Crystal structures and luminescence properties of two Cd(II) complexes based on 2-(1H-imidazol-1-methyl)-6-methyl-1H-benzimidazole

2.1 General information and materials

All chemicals were of AR grade and used without purification (Sinopharm Chemical Reagent Beijing Co., Ltd., Beijing, China). IR data were recorded on a BRUKER TENSOR 27 spectrophotometer (Bruker Corporation, Billerica, MA, USA) with KBr pellets in the region from 4000 to 400 cm^–1. Elemental analyses (C, H, and N) were carried out on a FLASH EA 1112 elemental analyzer (Thermo Fisher Scientific Company, Waltham, MA, USA). Steady state fluorescence measurements were performed using an F-7000 fluorescence spectrophotometer (Hitachi, Ltd., Chiyoda-ku, Tokyo, Japan) at room temperature in the solid state. TG measurements were performed by heating the sample from 30 °C to 800 °C at 10 °C min^–1 in air on a NETZSCH STA 409 PC/PG differential thermal analyzer (NETZSCH Group, Selb, Bavaria, Germany). Powder X-ray diffraction (PXRD) patterns were recorded using CuK_α radiation on a PANalytical X’Pert PRO diffractometer (PANalytical B.V., Almelo, Overijssel, Netherlands). immb was synthesized according to the literature method [41] with the following modification. 4-Methyl-benzene-1,2-diamine and imidazole-1-ylacetic acid were used instead of O-phenylenediamine and tetranitrazoleacetic acid, respectively, leaving the other experimental conditions unchanged.

2.2 Synthesis of {[Cd(immb)(I)₂]·DMF}_n (1)

CdI₂ (0.05 mmol) and immb (0.05 mmol) were dissolved in a H₂O-CH₃OH-DMF mixed solvent (7 mL, 5:1:1 [v/v/v]). Then the resultant solution was poured into a 25 mL Teflon-lined stainless steel vessel. The vessel was sealed and heated at 100 °C for 72 h. After cooling to room temperature at a rate of 10 °C h^–1, light yellow crystals of 1 suitable for X-ray diffraction were obtained. Yield: 41 %. − Anal. for C₁₅H₁₈CdI₂N₅O (650.54): calcd. C 27.69, H 2.79, N 10.77; found: C 26.96, H 2.65, N 10.84. − FT-IR (KBr, cm^–1): ν = 3442(m), 3115(m), 2923(w), 1654(s), 1519(s), 1492(m), 1434(m), 1387(m), 1093(m), 741(m), 599(m).

2.3 Synthesis of {[Cd₃(immb)(btc)₂]·H₂O}_n (2)

The preparation of 2 was similar to that of 1 except for the extra addition of Na₃btc (0.05 mmol). Light yellow crystals of {[Cd₃(immb)(btc)₂]·H₂O}_n suitable for X-ray analysis were collected. Yield: 45 %. − Anal. for C₃₀H₂₀Cd₃N₄O₁₃ (981.70): calcd. C 36.70, H 2.05, N 5.71; found: C 37.37, H 1.93, N 5.92. − FT-IR (KBr, cm^–1): ν = 3412(m), 3140(m), 2987(w), 1609(s), 1560(s), 1458(m), 1373(m), 1092(s), 773(s), 716(s), 536(w).

2.4 Single-crystal structure determinations

A suitable single crystal of each complex was carefully selected and glued to a thin glass fiber. Intensity data were collected a Rigaku Saturn 724 CCD area detector (Rigaku Corporation, Akishima, Tokyo, Japan) with a graphite monochromator as the X-ray source (MoK_α radiation, λ = 0.71073 Å) operating at 50 kV and 40 mA. The data were collected by the ω-scan mode at 293(2) K; the crystal-to-detector distance was 45 mm. An empirical absorption correction was applied. The data were corrected for Lorentz-polarization effects. The structures were solved by Direct Methods and refined by full-matrix least squares based on F² and difference Fourier techniques using the programs Shelxs/l-97 [42]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms bound to carbon and nitrogen atoms were positioned geometrically and refined using a riding model. In complex 1, the disordered methyl group was modeled by splitting it into two parts. For complex 2, attempts were made to find the hydrogen atoms associated with the water molecule O13 from difference Fourier maps. They could not be located unambiguously, however. All hydrogen atoms were included in the final refinement. Crystal data and numbers pertinent to data collection and structure refinement are summarized in Table 1. Selected bond lengths and bond angles are listed in Table 2, and hydrogen bond parameters in Table 3.

