Two 2D Co(II)/Mn(II) coordination polymers based on the quinoline-2,3-dicarboxylate ligand: synthesis, crystal structure, and fluorescence properties

2.1 Materials and physical methods

All chemicals were commercially available and used as received without further purification. The elemental analyses (C, H, N, and S) were performed on a Perkin-Elmer 240C apparatus. FT-IR spectra were recorded in the range of 4000–450 cm⁻¹ on a Perkin Elmer Frontier spectrometer. Thermogravimetric analyses (TG) were performed under nitrogen with a heating rate of 10 K min⁻¹ using a Perkin Elmer Thermogravimetric Analyzer TGA4000. Luminescent spectra were recorded with a Varian Cary Eclipse fluorescence spectrophotometer using a xenon arc lamp as the light source.

2.2 Synthesis of [Co(2,3-qldc)(H₂O)] _n (1)

A mixture of Co(CH₃COO)₂·4H₂O (0.1 mmol), 2,3-H₂qldc (0.1 mmol), NaOH (0.2 mmol), and aqueous ethanol (5/5 mL) was added to a 25 mL Teflon-lined stainless steel reactor and heated at 140 °C for three days, and then slowly cooled to room temperature. Red block single crystals suitable for X-ray data collection were obtained by filtration. Yield: 75% (based on Co). – Anal. for C₁₁H₇CoNO₅ (292.11): calcd. C 45.19, H 2.40, N 4.79; found C 45.17, H 2.43, N 4.78. – IR (KBr): ν = 3357(w), 1618(s), 1549(m), 1456(s), 1370(m), 1278(s), 1187(m), 1154(m), 913(m), 854(m), 778(m), 746(w), 685(w), 537(w), 455(w) cm⁻¹.

2.3 Synthesis of [Mn(2,3-qldc) (H₂O)] _n (2)

The synthesis of 2 was similar to that for 1 except for Mn(CH₃COO)₂·4H₂O (0.1 mmol) substituted for Co(CH₃COO)₂·4H₂O (0.1 mmol). Light yellow block single crystals suitable for X-ray data collection were obtained by filtration. Yield: 60% (based on Mn). – Anal. for C₁₁H₇MnNO₅ (288.12): calcd. C 45.81, H 2.43, N 4.86; found C 45.83, H 2.40, N 4.88. – IR (KBr): ν = 3225(w), 1613(s), 1554(m), 1453(s), 1372(m), 1244(s), 1179(m), 1134(m), 903(w), 852(m), 793(m), 786(m), 662(w), 627(w), 498(w), 459(w) cm⁻¹.

2.4 X-ray crystallographic studies

Single-crystal data collections were performed on a Bruker Smart Apex II CCD diffractometer with graphite-monochromatized MoKα radiation (λ = 0.71073 Å) at T = 298(2) K. Using Olex2 [34], the structure was solved with the ShelxT structure solution program using Intrinsic Phasing [35] and refined with the ShelxL refinement package using least-squares minimization [36]. All non-hydrogen atoms were refined with anisotropic displacement parameters. Carbon-bound H atoms were placed in calculated positions (d _C–H = 0.93 Å) and were included in the refinement in the riding model approximation, with U _iso(H) set to 1.2U _eq(C). The H atoms of coordinated H₂O in 1 and 2 were also positioned geometrically and refined as riding atoms, with d _O–H = 0.85 Å and U _iso(H) = 1.5U _eq(O). The structures were examined using the ADDSYM subroutine of Platon [37] to ensure that no additional symmetry could be applied to the models. Pertinent crystallographic data collection and refinement parameters are collated in Table 1. Selected bond lengths and angles in 1 are given in Table 2. Hydrogen bond geometry data are presented in Table 3.

CCDC 2096886 (1) and 2096887 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data request/cif.

