Two novel copper(II) supramolecular complexes: synthesis, crystal structures, and Hirshfeld surface analysis

2.1 Reagents and instruments

All reagents and chemicals used in this study were commercially sourced and utilized without further purification. Elemental analysis was performed using a Perkin-Elmer 240C analyzer. Powder X-ray diffraction (PXRD) patterns were collected using a Rigaku Dmax 2000 X-ray diffractometer equipped with graphite monochromatized CuKα radiation (λ = 0.154 nm). Thermogravimetric analysis (TGA) was carried out using a TGS DT2960 analyzer over the temperature range of 25–800 °C. The IR spectrum was recorded in the range of 4,000–400 cm⁻¹ on a Perkin-Elmer Spectrum One FTIR spectrometer using a KBr pellet technique. UV–vis absorption spectra of finely ground samples were measured on a Shimadzu UV-2550 spectrophotometer, while emission spectra were recorded using a Renishaw in Via Raman microscope.

2.2 X-ray crystallography

Single crystals of complexes (1) and (2) were carefully selected for size, and single crystal X-ray diffraction analyses were performed at 273(2) K and 296(2) K on a Bruker-AXS Smart CCD diffractometer equipped with graphite-monochromatized MoKα (λ = 0.71073 Å) radiation using an ω-ϕ scan method. The structures were solved directly by SIR2014 ²⁸ and refined by the SHELXL2018/3 ²⁹ program based on F²-based full matrix least-squares technique. All hydrogen atoms were located using difference Fourier methods and refined as riding models, while the non-hydrogen atoms were refined with anisotropic displacement parameters. The selected bond distances and angles were gathered in Table 1, and X-ray crystallographic and refinement parameters are summarized in Table 2.

2.3 Synthesis of biaqua-bis[(μ₂-3-carboxylato-1-carboxylatomethyl-2-oxidopyridinium-κ³ O,O′:O″)-(2,2′-bipyridine-κ² N,N′) copper(II)] tetrahydrate (1)

CuCl₂·2H₂O (0.05 mmol, 0.099 g), H₂L (0.05 mmol, 0.0099 g), and 2,2′- bipyridine (0.1 mmol, 0.0156 g) were added to H₂O (2.5 mL) and DMF (3 mL) in a sealed glass vial. The resulting mixture was stirred continuously at room temperature for 15 min to get a mixed solution, followed by heating at 95 °C for 96 h. Upon cooled to room temperature, blue bulk crystal of complex (1) were obtained. Single crystals suitable for X-ray diffraction analysis were obtained. (1): trace amount (yield∼43 %, based on Cu). C₃₆H₃₈Cu₂N₆O₁₆ (%): calcd. C 46.11, H 4.08, N 8.96; Found C 45.27, H 4.03, N 8.73.

2.4 Synthesis of poly{aqua-(μ₂-3-carboxylato-1-carboxylatomethyl-2-oxidopyridinium-κ³ O,O′:O″)-hemi(μ₂-4,4′-bipyridine-κ² N:N′) copper(II)] bihydrate}(2)

CuCl₂·2H₂O (0.1 mmol, 0.0170 g), H₂L (0.05 mmol, 0.0099 g), and 4,4′- bipyridine (0.1 mmol, 0.0160 g) were dissolved in a mixed solvent of H₂O/DMF (5 mL, 1:1 = v/v) in a glass vial. The mixture was stirred for 15 min at room temperature to ensure thorough mixing. It was then heated at 93 °C for 96 h and subsequently cooled to room temperature, yielding blue block-shaped crystals of complex (2). The crystals of complex (2) were washed with distilled water and dried under ambient conditions. Single crystals suitable for X-ray diffraction analysis were obtained. (2): trace amount (yield∼38 %, based on Cu). C₁₃H₁₅CuN₂O₈ (%): calcd. C 39.91, H 3.38, N 7.16; Found C 39.43, H 3.82, N 7.10.

