Six cobalt(II), zinc(II), nickel(II) and copper(II) complexes based on bis-benzimidazolyl bidentate ligands with phenolyl ether linkers: synthesis, structural studies and recognition of HSO₄⁻†

Zhixiang Zhao, Yue Yang, Linhai Hu, Jianhua Wang, Jie Yu and Qingxiang Liu*
Tianjin Key Laboratory of Structure and Performance for Functional Molecules, College of Chemistry, Tianjin Normal University, Tianjin 300387, P. R. China. E-mail: tjnulqx@163.com

First published on 12th August 2024

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1. Introduction

Transition metals and pre-designed organic ligands can self-assemble into the desired functional metal–organic frameworks (MOFs) by controlling the crystallization process. There are many factors affecting the construction of MOFs, such as ligands, metals, solvents, temperature, etc.^1–3 Among these factors, the organic ligand is one of the most important factors. Ligands bearing N-donors play an important role, in which the structure unit with nitrogen atoms mainly includes pyridine, imidazole, benzimidazole, triazole and pyrrole.^4–7 These ligands can use nitrogen atoms to coordinate with various transition metals to form 1D to 3D MOFs. MOFs have been used in some fields, for example, magnetism, adsorption and luminescence.^8–12 In addition, the application of MOFs as hosts in host–guest chemistry has also attracted the attention of researchers.^13–16 Compared to other types of hosts, MOFs contain multi-binding sites for guests, such as nitrogen atoms, π systems, metal ions and active hydrogen.^17–21 The advantages of MOFs are that they are easy to prepare and have good stability to air or moisture. HSO₄⁻ as a common ion in everyday life takes part in many life processes of humans, animals and plants. For example, cell growth and organism development can be promoted by bisulfate.^22,23 Bisulfate at low pH can produce sulfuric acid that has an irritating effect on the eyes, skin and respiratory tract.^24,25 Bisulfate at high pH can produce sulfate ions that pollute the environment.^26–28 Therefore, the detection of HSO₄⁻ is of great importance in life science and environmental science. Although some detection methods for HSO₄⁻ have been reported,^29–32 the development of new methods remains desirable.

In this paper, we reported three new bis-benzimidazolyl bidentate ligands with phenolyl ether linkers (Scheme 1), 1,n-bis[2-(1H-1,3-benzimidazol-1-ylmethyl)phenoxy]alkane (L₁: n = 1, L₂: n = 3, L₃: n = 4), and their six metal complexes (two cobalt(II) complexes ([Co(L₁)(NO₃)₂] (1) and [Co(L₃)(L_A)]_n (6) (L_A = maleate)), two copper(II) complexes ([Cu(L₁)(SO₄)]₂ (4) and [Cu(L₃)(SO₄)]₂ (5)), one zinc(II) complex ([Zn(L₁)(NO₃)₂] (2)) and one nickel(II) complex ([Ni(L₂)(NO₃)₂] (3)). The conformations of complexes 1–6 were analyzed. Additionally, the recognition performance of complex 1 for HSO₄⁻ was studied through fluorescence spectra, UV/vis spectra, ¹H NMR titrations, HRMS spectra and IR spectra.

2. Experimental

2.1. Preparation of 1,1-bis[2-(1H-1,3-benzimidazol-1-ylmethyl)phenoxy]methane (L₁)

Diiodomethane (10.000 g, 0.036 mol), salicylaldehyde (10.552 g, 0.087 mol) and K₂CO₃ (11.942 g, 0.087 mol) were added to 80 mL of DMF, and the suspension was kept at 80 °C in N₂ with stirring for 24 hours. After the reaction ended, the suspension was poured into 0.5 L of cold water to precipitate the solid. A light yellow powder of 1,1-bis[2-(formyl)phenoxy]methane was obtained through filtering. ¹H NMR (400 MHz, CDCl₃): δ 10.46 (s, 2H, CHO), 7.85–7.88 (m, 2H, PhH), 7.61–7.63 (m, 2H, PhH), 7.37 (d, J = 8.4 Hz, 2H, PhH), 7.17 (t, J = 7.5 Hz, 2H, PhH), 6.01 (s, 2H, OCH₂O). Yield: 8.362 g (87%). M.p: 132–133 °C.

1,1-Bis[2-(formyl)phenoxy]methane (5.000 g, 19.5 mmol) was dissolved in 150 mL of mixed solvent (THF/C₂H₅OH = 1:1), and then sodium borohydride (1.919 g, 50.7 mmol) was added to the above solution within 5 times. Because the process was exothermic, an ice bath was required. After the solution was refluxed for 10 hours, it was poured into 400 mL of water to precipitate a white solid. The solid was collected by filtration to give a white powder of 1,1-bis[2-(hydroxymethyl)phenoxy]methane. ¹H NMR (400 MHz, CDCl₃): δ 7.29–7.35 (m, 4H, PhH), 7.19 (d, J = 8.1 Hz, 2H,), 7.06 (t, J = 7.4 Hz, 2H, PhH), 5.87 (s, 2H, OCH₂), 4.63 (d, J = 3.4 Hz, 4H, CH₂OH), 2.04 (s, 2H, CH₂OH). Yield: 4.688 g (92%). M.p: 113–114 °C.