3.1 Description of the structure of {[Co(2,3-qldc)(H₂O)]} _n (1)

Single crystal X-ray diffraction data have revealed that 1 and 2 are a pair of 2D isostructural CPs and crystallize in the same monoclinic space group P2₁/n with similar cell parameters (Table 1). Thus only 1 represents 1 and 2 for the detailed structural description given here. There are one divalent Co ion, one fully deprotonated qldc²⁻ ligand, and one coordinated H₂O molecule in the asymmetric unit (Figure 1(a)). Each Co atom is hexa-coordinated with five O atoms (O1, O2ⁱ, O3ⁱ, O4ⁱⁱ, and O5, symmetry codes i and ii seen in Table 2) and one N atom (N1) from three symmetry-related 2,3-qldc^2– ligands and one H₂O molecule, resulting in a slightly distorted octahedral configuration, as shown in Figure 1(b). The Co–N bond length is 2.1419(17) Å, and the Co–O bond lengths are in the range of 2.0432(14)–2.1252(15) Å (Table 2). All the bond lengths are in agreement with those reported in other CPs based on the 2,3-H₂qldc ligand [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33].

Figure 1:

(a) View of the asymmetric unit of 1; (b) The coordination environment for Co²⁺ in 1 (symmetry codes: (i) −x+1, y−1/2, −z+3/2; (ii) −x+1, −y+1, −z+1).

The detailed coordination mode of the 2,3-qldc²⁻ ligand is shown in Figure 2(a). Notably, two carboxylate groups in 2,3-qldc^2– adopt the same type of coordination mode μ ₂-η ¹:η ¹. Four 2,3-qldc^2– ligands bridge four Co²⁺ cations by the above-mentioned fashion through their two carboxylate O atoms, resulting in quadrilateral substructural blocks {[Co(2,3-qldc²⁻)]₄} or {[Co(COO)]₄} with Co···Co separations of 5.548(5) and 5.588(5) Å, respectively (Figure 2(b) and (c)). These neighboring {[Co(2,3-qldc^2–)]₄} units are further interconnected through the 2,3-qldc^2– ligands to form a 2D framework (Figure 2(d)).

Figure 2:

(a) View of the coordination mode of 2,3-qldc^2– in 1; (b) The fragment of [Co(2,3-qldc)]₄ in 1; (c) The fragment of [Co(COO)]₄ in 1; (d) View of the 2D metal-organic framework (MOF) in 1; (e) The simplified 2D MOF in 1; (f) The 4-connected {4⁴·6²} topology in 1.

From the topological point of view, each {Co} can be considered to act as a connected node, and {COO} can be considered as a linker in 1 (Figure 2(e)). The alternatingly repeating {[Co(COO)]₄} clusters and ligands are connected in an infinite 2D 4-connected uninodal sql topology with a Schläfli symbol {4⁴·6²} and a Shubnikov tetragonal plane net (Figure 2(f)) as analyzed with the TOPOS 4.0 program [38]. Interestingly, each Co core unit acts as a 3-connected node, linked by three 2,3-qldc^2– ligands, and each 2,3-qldc^2–ligand chelates or bridges three Co atoms acting as a tridentate linker (Figure 3(a)–(d)). The 2D structure of 1 can be simplified as a 3-c unimodal fes topology with a Schläfli symbol {4·8²} and a Shubnikov plane net (Figure 4).

Figure 3:

(a) View of a Co²⁺ coordination with three 2,3-qldc^2– ligands in 1; (b) View of the simplified coordination of Co²⁺ with three 2,3-qldc^2– ligands in 1 (Pink ball represents Co²⁺, and green ball represents the core of 2,3-qldc); (c) View of the coordination mode of 2,3-qldc^2– in 1; (d) View of the simplified coordination mode of 2,3-qldc^2– in 1 (Pink ball represents Co²⁺, and green ball represents the core of 2,3-qldc).

Figure 4:

The 3-connected {4·8²} topology in 1.

In the crystal, the 2D polymeric arrays of structure 1 are further associated by a pair of hydrogen bonding interactions, involving the coordinated H₂O molecule and the carboxylate groups. Intermolecular O–H···O hydrogen bonds constitute one R 2 8 and two R 2 6 rings with the coordinated H₂O molecule (O5, donor, two-center linear) [39] and only one of the two η ¹:η ¹-bridged mode carboxyl moieties (O1 and O3, acceptor) (Figure 5(a) and (b) and Table 3). In addition a pair of intermolecular non-classical hydrogen bonds C–H···O also plays an important role in assembling the substructural unit [Co(2,3-qldc)(H2O)] into a 2D supramolecular network with carboxylate O atoms (O2 and O4 as acceptors), i.e., C8–H8···O2i and C10–H10···O4vi (Table 3). These hydrogen bonds inter-lock the 2D network in reversely alternating parallel arrangement (Figure 5(a) and (b)).