CCDC 998647 (1) and 998648 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre viahttp://www.ccdc.cam.ac.uk/data_request/cif.

3.1 Crystal and molecular structure of {[Cd(immb)I₂]·DMF}_n (1)

The result of the X-ray diffraction analysis has revealed that complex 1 crystallizes in the monoclinic system with space group P2₁/c. As is shown in Fig. 1a, the Cd(II) ion adopts a distorted tetrahedral coordination geometry. Each Cd(II) ion is four-coordinate with two nitrogen atoms (N1, N4^#1) from two immb ligands and two terminal iodide ions (I1, I2). The Cd–N bond lengths (Cd1–N1 = 2.255(7) and Cd1–N4^#1 = 2.240(8) Å) are close to those in [CdI₂(pbbm)]₂ (ppbm = 1,1′-(1,5-pentanediol)bis-1H-benzimidazole, Cd–N: 2.247(3) and 2.299(3) Å) [43]. The bond lengths Cd1–I1 and Cd1–I2 are 2.7019(12) and 2.7447(12) Å, normal for terminal iodide ions. As shown in Fig. 1b, the immb ligands bridge Cd(II) ions to form a ···Cd-immb-Cd-immb··· chain and the intrachain Cd···Cd distance separated by immb is 8.2523(18) Å. The dihedral angle between the benzimidazole and imidazole rings is ca. 72.8°. As shown in Table 3 and Fig. 1c, there are three kinds of hydrogen bonds: N–H···O hydrogen bonds between NH of the benzimidazole rings and O from DMF with a bond length of 2.730(11) Å and a bond angle of 163.8°, C–H···O hydrogen bonds between CH of the imidazole rings and O from DMF with a bond length of 3.257(12) Å and a bond angle of 131.4°, and C–H···I hydrogen bonds between a CH unit of the methyl group and a terminal I with a bond length of 3.62(3) Å and a bond angle of 114°. The chains are also connected by π–π interactions between benzimidazole rings which are parallel to each other and have an interplanar distance of 3.7482(9) Å leading to the 2D structure. As depicted in Fig. 1d, adjacent layers are stacked into 3D supramolecular architecture in the solid state through van der Waals forces.

Fig. 1:

(a) Coordination environment of the Cd(II) center in 1 with the atom numbering scheme, showing 30 % probability ellipsoids. Hydrogen atoms, uncoordinated DMF molecules, and one alternative of the disordered methyl group are omitted for clarity. Symmetry codes: ^#1x, –y + 1/2, z + 1/2. (b) View of a chain of 1 extending along the crystallographic c direction. (c) View of a layer of 1 supported by hydrogen bonds and interchain π–π interactions. Intralaminar hydrogen bonds in 1 are indicated by dashed lines, but part of the hydrogen atoms are omitted for clarity. (d) View of the 3D supramolecular framework of 1.

In previous work [28] we reported a complex formulated as {[Cd(imb)I₂]·DMF}_n. In this complex the Cd(II) ions are located in a tetrahedral coordination geometry and bridged by imb ligands forming ···Cd-imb-Cd-imb··· chains and the intrachain Cd···Cd distance separated by an imb ligand is 8.2123(18) Å. The dihedral angle between the benzimidazole and imidazole rings is ca. 71.0°. As a matter of fact, the crystal and molecular structures of 1 and {[Cd(imb)I₂]·DMF}_n [28] closely resemble each other which extends to the same crystal system and space group and similar cell constants. This indicates that the introduction of a methyl substituent on the benzimidazole unit of the imb ligand in 1 (leading to the immb ligand) has no significant influence on the crystal structures of the complexes. This is also borne out by the observed disorder of this methyl group in the crystal structure of 1. It should be noted at this point that there is a second polymorph of {[Cd(imb)I₂]·DMF}_n reported in [24].