SOCl₂ (9.142 g, 76.8 mmol) was slowly added to a CHCl₃ (90 mL) solution of 1,1-bis[2-(hydroxymethyl)phenoxy]methane (5.000 g, 19.2 mmol) via a constant pressure drip funnel, and then the solution reacted at 65 °C for 12 hours. The resulting yellow solution was poured into ice water (300 mL), and then saturated sodium carbonate was added to remove HCl and excess SOCl₂. The CHCl₃ layer was separated through a separatory funnel and washed three times with water and dried with anhydrous MgSO₄. After the solvent was removed, a white powder of 1,1-bis[2-(chloromethyl)phenoxy]methane was obtained. ¹H NMR (400 MHz, CDCl₃): δ 7.32–7.37 (m, 6H, PhH), 7.02–7.06 (m, 2H, PhH), 5.88 (s, 2H, OCH₂), 4.63 (s, 4H, CH₂Cl). Yield: 4.635 g (81%). M.p: 106–108 °C.

KOH (1.510 g, 26.9 mmol), benzimidazole (1.740 g, 14.8 mmol) and TBAB (0.217 g, 0.7 mmol) were added to 80 mL of acetonitrile, and the solution was heated to 82 °C for 2 hours. The acetonitrile solution (70 mL) of 1,1-bis[2-(chloromethyl)phenoxy]methane (2.000 g, 6.7 mmol) was added to the above solution and stirred for 72 hours at 82 °C. After the reaction was completed, the solution was poured into 400 mL of water to precipitate a white solid. A white powder of 1,1-bis[2-(1H-1,3-benzimidazol-1-ylmethyl)phenoxy]methane (L₁) was obtained by filtration. ¹H NMR (400 MHz, DMSO-d₆): δ 5.37 (s, 4H, PhCH₂), 6.05 (s, 2H, OCH₂), 6.97–7.01 (m, 2H, ArH), 7.09–7.15 (m, 4H, ArH), 7.18 (d, J = 4.0 Hz, 2H, PhH), 7.27–7.30 (m, J = 4.0 Hz, 2H, bimiH), 7.42 (d, J = 8.0 Hz, 2H, bimiH), 7.62 (d, J = 8.0 Hz, 2H, bimiH), 8.28 (s, 2H, 2-bimiH). IR (KBr, cm⁻¹): 3423w, 3075w, 2971w, 2913w, 2360m, 1648w, 1606w, 1495vs, 1457s, 1362m, 1222m, 1286m, 1206m, 1187m, 1023s, 769s, 738s, 474w. Yield: 2.341 g (90%). M.p: 165–167 °C. Anal. calcd. for C₂₉H₂₄N₄O₂: C, 75.63; H, 5.25; N, 12.16. Found: C, 75.84; H, 5.59; N, 12.54.

2.2. Preparation of 1,3-bis[2-(1H-1,3-imidazol-1-ylmethyl)phenoxy]propane (L₂)

The synthesis method of 1,3-bis[2-(1H-1,3-benzimidazol-1-ylmethyl)phenoxy]propane (L₂) was similar to L₁, only with 1,3-dibromopropane instead of diiodomethane. ¹H NMR (400 MH_Z, DMSO-d₆): δ 2.21 (t, J = 5.8, 2H, CCH₂C), 4.08 (t, J = 5.6, 4H, OCH₂), 5.44 (s, 4H, PhCH₂), 6.89 (t, J = 7.4, 2H, bimiH), 6.96 (d, J = 8.0, 2H, bimiH), 7.158 (q, J = 4.7, 4H, bimiH), 7.21 (d, J = 7.6, 2H, PhH), 7.26 (t, J = 7.6, 2H, PhH), 7.49 (t, J = 4.4, 2H, PhH), 7.64 (t, J = 4.4, 2H, PhH), 8.27 (s, 2H, 2-bimiH). IR (KBr, cm⁻¹): 3434m, 3058w, 2934w, 2360m, 1604m, 1498vs, 1457s, 1364m, 1285s, 1252s, 1120m, 1052 m, 750s, 735vs, 420m. Yield: 2.743 g (91%). M.p.: 170–172 °C. Anal. calcd. for C₃₁H₂₈N₄O₂: C, 76.20; H, 5.77; N, 11.46. Found: C, C, 76.53; H, 6.11; N, 11.85.

2.3. Preparation of 1,4-bis[2-(1H-1,3-benzimidazol-1-ylmethyl)phenoxy]butane (L₃)

The synthesis method of 1,4-bis[2-(1H-1,3-benzimidazol-1-ylmethyl)phenoxy]butane (L₃) was similar to L₁, only with 1,4-dibromobutane instead of diiodomethane. ¹H NMR (400 MH_Z, DMSO-d₆): δ 1.91 (s, 4H, CCH₂CH₂C), 4.07 (s, 4H, OCH₂), 5.44 (s, 4H, PhCH₂), 6.88 (t, J = 7.4, 2H, bimiH), 7.03 (d, J = 8.0, 2H, bimiH), 7.14 (q, J = 8.3, 6H, bimiH, PhH), 7.26 (t, J = 7.8, 2H, PhH), 7.51 (d, J = 7.2, 2H, PhH), 7.62 (d, J = 7.6, 2H, PhH), 8.27 (s, 2H, 2-bimiH). IR (KBr, cm⁻¹): 3440w, 3050w, 2942w, 2361w, 1600m, 1491vs, 1455s, 1367m, 1285s, 1254s, 1197w, 1109m, 993m, 749vs. Yield: 2.640 g (89%). M.p.: 185–187 °C. Anal. calcd. for C₃₂H₃₀N₄O₂: C, 76.47; H, 6.02; N, 11.15. Found: C, 76.09; H, 6.38; N, 11.36.