3.1 Crystal structure description for complex (1)

The single-crystal X-ray diffraction analysis reveals that complex (1) crystallizes in the space group P2₁/n. The asymmetric unit consists of one crystallographically independent Cu(II) ion, one 2,2′-bipy ligand, one L anion, one coordinated H₂O molecule, and two free H₂O molecules. As shown in Figure 1, the coordination environment of Cu(II) ions, which exhibits a six-coordinate distorted octahedral geometry, is completed by two nitrogen atoms N(2) and N(3) (N(2)–Cu(1) = 1.986(2) Å, N(3)–Cu(1) = 1.997(2) Å) from one 2,2′-bipy ligand, three oxygen atoms O(2), O(3) and O(1)^vi (O(2)–Cu(1) = 1.9070(18) Å, O(3)–Cu(1) = 1.9357(18) Å, Table 1) from two different L ligand anions, as well as one oxygen atom O(1W) (O(1W)–Cu(1) = 2.328(2) Å) in the H₂O molecule.

Figure 1:

Coordination center of Cu(II) in complex (1) (Symmetric codes: ^vi 1 − x, −y, −z).

The deprotonated L ligand anion adopts a μ₂-η¹:η¹:η¹:η⁰:η⁰ linking mode, where the carboxylate oxygen atoms from the L ligand take the coordination mode of monodentate with adjacent Cu(II) ions, thereby forming a binuclear [CuL]₂ structural unit (Figure 2). The distance between adjacent Cu⋯Cu is 5.200 Å. Subsequently, adjacent binuclear [CuL]₂ clusters are interconnected through π-π stacking interactions (N(2)/C(9)–C(13), N(2)^vi/C(9)^vi–C(13)^vi; Symmetric code: ^vi 1 − x, −y, −z, Table 3) between the pyridine rings of the 2,2′-bipy ligands to form a supramolecular chain. ³⁰

Figure 2:

Complex (1) forms a chain through π-π interactions.

Interestingly, additional π-π stacking interactions (N(3)/C(14)–C(18), N(1)^vii/C(1)^vii–C(5)^vii; Symmetric code: ^vii −1/2 − x, 1/2 − y, −1/2 − z) are observed between the pyridine ring of the 2,2′-bipy ligand and the aromatic ring of the H₂L ligand anion, which further extend the adjacent chains into supramolecular layers (Figure 3). The stability of the layered structure is enhanced due to the presence of O–H⋯O hydrogen bonds.

Figure 3:

The layered structure of complex (1) formed based on another kind of π-π stacking.

3.2 Crystal structure description for complex (2)

Single crystal X-ray diffraction analysis indicates that complex (2) crystallizes in the triclinic system with the P 1 ‾ space group, exhibiting a similar layered network structure to that of complex (2). As shown in Figure 4, a Cu(Ⅱ) ion, a L ligand anion, half of one 4,4′-bipy ligand, a crystalline water molecule, and two free water molecules constitute the asymmetric unit structure of complex (2). Each Cu(II) ion coordinates with one N atom (Cu(1)–N(2) = 2.0222(18) Å) from 4,4′-bipy, two carboxylate oxygen atoms (Cu(1)–O(2) = 1.9203(17) Å, Cu(1)–O(5)ⁱⁱ = 1.9668(16) Å), and one hydroxyl oxygen atom (Cu(1)–O(3) = 1.9533(16) Å) from one L ligand anion, along with one oxygen atom (Cu(1)–O(1W) = 2.2342(19) Å) from a water molecule, thereby forming a five-coordinate [CuO₄N] distorted square pyramidal geometry.

Figure 4:

Coordination environment diagram of complex (2).

As shown in Figure 5, L ligand anion connects adjacent Cu(II) ions via a μ₂-η⁰:η¹:η¹:η⁰:η¹ coordination mode, forming a dinuclear [CuL]₂ structure with a Cu⋯Cu distance of 6.710 Å. Furthermore, 4,4′-bipy bridges adjacent dinuclear [CuL]₂ units, generating a chain structure with an extended Cu⋯Cu distance of 11.134 Å.

