Influence of metal ions on the formation of new metal complexes constructed from tetrachlorophthalic acid

2.1 Synthesis and general characterization

Complexes 1–3 were prepared by using 1,2-H₂BDC-Cl₄ and Cu^II, Cd^II, and Ni^II salts in DMF/H₂O under similar conditions. All three complexes are stable to air and moisture and are insoluble in common solvents such as water, alcohol, and acetonitrile. The IR spectra of 1–3 show features attributable to the components of the complexes. The broad bands centered at ca. 3407–3444 cm^–1 indicate the O–H stretching of hydroxyl or water. The strong absorption bands at 1718 cm^–1 for 1 and 1720 cm^–1 for 2 reveal the presence of carboxyl groups, while there is no absorption band around 1720 cm^–1 for 3, indicating the complete deprotonation of the 1,2-H₂BDC-Cl₄ ligand, which is consistent with the crystal structure determination. All three IR spectra display strong peaks in the ranges of 1601–1645 and 1386–1433 cm^–1, which can be attributed to the antisymmetric and symmetric stretching vibrations of carboxylate groups. The elemental analysis results of these complexes match well with their molecular formulae evaluated by X-ray diffraction analyses.

2.2 Structural description of complexes 1–3

2.2.1 [Cu(1,2-HBDC-Cl₄)₂(DMF)₂] (1)

Single-crystal X-ray diffraction has revealed that crystals of 1 are orthorhombic with space group Pbca and the asymmetric unit contains half a Cu^II ion, one 1,2-HBDC-Cl₄ anion, and one DMF ligand. As shown in Fig. 1a, the Cu^II ion is four-coordinated by two oxygen atoms from two 1,2-HBDC-Cl₄ ligands with a Cu–O distance of 1.930(1) Å and two oxygen atoms from two DMF ligands with a Cu–O distance of 1.949(1) Å. In this complex, the acid 1,2-H₂BDC-Cl₄ is mono-deprotonated to the 1,2-HBDC-Cl₄ anion and adopts a monodentate terminal coordination mode, giving rise to a discrete structure. Each [Cu(1,2-HBDC-Cl₄)₂(DMF)₂] unit is linked to four neighboring ones by strong intermolecular O3–H3···O2ⁱ hydrogen bonding interactions (H···O/O···O distance: 1.807/2.626(2) Å, angle: 178.8(1)°, i = –x– 1/2,y– 1/2, z) between the uncoordinated carboxylate O2 atoms and H atoms of the carboxyl group, affording a supramolecular layer (Fig. 1b). These layers are further assembled into a 3D supramolecular framework via intermolecular Cl1···Cl4ⁱⁱ interactions (Cl···Cl distance = 3.654 Å, ii = –x, y+ 1/2, –z+ 1/2) (Fig. 1c). It should be pointed out that the Cl···Cl distance is shorter than twice Pauling’s van der Waals radius of the chlorine atom (3.76 Å) [31], and is comparable to that stated by Bondi (3.52 Å) [32].

Fig. 1:

Views of (a) the coordination environment of the Cu^II center in 1 (symmetry code: #1 –x, –y, –z), (b) the 2D supramolecular layer with O–H···O hydrogen bonds highlighted as dashed lines, and (c) the 3D supramolecular architecture (Cl···Cl and O–H···O hydrogen bonds are shown as dashed lines).

2.2.2 {[Cd(1,2-HBDC-Cl₄)₂(H₂O)₄]·2DMF} (2)

X-ray analysis reveals that 2 crystallizes in the space group Pbcn and the asymmetric unit contains half a Cd^II ion, one 1,2-HBDC-Cl₄ anion, two aqua ligands, and one DMF guest molecule. The Cd^II ion is six-coordinated by two oxygen atoms from two 1,2-HBDC-Cl₄ anions and four oxygen atoms from four aqua ligands to constitute an octahedral geometry (Fig. 2a). The Cd–O bond lengths range from 2.282(2) to 2.305(2) Å, and the O–Cd–O bond angles are in the range of 82.4(1)–176.8(1)°. Similar to 1, the mono-deprotonated 1,2-HBDC-Cl₄ units of 2 are coordinated to Cd^II ion via a monodentate terminal coordination mode. The analysis of the molecular packing of 2 in the unit cell shows that there exist multiple hydrogen bond interactions, resulting in a 2D supramolecular network. As illustrated in Fig. 2b, all carboxylate oxygen atoms are involved in the formation of O–H···O hydrogen bonding with water ligands or carboxyl groups (hydrogen bond parameters are listed in Table 1) to result in a chain along the crystallographic c axis. These chains are further stabilized by intermolecular O5–H5A···O5ⁱ hydrogen bonding interactions (H···O/O···O distance: 2.53/3.107(3) Å, angle: 128°, i = –x+1, –y, –z) between coordinated water ligands. With respect to the non-binding DMF guest, it works as a linker to fabricate a supramolecular layer in the crystallographic bc plane connecting the above-mentioned 1D motifs through intermolecular O5–H5B···O7 and C10–H10C···Cl1 interactions (see Table 1 for details), as is depicted in Fig. 2c.

