Triarylazoimidazole-Zn^II, Cd^II, and Hg^II Complexes: Structures, Photophysics, and Antibacterial Properties

Abstract

Novel triarylazoimidazoles containing strong electron donors (p-NEt₂) or acceptors (p-NO₂) by the azoaryl group, and their group 12 metal complexes were synthesized and fully characterized, including X-ray analysis for several complexes. Novel complexes exhibit red photo-luminescence emission (Φ up to  0.21) in a solution. Moreover, the antibacterial activity of complexes was tested against Gram-positive microorganism S. aureus and Gram-negative microorganism E. coli.

1. Introduction

Imidazole function is an important group in biology, often playing a role as a supporting ligand in metal-containing systems [1,2,3]. This stimulates the synthesis and structural characterization of imidazole metal complexes to better understand the role of these ligands. Azoimidazoles constitute an important class of photoactive materials, which were intensively studied earlier [4,5,6,7,8,9]. In addition, they are interesting chelating ligands, which electronics could be easily modulated via facile structural tuning [6,8,10]. Recently we started exploring triarylazoimidazoles and their coordination compounds [2,11,12]. Surprisingly, triarylazoimidazoles received almost no attention in the literature. In our recent publication, we showed that coordination of some triarylazoimidazoles to Zn^II, Cd^II, and Hg^II led to the formation of bright red emissive complexes (quantum yields up to 44%), which could be excited with a visible light [2]. Here, we report the synthesis and structural characterization of several novel triarylazoimidazoles and their emissive group 12 metal complexes (Φ up to 0.21). Introduction of the p-bromo substituent to the azoaryl substituent of the Zn-coordinated triarylazoimidazole shifts an emission maximum to 677 nm relatively to the unsubstituted triarylazoimidazole complex. In addition, we report antibacterial properties of novel complexes.

2. Materials and Methods

General remarks. Unless stated otherwise, all the reagents used in this study were obtained from the commercial sources (Aldrich (Schnelldorf, Germany), TCI-Europe (Zwijndrecht, Belgium), Strem (Bischheim, France), and ABCR (Karlsruhe, Germany)). NMR spectra were recorded on a Bruker Avance III (Karlsruhe, Germany) (¹H: 400 MHz); chemical shifts (δ) are given in ppm relative to TMS, coupling constants (J) in Hz. The solvent signals were used as references (CDCl₃: δ_C = 77.16 ppm; residual CHCl₃ in CDCl₃: δ_H = 7.26 ppm; CD₂Cl₂: δ_C = 53.84 ppm; residual CHDCl₂ in CD₂Cl₂: δ_H = 5.32 ppm); ¹H and ¹³C assignments were established using NOESY, HSQC, and HMBC experiments; numbering schemes as shown in the inserts. C, H, and N elemental analyses were carried out on a Euro EA 3028HT CHNS/O analyzer (Pavia, Italy). Mass-spectra were obtained on a Bruker micrOTOF spectrometer equipped with the electrospray ionization (ESI) source (Bremen, Germany); MeOH, CH₂Cl₂, or MeOH/CH₂Cl₂ mixture was used as a solvent. Solvents were purified by distillation over the indicated drying agents and were transferred under Ar: Et₂O (Mg/anthracene), CH₂Cl₂ (CaH₂), hexane (Na/K). Flash chromatography: Merck Geduran^® Si 60 (40–63 μm). Absorption spectra were measured in a 3.8 mL quartz cuvette using a UV-VIS spectrometer (UV-3600, Shimadzu, Kyoto, Japan) and photoluminescence emission and excitation spectra in the same cuvette, using a spectrofluorometer (RF-5031PC, Shimadzu). The luminescence quantum yield was determined using the slope method relative to the reference fluorophore, which the ethanol solution of Nile Blue dye (Φ = 0.27) excited at 590–625 nm, using a series of ethanol solutions of the sample with varying concentrations.

Synthesis of 9. Methanol solution of NaOMe (30 wt %, 1.9 mmol, 356 µL) and a solution of p-diethylaminophenyldiazonium tetrafluoroborate (1.9 mmol, 500.0 mg) in 2 mL MeCN were sequentially added to a solution of diphenylimidazole 1 (1.9 mmol, 418.7 mg) in EtOH (10 mL). The resulting mixture was stirred for 1 h, evaporated, redissolved in CH₂Cl₂, filtered, evaporated again, washed with Et₂O (3 × 3 mL), and dried under vacuum. Yield: 674.2 mg (90%) Elem. anal. calcd. for C₂₅H₂₅N₅: C, 75.92; H, 6.37; N, 17.71. Found: C 76.13; H 6.54; N 17.58. ¹H NMR (400 MHz, CDCl₃): δ 9.74 (s, br, 1H), 7.90 (d, J = 9.1 Hz, 2H), 7.64 (m, 4 H), 7.38–7.31 (m, 6 H), 6.71 (d, J = 9.1 Hz, 2H), 3.47 (q, J = 7.0 Hz, 4H), 1.24 (t, J = 7.0 Hz, 6H). ¹³C{¹H} NMR (151 MHz, CDCl₃): δ 211.8, 154.6, 150.9, 142.8, 135.5, 128.7, 128.5, 128.0, 126.2, 111.3, 44.9, 12.8. UV/Vis (CH₂Cl₂): λ_max = 493 nm, ε = 3.83 × 10⁴ M⁻¹ cm⁻¹.