2.4. Complex [Co(L₁)(NO₃)₂] (1)

The DMF (5 mL) solution of L₁ (0.050 g, 0.1 mmol) and the CH₃OH (15 mL) solution of Co(NO₃)₂·6H₂O (0.057 g, 0.3 mmol) were mixed, and the mixture was stirred for 60 minutes at 40 °C. After filtration, the filtrate was placed in a small beaker. The small beaker was sealed with a sealing film and several air holes were poked in the sealing film. The single crystals of 1 were obtained by slowly evaporating the solvents. Anal. calcd. for C₂₉H₂₄CoN₆O₈: C, 54.13; H, 3.76; N, 13.06%. Found: C, 54.43; H, 3.58; N, 13.34%. IR (KBr, cm⁻¹): 3435m, 3116w, 2357m, 2336m, 1615m, 1512vs, 1385s, 1290s, 1221m, 1190m, 1084m, 1115w, 1015s, 743s, 424w. Yield: 0.043 g (62%). M.p.: 246–248 °C.

The synthesis methods of complexes 2–5 were analogous to the synthesis method of 1, and the differences were that the different ligands and metal salts were used (L₁ and Zn(NO₃)₂·6H₂O for 2, L₂ and Ni(NO₃)₂·6H₂O for 3, L₁ and CuSO₄·5H₂O for 4, and L₃ and CuSO₄·5H₂O for 5).

2.5. Complex [Zn(L₁)(NO₃)₂] (2)

Anal. calcd. for C₂₉H₂₄N₆O₈Zn: C, 53.59; H, 3.72; N, 12.93%. Found: C, 53.68; H, 3.53; N, 12.72%. IR (KBr, cm⁻¹): 3447s, 3120w, 2357m, 1652m, 1480s, 1384vs, 1295s, 1221m, 1192m, 1113w, 1014m, 752m, 466w. Yield: 0.041 g (59%). M.p.: 280–282 °C.

2.6. Complex [Ni(L₂)(NO₃)₂] (3)

Anal. calcd. for C₆₂H₅₆N₁₂O₁₆Ni₂: C, 55.46; H, 4.20; N, 12.51%. Found: C, 55.32; H, 4.61; N, 12.41%. IR (KBr, cm⁻¹): 3440m, 3124w, 2942w, 2361m, 1598m, 1511vs, 1483vs, 1385vs, 1264s, 1188m, 1121m, 1051m, 1017m, 748s, 719m, 495m, 424 m. Yield: 0.079 g (58%). M.p.: 268–270 °C.

2.7. Complex [Cu(L₁)(SO₄)]₂ (4)

Anal. calcd. for C₃₂H₃₁N₅O₇SCu: C, 55.44; H, 4.50; N, 10.10%. Found: C, 55.32; H, 4.43; N, 10.05%. IR (KBr, cm⁻¹): 3523m, 3444m, 3097s, 2799m, 2482m, 1599m, 1461s, 1174s, 1064vs, 1024vs, 965vs, 885s, 836s, 796s, 747s, 647s, 588vs, 509s. Yield: 0.041 g (55%). Anal. M.p.: >320 °C.

2.8. Complex [Cu(L₃)(SO₄)]₂ (5)

Anal. calcd. for C₃₃H₃₄N₄O₇SCu: C, 57.09; H, 4.93; N, 8.07%. Found: C, 57.41; H, 4.73; N, 8.32%. IR (KBr, cm⁻¹): 3434s, 3116m, 2929m, 2681w, 2494w, 2361m, 1635m, 1602m, 1513s, 1458m, 1387m, 1300 m, 1241s, 1136vs, 1042m, 976m, 939m, 753s, 657m, 611m, 574m, 499w. Yield: 0.047 g (59%). M.p.: 260–264 °C.

2.9. Complex [Co(L₃)(L_A)₂] (6)

The synthesis method of complex 6 was analogous to the synthesis method 1, and the difference was that L₃ and CoSO₄·7H₂O were used. Additionally, maleic acid (H₂L_A) was also added as a co-ligand, and triethylamine was used to control the pH at 7. Anal. calcd. for C₃₆H₃₂N₄O₆Co: C, 64.00; H, 4.77; N, 8.29%. Found: C, 64.31; H, 4.52; N, 8.44%. IR (KBr, cm⁻¹): 3440m, 3100w, 3058w, 2942m, 2871w, 2361m, 1613vs, 1557s, 1461s, 1340vs, 1255s, 1188m, 1026m, 976m, 750s, 686m, 499w. Yield: 0.042 g (63%). M.p.: >320 °C.

2.10. Fluorescence titrations

An RF-5301PC fluorescence spectrophotometer was used to record emission spectra from 270 nm to 380 nm. The test solutions were prepared in CH₃CN/H₂O (v/v = 1:1). Complex 1 (5 × 10⁻⁶ mol L⁻¹) was added into a 4 mL cuvette, and guest HSO₄⁻ (0–15 × 10⁻⁵ mol L⁻¹) was gradually added applying a microsyringe for fluorescence titrations. The excitation wavelength was 248 nm (the emission and excitation slits: 3 nm and 5 nm). After each addition, it was left for 8–10 min, and then the fluorescence intensity was recorded. Origin 8.0 was used for statistical analysis of the data.

2.11. UV/vis titrations

A JASCO-V570 spectrometer was used to record absorption spectra from 200 nm to 400 nm in CH₃CN/H₂O (v/v = 1:1). The test solutions in the UV experiments were prepared in a similar way to that of the fluorescence experiments. The concentrations of complex 1 and guest HSO₄⁻ in the test solutions were 5.0 × 10⁻⁶ mol L⁻¹ and 0–35 × 10⁻⁵ mol L⁻¹, respectively. After each addition, it was left for 8–10 min, and then the absorption spectra were recorded. Origin 8.0 was used to process the data.