Fig. 2:

Views of (a) the coordination environment of the Cd^II center in 2 (symmetry code: #1 –x + 1, y + 1, –z + 1/2), (b) the hydrogen bonding chain arrangement of 2, and (c) the 2D supramolecular network constructed via O–H···O and C–H···Cl interactions.

Table 1

Hydrogen bond parameters in the crystal structures of 1–3.

Complex	D–H···A	H···A (Å)	D···A (Å)	D–H···A (deg)	Symmetry code
1	O3–H3···O2	1.81	2.626(2)	179	–x– 1/2, y– 1/2,z
2	O4–H4···O1	1.78	2.581(3)	166	x, –y, z– 1/2
	O5–H5A···O2	2.53	3.029(3)	120	–x+1, –y, –z
	O5–H5A···O3	2.58	3.155(3)	129	–x+1, –y, –z
	O5–H5A···O5	2.53	3.107(3)	128	–x+1, –y, –z
	O5–H5B···O7	1.84	2.644(3)	165	x,y– 1, z
	O6–H6A···O3	2.25	3.032(4)	159	–x+1, –y, –z
	C10–H10C···Cl1	2.81	3.575(5)	138
	O6–H6B···O2	2.23	2.983(4)	153
3	O6–H6A···O11	2.01	2.849(7)	172
	O6–H6B···O2	1.83	2.672(4)	170	x–1, y,z
	O7–H7A···O5	1.85	2.703(5)	176
	O7–H7B···O2	2.00	2.835(5)	163	–x+ 1, –y, –z+ 1
	O8–H8A···O5	1.90	2.734(5)	164	–x, –y, –z+ 1
	O8–H8B···O4	1.78	2.618(5)	167	–x+ 1, –y+ 1, –z+ 1
	O9–H9A···O3	1.93	2.780(5)	164	–x+ 1, –y+ 1, –z+ 1
	O9–H9B···O2	1.99	2.752(4)	145
	O10–H10A···O3	1.99	2.836(5)	167
	O10–H10B···O8	1.98	2.826(5)	176	–x, –y+ 1, –z+ 1
	O11–H11D···O4	2.03	2.849(7)	160	x–1, y,z
	O11–H11E···Cl3	2.30	3.121(6)	162	–x+ 1, –y+ 1, –z

2.2.3 {[Ni(1,2-BDC-Cl₄)(H₂O)₅]·DMF·H₂O} (3)

Complex 3 forms monoclinic crystals with space group P1̅. The asymmetric unit consists of one Ni^II ion, one dianionic 1,2-BDC-Cl₄ ligand, five coordinated water molecules, one free water molecule, and one free DMF molecule. In contrast to the mono-deprotonated 1,2-HBDC-Cl₄ ligands of 1 and 2, 1,2-H₂BDC-Cl₄ in 3 is fully deprotonated. One carboxylate group is coordinated to one Ni^II center with a monodentate coordination mode, while the other is uncoordinated. As shown in Fig. 3a, the Ni^II ion is six-coordinated by one oxygen atom from one 1,2-BDC-Cl₄ dianion and five oxygen atoms from five aqua ligands, taking on a slightly distorted octahedral geometry. The Ni–O bond lengths vary from 2.009(3) to 2.070(3) Å, and the O–Ni–O bond angles range from 84.6(1) to 179.2(1)°. Each mononuclear [Ni(1,2-BDC-Cl₄)(H₂O)₅] unit is connected by a pair of strong intermolecular O10–H10B···O8 hydrogen bonding interactions (see Table 1 for details) between coordinated O10 and O8 water molecules to constitute a dimeric [Ni(1,2-BDC-Cl₄)(H₂O)₅]₂ subunit with a Ni···Ni separation of 5.161(1) Å. The interstitial DMF molecules connect the neighboring dimeric units by O7–H7A···O5 and O8–H8A···O5 interactions (see Table 1) to afford a chain-like supramolecular array along the crystallographic b axis (see Fig. 3b). These adjacent 1D motifs are further connected through intermolecular O6–H6B···O2 hydrogen bond interactions (see Table 1) between the coordinated water molecules of O6 and uncoordinated carboxylate O2 atoms of 1,2-BDC-Cl₄, leading to the formation of a 2D supramolecular sheet extending along the bc plane (see Fig. 3c).