3.2 Crystal and molecular structure of {[Cd₃(immb)(btc)₂]·H₂O}_n (2)

The crystal architecture of 2 is different from that of 1. The crystal structure determination reveals that complex 2 crystallizes in the triclinic system with space group P1̅. There are one immb ligand, two btc^3– anions, three Cd(II) cations (Cd1, Cd2, and Cd3), and one solvent water molecule in the asymmetric unit. As depicted in Fig. 2a, there are three kinds of crystallographically independent Cd(II) ions. Cd1 is six-coordinated with a distorted octahedral geometry comprising six oxygen atoms (Cd1–O4 = 2.243(3) Å, Cd1–O5 = 2.298(3) Å, Cd1–O7 = 2.274(3) Å, Cd1–O9 = 2.247(3) Å, Cd1–O2^#1 = 2.274(3) Å, Cd1–O11^#2 = 2.298(3) Å) from four btc^3– ligands. The equatorial plane is formed by the four oxygen atoms O4, O5, O9, and O2^#1 (mean deviation from the common best plane, 0.1101 Å), while the vertices are occupied by the two oxygen atoms O7 and O11^#2 with the O7–Cd1–O11^#2 bond angle of 168.69(9)°. Different from Cd1, Cd2 is hepta-coordinated by one nitrogen atom (Cd2–N3 = 2.290(3) Å) of a benzimidazole ring and six oxygen atoms from three chelating carboxylates (Cd2–O3 = 2.542(3) Å, Cd2–O4 = 2.358(3) Å, Cd2–O7^#1 = 2.478(3) Å, Cd2–O8^#1 = 2.294(3) Å, Cd2–O11^#2 = 2.593(3) Å, Cd2–O12^#2 = 2.353(3) Å), forming an irregular CdO₆N coordination geometry. As depicted in Table 2, most of the bond angles at Cd2 deviate dramatically from the ideal angles expected for a pentagonal bipyramidal geometry, as exemplified by the angles of O12^#2–Cd2–O11^#2, O8^#1–Cd2–O3, O12^#2–Cd2–O4, and O8^#1–Cd2–O7^#1 (52.45(9)°, 125.18(12)°, 110.54(10)°, 53.32(10)°, respectively). The coordination environment of Cd3 is similar to that of Cd2. Cd3 is seven-coordinated by one nitrogen atom (Cd3–N1^#3 = 2.236(3) Å) of an imidazole ring, and six oxygen atoms (Cd3–O9 = 2.341(3) Å, Cd3–O10 = 2.512(3) Å, Cd3–O1^#1 = 2.327(3) Å, Cd3–O2^#1 = 2.586(3) Å, Cd3–O5^#2 = 2.339(3) Å, Cd3–O6^#2 = 2.496(3) Å) from three chelating carboxylates, forming an irregular CdO₆N coordination geometry. As depicted in Table 2, most of the bond angles at the Cd3 ion also deviate dramatically from the ideal angles expected for an pentagonal bipyramidal geometry, as exemplified by the angles of O5^#2–Cd3–O6^#2, O9–Cd3–O10, O1^#1–Cd3–O9, and O1^#1–Cd3–O5^#2 (52.93(9)°, 53.48(9)°, 112.00(10)°, 116.60(10)°, respectively). These Cd–O and Cd–N bond lengths are close to those in the reported complexes [Cd(BDC)(phen)·DMF] (H₂BDC = benzene-1,4-dicarboxylic acid, phen = 1,10-phenanthroline) [44], and [Cd(tmb)₂(Cl)₂(H₂O)₂]·4H₂O (tmb = 2-((1H-1,2,4-triazol-1-yl)methyl)-1H-benzimidazole) [45]. As depicted in Fig. 2b, two kinds of btc^3– ligands adopt the same coordination mode (μ₂-η₂:η₁), and each btc^3– links five Cd(II) ions. One kind of btc^3– ligand (dark red) connects two Cd1 ions, two Cd2 ions, and one Cd3 ion, and the other (yellow) joins two Cd1 ions, two Cd3 ions, and one Cd2 ion. Thus, the μ₅-btc^3– ligands connect the Cd(II) ions to form an infinite chain along the a direction. As depicted in Fig. 2c, the chains are further interlinked through bridging immb ligands to form layers along the ac plane, and the Cd···Cd distance separated by a bridging immb is 8.9549(25) Å. Adjacent layers are further stacked to the 3D structure through van der Waals forces (Fig. 2d). N–H···O hydrogen bonds between the benzimidazole groups and uncoordinated water molecules, and O–H···O hydrogen bonds between uncoordinated water molecules and carboxylate groups further stabilize the molecular structure.