Synthesis of 10. Methanol solution (2 mL) of potassium tert-butoxide (189.4 mg, 1.69 mmol) and a solution of p-nitrophenyldiazonium tetrafluoroborate (1.69 mmol, 401.7 mg) in MeCN (10 mL) were sequentially added to a solution of di-p-methoxyphenylimidazole (1.69 mmol, 473.3 mg) in MeOH (20 mL). The resulting mixture was stirred for 1 h, evaporated, redissolved in CH₂Cl₂, filtered, evaporated again, washed with Et₂O (3 × 3 mL), and dried under vacuum. Yield: 374.1 mg (52%) Elem. anal. calcd. for C₂₃H₁₉N₅O₄: C, 64.33; H, 4.46; N, 16.31. Found: C 64.58; H 4.69; N 16.22. ¹H NMR (400 MHz, CDCl₃): δ 10.05 (s, br, 1H), 8.35 (d, J = 9.0 Hz, 2H), 8.01 (d, J = 9.0 Hz, 2H), 7.59 (m, 4H), 6.93 (d, J = 8.0 Hz, 4H), 3.85 (s, 6H). ¹³C{¹H} NMR (151 MHz, CDCl₃): δ 160.1, 159.4, 155.5, 153.9, 148.2, 129.7, 129.3, 124.9, 123.0, 114.3, 55.4. UV/Vis (CH₂Cl₂): λ_max = 512 nm, ε = 1.92 × 10⁴ M⁻¹ cm⁻¹.

Synthesis of 11. Methanol solution (2 mL) of potassium t-butoxide (165.6 mg, 1.49 mmol) and a solution of p-bromophenyldiazonium tetrafluoroborate (1.49 mmol, 403.5 mg) in MeCN (10 mL) were sequentially added to a solution of di-p-methoxyphenylimidazole (1.49 mmol, 413.5 mg) in MeOH (20 mL). The resulting mixture stirred for 1 h, evaporated, redissolved in CH₂Cl₂, filtered, evaporated again, washed with Et₂O (3 × 3 mL), and dried under vacuum. Yield: 374.1 mg (62%). Elem. anal. calcd. for C₂₃H₁₉N₅O₄: C 49.14; H 3.37; N 10.42. Found: C 49.56; H 3.74; N 10.11. ¹H NMR (400 MHz, CDCl₃): δ 9.86 (s, br, 1H), 7.82 (d, J = 8.8 Hz, 2H), 7.63 (d, J = 8.8 Hz, 4H), 7.46 (m, 2H), 6.91 (m, 4H), 3.84 (s, 6H). ¹³C{¹H} NMR (151 MHz, CD₃CN): δ 159.5, 158.3, 154.0, 143.0, 132.8, 130.2, 130.0, 124.3, 114.5, 55.9. UV/Vis (CH₂Cl₂): λ_max = 483 nm, ε = 1.86 × 10⁴ M⁻¹ cm⁻¹.

General procedure for the synthesis of 7Zn–11Zn, 7Cd–11Cd, and 7Hg–11Hg. Methanol solution of the ligand 9–11 (2 mL) was added to a solution of an appropriate metal halide (1 eq.) in 1 mL MeOH. The resulting mixture was kept without stirring for 24 h, the formed precipitate was filtered, washed with MeOH (3 × 1 mL), Et₂O (3 × 3 mL), and dried under vacuum.

9Zn. 9 (1 eq, 0.13 mmol, 50.4 mg), ZnCl₂ (1 eq., 0.13 mmol, 17.8 mg) were used. Yield: 32.6 mg (48.2%). ¹H and ¹³C NMR spectra were not obtained due to the low solubility of the compound in the common deuterated solvents. Elem. anal. calcd. for C₂₅H₂₅Cl₂N₅Zn: C, 56.46; H, 4.74; N, 13.17. Found: C 56.75; H 7.86; N 13.05. Crystals, suitable for X-ray analysis, were obtained from the reaction mixture. UV/Vis (CH₂Cl₂): λ_max = 614 nm, ε = 3.19 × 10⁴ M⁻¹ cm⁻¹.

9Cd. 9 (1 eq, 0.13 mmol, 50.2 mg), CdBr₂ 4H₂O (1 eq., 0.13 mmol, 44.0 mg) were used. Yield: 46.4 mg (54.9%). ¹H and ¹³C NMR spectra were not obtained due to the low solubility of the compound in the common deuterated solvents. Elem. anal. calcd. for C₂₅H₂₅Br₂N₅Cd: C, 44.97; H, 3.77; N, 10.49. Found: C 45.14; H 3.98; N 10.38. UV/Vis (CH₂Cl₂): λ_max = 599 nm, ε = 3.03 × 10⁴ M⁻¹ cm⁻¹.

9Hg. 9 (1 eq, 0.13 mmol, 50.0 mg), HgCl₂ (1 eq., 0.13 mmol, 35.1 mg) were used. Yield: 46.0 mg (54.5%). ¹H and ¹³C NMR spectra were not obtained due to the low solubility of the compound in the common deuterated solvents. Elem. anal. calcd. for C₂₅H₂₅Cl₂N₅Hg: C, 45.02; H, 3.78; N, 10.50. Found: C 45.31; H 3.87; N 10.46. Crystals, suitable for X-ray analysis, were obtained from the reaction mixture. UV/Vis (CH₂Cl₂): λ_max = 517 nm, ε = 3.13 × 10⁴ M⁻¹ cm⁻¹.