2.12. Job's plot

In the Job's plot experiment, a JASCO-V570 spectrometer was used to record absorption spectra from 200 nm to 400 nm in CH₃CN/H₂O (v/v = 1:1). The overall concentration was maintained at 5.0 × 10⁻⁶ mol L⁻¹, and the molar fraction of guest HSO₄⁻ was changed from 0 to 1. In the procedure of preparation of test solutions, the different amounts of host 1 and guest HSO₄⁻ solutions were placed into a 10 mL volumetric flask using a microsyringe, and then diluted to 10 mL. After each mixture, it was left for 8–10 min, and then the absorption spectra were recorded. The Job's plot working curve was obtained by fitting the data by using Origin 8.0.

3. Results and discussion

3.1. Synthesis and general characterization of ligands L₁–L₃ and complexes 1–6

Salicylic aldehyde was reacted with CH₂I₂ or Br(CH₂)_nBr (n = 3 and 4), and then NaBH₄ was used to reduce the aldehyde groups to give 1,n-bis(2-hydroxymethylphenoxy)alkane. After chlorination of hydroxyl groups, 1,n-bis(2-(chloromethyl)phenoxy)alkane was obtained. Finally, benzimidazole was reacted with 1,n-bis(2-(chloromethyl)phenoxy)alkane to give ligands L₁–L₃ (Scheme 2).

Complexes 1–6 were synthesized through the reactions of the corresponding ligands and metal salts in DMF/CH₃OH (v/v = 1:3) (L₁ and Co(NO₃)₂·6H₂O for [Co(L₁)(NO₃)₂] (1), L₁ and Zn(NO₃)₂·6H₂O for [Zn(L₁)(NO₃)₂] (2), L₂ and Ni(NO₃)₂·6H₂O for [Ni(L₂)(NO₃)₂] (3), L₁ and CuSO₄·5H₂O for [Cu(L₁)(SO₄)]₂ (4), and L₃ and CuSO₄·5H₂O for [Cu(L₃)(SO₄)]₂ (5). Complex [Co(L₃)(L_A)]_n (6) was prepared by the reaction of L₃ and CoSO₄·7H₂O; meanwhile, a maleate group (L_A) was used as a co-ligand. The single crystals of 1–6 were obtained by slowly evaporating the solvents.

3.2. Structures of complexes 1–6

The crystal systems of 1 and 2 were found to be monoclinic, and the corresponding data corresponded with the P2₁/c space group. In the molecular structures of 1 or 2 (Fig. 1(a) and 2(a)), each of them contained one 16-membered macrocycle formed by one ligand L₁ and one metal ion (Co(II) ion for 1 and Zn(II) ion for 2). In 1 or 2, each metal ion (Co(II) or Zn(II)) was hexa-coordinated with two nitrogen atoms from two benzimidazole rings and four oxygen atoms from two NO₃⁻ ions to adopt an octahedral geometry. Two benzimidazole rings formed the dihedral angles of 89.1(2)° for 1 and 89.2(2)° for 2, and the dihedral angles of two couples of the adjacent benzimidazole ring and benzene ring were 79.9(2)° and 73.8(1)° for 1, and 82.0(1)° and 74.4(1)° for 2.

In the Co(II)-centered octahedron for 1, the axial position was occupied by O(7) and N(4), and the axial distances were 2.048(2) Å and 2.214(2) Å. The N(4)–Co(1)–O(7) angle was 152.5(8)°. N(1), O(6), O(4) and O(3) were located at the equatorial plane. The Co–N and Co–O distances were 2.032(2)–2.048(2) Å and 2.103(2)–2.267(3) Å. The O–Co–N and O–Co–O angles were 93.3(9)–152.5(7)° and 55.7(1)–137.0(9)°. These values were analogous to the values of reported Co(II) complexes.^33,34

In the Zn(II)-centered octahedron for 2, the axial positions were occupied by O(7) and N(1), and the axial distances were 2.035(1) Å and 2.168(1) Å. The N(1)–Zn(1)–O(7) angle was 148.7(7)°. N(4), O(3), O(4) and O(6) were located at the equatorial plane. The distances of Zn–N and Zn–O were 2.012(1)–2.032(1) Å and 2.032(1)–2.691(2) Å. The O–Zn–O and N–Zn–O angles were 58.4(6)–103.6(8)° and 90.3(7)–148.7(7)°. These values were analogous to the values of reported Zn(II) complexes.^33,35

Complex 3 contained one 18-membered macrocycle formed by one ligand L₂ and one Ni(II) ion (Fig. 3(a)). The Ni(II) ion was hexa-coordinated with two nitrogen atoms from two benzimidazole rings and four oxygen atoms from two NO₃⁻ ions to adopt an octahedral geometry. The crystal system of 3 was triclinic, and the corresponding data corresponded with the P space group. Two benzimidazole rings in 3 formed a dihedral angle of 78.4(1)°. Two benzene rings in the same ligand formed a dihedral angle of 79.1(1)°. The dihedral angles of two couples of the adjacent benzimidazole ring and benzene ring were 77.5(1)° and 72.3(9)°. Four O atoms from two NO₃⁻ and two N atoms from two benzimidazoles coordinated with the Ni(II) ion to form an octahedron. In this octahedron, the axial positions were occupied by O(7) and N(1), and the axial distances were 2.016(1) Å and 2.199(1) Å, and the O(7)–Ni(1)–N(1) angle was 155.9(6)°. N(4), O(3), O(6) and O(4) were located in the equatorial plane. The bond distances of Ni–N and Ni–O were 2.016(1)–2.019(1) Å and 2.062(1) Å–2.199(1) Å. The O–Ni–O and N–Ni–O bond angles were 60.6(5)–153.4(5)° and 88.8(6)–155.9(6)°. These values were analogous to the values of reported Ni(II) complexes.³⁶