Fig. 2:

(a) Coordination environment of the Cd(II) centers in 2 with the atom numbering scheme, showing 30 % probability ellipsoids. Hydrogen atoms and uncoordinated water molecules are omitted for clarity. Symmetry codes: ^#1 –x + 1, –y – 1, –z – 1; ^#2 –x + 2, –y – 1, –z – 1; ^#3x, y, z – 1. (b) View of the 1D structure along the a-axis and coordination modes of btc^3– found in 2. (c) View of the 2D structure. (d) View of the supramolecular framework.

3.3 PXRD patterns and TG analyses

PXRD patterns for 1 and 2 were recorded to confirm the phase purity of the samples. They were found comparable to the corresponding simulated ones calculated from the single-crystal diffraction data (Fig. 3), indicating good phase purity of each bulk sample.

Fig. 3:

Measured and simulated PXRD patterns of complexes 1 and 2.

Thermogravimetric analysis of 2 was performed by heating the sample from 30 °C to 800 °C in air (Fig. 4). The TG curve of 2 reveals the first mass loss of 1.9 % between 145 °C and 260 °C, which can be assigned to the release of water molecules (calcd. 1.84 %). Continuous weight loss from 385 °C to 580 °C corresponds to the decomposition of immb and 1,2,3-benzenetricarboxylate. A plateau is observed from 580 °C to 800 °C. The residue amounts to 40.4 %, which is attributed to CdO (calcd. 39.24 %). The corresponding exothermic peaks (421, 456 and 526 °C) in the differential scanning calorimetry (DSC) curve also record the processes of weight loss. TG and DSC analyses of complexes containing halogens cannot be performed on a NETZSCH STA 409 PC/PG differential thermal analyzer. As a consequence, corresponding thermogravimetric analyses of 1 have not been carried out.

Fig. 4:

TG and DSC curves of complex 2.

3.4 Luminescence properties

The luminescence properties of complexes with d¹⁰ metal centers have attracted interest for potential applications in chemical sensors, photochemistry, and light-emitting diode displays [46–48]. In this study, we have investigated the fluorescence of free immb, Na₃btc, 1, and 2 in the solid state at room temperature. As is shown in Fig. 5, immb shows an emission band with a maximum at 310 nm (λ_ex = 268 nm) and Na₃btc gives an emission band at 420 nm upon excitation at 321 nm. Complex 1 displays an emission band at 316 nm when excited at 273 nm. Complex 2 shows emission bands at 310 and 408 nm when excited at 272 nm. Obviously, the emissions observed in complexes 1 and 2 are neither MLCT (metal-to-ligand charge transfer) nor LMCT (ligand-to-metal charge transfer) since the Cd²⁺ ions are difficult to oxidize or reduce due to their d¹⁰ configuration [49]. As a result, the emission band of complex 1 can be assigned to the intraligand transition (LLCT) of immb ligands since similar emission is also observed for free immb. It is different from complex 1; both the immb ligands and 1,2,3-benzenetricarboxylates make contribution to the fluorescence emission of complex 2.

Fig. 5:

Solid-state emission spectra of immb, Na₃btc, and complexes 1 and 2.