine) [24], [ZnL₆][B₁₂H₁₂] (L is 1-vinylimidazole) and [ZnL₄][B₁₂H₁₂] (L is 1-allylimidazole) [25], [EtZn][An] and Zn[An]₂ (An is [Me₃NB₁₂Cl₁₁]⁻, [Pr₃NB₁₂H₅Cl₆]⁻, [Bu₃NB₁₂H₄Cl₇]⁻, and [Hex₃NB₁₂H₅Cl₆]⁻) [26], [ZnL₂(ƞ²-O₂NO)]₂[L₂Zn(μ-NO₃)₂ZnL₂][B₁₂H₁₂]₂ (L is phen) and [Cd(bpa)₂(CH₃CN)₂][B₁₂H₁₂] [23], [ML₃][B₁₂H₁₂] (M is Zn(II), Cd(II) and L is bipy, phen [23]; M is Zn(II) and L is 1-methylbenzimidazo-2-yl-methyleneaniline or 1-ethyl-2-(4-methoxyphenyl)-azobenzimidazole) [27]) have been isolated.

Only three cadmium(II) complexes are currently known to feature the [B₁₂H₁₂]^2– anion coordinated by a metal. These include unstable binuclear complex [Cd(bpa)₂[μ-B₁₂H₁₂]]₂ [23] and mixed-ligand complexes with benzimidazole derivatives [CdL₂[B₁₂H₁₂]] (L is 1-methylbenzimidazo-2-yl-methyleneaniline) and [Cd(L²)₂(CH₃CN)[B₁₂H₁₂]] (L is 1-ethyl-2-(4-methoxyphenyl)-azobenzimidazole) [27]. This fact allows us to assume that the formation of complexes with the coordinated closo-dodecaborate anion being a “soft” inorganic base is possible even for metals of “intermediate” acidity, according to Pearsonʹs concept. The crucial thing is to choose the right reagents and synthesis conditions.

In our current study, we have investigated the complexation of zinc(II) and cadmium(II) with the [B₁₂H₁₂]^2– anion in the presence of Schiff base derivatives—specifically N-(4-methoxyphenyl)-1-(1-methylbenzimidazol-2-yl)methanimine (L¹), 4-methoxy-N-[(1-methylbenzimidazol-2-yl)methyl]aniline (L²), and 2-[(E)-(1-propylbenzimidazol-2-yl)iminomethyl]phenol (L³).

2. Materials and Methods

2.1. Materials

Acetonitrile (HPLC grade) was procured from Merck KGaA, Darmstadt, Germany and utilized without the need for additional purification. Complexes [M(DMF)₆][B₁₂H₁₂] (M = Zn(II), Cd(II)) were synthesized according to the procedure outlined [23].

2.2. Synthesis of Organic Ligands L¹–L³

2.2.1. N-(4-Methoxyphenyl)-1-(1-methylbenzimidazol-2-yl)methanimine (L¹)

A solution containing para-anisidine (0.62 g, 5 mmol) and 1-methyl-2-formylbenzimidazole (0.8 g, 5 mmol) in benzene (15 mL) (Warning! Highly toxic and carcinogenic) was heated to boiling for 2 h while concurrently distilling off water using a Dean-Stark apparatus. Upon cooling the solution, pale yellow crystals were formed, filtered, dried in air, and subsequently recrystallized from heptane. Melting point (mp) is 139–140 °C. Yield, 0.60 g (45%). IR (NaCl, cm^–1): ν(C=N) 1627, 1612, 1592, 1575; ¹H NMR (600 MHz, CDCl₃, ppm): 3.82 (3H, s, OCH₃), 4.27 (3H, s, NCH₃), 6.94 (2H, d, J = 9.0 Hz, H3′, H5′), 7.30 (1H, d, J = 7.1 Hz, H4 or H7), 7.32 (2H, d, J = 9.0 Hz, H2′, H6′), 7.36 (1H, t, J = 7.5 Hz, H5), 7.41 (1H, d, J = 8.1 Hz, H7 or H4), 7.82 (1H, d J = 8.4 Hz, H6), 8.75 (1H, s, CH=N); ¹³C NMR (150 MHz, CDCl₃, ppm): 32.08, 55.49, 109.84, 114.59, 120.70, 122.51, 124.57, 137.25, 143.04, 148.06, 149.93, 159.33; elemental analysis calculated (%) for C₁₆H₁₅N₃O (265.31): C 72.43, H 5.70, N 15.84; found: C 72.51, H 5.68, N 15.73.