The crystal systems of 4 and 5 were triclinic, and the corresponding data corresponded with the P space group. Complexes 4 and 5 had similar molecular structures (Fig. 4(a) and 5(a)), where two macrometallocycles (16-membered rings for 4 and 19-membered rings for 5) were linked by two SO₄²⁻. Each macrometallocycle was constructed from one ligand (L₁ for 4 and L₃ for 5) and one Cu(II) ion, and an inversion center was observed in each of them. In 4 or 5, the Cu(II) atom was penta-coordinated with two nitrogen atoms from two benzimidazoles and three oxygen atoms from two SO₄²⁻ (one SO₄²⁻ provided two oxygen atoms, while the other SO₄²⁻ provided one oxygen atom) to adopt a pentagonal bipyramid. In 4 or 5, the axial positions were occupied by O(4) and N(4), and the axial distances were 1.963(8) Å and 2.005(8) Å for 4 and 1.984(2) Å and 2.008(1) Å for 5. The angles of (4)–Cu(1)–O(4) were 165.1(4)° for 4 and 162.0(9)° for 5. N(1), O(3) and O(5) atoms for 4 and N(1), O(3) and O(6) atoms for 5 occupied the equatorial plane. The distances of Cu–N and Cu–O were 1.963(8)–2.031(9) Å and 1.997(1) Å–2.296(2) Å. The O–Cu–O, N–Cu–N and N–Cu–O bond angles were 70.5(3)–122.3(3)°, 93.9(3)–100.1(9)° and 84.5(8)–165.1(4)°. The Cu⋯Cu separation was 4.181(3) Å for 4 and 4.380(8) Å for 5. These values were analogous to the values of reported Cu(II) complexes.^33,37 The dihedral angle between benzimidazole rings in the same ligand was 84.0(1)° for 4 and 65.4(3)° for 5. In the same ligand, the dihedral angle between two benzene rings was 84.2(1)° for 4 and 65.2(2)° for 5, and the dihedral angles of two couples of the adjacent benzimidazole ring and benzene ring were 87.0(1)° and 85.6(7)° for 4 and 74.5(2)° and 76.7(3)° for 5.

The crystal system of 6 was found to be monoclinic, and the corresponding data corresponded with the P2₁/n space group. Analysis of the crystal structure of complex 6 showed that 19-membered macrometallocycles formed by one Co(II) ion and one L₃ were connected together by maleate groups to form a 1D polymeric chain (Fig. 6(a)). In each 19-membered macrometallocycle, two benzimidazole rings formed a dihedral angle of 58.3(1)°, and two benzene rings formed a dihedral angle of 51.0(2)°. The dihedral angles of two couples of the adjacent benzimidazole ring and benzene ring were 89.2(1)° and 70.8(1)°. In 6, the Co(II) ion was penta-coordinated with two nitrogen atoms from two benzimidazole rings and three oxygen atoms from two maleate groups (one maleate group provided two oxygen atoms, while the other maleate group provided one oxygen atom) to adopt a pentagonal bipyramidal geometry. In the pentagonal bipyramid, the axial positions were occupied by O(4) and N(1), and the axial distances were 2.054(1) and 2.366(1) Å, and the O(4)–Co(1)–N(1) angle was 160.8(5)°. The distances of Co–O were from 1.939(1) Å to 2.028(1) Å), and the N–Co distances were 2.023(1)–2.054(1) Å. The O–Co–N and O–Co–O angles were 84.1(6)–160.8(5)° and 59.2(5)–113.5(6)°. The N(4)–Co(1)–N(1) angle was 101.4(6)°. These values were analogous to the values of reported Co(II) complexes.^33,34

3.3. The crystal packings of complexes 1–6

The 3D architecture of 1 (Fig. 1(b)) was formed via C–20H⋯O H-bonds³³ and π–π stacking interactions³⁸ between benzimidazoles (Tables S3 and S4 in the ESI†). In the H-bonds, oxygen atoms were from NO₃⁻ and H atoms were from CH₂ of ether chains.

In the crystal packing of complex 2 (Fig. 2(b)), C–H⋯O H-bonds and two types of π–π stacking interactions were the main bonding forces for the formation of the 2D supramolecular layer. The first type of π–π interaction was between benzimidazoles, and the second type was between benzenes. In the H-bonds, oxygen atoms were from NO₃⁻ and H atoms were from CH₂ of ether chains.

In the crystal packing of 3 (Fig. 3(b)), the 3D network was constructed via C–H⋯O H-bonds and π–π stacking interactions between benzimidazoles. In the H-bonds, oxygen atoms were from NO₃⁻ and H atoms were from CH₂ of ether chains.

In the crystal packing of 4, the 2D layer (Fig. 4(b)) was constructed via π–π stacking interactions between benzimidazoles. The 2D layer was further expanded to become a 3D architecture through the C–H⋯π contacts³⁹ (the distance of H⋯π was 2.751(1) Å and the angle of C–H⋯π was 128.8(7)°) (Fig. 4(c)). In the C–H⋯π contacts, the hydrogen atoms were from CH₂ and the π systems were from benzene rings.