10Zn. 10 (2 eq., 0.17 mmol, 75.0 mg), ZnCl₂ (1 eq., 0.09 mmol, 12.2 mg) were used. Yield: 48.4 mg (55.5%). ¹H and ¹³C NMR spectra were not obtained due to the low solubility of the compound in the common deuterated solvents. Elem. anal. calcd. for C₂₃H₁₉Cl₂N₅O₄Zn: C, 48.83; H, 3.39; N, 12.38. Found: C 49.11; H 3.56; N 12.29. UV/Vis (CH₂Cl₂): λ_max = 516 nm, ε = 7.70 × 10⁴ M⁻¹ cm⁻¹.

10Cd. 10 (2 eq., 0.17 mmol, 74.9 mg), CdBr₂ 4H₂O (1 eq., 0.09 mmol, 30.6 mg) were used. Yield: 66.6 mg (67.2%). ¹H and ¹³C NMR spectra were not obtained due to the low solubility of the compound in the common deuterated solvents. Elem. anal. calcd. for C₂₃H₁₉Br₂N₅O₄Cd: C, 39.37; H, 2.73; N, 9.98. Found: C 39.71; H 2.94; N 10.05. Crystals, suitable for X-ray analysis, were obtained from the reaction mixture. UV/Vis (CH₂Cl₂): λ_max = 516 nm, ε = 6.67 × 10⁴ M⁻¹ cm⁻¹.

10Hg. 7 (2 eq., 0.17 mmol, 74.9 mg), HgCl₂ (1 eq., 0.09 mmol, 23.4 mg) were used. Yield: 45.0 mg (45.8%). ¹H and ¹³C NMR spectra were not obtained due to the low solubility of the compound in the common deuterated solvents. Elem. anal. calcd. for C₂₃H₁₉Cl₂N₅O₄Hg: C, 39.41; H, 2.73; N, 9.99. Found: C 39.45; H 2.78; N 10.07. UV/Vis (CH₂Cl₂): λ_max = 517 nm, ε = 6.31 × 10⁴ M⁻¹ cm⁻¹.

11Zn. 11 (1 eq., 0.18 mmol, 50.3 mg), ZnCl₂ (1 eq., 0.18 mmol, 14.9 mg) were used. Yield: 26.3 mg (40.5%). ¹H and ¹³C NMR spectra were not obtained due to the low solubility of the compound in the common deuterated solvents. Elem. anal. calcd. for C₂₃H₁₉BrCl₂ZnN₄O₂: C, 46.07; H, 3.19; N, 9.34. Found: C 45.87; H 3.08; N 9.36. UV/Vis (CH₂Cl₂): λ_max = 527 nm, ε = 2.14 × 10⁴ M⁻¹ cm⁻¹.

11Cd. 11 (1 eq., 0.11 mmol, 50.2 mg), CdBr₂⸱4H₂O (1 eq., 0.11 mmol, 37.2 mg) were used. Yield: 31.4 mg (39.7%). ¹H and ¹³C NMR spectra were not obtained due to the low solubility of the compound in the common deuterated solvents. Elem. anal. calcd. for C₂₃H₁₉Br₃CdN₄O₂: C, 37.56; H, 2.60; N, 7.62. Found: C 37.64; H 2.66; N 7.81. UV/Vis (CH₂Cl₂): λ_max = 483 nm, ε = 1.03 × 10⁴ M⁻¹ cm ¹.

11Hg. 11 (1 eq., 0.11 mmol, 50.0. mg), HgCl₂ (1 eq., 0.11 mmol, 29.9 mg) were used. Yield: 33.2 mg (41.62%). ¹H and ¹³C NMR spectra were not obtained due to the low solubility of the compound in the common deuterated solvents. Elem. anal. calcd. for C₂₃H₁₉BrCl₂HgN₄O₂: C, 37.59; H, 2.61; N, 7.62. Found: C 37.81; H 2.54; N 7.58. UV/Vis (CH₂Cl₂): λ_max = 503 nm, ε = 2.24 × 10⁴ M⁻¹ cm ¹.

2.1. X-ray Diffraction Studies

XRD data for 9Hg’·2CH₃OH, 8·solvate and 10Cd·1.75CH₃OH·0.75H₂O were collected using equipment of the Center for Molecular Studies of the Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences. The X-ray diffraction studies for 9Zn·CH₃OH, 9Hg·CH₃OH and 10 were performed at the Shared Equipment Center of the Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences. The X-ray measurements were performed using shared experimental facilities supported by INEOS RAS and IGIC RAS state assignment.