2.2.2. 4-Methoxy-N-[(1-methylbenzimidazol-2-yl)methyl]aniline (L²)

Sodium borohydride (0.28 g, 7.5 mmol) was introduced into a solution of L¹ in methanol (20 mL), and the mixture was stirred at room temperature for 2 h. Subsequently, water (10 mL) was added and methanol was evaporated. The resulting white precipitate was separated by filtration, washed with water, dried, and subjected to recrystallization from ethyl acetate. Melting point (mp) is 113–114 °C. Yield, 1.22 g (91%). IR (NaCl, Nujol mull, cm^–1): ν(NH) 3260; ν(C=N) 1612, 1605, 1586; ¹H NMR (600 MHz, CDCl₃, ppm): 3.72 (3H, s, OCH₃), 3.73 (3H, s, NCH₃), 4.32 (1H, s, NH), 4.45 (2H, d, J = 4.1 Hz, CH₂), 6.72 (2H, d, J = 8.9 Hz, H2′, H6′), 6.79 (2H, d, J = 8.9 Hz, H3′, H5′), 7.23–7.30 (3H, m, H5, H6, H4 or H7), 7.74 (1H, d, J = 8.9 Hz, H7 or H4); ¹³C NMR (150 MHz, CDCl₃, ppm): 29.20, 42.13, 55.24, 108.55, 114.18, 118.98, 121.53, 122.10, 135.62, 141.23, 141.76, 151.36, 152.30; elemental analysis calculated (%) for C₁₆H₁₇N₃O (267.33): C 71.89, H 6.41, N 15.72; found: C 71.91, H 6.48, N 15.68.

2.2.3. 2-[(E)-(1-Propylbenzimidazol-2-yl)iminomethyl]phenol (L³)

A solution containing 1-propyl-2-aminobenzimidazole (1.75 g, 10 mmol) and salicylaldehyde (1.22 g, 10 mmol) in benzene (20 mL) was subjected to boiling for 1 h while simultaneously distilling off water using a Dean-Stark apparatus. Following this, benzene was removed via rotary evaporation. The resulting oily product was subsequently recrystallized from hexane, yielding yellow crystals, which were then filtered, air-dried, and collected. Melting point (mp) is 75–76 °C. Yield, 2.2 g (79%). IR (NaCl, Nujol mull, cm^–1): ν(OH) ~3380–3250; ν(C=N) 1670, 1620, 1605, 1571, 1533. ¹H NMR (600 MHz, CDCl₃, ppm): 0.96 (3H, t, J = 7.4 Hz, CH₃), 1.87–1.93 (2H, m, CH₂CH₂CH₃), 4.24 (2H, t, J = 7.1 Hz, CH₂CH₂CH₃), 6.98 (1H, t, J = 7.5 Hz, H3′), 7.03 (1H, d, J = 8.3 Hz, H6′), 7.25–7.29 (2H, m, H4′, H5′), 7.35 (1H, d, J = 9.0 Hz, H4), 7.44 (1H, t, J = 8.6 Hz, H5), 7.52 (1H, d, J = 9.1 Hz, H6), 7.72 (1H, d, J = 8.8 Hz, H7), 9.60 (1H, s, CH=N), 12.55 (1H, s, OH); ¹³C NMR (150 MHz, CDCl₃, ppm):10,92, 22.67, 43.87, 108.91, 116.78, 118.42, 119.34, 122.18, 133.45, 134.34, 141.11, 152.26, 161.21, 166.57; elemental analysis calculated (%) for C₁₇H₁₇N₃O (279.34): C 73.10, H 6.13, N 15.04; found: C 73.15, H 6.10, N 15.10.

2.3. Synthesis of Zinc(II) and Cadmium(II) Complexes

A solution of organic ligand L (L¹, L², or L³) (6 mmol) in CH₃CN (10 mL) was added to a solution of [M(DMF)₆][B₁₂H₁₂] (M = Zn(II), Cd(II)) (3 mmol) in the same volume of the same solvent. The precipitate formed after standing the reaction mixture in air was filtered, air-dried, and collected.

2.3.1. [Cd(L¹)₂[B₁₂H₁₂]] (Compound 1)

Yellow precipitate of 1 was obtained by allowing the reaction solution containing [Cd(DMF)₆][B₁₂H₁₂]/L¹ to evaporate for 48 h. Yield, 69%. IR (NaCl, Nujol mull, cm^–1): ν(BH) 2506, 2477; ν(BH)_MHB 2407; ν(C=N) 1650, 1623, 1609, 1579; elemental analysis calculated (%) for CdC₃₂H₄₂N₆O₂B₁₂ (784.86): Cd 14.32, C 48.97, H 5.39, N 10.71, B 16.5; found: Cd 14.39, C 48.91, H 5.47, N 10.62, B 16.1.