Fig. 3:

Views of (a) the coordination environment of the Ni^II center in 3, (b) the 1D hydrogen bonding arrangement of 3 containing [Ni(1,2-BDC-Cl₄)(H₂O)₅]₂ dimeric subunits, and (c) the 2D supramolecular layered structure.

In comparison with our previously reported 1D chiral Mn^II complex [30], complexes 1–3 all crystallize in different achiral space groups and display mononuclear structures. The Cu^II ion in 1 takes on a square planar coordination geometry completed by two anionic 1,2-HBDC-Cl₄ ligands and two DMF ligands, the Cd^II ion in 2 adopts a distorted octahedral geometry comprising two anionic 1,2-HBDC-Cl₄ ligands and four water ligands, while the Ni^II ion in 3 is octahedrally coordinated by one dianionic 1,2-BDC-Cl₄ ligand and five water ligands. The carboxyl groups of 1,2-H₂BDC-Cl₄ are mono-deprotonated in 1 and 2, which are different from the completely deprotonated ligand in 3. The coordination preference of the metal ions clearly plays an important role in the formation of their complexes since the only difference in the synthetic conditions employed for these complexes is the transition metal components.

2.3 Thermal stability

To investigate the thermal stability of complexes 1–3, thermogravimetric (TGA) measurements were carried out from room temperature to 800 °C and the corresponding curves are shown in Fig. 4. No decomposition of 1 is observed below 95 °C, and the first weight loss of 17.2% in the range of 95 °C–170 °C corresponds to the loss of coordinated DMF ligands (calculated: 17.9%). The residual solid starts to decompose near 230 °C, which is complete at 800 °C. The TGA curve of 2 shows the initial weight loss of 22.9% in the range of 75 °C–190 °C, which can be ascribed to the removal of four water ligands and two lattice DMF molecules (calculated: 23.3%). The weight loss continues until the temperature reaches 600 °C. The remaining weight indicates that the final residue possibly is CdO (calculated: 13.7%, found: 14.5%). In the case of 3, the weight loss of 33.1% (calculated: 32.3%) in the range of 50 °C–210 °C is in accordance with the removal of ten water ligands and two lattice DMF molecules as well as one interstitial water molecule. Subsequently, pyrolysis of the remaining substance is observed upon heating to 275 °C, and further heating to 800 °C reveals a further gradual weight loss of the sample. The final solid holds a weight of 14.6% (calculated: 13.8%) of the total sample which corresponds to NiO.

Fig. 4:

Solid-state emission spectra of the ligand 1,2-H₂BDC-Cl₄ and complexes 1–3.

2.4 Photoluminescence properties

The emission spectra of complexes 1–3 and of the acid 1,2-H₂BDC-Cl₄, under excitation of 336 nm in the solid state at room temperature, are shown in Fig. 4. The strongest emission peak for the free acid appears at 474 nm, corresponding to n → π* transitions. For 1–3, the maximum emission peaks are similarly observed in the blue region (1, 476 nm; 2, 475 nm; and 3, 476 nm), which are also attributed to ligand-centered transitions. Weaker shoulders around 510 nm can also be assigned to intraligand transitions because a similar peak at about 508 nm appears for 1,2-H₂BDC-Cl₄. There is a considerable enhancement of the emission intensity of complex 2 compared with free 1,2-H₂BDC-Cl₄, which may result from the increased rigidity of the organic ligand when binding to Cd^II, effectively reducing the loss of the energy through radiationless pathways [33, 34]. This result reveals that no significant effect of metal ions on the fluorescence of the three complexes has been observed.