Single crystals of 9Zn·CH₃OH, 9Hg·CH₃OH, 9Hg’·2CH₃OH, 8·solvate, 10 and 10Cd·1.75CH₃OH·0.75H₂O were obtained from reaction mixtures. Intensities of the reflections for these crystals were collected with Bruker D8 Venture (9Zn·CH₃OH, 9Hg·CH₃OH, 10) and Bruker Quest (9Hg·2CH₃OH, 8·solvate and 10Cd·1.75CH₃OH·0.75H₂O) diffractometers (Bruker AXS, Inc., Madison, WI, USA) at 100.0(2) K (MoKα-radiation, λ = 0.71073 Å). The structures were solved by the SHELXT method [13] and refined by full-matrix least squares against F². Non-hydrogen atoms were refined anisotropically. H(N) and H(O) atoms were located on difference Fourier maps, and those of H(C) atoms were calculated. The unit cell of 8·solvate contains two solvent molecules (each occupies 186 ų and has 50 counts of electrons), which were treated as a diffuse contribution to the overall scattering without specific atom positions by the SQUEEZE/PLATON procedure [14]. A similar procedure was applied to treat some of the highly disordered solvent molecules in 10Cd; two voids with volumes 624 and 199 ų and 399 and 106 counts of electrons were found. All hydrogen atoms were included in a refinement by the riding model with U_iso(H) = 1.5U_eq(X) for methyl and hydroxy groups and 1.2U_eq(X) for the other atoms. All calculations were made using the SHELXL2014 [15] and OLEX2 [16] program packages. Crystallographic parameters and refinement details for all complexes are listed in Table S1. CCDC 2166661–2166666 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/structures/ (accessed on 10 April 2022).

2.2. Computational Details

The DFT calculations based on the experimental X-ray geometries of 9Hg, 9Hg’, 9Zn, and 10Cd were carried out with the help of the Gaussian-09 [17] program package. The topological analysis of the electron density distribution was performed by using the Multiwfn program (version 3.7) [18]. The Cartesian atomic coordinates for model supramolecular associates are presented in the attached xyz-files, Supplementary Materials.

3. Results and Discussion

3.1. Synthesis and Structural Characterization

Novel triarylazoimidazoles 9–11 were readily synthesized by the coupling between the corresponding diazonium salts 4–6 and diarylimidazoles 1 or 2 (Scheme 1). The synthesis of similar triarylazoimidazoles 7 and 8 we reported earlier [2,12].

Scheme 1. Synthesis of triarylazoimidazoles 7–11.

Triarylazoimidazoles 7–11 appear as bright red solids, 7, 8, 10, and 11 [11] could be recrystallized from saturated MeOH solutions to produce crystals, suitable for analysis by single crystal X-ray crystallography (Figure 1). In addition, we obtained single crystals of 8 (Figure 1), which synthesis we reported earlier [2,12].

Figure 1. (a) Molecular structures of 8 and 10 in representation of atoms with thermal ellipsoids (given at p = 50%) and (b) comparison of their conformations with that of 11. Solvent molecules are omitted. Overlaid atoms of 8 (magenta), 10 (blue), and 11 (red) belong to the imidazole ring.

Data about molecular and crystal structures of pure triarylazoimidazoles are limited with those for 2-(p-bromophenyl)diazen-1-yl-containing imidazole 11 [11]. Herein we compare its geometry with that of p-methoxy- and p-nitrosubstituted analogs. The main geometrical parameters of these molecules are listed in Table 1. Bond distances are very close for these molecules (Figure 1, Table 1). All molecules adopt a trans conformation with respect to the double N3=N4 bond. The azo double bond is elongated as compared with that in azobenzene, and the imidazole ring is nearly coplanar with the aryl group attached to the azo group (Figure 1). This fact indicates a prominent electron delocalization within the molecule. The two other substituents at carbon atoms of the five-membered ring can freely rotate along single C–C bonds, visualized in Figure 1.

Table 1. Selected geometrical parameters (Å, °) for azoimidazoles 7–11 and their group 12 metal complexes.

	9Zn	9Hg	9Hg’	8	10	10Cd	11
M-Hal	2.212(2)–2.221(1)	2.3905(9)–2.4210(8)	2.382(2)	-	-	2.595(1)–2.640(1)	-
M–N1Pz	2.035(2)	2.280(2)	2.156(7)–2.220(6)	-	-	2.305(5)–2.392(5)	-
M–N4	2.137(2)	2.430(2)	2.430(8)	-	-	2.556(5)–2.885(6)	-
N–M–N	79.00(7)	70.08(8)	69.8(2)	-	-	62.2(2)–65.8(2)	-
Hal–M–Hal	115.15(5)	121.99(3)		-	-	103.8(3)–111.5(4)	-
N1–C3	1.386(2)	1.380(3)	1.358(10)	1.3868(1)	1.35887(6)	1.348(7)–1.384(8)	1.372(3)
N1=C1	1.346(2)	1.347(3)	1.321(10)	1.3595(1)	1.35986(8)	1.335(8)–1.373(8)	1.360(4)
C1–N2	1.337(2)	1.384(3)	1.338(10)	1.3253(1)	1.32652(7)	1.321(8)–1.352(8)	1.326(3)
C1–N3	1.378(3)	1.381(3)	1.354(10)	1.3904(1)	1.38415(7)	1.352(9)–1.389(8)	1.392(4)
N2–C2	1.387(2)	1.332(3)	1.356(10)	1.3806(1)	1.37085(8)	1.350(8)–1.395(8)	1.375(4)
C2=C3	1.392(3)	1.395(3)	1.385(11)	1.3962(1)	1.39362(7)	1.394(9)–1.421(9)	1.388(4)
N3=N4	1.296(2)	1.295(3)	1.252(9)	1.2742(1)	1.26990(7)	1.271(7)–1.299(8)	1.274(3)
Ar–C	1.470(3)–1.471(3)	1.472(3)–1.473(3)	1.454(10)–1.463(10)	1.4664(1)–1.4528(1)	1.46587(8)–1.48591(9)	1.443(8)–1.485(10)	1.474(3)–1.477(3)
ArvN4	1.380(2)	1.383(3)	1.386(10)	1.4224(1)	1.42510(7)	1.418(8)–1.437(8)	1.426(4)

Subsequent addition of group 12 metal chlorides to 7–11 resulted in the formation of chelates 7Zn–11Zn, 7Cd–11Cd, and 7Hg–11Hg (Scheme 2), which precipitated from the reaction mixtures as dark purple microcrystalline solids. The crystals of complexes 9Zn, 9Hg, and 10Cd were suitable for analysis by single crystal X-ray crystallography. The structural investigations proved the formation of azoimidazole chelate complexes (Figure 2). The Cd complex 10Cd, which contained p-NO₂ substituent by the azoaryl group, contained two chelating azoimidazoles in contrast to 9Zn and 9Hg.