Figure 5:

Chain structure of complex (2).

Moreover, the O(1W)–H(1A)⋯O(1)^v hydrogen bonds facilitate the connection of chains into a layered structure (Table 4), while additional hydrogen bonds contribute significantly to the stabilization of this layered architecture (Figure 6).

Figure 6:

Structure of a complex (2).

3.3 Powder XRD analysis of complexes (1) and (2)

The powder X-ray diffraction (PXRD) patterns of complexes (1) and (2) are presented in Figure 7. A direct comparison reveals that the experimental diffraction peaks are in reasonable agreement with those derived from the simulated patterns, providing strong evidence for phase purity of both samples.

Figure 7:

Experimental and simulated PXRD patterns of complexes (1) (a) and (2) (b).

3.4 FT-IR spectroscopy of complexes (1) and (2)

As shown in Figure 8, infrared spectroscopy (IR) measurements of complexes (1) and (2) were conducted within the frequency range of 400–4,000 cm⁻¹ using KBr as the background. Broad absorption peaks observed between 3,441 and 3,074 cm⁻¹ are predominantly attributed to the hydroxyl O–H stretching vibrations of water molecules in complex (1). The peak at 2,364 cm⁻¹ corresponds to the stretching vibration of CO₂ from the air. ³¹ The characteristic peaks for C–N and C=N stretching vibrations are located at 1,653 cm⁻¹ and 1,532 cm⁻¹, respectively. Additionally, two peaks at 1,592 cm⁻¹ and 1,374 cm⁻¹ correspond to the asymmetric ν_as(COO) and symmetric ν_s(COO) vibrations of the carboxylate groups in the L ligand anions. Peaks at 1,244 cm⁻¹ and 1,183 cm⁻¹ are assigned to the in-plane bending vibrations of C–H bonds.

Figure 8:

IR spectroscopy of complexes (1) (a) and (2) (b).

In comparison with complex (1), the peaks at 1,420 cm⁻¹ and 1,611 cm⁻¹ in complex (2) can be attributed to the symmetric (ν_s(COO)) and asymmetric (ν_as(COO)) stretching vibrations of the carboxyl group in the L ligand anions. Furthermore, the peaks at 1,648 cm⁻¹, 1,532 cm⁻¹, and 1,304 cm⁻¹ are associated with the stretching vibrations of ν(C=N) and ν(C–N). The in-plane bending and out-of-plane stretching vibrations of C–H bonds are identified at 1,225 cm⁻¹ and 784 cm⁻¹, respectively. Lastly, the characteristic absorption peak of Cu–O is observed at 594 cm⁻¹. ³²

3.5 Thermogravimetric analysis of complex (1) and (2)

The thermal stabilities of complexes (1) and (2) were investigated under a temperature range from 25 to 800 °C with a heating rate of 10 K min⁻¹. The weight loss profiles of complexes (1) and (2) are illustrated in Figure 9. For complex (1), the first stage of weight loss occurred within the temperature range of 30–180 °C, with an actual slight weight loss of approximately 10.5 % (theoretical value: 11.51 %), which corresponded to the loss of two coordinated water molecules and four free water molecules. The second stage of weight loss began at 208 °C, where the pronounced downward trend of the thermogravimetric curve indicated the collapse of the framework structure of complex (1). The curve plateaued around 550 °C, indicating that complex (1) was fully decomposed, leaving CuO as the final residual product. Throughout the entire process, the total weight loss amounted to 76.4 % of the initial mass. Regarding complex (2), the initial weight loss observed within the temperature range of 63–171 °C can be attributed to the loss of water molecules. A subsequent substantial weight loss was recorded between 171 and 475 °C, which is likely associated with the breakdown of the structure. The thermogravimetric curve gradually leveled off at approximately 475 °C, demonstrating that complex (2) had undergone complete decomposition.

Figure 9:

TG curves of the complexes (1) (a) and (2) (b).