In summary, three new discrete Cu^II, Cd^II, and Ni^II complexes with perchlorinated phthalate ligands have been synthesized and structurally characterized. The results reveal that the coordination geometries of the transition metal ions play an important role in governing the structural features of these metal-organic coordination architectures. The investigation of the packing of the molecules of such complexes suggests that intermolecular O–H···O hydrogen bonding and Cl···Cl (or C–H···Cl) interactions are among the driving forces in linking the discrete entities into high-dimensional supramolecular networks. The complexes exhibit blue emissions in the solid state at room temperature, not markedly different from that of tetrachlorophthalic acid.

3.1 Synthesis of [Cu(1,2-HBDC-Cl₄)₂(DMF)₂] (1)

A solution of CuCl₂·2H₂O (51.1 mg, 0.3 mmol) in H₂O (8 mL) was added to a solution of 1,2-H₂BDC-Cl₄ (91.2 mg, 0.3 mmol) in DMF (8 mL). After stirring for ca. 15 min, the resultant solution was filtrated and left to stand at room temperature. Blue block-shaped single crystals of 1 suitable for X-ray diffraction were obtained after 1 week by slow evaporation of the solvents in ca. 70% yield (85.6 mg; based on 1,2-H₂BDC-Cl₄). – Analysis for C₂₂H₁₆Cl₈CuN₂O₁₀ (%): calcd. C 32.4, N 3.4, H 2.0; found C 33.2, N 3.2, H 1.9. – IR (cm^–1): 3423 br, 2959 m, 2927 m, 1718 s, 1643 s, 1577 s, 1433 s, 1395 s, 1344 s, 1239 m, 1123 m, 1056 w, 905 s, 831 m, 667 s, 603 m.

3.2 Synthesis of {[Cd(1,2-HBDC-Cl₄)₂(H₂O)₄]· 2DMF} (2)

The same synthetic procedure as that for 1 was used except that CuCl₂·2H₂O was replaced by CdCl₂ (55.0 mg, 0.3 mmol), giving colorless sheet-like single crystals of 2 in ca. 40% yield (56.2 mg, based on 1,2-H₂BDC-Cl₄). – Analysis for C₂₂H₂₄CdCl₈N₂O₁₄ (%): calcd C 28.2, N 3.0, H 2.6; found C 27.6, N 3.3, H 2.5. – IR (cm^–1): 3444 br, 2958 m, 2931 m, 1720 s, 1610 s, 1573 s, 1426 s, 1386 s, 1261 s, 1120 m, 1082 w, 935 w, 904 m, 828 m, 643 m, 538 m.

3.3 Synthesis of {[Ni(1,2-BDC-Cl₄)(H₂O)₅]₂· 2DMF·H₂O} (3)

The same synthetic procedure as that for 1 was used except that CuCl₂·2H₂O was replaced by NiCl₂·6H₂O (71.3 mg, 0.3 mmol), affording green block-shaped single crystals of 3 in ca. 50% yield (79.9 mg, based on 1,2-H₂BDC-Cl₄). – Analysis for C₂₂H₃₆Cl₈N₂Ni₂O₂₁ (%): calcd. C 24.4, N 2.6, H 3.5; found C 24.2, N 2.7, H 3.4. – IR (cm^–1): 3407 br, 2967 m, 2925 m, 1645 s, 1601 s, 1422 s, 1392 s, 1341 s, 1264 s, 1123 m, 1104 m, 938 w, 812 w, 738 m, 667 m, 621 w.

3.4 X-ray structure determinations

Single-crystal X-ray diffraction measurements were performed on a Bruker Apex II CCD diffractometer at ambient temperature with MoK_α radiation (λ = 0.71073 Å). In each case, a semiempirical absorption correction was applied using sadabs [35], and the program saint was used for integration of the diffraction profiles [36]. The structures were solved by direct methods using the shelxs program of the shelxtl package and refined anisotropically for all non-hydrogen atoms by full-matrix least squares on F² with shelxl [37–40]. Carbon-bound hydrogen atoms were placed in geometrically calculated positions using a riding model. Oxygen-bound hydrogen atoms were firstly localized by difference Fourier maps and then fixed geometrically with isotropic displacement parameters. Crystallographic data and structural refinement parameters are summarized in Table 2, and selected bond lengths and angles are listed in Table 3.

CCDC 1025077, 1025078, and 1025079 contain the supplementary crystallographic data for complexes 1–3, respectively. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.