Scheme 2. Synthesis of triarylazoimidazole complexes 7Zn–11Zn, 7Cd–11Cd, and 7Hg–11Hg.

Figure 2. Asymmetric units of 9Zn, 9Hg, and 10Cd in representation of atoms with thermal ellipsoids (given at p = 50%). Solvent molecules are omitted for 10Cd.

Metal atoms in 9Zn and 9Hg complexes coordinate one azaimidazole ligand in a bidentate-chelate mode, and two terminal chlorine atoms to form a distorted tetrahedral MN₂Cl₂ coordination polyhedron. Asymmetric unit of 10Cd contains two independent complexes. Their conformations are compared in Figure S2; they differ mainly in disposition of phenylmethoxy and nitrophenyl groups in respect to azoaimidazole fragments. In 10Cd, the large Cd²⁺ atom binds two chelating aza ligands and two terminal bromine atoms to form an octahedral CdN₄Br₂ coordination polyhedron with the nitrogen atoms of the azo group being in axial positions. Lengths of M–Cl, M–N1, and M–N4 bonds increase from Zn²⁺ to Hg²⁺ and to Cd²⁺ and N-M–N chelate angles decrease in the same raw (Table 1). Delocalization within the imidazole ring at coordination becomes less pronounced as it follows from elongation of single and shortening of double bonds; however, coordination of the azoaryl moiety results in elongation of the N=N bond and shortening of the N4–Ar and C1–-N3 bonds. For all three complexes, all azoimidazoles retain upon coordination the trans configuration at diazo groups, and nearly planar configuration of the azoimidazole moieties. However, the N4 atom of the azo bond is situated on the same side as the N2 atom of the imidazole ring in respect to the C1–N3 bond, while for neutral ligands, it is always situated on the same side as the N1(H) atom of the ring. Interestingly, the N atom of the diethylamino group is also almost planar with CNC plane being coplanar with the plane of the azoimidazole moiety, which indicates the significant electronic conjugation in the whole system.

Overall, molecular geometry of triarylazoimidazoles in 5, 7–10 is similar to those reported for structurally relevant azo-compounds [5,9,19,20,21,22,23,24,25] and imidazole derivatives [26,27].

Interestingly, the reaction of 9 with HgCl₂ resulted in the formation of a very small batch of crystals of 9Hg’ (Figure 3 and Figure S1) along with the main product 9Hg. Although we were not able to reproduce the synthesis of 9Hg’, here we give a description of the crystal structure of 9Hg’ since it is a remarkable one-dimensional coordination polymer. In this complex, chlorine atoms act as terminal ligands, and deprotonated azaimidazole 9 acts as a tridentate bridging and chelating ligand, which coordinates by both N atoms of the imidazole ring and the N4 atom of the azo group. Coordination bonds for the resulting tetrahedral HgN₂Cl₂ coordination polyhedron are very close to the 9Hg, while delocalization within the five-membered ring becomes even more pronounced than in the neutral uncoordinated ligand (Table 1). Overall complex is a chain coordination polymer parallel with crystallographic axis c (Figure 3).

Figure 3. Ball-and-stick representation of the coordination polymer 9Hg’ in the crystal. Solvent molecules and hydrogen atoms are omitted. Color code: C–grey, Cl–green, H–light grey, Hg–olive green, N–blue, O–red.

Presence of acidic hydrogen atoms in 8, 10, 9Zn, 9Hg, and 10Cd as well as a number of solvent water and MeOH molecules in solid 9Zn, 9Hg, 9Hg’, and 10Cd allows formation of strong H-bonds in these solids. Presence of some of these hydrogen bonds in metal complexes was confirmed using DFT calculations of isolate clusters followed by analysis of electron density distribution within the QTAIM approach (see Section 3.4) [28]. Herein we should note that in pure ligand 10 H-bonded dimers (see Figure S3) connected by two N-H…O bonds with a methoxy group were found (r(N…O) = 2.921(3)Å, NHO = 165.7°), while in 8, one can propose that H(N) atom should be involved in H-bonding with acceptor groups of solvent molecules removed from refinement using the SQUEEZE procedure. In isostructural 9Zn and 9Hg H-bonded dimers (Figure S3) can be found due to N-H…O and O-H…Cl interactions with a hydroxy group of solvent molecule (r(N…O) = 2.747(3)–2.750(3)Å, NHO = 165.6–167.5°; r(O…Cl) = 3.104(3)–3.129(3) Å, OHCl = 163.1–165.0°). In 9Hg’, the amino group is absent, but methanol molecules are connected to the coordination polymer through O-H…O and O-H…Cl interactions (Figure S3; r(O…O) = 2.716(16) Å, OHO = 130.3°; r(O…Cl) = 3.124(10) Å, OHCl = 162.6°). In 10Cd, one of the amino groups interacts with a bromine atom of a neighboring complex, two others with methanol molecules, and the fourth one with a water molecule. Solvent molecules in 10Cd also take part in O-H…O and O-H…Br bonding. Besides a strong hydrogen bond, C-H…Hal and C-H…O interactions can be found in solid 9Zn, 9Hg, 9Hg’, and 10Cd. Planar configuration of the ligand also allows realization of hydrophobic C–H…π and stacking interactions in these compounds. Particularly, in H-bonded dimers in 9Zn and 9Hg the mean plains of ligands are situated parallel at distances 3.298(3) and 3.308(2) Å, respectively. Thus, geometry of these complexes allows realization of both hydrophilic and hydrophobic interactions in solids and ligand–receptor complexes.