3.6 UV–vis spectroscopy of complex (1) and (2)

Under ambient temperature conditions, the solid-state UV–Vis absorption spectra of complexes (1) and (2), as well as their corresponding three ligands (H₂L, 2,2′-bipy and 4,4′-bipy), were measured. As shown in Figure 10(a), the 2,2′-bipy ligand exhibited high and broad absorption bands in the wavelength range of 200–267 nm, while the H₂L ligand showed similar characteristics in the 200–366 nm region. Meanwhile, the 4,4′-bipy ligand also demonstrated relatively strong absorption bands in the 200–300 nm region (Figure 10(b)). Analogous absorption features were observed for complexes (1) and (2): complex (1) displayed absorption bands in the 200–339 nm range, whereas complex (2) exhibited absorption in the narrower 200–258 nm region. These absorption bands are predominantly attributed to π-π^∗ and n-π^∗ transitions within the ligands. Furthermore, the absorption bands of complex (1) in the 470–800 nm region, along with the strong absorption band of complex (2) within the same spectral range, can be ascribed to the d-d transitions of the Cu²⁺ centers. ³³

Figure 10:

UV–vis spectroscopy of complexes (1) (a) and (2) (b).

3.7 Photoluminescence spectroscopy of complex (1) and (2)

The solid-state photoluminescence characteristics of complexes (1) and (2), along with the organic ligands H₂L, 2,2′-bipy and 4,4′-bipy, were recorded. As depicted in Figure 11(a), the H₂L ligand exhibited a pronounced emission peak at 383 nm under an excitation wavelength of 256 nm, whereas the 2,2′-bipy ligand showed emission peaks at 377 nm (λ_ex = 330 nm). Remarkably, complex (1) displayed a robust emission at 378 nm (λ_ex = 327 nm), which closely overlapped with the emission profile of the 2,2′-bipy ligand. This spectral congruence implies that the fluorescence emission of complex (1) likely arises from intraligand charge transfer within the 2,2′-bipy ligand. In Figure 11(b), the 4,4′-bipy and H₂L ligands exhibited emission bands at 422 nm (λ_ex = 256 nm) and 383 nm (λ_ex = 329 nm), respectively. Under 329 nm excitation, complex (2) displayed a strong emission band at 376 nm, closely matching the H₂L ligand’s profile, which indicates intraligand charge transfer (ILCT) within the H₂L ligand. ³⁴

Figure 11:

The solid-state photoluminescence spectra: complex (1), 2,2′-bipy and H₂L ligand (a); complex (2), 4,4′-bipy and H₂L ligand (b).

3.8 Hirshfeld surface analysis of complex (1)

Hirshfeld surfaces analyzed for complex (1) were generated using CrystalExplorer21.5 ³⁵ and the d_norm, shape-index, and curvedness parameters were obtained, complemented by fingerprint plot analysis. The d_norm color mapping reflects interatomic distances: red regions denote intermolecular distances shorter than the sum of van der Waals radii, white areas represent approximate van der Waals contacts, and blue zones indicate distances exceeding the van der Waals sum. Figure S1 shows multiple bright and light-red spots on the d_norm surface, attributed to hydrogen-bonding interactions. The shape-index surface reveals distinct π-π stacking interactions, characterized by adjacent red-yellow and blue-green triangular patterns alongside back-to-back diamond motifs. The curvedness analysis highlights surface flatness correlated with molecular packing efficiency.

The two-dimensional fingerprint plots in Figure S2 depict the corresponding percentage contributions of intermolecular contacts within complex (1). Among these, H⋯H intermolecular contacts exhibit the highest contribution at 37.9 %, while O⋯H/H⋯O interactions account for a significant 29.9 % of the overall crystal packing. C⋯H/H⋯C interactions contribute 14.7 % to the surface area. Other weak intermolecular contacts make minor contributions, including C⋯C (7.6 %), H⋯N/N⋯H (3.7 %), C⋯O/O⋯C (1.9 %), O⋯Cu/Cu⋯O (1.5 %), O⋯O (1.1 %), and N⋯O/O⋯N (1.1 %).