2.3.2. [Cd(L²)₂(CH₃CN)(H₂O)][B₁₂H₁₂] (Compound 2)

Colorless single crystals of 2 suitable for X-ray diffraction studies were obtained by allowing the reaction solution containing [Cd(DMF)₆][B₁₂H₁₂]/L² to evaporate for 48 h. Yield, 65%. IR (NaCl, Nujol mull, cm^–1): ν(OH)_H2O 3583, ~3513; ν(NH) 3257; ν(BH) 2479, 2461, 2436; ν(C=N) 1651, 1618, 1597; elemental analysis calculated (%) for CdC₃₄H₅₁N₇O₃B₁₂ (847.95): Cd 13.26, C 48.16, H 6.06, N 11.56, B 15.3; found: Cd 13.32, C 48.09, H 6.14, N 11.61, B 14.8.

2.3.3. [Zn(Bz-NH₂)₂(CH₃COO)₂] (Compound 3)

Light yellow single crystals of 3 suitable for X-ray diffraction studies were obtained by allowing the reaction solution containing [Zn(DMF)₆][B₁₂H₁₂]/L³ to evaporate for 48 h. Yield, 62%. IR (NaCl, Nujol mull, cm^–1): ν_as(NH)_NH2 3333, ν_s(NH)_NH2 3189; ν(CO)_COO 1663; ν(C=N) + δ(NH₂) 1617, 1562; elemental analysis calculated (%) for ZnC₂₄H₃₂N₆O₄ (533.94): Zn 12.25, C 53.99, H 6.04, N 15.74; found: Zn 12.31, C 53.92, H 5.98, N 15.79.

2.3.4. (HBz-NH₂)₂[B₁₂H₁₂]∙2CH₃CN (Compound 4∙2CH₃CN) + Cadmium(II) Complex with Salicylaldehyde

An inhomogeneous white precipitate containing colorless single crystals of salt 4∙2CH₃CN suitable for X-ray diffraction studies was obtained by allowing the reaction solution containing [Cd(DMF)₆][B₁₂H₁₂]/L³ to evaporate for 48 h. Yield, ~55%. IR (NaCl, Nujol mull, cm^–1): ν(OH) 3605, 3566_; ν(NH)_NH+NH2 3340, 3334, 3298, 3225; ν(BH) 2470; ν(C=O) 1663; ν(C=N) + δ(NH₂) 1640, 1542.

2.4. 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).

2.5. 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).

2.6. UV Spectroscopy

UV-vis absorption spectra of ligands L¹–L³ 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).

2.7. ¹H and ¹³C NMR Spectroscopy

¹H and ¹³C NMR spectra of organic ligands L¹–L³ were measured on a Bruker AVANCE-600 spectrometer (Bruker AXC, Inc., Karlsruhe, Germany) in CDCl₃ at 600 and 150 MHz, respectively, with internal deuterium stabilization. The chemical shifts of the ¹H nuclei are given relative to the residual signals of the deuterosolvent. Tetramethylsilane was used as an external standard. ¹H and ¹³C NMR spectra are present in Supplementary Materials (Figures S8–S13).

2.8. X-ray Diffraction

The single-crystal X-ray diffraction data for compounds 2–4∙2CH₃CN and ligand L² 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 F² 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 CH₃-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 CH₃CN 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.

Crystallographic data for all compounds prepared have been deposited with the Cambridge Crystallographic Data Center, CCDC nos. 2286255–2286257 (for compounds 2, 3, 4∙2CH₃CN) and 2291145 (for L²). Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge CHB2 1EZ, UK (Fax: +44-1223-336033; e-mail: [email protected] or www.ccdc.cam.ac.uk, accessed on 2 August 2023).

2.9. 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.

3. 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 [B_nH_n]^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 L¹–L³ in the presence of the [B₁₂H₁₂]^2– anion, utilizing these compounds as our starting materials.

3.1. System [M(DMF)₆][B₁₂H₁₂] (M = Zn(II), Cd(II))/L (L = L¹, L²)

The reaction pathways for zinc(II) and cadmium(II) complexation with ligand L¹ containing the –CH=N– linker group and ligand L² 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 L¹, the mixed-ligand complex [Cd(L¹)₂[B₁₂H₁₂]] was isolated with a boron cluster anion coordinated by the metal. However, for ligand L², we obtained a mixed-ligand cationic complex [Cd(L²)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 U_eq the organic ligand used. It’s worth noting that upon evaporating a solution containing [Zn(DMF)₆][B₁₂H₁₂] and ligand L², single crystals of ligand L² 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 [B₁₂H₁₂]^2– anion was described [27]. The authors isolated the mixed-ligand complex [Cd(L′)₂[B₁₂H₁₂]] with the coordinated closo-dodecaborate anion and determined its structure by X-ray diffraction.

The structure of complex 2 was determined using X-ray diffraction. The crystallographically 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 (N_im), amino atoms (N_amin) of two azoligands, a nitrogen atom of acetonitrile (N_CH3CN), and one oxygen atom of a water molecule. The Cd–N_im bond lengths are 2.339(3) Å and 2.535(3) Å, and the Cd–N_amin bonds are 2.535(3) Å, 2.536(3) Å, Cd–N_CH3CN 2.536(3) Å, and Cd–O 2.339(3) Å.