3.2. Absorption and Emission Profiles

Ligands 7–10 exhibited a red color in the dichloromethane solution and the main absorption band with the absorption maximum close to 500 nm and additional absorption bands in the UV range (Figure 4, Table 2).

Figure 4. UV-Vis absorptions spectra of the ligand 9 and its complexes (a), ligand 10 and its complexes (b), ligand 11 and its complexes (c) in dichloromethane.

Table 2. UV-Vis absorption characteristics of the ligands 9–11 in dichloromethane solution.

	Absorption Maximum, nm	εmax, M−1 cm−1
9	493	38,300
10	512	19,200
11	483	18,600

Arylazoimidazole compounds have the absorption maximum in the near UV range around 360 nm corresponding to intraligand π–π* charge transfer (ILCT) transitions [29,30], the bathochromic shift of this maximum to 480–510 nm in ligands 9–11 can be partly attributed to electron delocalization effect due to the presence of two additional aromatic rings. Position and intensity of the ILCT absorption band is also affected by electron-donating and electron-withdrawing groups at the para-position of the aryl rings. In ligands 7 and 9, the electron density shifts upon photoexcitation from the electron-donating OMe (Et₂N) group to the imidazole ring (electron acceptor). Correspondingly, in agreement with the previous findings [31] a stronger electron donor in the R₁ position gives 9 a higher absorption maximum (493 vs. 441 nm) and a larger extinction coefficient (38,300 vs. 28,300 M⁻¹ cm⁻¹) compared to 7 [2]. Planar geometry of the arylazoimidazole fragment facilitates ILCT, thus producing large values of the extinction coefficients of 7 and 9. In 8 and 10–11, electron-donating methoxy groups at the R₂ position induce additional charge transfer from the aryl groups attached to the imidazole ring. This effect is further strengthened by the electron-withdrawing Br and NO₂ groups at the R₁ position, resulting in increase of absorption maximum from 466 (8) to 483 (11) and 512 nm (10). Lack of planarity between imidazole-attached aryl rings and imidazole hinders intraligand charge transfer and results in smaller values of extinction coefficients of the 8, 10, and 11 ligands compared with 7 and 9. Extinction coefficients of 9–11 ligands corresponded to 22–28,000 M⁻¹ cm⁻¹ values reported for similar arylazoimidazole and triarylazoimidazole compounds [2].

The ILCT absorption bands of the ligands 9 and 11 exhibited a bathochromic shift, which was especially strong in case of Zn and Cd complexes of 9 giving them intensive blue colors (Figure 4, Table 2). The same coordination-induced spectral shift was previously discovered for the complexes of 7 and 8 [2]. This shift is attributed to the electron-withdrawing effect of metal ions coordinated to the azo group and imidazole ring and depends on the LUMO at the N1 and N4 nitrogen atoms. Thus, the strongest coordination-induced absorption shift was observed for complexes of 9 (from 493 to ca. 590 nm in 9Zn), where electron density in the excited state was localized at the imidazole ring as a result of charge transfer induced by the Et₂N substituent. At the same time, in ligand 10, the nitro group itself exerted a strong electron-withdrawing effect via the aromatic ring and attracted electron density from the azo and imidazole groups; as a result the bathochromic shift of the absorption maximum induced by coordination was small (from 512 to 516 nm). The complexes of 11 with a weakly electron-withdrawing Br atom at the R₁ position exhibited an intermediate shift (from 483 to 527 nm). Coordination-induced bathochromic shift of the ILCT band also depended on the metal ion and followed the trend λ_maxILCT(Hg) < λ_maxILCT(Cd) < λ_maxILCT(Zn) [2]. Additional absorption bands appeared in the UV-Vis spectra of complexes in the spectral region between 580 and 620 nm and is attributed to metal-to-ligand charge transfer (MLCT) transition. In case of complexes 9Zn and 9Cd, this coordination-induced MLCT band was even stronger than the ILCT band at 550–560 nm, for others it appeared as a shoulder on the absorption spectrum.

Whereas ligands 9–11 were nonluminescent, all complexes exhibited red photoluminescence emission in dichloromethane solutions with emission maximum between 650 and 680 nm (Figure 4, Table 3), similar orange or red photoluminescence with emission maximum between 600 and 660 nm was previously observed for the metal complexes of 7 and 8 [2].

Table 3. UV-Vis absorption and photoluminescence characteristics of the complexes 9Zn–11Zn, 9Cd–11Cd, and 9Hg–11Hg in dichloromethane solution.

