A new cadmium(ii) coordination polymer with 1,4-cyclohexanedicarboxylate acid and phenanthroline derivate: Synthesis and crystal structure

Over the past decades, more and more researchers have focused on the design and synthesis of coordination polymers, because of their unique structures and their potential applications in magnetism, fluorescence, biological, gas storage, and so forth (Guillem et al., 2012; Liu et al., 2021; Song et al., 2021; Wang et al., 2020; Yang et al., 2010). Many different factors such as solvent, organic ligands, temperatures, pH, metal ions, and non-covalent interactions (aromatic stacking interactions) have influence on the final structures and properties of coordination polymers (Li et al. 2021; Martin et al. 2008). So far, most of the coordination polymers have been constructed from carboxylic acid ligands, nitrogen-containing organic ligands, and transition metal ions through intermolecular interactions or coordination bonds (Shimomura et al., 2010; Tiwari et al., 2021; Zhang et al., 2021). Phenanthroline derivative ligands are the typical N-donor ligands, which could adopt diverse conformations in the coordination of the center metal (Ma et al., 2009). Phenanthroline derivatives are rigid structures due to their central ring resulting in the two N-atoms being held in juxtaposition; so they can coordinate with transition metal ions quickly (Hu et al., 2021; Llabrés i Xamena et al., 2008; Song et al., 2021; Wei et al., 2020). In our research, nitrogen-containing organic ligands formed the π–π stacking interactions, and the existences of these π–π stacking interactions contribute considerably to the stability of coordination polymers (Wang et al., 2011).

With the above considerations, Cd(ii) coordination polymers with derivative 1-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)naphthalen-2-ol ligands (L) and the 1,4-cyclohexanedicarboxylate acid (1,4-H₂chdc), formulated as [Cd(1,4-chdc)(L)] _n (1) was constructed under hydrothermal conditions.

X-ray crystallographic analysis reveals that the asymmetric unit of 1 contains one Cd(ii) atom, one 1,4-chdc anion and one L ligand. Each Cd(ii) atom is coordinated by five carboxylate oxygen atoms from three 1,4-chdc anions (Cd(1)–O(2) = 2.301(2), Cd(1)–O(3) = 2.413(2), Cd(1)–O(4)ⁱ = 2.487(2), Cd(1)–O(5)ⁱ = 2.362(2), Cd(1)–O(5)ⁱⁱ = 2.400(2) Å) and two nitrogen atoms from one chelating L ligand (Cd(1)–N(1) = 2.292(2), Cd(1)–N(2) = 2.405(3) Å) in a distorted [CdN₂O₅] pentagonal bipyramidal geometry (Figure 1). The distances of the Cd-O bonds range from 2.301(2) to 2.487(2) Å, which are close to those of [Cd(L-pheA)(Ac)] _n (from 2.248(14) to 2.426(13) Å) (L-pheHA = L-phenylalanine hydroxamic acid) (Chen et al., 2020). The full deprotonated 1,4-chdc anion serves as a µ₃-bridge linking three Cd(ii) ions, in which one carboxylic group adopts chelating mode to connect one Cd(ii) ions; the other carboxylic group adopts μ₂-η¹:η²-chelating/bridging mode to coordinate with two Cd(ii) ions. The 1,4-chdc ligand, linking three Cd(ii) ions to form a 2D layer, is shown in Figure 2, and the farthest Cd···Cd distance is 11.277 Å.

Figure 1

View of the coordination environment of the Cd(ii) atom of 1 (the central ring of the 1,4-chdc is disordered over two positions, symmetric code: ⁽ⁱ⁾ x, –y + 5/2, z–1/2; ⁽ⁱⁱ⁾ –x + 1, y + 1/2, –z + 1/2).

Figure 2

View of the 2D layer structure of 1 constructed by the 1,4-chdc.

Strikingly, the secondary N-donor L ligands are situated on both sides of the 2D layer (Figure 3), which allow the formation of π–π stacking between the pyridine ring and benzene ring of the L ligands with the centroid-to-centroid distance of 3.633(2) Å and face-to-face distance of 3.6158(12) Å, and the dihedral angle between the two planes is ca. 2.78(16)^o (the pyridine ring and benzene ring are composed of N(2)/C(6)–C(10) and C(18)^viii–C(23)^viii, respectively; symmetry code: ^viii –x, –y + 1, –z + 1) (Figure 4). These π–π stacking interactions connected the neighboring layers into a 3D supramolecular architecture (Figure 5). Additionally, as illustrated in Figure 6, O–H⋯N hydrogen bond (O(1)–H(1 A)⋯O(3)^vii, symmetric code: ^vii –x, –y + 2, –z + 1, as seen in Table 1,) and N–H⋯O hydrogen bonds (N(4)–H(4)⋯O(1) and N(4)–H(4)⋯O(4)^vi, symmetric code: ^vi –x, y–1/2, –z + 1/2, as illustrated in Table 1) further consolidate the 3D supramolecular structure of 1.

Figure 3

View of the 2D layer structure of 1 constructed by the 1,4-chdc and L ligands.

Figure 4

View of the π–π interactions between two L ligands of neighboring layers.

Figure 5

View of the 3D supramolecular structure of 1 formed by π–π interactions.

Figure 6

View of the O–H⋯N and N–H⋯O hydrogen bonds interactions (symmetric code: ^vi –x, y–1/2, –z + 1/2, ^vii –x, –y + 2, –z + 1).

For the IR spectrum of 1 (Figure 7), one strong and wide peak at 3,421 cm⁻¹ was observed, which is characteristic of the O–H stretching frequency of the L ligand. Two peaks at 1,626 and 1,545 cm⁻¹ probably correspond to the vas(COO–) and vs(COO–) stretching vibration of dicarboxylate in the 1,4-chdc anion. Peaks at 1,413 and 1,072 cm⁻¹ could be assigned to v(C–N) and v(C═N) stretching vibration of the L ligand.

Figure 7

IR spectroscopy curve of 1.

In summary, we report a 2D coordination polymer [Cd(1,4-chdc)(L)] _n (1) obtained under hydrothermal conditions with 1-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)naphthalen-2-ol (L) and 1,4-cyclohexanedicarboxylate acid (1,4-H₂chdc). The central Cd(ii) atom is coordinated by 1,4-chdc anion and L ligand in a distorted [CdN₂O₅] pentagonal bipyramidal coordination environment to generate a 2D layer. The adjacent 2D layers are further expanded into 3D supramolecular structure via π–π stacking interactions.