The 2D layer of 5 was formed through the C–H⋯O H-bonds and π–π stacking interactions between benzenes. In the H-bonds (Fig. 5(b)), oxygen atoms were from SO₄²⁻ and H atoms were from benzimidazole rings.

The 2D layer of 6 (Fig. 6(b)) was formed by the π–π stacking interactions between benzimidazoles, and the 2D layer was further extended into a 3D network via the C–H⋯O H-bonds (Fig. 6(c)). In the H-bonds, oxygen atoms were from maleate and H atoms were from CH₂ of the benzene ring.

3.4. The conformations of complexes 1–6

Complexes 1–6 mainly contained three types of conformations (Scheme 3), namely, (1) the macrometallocycle (I) formed by one bis-benzimidazolyl bidentate ligand and one metal ion (like complexes 1–3); (2) the dimer (II) formed by two macrometallocycles via using sulfate as a bridge, where each macrometallocycle was formed by one bis-benzimidazolyl bidentate ligand and one metal ion (like complexes 4 and 5); (3) the 1D polymeric chain (III) formed by macrometallocycles via using maleate groups as bridges (like complex 6). On the whole, the different conformations of complexes 1–6 were mainly related to the length of the ether chain, metal ions, counter-ions and the steric hindrance around metal ions.

3.5. The solid-state fluorescence spectra of ligands L₁–L₃ and complexes 1–6

The solid-state fluorescence spectra of ligands L₁–L₃ and complexes 1–6 were measured at room temperature as indicated in Fig. S1–S3† (λ_ex = 300 nm). Ligands L₁–L₃ showed strong emission bands in the regions of 325–550 nm, which may be attributed to intraligand n–π* and π–π*.^40–42 The fluorescence intensity of complexes 1–6 was reduced obviously compared with their ligands, which may be ascribed to the metal perturbing intraligand processes.^43,44

3.6. IR spectra analysis of complexes 1–6

In complexes 1–6, the bands in 1260–1230 cm⁻¹ may result from ν_s(C–C) or ν_s(C–N) of benzene rings or imidazole rings. The strong absorption bands at around 1280–950 cm⁻¹ originated from ν(C–O) of ether chains. The bands at around 1620–1600 cm⁻¹ may result from ν(CN) of benzimidazole rings. The bands in the 1680–1500 cm⁻¹ region may be ascribed to the CC stretching vibrations. The absorption bands at around 3100–2850 cm⁻¹ can be assigned to aromatic ν(C–H) modes. These values were analogous to their ligands. The bands at 3447–3423 cm⁻¹ may be attributed to ν(O–H) of coordinated H₂O. The peaks at around 1650 cm⁻¹ may be attributed to ν(N–O) of NO₃⁻ for 1–3. The peaks at 1074 cm⁻¹ and 1109 cm⁻¹ may be ascribed to the stretching vibrations of S–O of SO₄²⁻ for 4 and 5. The strong band of CO at 1617 cm⁻¹ was observed for 6.

3.7. Powder X-ray diffraction of complexes 1–6

In order to establish the crystalline phase purity of complexes 1–6, powder X-ray diffraction (PXRD) experiments were carried out (Fig. S4–S9†). The agreement between the experimental PXRD patterns of the bulk samples 1–6 and the patterns simulated from the single-crystal data of 1–6 proved the crystalline phase purity of these complexes.

3.8. Thermogravimetric analyses for complexes 1–6

To examine the thermal stability of 1–6, thermogravimetric (TG) analyses for crystal samples were performed under a flowing nitrogen atmosphere with a heating rate of 20 °C min⁻¹ from ambient temperature up to 800 °C (Fig. S10–S15†). The TG analysis for 1 (Fig. S10†) indicated that complex 1 kept stable up to 182.1 °C, and then experienced two steps of thermal decomposition of the organic components from 182.1 °C to 297.6 °C with a weight loss of 23.84%, which did not stop until heating ended at 800 °C. The TG analyses for 2, 3 and 6 (Fig. S11, S12 and S15†) exhibited similar thermal behavior. These three complexes had high thermal stability, and they remained stable up to 272.8 °C for 2, 280.3 °C for 3 and 320.5 °C for 6, and they experienced a one-step weight loss of 19.93% from 272.8 °C to 389.5 °C for 2, 38.17% from 280.3 °C to 465.9 °C for 3 and 80.79% from 320.5 °C to 486.8 °C for 6. These weight losses were attributed to the thermal decomposition of the organic components, which did not stop until heating ended at 800 °C. The TG curve of 4 (Fig. S13†) revealed a weight loss of 1 equivalent of DMF (calcd: 10.54%, found: 10.19%) from 267.4 °C to 312.9 °C. This complex experienced a weight loss of 25.91% from 312.9 °C to 512.3 °C, attributed to the thermal decomposition of the organic components, which did not stop until heating ended at 800 °C. The TG curve of 5 (Fig. S14†) revealed a weight loss of 4.31% from 62.5 °C to 100.2 °C, which corresponded to the loss of 1 equivalent of CH₃OH (calcd: 4.61%). This complex experienced multiple steps of thermal decomposition of the organic components from 100.2 °C to 517.7 °C with a weight loss of 50.37%. The thermal decomposition of the organic components did not stop until heating ended at 800 °C.