	Absorption Maximum, nm	εmax, M−1 cm−1	Excitation Maximum, nm	Emission Maximum, nm	PL Quantum Yield, %
9Zn	614	31,900	626	662	1.13
9Cd	599	30,300	613	653	0.60
9Hg	517	31,300	596	650	0.2
10Zn	516	77,000	621	661	16.7
10Cd	516	66,700	614	662	4.7
10Hg	517	63,100	609	661	2.6
11Zn	527	21,400	590	677	21.0
11Cd	483	10,250	580	668	18.8
11Hg	503	22,400	549	657	0.01

Excitation and emission maximum wavelengths followed the general trend λ_ex(em) (Hg) < λ_ex(em) (Cd) < λ_ex(em) (Zn), and the luminescence quantum yields–Φ (Hg) < Φ (Cd) < Φ (Zn) in accordance with the trend found for metal complexes of 7 and 8 [2]. Luminescence excitation maxima coincided with the positions of MLCT absorption bands on UV-Vis spectra of complexes, which confirms that emission is excited via coordination-related MLCT transitions. R₁ and R₂ substituents in 9–11 strongly affected the luminescence quantum yield (Table 3). The maximal quantum yield (21% for 11Zn) is smaller than the yield of 7 and 8 metal complexes (up to 44%) [2] but was still significant.

3.3. Antibacterial Tests

The in vitro antibacterial activity of the 7–11 against Gram-positive microorganism S. aureus and Gram-negative microorganism E. coli was estimated using the agar-well diffusion method. The antibacterial properties of 7–11 (Table 4) were compared with those for the commercially available conventional antibiotics ampicillin and gentamicin. The antibacterial activities of the mentioned organic compounds increase in the row 7 < 8~10 < 11 < 9, where 9 is the undoubted leader (see the corresponding inhibition zones and, especially, MIC values). We assume that the antibacterial pharmacophore is an azaimidazole moiety, which can be highly protonated at physiological pH. The resulting cation interacts with the anionic fragments of bacterial cell surface, which causes an osmotic imbalance, damages of the bacterial cell membrane, and leakage of the cell content into the external environment. The cascade of these unfavorable events for the bacterial cell inevitably leads to the death of the bacterium. The intracellular components of bacteria are characterized by strong absorption at 260 nm. This fact was used in previous studies to monitor the integrity of the bacterial cell membrane by spectrophotometry. We used this approach within the frames of the current work to study the effect of 7–11 on the integrity of the cell membranes of S. aureus and E. coli. We found that treatment of the bacterial suspension with 7–11 caused a dramatic increase in optical density from 0.5 to 0.9 in 6 min (9), 10 min (8, 10, 11), or 20 min (7). Thus, the tested organic compounds effectively damaged the bacterial cell membrane, but the effect of 9 most rapidly developed. The highest antibacterial effect of 9 can be explained by the presence of a larger number of N-atoms that can be protonated at a physiological pH value.

Table 4. The in-vitro antibacterial activity of the ligands 7–11 and their complexes against Gram-positive microorganism S. aureus and Gram-negative microorganism E. coli.

	S. aureus	E. coli
	Inhibition Zone, mm/MIC
7	20.4 ± 0.1/1.11	18.3 ± 0.2/1.13
8	22.3 ± 0.2/0.97	19.1 ± 0.3/0.99
9	23.6 ± 0.1/0.81	20.1 ± 0.3/0.79
10	22.1 ± 0.3/0.97	19.1 ± 0.2/0.99
11	22.6 ± 0.1/0.95	19.1 ± 0.2/0.95
7Zn	23.8 ± 0.2/0.87	19.9 ± 0.2/0.81
7Cd	25.2 ± 0.1/0.63	18.9 ± 0.3/0.75
7Hg	28.7 ± 0.3/0.34	20.2 ± 0.2/0.63
8Zn	24.5 ± 0.3/0.77	19.7 ± 0.1/0.83
8Cd	26.9 ± 0.3/0.51	20.4 ± 0.2/0.63
8Hg	30.1 ± 0.2/0.19	21.0 ± 0.2/0.51
9Zn	25.0 ± 0.1/0.63	20.9 ± 0.2/0.58
9Cd	28.5 ± 0.2/0.32	21.8 ± 0.3/0.43
9Hg	33.4 ± 0.3/0.11	22.9 ± 0.1/0.22
10Zn	24.8 ± 0.1/0.74	19.8 ± 0.4/0.82
10Cd	26.5 ± 0.1/0.52	20.9 ± 0.1/0.65
10Hg	30.4 ± 0.2/0.18	21.0 ± 0.3/0.50
11Zn	24.4 ± 0.2/0.75	19.5 ± 0.1/0.80
11Cd	27.3 ± 0.3/0.49	20.6 ± 0.1/0.62
11Hg	30.0 ± 0.3/0.20	21.2 ± 0.3/0.51
Ampicillin	30.2 ± 0.2/0.12	─
Gentamicin	─	22.1 ± 0.2/0.21

The antibacterial activity of Zn(II), Cd(II), and Hg(II) complexes derived from 7–11 is substantially higher than that of the staring 7–11 (Table 1). Provided the same metal center, the complexes derived from 9 are characterized by the highest antibacterial effect, while the complexes derived from 7 are the least active. Moreover, provided the same ligand, the antibacterial activity of complexes increases in the rows Zn(II), Cd(II), Hg(II), and this corresponds to the previously reported data about antibacterial activity of Zn(II), Cd(II), and Hg(II) complexes with glibenclamide or 2,6-bis(benzimidazol-2-yl) pyridine ligands [32,33]. Thus, the most effective antibacterial complex proved to be 9Hg. The antibacterial activity of this complex is almost equal to that of the reference antibiotics ampicillin and gentamicin.