Figure 5:

(a) The 2D O–H···O hydrogen bonded framework in 1; (b) View of one R 2 8 and two R 2 6 O–H···O hydrogen bonded rings derived of 2,3-qldc^2– and a coordinated H₂O molecule in 1 (symmetry codes: (i) −x+1, y−1/2, −z+3/2; (iv) x, −y+3/2, z+1/2; (v) −x+1, −y+1, −z+2).

Furthermore, between adjacent 2D nets, two π···π stacking interactions are observed between the symmetry-related pyridine (N1–C2–C3–C5–C6–C7, Cg2)/benzene (C6–C7–C8–C9–C10–C11, Cg3) rings (Cg2 and Cg3^vii, symmetry code: vii −x, 1−y, 1−z), and benzene/benzene rings (Cg3 and Cg3^vi) (face-to-face). Interestingly, one Cg3-Cg3^vi and two Cg2-Cg3^vii interactions form a capital “N” shape (Figure 6). The dihedral angle between the two planes are 3.55(10) and 0.04(10)° with centroid-to-centroid distances of 3.7566(12), 3.8910(13) Å and perpendicular distances of 3.2604(8), 3.3254(9) Å, respectively. Thus, these layers are extended into an interwoven 3D supramolecular architecture through O–H···O, C–H···O, and π···π interactions.

Figure 6:

The scheme of the 2D MOF assembled into an interwoven 3D supramolecular architecture through π···π interactions.

3.2 Infrared spectra of 1 and 2

Comparing infrared spectra of the free ligand to those of its complexes provides information about the coordination of the ligand. Complexes 1 and 2 show similar infrared bands in the range 4000–450 cm⁻¹, which are different from the free ligand. The broad band centered at 3357 cm⁻¹ for 1 and 3225 cm⁻¹ for 2 reveal the O–H characteristic stretching vibration of the coordinated H₂O. Bands assigned to ν _as(COO⁻) and ν _s(COO⁻), which are observed for the free ligand at 1728 and 1407 cm⁻¹, respectively, are shifted to 1549 for 1, 1554 cm⁻¹ for 2 and 1456 for 1, 1453 cm⁻¹ for 2, indicating that deprotonation of the two carboxy-late groups occurred upon coordination. The differen-ces between the asymmetric and symmetric stretches, Δ(ν _as(COO⁻) − ν _s(COO⁻)) are smaller than 200 cm⁻¹, indicating bidentate-bridged carboxylates. All these spectroscopic features of 2,3-H₂qldc and 1–2 are consistent with the results of the crystal structure determinations.

3.3 PXRD and thermal analysis

In order to confirm the purity of 1 and 2, the PXRD patterns were checked at room temperature. The experimental PXRD patterns are almost identical with the simulated pattern based on the single-crystal data, confirming that 1 and 2 have been obtained as a pure crystalline phase (Figure 7). In order to explore the thermal stability of 1 and 2, TG studies have been performed in a nitrogen atmosphere at a heating rate of 10 K min⁻¹ between 50 and 800 °C. Compounds 1 and 2 have similar TG curves, the first observed weight loss from 150–210 °C corresponds to the coordinated water (found 6.25% and calculated 6.16% for 1, found 6.34% and calculated 6.25% for 2). Then a continuous decomposition of the framework occurs above 325 °C for 1 and, 415 °C for 2 (Figure 8).

Figure 7:

The simulated and experimental PXRD patterns of 1 and 2.

Figure 8:

The TG curves for 1 and 2.

3.4 Luminescent properties

Luminescent CPs have received much interest due to their potential applications in photocatalysis, biomedical imaging, and fluorescent sensors. The luminescence properties of 1 and 2 have been investigated in the solid state at room temperature (Figure 9). A broad emission peak of 1 is observed at 387 nm (λ _ex = 337 nm), which may be attributed to π* → n or π* → π transitions [11, 18]. The CPs 2 shows emission bands with a maximum at 422 nm (λ _ex = 293 nm). Compared with 1, the emissions of 2 are red-shifted by 35 nm, which may be assigned to different metal-to-ligand charge transfer processes (MLCT). Similar red shifts have been reported previously [40], [41], [42].

Figure 9:

The solid state fluorescence spectra of 1 and 2.