3.9. Recognition of HSO₄⁻ using 1 as a chemosensor

A solution of 5.0 × 10⁻⁶ mol L⁻¹ complex 1 was prepared in H₂O/CH₃CN (v/v = 1:1) at 25 °C. As shown in Fig. 7, complex 1 had a strong fluorescence emission at 275–360 nm (λ_ex = 248 nm, 3 nm/5 nm). When 20 equivalents of Br⁻, F⁻, Cl⁻, I⁻, H₂PO₄⁻, NO₃⁻, OAc⁻ or HP₂O₇³⁻ (their cations being tetrabutylammonium (ⁿBu₄N⁺)) were added to the solution of 1, respectively, the fluorescence intensity (FI) of 1 did not significantly change. But the FI of 1 was quenched after 20 equivalents of HSO₄⁻ were added. We also tested the recognition ability of complexes 2–6 for anions, but complexes 3–6 did not have good selectivity for anions, and complex 2 and complex 1 had similar selectivity for anions. So we only choose complex 1 as a chemosensor to research its recognition ability for anions.

A UV screening experiment was carried out (Fig. S16†), and the UV absorption peak of complex 1 appeared at 240–290 nm. When 20 equivalents of Br⁻, F⁻, Cl⁻, I⁻, H₂PO₄⁻, NO₃⁻, OAc⁻, or HP₂O₇³⁻ were added to the solution of 1, respectively, the absorption of 1 did not obviously change. When 20 equivalents of HSO₄⁻ were added, the UV absorption of 1 had a significant change. These results indicated that complex 1 had good recognition ability towards HSO₄⁻.

Fluorescence titrations were carried out to observe the fluorescence changes when HSO₄⁻ was titrated into the solution of 1. As displayed in the inset of Fig. 8, when the ratio of the concentration of HSO₄⁻ and 1 was less than 18:1, there was a significant decrease in fluorescence intensity. When the ratio was in the range of 18:1 to 24:1, the reduction of fluorescence intensity became slow. When the ratio was greater than 24:1, the addition of HSO₄⁻ had little effect on the fluorescence intensity. This phenomenon followed the traditional Stern–Volmer equation (eqn (1)).⁴⁵

F0/F = 1 + KSVCHSO4−	(1)

In eqn (1), F and F₀ represent the fluorescence intensity without and with HSO₄⁻, C_HSO4⁻ is the concentration of HSO₄⁻, and K_SV is the association constant between 1 and HSO₄⁻. The association constant K_SV for 1·HSO₄⁻ was calculated to be 1.81 × 10⁴ M⁻¹ (R = 0.999) (Fig. S17†), which indicated that there was a strong bonding force between 1 and HSO₄⁻. The detection limit of 1 to HSO₄⁻ was calculated to be 1.40 × 10⁻⁷ mol L⁻¹ (Fig. S18†). This value was in the middle of the values reported in the literature (3.84 × 10⁻⁵–7.5 × 10⁻⁹ mol L⁻¹).^31,46–48

In the UV titration experiments (Fig. S19†), the UV absorption of 1 decreased gradually at 240–258 nm and increased gradually at 258–277 nm with the increase of HSO₄⁻ concentration. These changes corresponded to the Benesi–Hildebrand equation (eqn (2)).⁴⁹

A0/(A0 − A) = [εr/(εr − εc)](1/KCHSO4− + 1)	(2)

In eqn (2), A₀ and A are the UV absorption of 1 without and with HSO₄⁻, C_HSO4⁻ is the concentration of HSO₄⁻, and ε_r and ε_c are the molar extinction coefficients of 1 and 1FHSO₄⁻, respectively. The binding constant K between 1 and HSO₄⁻ was 1.06 × 10⁴ M⁻¹ (R = 0.995) (Fig. S20†), and it was similar to the K_SV from the fluorescence method.

To understand the bonding ratio between 1 and HSO₄⁻, a Job's plot experiment was performed (Fig. 9). The total concentration for both 1 and HSO₄⁻ was maintained at 5.0 × 10⁻⁶ mol L⁻¹. A plot was made with χ as the horizontal coordinate (χ represents the molar fraction of 1) and χΔA as the vertical coordinate (ΔA is the UV absorption difference with and without HSO₄⁻). From Fig. 9, it can be found that χΔA reached the highest point when χ = 0.5, which indicated that the stoichiometric ratio of 1 and HSO₄⁻ was 1:1.

To investigate the fluorescence response time of complex 1 to HSO₄⁻, we performed a fluorescence responsiveness experiment. The solution of 1 (5.0 × 10⁻⁶ mol L⁻¹) and HSO₄⁻ (1.0 × 10⁻⁴ mol L⁻¹) was tested at 0, 30, 60, 90, 120, 150, and 180 seconds, respectively. The fluorescence response time plot of 1 to HSO₄⁻ was drawn using F/F₀ as the vertical coordinate and time (T) as the horizontal coordinate.⁵⁰ From Fig. S21,† we can see that the fluorescence response time of 1 to HSO₄⁻ can reach stability in about 30 seconds, indicating that the action of 1 and HSO₄⁻ was very fast and 1 can be used for the rapid detection of HSO₄⁻.

Interference experiments were carried out to understand the interference of other ions. Firstly, a CH₃CN/H₂O (v/v = 1:1) standard solution of 1 (5.0 × 10⁻⁶ mol L⁻¹) was prepared, and then 20 equivalents of HSO₄⁻ were added to the standard solution. Next, 20 equivalents of F⁻, Cl⁻, I⁻, Br⁻, OAc⁻, H₂PO₄⁻, NO₃⁻, or HP₂O₇³⁻ were added, respectively. We can see from Fig. S22† that the presence of other ions did not significantly interfere with the recognition of 1 towards HSO₄⁻.