3.4. Theoretical Studies of Intermolecular Hydrogen Bonding Interactions N–H···O, O–H···Cl, and N–H···Br in 9Hg, 9Hg’, 9Zn, and 10Cd

In order to confirm or disprove the hypothesis on the existence of various most interesting intermolecular hydrogen bonding interactions N–H···O, O–H···Cl, and N–H···Br in 9Hg, 9Hg’, 9Zn, and 10Cd, and approximately quantify the strength of these supramolecular contacts from a theoretical viewpoint, the DFT calculations followed by the topological analysis of the electron density distribution within the QTAIM approach [28] were carried out at the ωB97XD/SDD level of theory for model supramolecular associates (see computational details and attached xyz-files in Supplementary Materials). Results of QTAIM analysis summarized in Table 5, the contour line diagrams of the Laplacian of electron density distribution ∇²ρ(r), bond paths, and selected zero-flux surfaces, visualization of electron localization function (ELF), and reduced density gradient (RDG) analyses for the selected intermolecular hydrogen bonding interactions N–H···O, O–H···Cl, and N–H···Br in 9Hg and 10Cd are shown in Figure 5 for illustrative purposes.

Table 5. Characteristics of H-bonds in 9Hg, 9Hg’, 9Zn, and 10Cd *.

Contact	r(A…D)	AHD	ρ(r)	∇2ρ(r)	λ2	Hb	V(r)	G(r)	Eint
9Hg
N–H···O	2.747(3)	167.5	0.031	0.135	−0.031	0.002	−0.030	0.032	9.4
O–H···Cli	3.129(3)	163.1	0.020	0.063	−0.020	0.000	−0.016	0.016	5.0
9Hg’
O–H···Cl	3.124(10)	162.6	0.020	0.064	−0.020	0.000	−0.016	0.016	5.0
9Zn
N–H···O	2.750(3)	165.6	0.030	0.124	−0.030	0.002	−0.028	0.030	8.8
Ni–Hi···Oi	2.750(3)	165.6	0.031	0.129	−0.031	0.002	−0.029	0.031	9.1
O–H···Cli	3.104(3)	165.0	0.022	0.068	−0.022	0.000	−0.017	0.017	5.3
Oi–Hi···Cl	3.104(3)	165.0	0.020	0.064	−0.020	0.000	−0.016	0.016	5.0
10Cd
N–H···Br	3.282(5)	173.7	0.020	0.061	−0.020	0.000	−0.015	0.015	4.7

* r(A…D)—bond distance (Å); AHD—bond angle (°); ρ(r)—values of the density of all electrons, ∇²ρ(r)—Laplacian of electron density, λ₂—appropriate eigenvalues, H_b—energy density, V(r)—potential energy density, G(r)—Lagrangian kinetic energy (a.u.) at the bond critical points (3, –1), corresponding to intermolecular hydrogen bonding interactions and strength for these contacts E_int (kcal/mol) estimated as −V(r)/2 [34]. Symmetry codes to generate equivalent atoms in crystal structures: (i) 1–x, 1–y, 1–z.

Figure 5. Contour line diagram of the Laplacian of electron density distribution ∇²ρ(r), bond paths, and selected zero-flux surfaces (left panel), visualization of electron localization function (ELF, center panel), and reduced density gradient (RDG, right panel) analyses for intermolecular hydrogen bonding interactions N–H···O and O–H···Cl in 9Hg. Bond critical points (3, –1) are shown in blue, nuclear critical points (3, –3)—in pale brown, ring critical points (3, +1)—in orange, cage critical points (3, +3)—in light green, bond paths are shown as pale brown lines, length units—Å, and the color scale for the ELF and RDG maps is presented in a.u.

The QTAIM analysis of model supramolecular associates demonstrates the presence of intermolecular hydrogen bonding interactions N–H···O, O–H···Cl, and N–H···Br in 9Hg, 9Hg’, 9Zn, and 10Cd (Table 5). The low magnitude of the electron density (0.020–0.031 a.u.), positive values of the Laplacian of electron density (0.061–0.135 a.u.), and zero or very close to zero positive energy density (0.000–0.002 a.u.) in these bond critical points (3, –1) as well as the estimated strength of appropriate supramolecular contacts (4.7–9.4 kcal/mol) are typical for hydrogen bonds in such chemical systems [23,35,36,37]. The balance between the Lagrangian kinetic energy G(r) and potential energy density V(r) at the bond critical points (3, –1) reveals that a covalent contribution in all discussed intermolecular hydrogen bonding interactions N–H···O, O–H···Cl, and N–H···Br in 9Hg, 9Hg’, 9Zn, and 10Cd is absent, and a sign of λ₂ indicates that these supramolecular contacts are attractive.

4. Conclusions

Overall, we reported the synthesis and characterization of novel triarylazoimidazoles and their group 12 metal complexes. Two triarylazoimidazoles (8 and 10) and four of their metal complexes (9Zn, 9Hg, 9Hg, 10Cd) were structurally characterized. Novel complexes exhibit red photoluminescence emission (Φ up to  0.21) in a solution. Moreover, the antibacterial activity of complexes was tested against Gram-positive microorganism S. aureus and Gram-negative microorganism E. coli.