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). The H(O)…H(B) and H(O)…B distances are 2.2858(7) Å and 2.716(4) Å, respectively, whereas the H(N)…H(B) and H(N)…B contacts fall in the range 1.8537(4)–2.4691(6) Å and 2.446(3)–2.763(4) Å, respectively.

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 L² 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 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 L¹ and L² 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 L¹ and L² 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.

3.2. System [M(DMF)₆][B₁₂H₁₂] (M = Zn(II), Cd(II))/L³

The zinc(II) and cadmium(II) complexation with ligand L³, which contains the –N=CH– linker group, follows a fundamentally different pathway in the presence of the closo-dodecaborate anion [B₁₂H₁₂]^2– compared to the processes described above. The study revealed the degradation of ligand L³, 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-NH₂)₂(CH₃COO)₂] (3) was isolated. Notably, Bz-NH₂ (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 metal-free salt (HBz-NH₂)₂[B₁₂H₁₂]∙2CH₃CN (4∙2CH₃CN), 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 oxygen-containing “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 L¹ and L², which maintain their structures without degradation, the reaction with zinc(II) does not occur because of the absence of oxygen-containing degradation fragments.

According to the X-ray diffraction data, complex 3 crystallizes in the monoclinic system (P2₁/n). The coordination environment of the zinc(II) atom is distorted and tetrahedral; it includes two iminium nitrogen atoms from two ligands Bz-NH₂, and two oxygen atoms from two acetates (Figure 3). The Zn–N_im distances are 2.0059(14) Å and 2.0139(13) Å, and the Zn–O distances are 1.9444(12) Å and 1.9620(12) Å

Both ligands Bz-NH₂ participate in intra- and intermolecular NH…O hydrogen bonds with acetate ions in complex 3 (

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

D–H–A	d(H–A)/Å	d(D–A)/Å	D–H–A/°
N3–H3A–O1	2.11	2.8771(19)	145.8
N6–H6A–O3	2.11	2.8976(18)	148.4
N6–H6B–O2 1	1.95	2.8131(18)	166.6

¹ 1 − x, 1 − y, 1 − z.

), 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°).

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 L³, 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 L³ 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-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 (H₂phen)[B₁₀H₁₀] and (Hbpa)₂[B₁₂H₁₁OH] during the cobalt(II) complexation in the CoCl₂/phen/[B₁₀H₁₀]^2– system and lead(II) complexation in the Cs₂[B₁₂H₁₁OH]/Pb(NO₃)₂/bpa system, respectively.

The crystallographically independent part of the triclinic unit cell (P-1) of salt 4∙2CH₃CN contains one HBz-NH₂ cation, half of the [B₁₂H₁₂]^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 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 L³ 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 L³ 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.

4. Conclusions

In this study, we investigated the process of zinc(II) and cadmium(II) complexation with a set of benzimidazole derivatives featuring different linker groups: N-(4-methoxyphenyl)-1-(1-methylbenzimidazol-2-yl)methanimine (L¹) (the –CH=N– linker group), 4-methoxy-N-[(1-methylbenzimidazol-2-yl)methyl]aniline (L²) (the –CH₂–NH– linker group), and 2-[(E)-(1-propylbenzimidazol-2-yl)iminomethyl]phenol (L³) (the –N=CH– linker group), all in the presence of the closo-dodecaborate anion. We observed that both the choice of the complexing metal and the nature of the linker group have significant effects on the course of complexation.

(i) The reaction between zinc(II), the “soft” inorganic anion [B₁₂H₁₂]^2–, and “soft” organic ligands L¹ and L², which retain their structures during the complexation, did not occur. Mixed-ligand complexes were exclusively formed with cadmium(II). Specifically, complexes [Cd(L¹)₂[B₁₂H₁₂]] with the coordinated [B₁₂H₁₂]^2– anion and [Cd(L²)₂(CH₃CN)(H₂O)][B₁₂H₁₂] with the boron cluster anion as a counterion were isolated. The latter complex was characterized by X-ray diffraction.

(ii) Ligand L³ was found to undergo degradation, yielding 1-propyl-2-aminobenzimidazole (Bz-NH₂) and salicylaldehyde. Consequently, we isolated zinc(II) complex [Zn(Bz-NH₂)₂(CH₃COO)₂] and salt (HBz-NH₂)₂[B₁₂H₁₂]∙2CH₃CN, and their structures were determined using X-ray diffraction.

Analysis of the electronic absorption spectra of the obtained compounds showed that the structures of ligands L¹ and L² are preserved in their complexes. In the case of ligand L³, cleavage of the linker bond –N=CH– during the complexation leads to the disappearance of π-conjugation of the electronic structure of the ligand.