Reversible binding of 1 with HSO₄⁻ was also performed in CH₃CN/H₂O (v/v = 1:1) (Fig. S23†). Adding 20 equivalents of Ba(NO₃)₂ into a mixed solution of 1 (5 × 10⁻⁶ mol L⁻¹) and HSO₄⁻ (1.0 × 10⁻⁴ mol L⁻¹) resulted in the increase of fluorescence intensity at 296 nm. At this time, the fluorescence intensity returned to the emission level of free 1. Upon addition of HSO₄⁻ again, the fluorescence intensity decreased, which meant the regeneration of 1·HSO₄⁻. The alternating addition of HSO₄⁻ and Ba(NO₃)₂ to the solution of 1 caused repeated changes of fluorescence intensity. With a slight loss of fluorescence efficiency, the “on–off–on” switching phenomenon could be repeated several times. These results indicated that 1 was an effective chemosensor for HSO₄⁻ with good reversibility.

3.10. Interactions of complex 1 with HSO₄⁻

To understand the bonding mode of complex 1 and HSO₄⁻, ¹H NMR titration experiments were performed, in which 0.25, 0.5, 0.75, 1.0 and 1.5 equivalents of HSO₄⁻ were respectively added to the solution of complex 1. From the ¹H NMR titration plot (Fig. 10), it was found that upon adding 1 equivalent of HSO₄⁻, the chemical shift of H_a on HSO₄⁻ moved 0.06 ppm to the high field, and the chemical shift of H_b–H_f on complex 1 moved 0.03–0.05 ppm to the low field. When HSO₄⁻ exceeded 1 equivalent, the chemical shift of H_a–H_f remained unchanged, which showed that the stoichiometric ratio of 1 and HSO₄⁻ was 1:1.

MS of 1·HSO₄⁻ was measured to further verify the binding ratio of complex 1 and HSO₄⁻. From Fig. S24,† it was found that the m/z of [1 + HSO₄⁻ + 2H⁺]⁺ was 742.0734, which further verified that the binding ratio between 1 and HSO₄⁻ was 1:1, which was consistent with the results from Job's plot analysis and ¹H NMR titrations.

In the infrared spectra (Fig. S25†), the ν(C–H) peak shifted from 754 cm⁻¹ to 748 cm⁻¹, the ν(C–N) peak shifted from 1224 cm⁻¹ to 1221 cm⁻¹, the ν(CN) peak shifted from 1656 cm⁻¹ to 1670 cm⁻¹, and the ν(CH₂) peak shifted from 2964 cm⁻¹ to 2961 cm⁻¹. The ν(S–O) peak of HSO₄⁻ moved from 884 cm⁻¹ to 887 cm⁻¹, the ν(SO) peak moved from 1239 cm⁻¹ to 1262 cm⁻¹, and the ν(O–H) peak moved from 3379 cm⁻¹ to 3470 cm⁻¹.

By comprehensively analyzing the structure of 1, ¹H NMR titrations, and MS and IR spectra, we could find that HSO₄⁻ was captured by 1 through C–H⋯O and O–H⋯O hydrogen bonds as shown in Scheme 4. When complex 1 was free, the photo-induced electron transfer (PET) process from benzene to benzimidazole was prohibited. As a result, complex 1 had a strong fluorescence emission. When HSO₄⁻ was held by 1, this PET process was switched on, resulting in the reduction of fluorescence intensity.⁵¹

4. Conclusions

In summary, three bis-benzimidazolyl bidentate ligands with phenolyl ether linkers, 1,n-bis[2-(1H-1,3-imidazol-1-ylmethyl)phenoxy]alkane (L₁: n = 1, L₂: n = 3, L₃: n = 4), and their six metal complexes ([Co(L₁)(NO₃)₂] (1), [Zn(L₁)(NO₃)₂] (2), [Ni(L₂)(NO₃)₂] (3), [Cu(L₁)(SO₄)]₂ (4), [Cu(L₃)(SO₄)]₂ (5) and [Co(L₃)(L_A)]_n (6) (L_A = maleate)) were prepared by the reaction of the respective ligands and metal salts. Complexes 1 and 2 had one 16-membered macrometallocycle, and complex 3 had one 18-membered macrometallocycle. In complexes 4 and 5, two macrometallocycles were linked together by two SO₄²⁻ (16-membered ring for 4 and 19-membered ring for 5). In complex 6, 19-membered macrometallocycles were connected together by maleate groups to form a 1D polymeric chain. In complexes 1–6, each macrometallocycle was formed by one metal ion and one ligand. 2D layers and 3D supramolecular architectures of complexes 1–6 were formed via weak intermolecular interactions, such as H-bonds, π–π interactions and C–H⋯π contacts. Three types of conformations of complexes 1–6 were also discussed. Besides, the recognition performance of complex 1 towards HSO₄⁻ was studied through fluorescence spectra, UV/vis spectra, ¹H NMR titrations, MS spectra and IR spectra. The large binding constant (1.81 × 10⁴ M⁻¹) showed the existence of a strong acting force between 1 and HSO₄⁻, and the low detection limit (1.40 × 10⁻⁷ mol L⁻¹) showed the high sensitivity of 1 to HSO₄⁻. Complex 1 had potential application value for the detection of HSO₄⁻ in life science and environmental science. Further studies on new complexes from the analogous ligands are underway.

Data availability

Crystallographic data for complexes 1–6 have been deposited at the CCDC under 2350976–2350979, 2350981 and 2350982.

Conflicts of interest

The authors have no conflicts of interest to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 21572159).

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Footnote

† Electronic supplementary information (ESI) available: CCDC 2350976–2350979, 2350981 and 2350982. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4ce00553h

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