Influence of Metal Ions on the Structural Complexity of Mixed-Ligand Divalent Coordination Polymers

Abstract

The reactions of the angular ligand 4,4′-oxybis(N-(pyridin-3-yl)benzamide) (L¹) and 1,4-naphthalenedicarboxylic acid (1,4-H₂NDC) with divalent metal salts yielded three distinct coordination polymers (CPs): {[Zn₂(L¹)(1,4-NDC)₂]·MeOH}_n, 1, {[Cu(L¹)(1,4-NDC)(H₂O)]·3H₂O}_n, 2, and {[Cd(L¹)(1,4-NDC)]·2H₂O}_n, 3. Complex 1 features a 2-fold interpenetrated 3D framework with the (4¹²·6³)-pcu topology, while complex 2 reveals a 1D triple-strained helical chain and complex 3 displays a 3-fold interpenetrated 3D framework with (6⁶)-dia topology. Additionally, the reactions of the flexible ligand N,N′-bis(3-methylpyridyl) adipoamide (L²) afforded {[Co₄(L²)_0.5(1,4-NDC)₃(H₂O)₃(µ₃-OH)₂]·EtOH·2H₂O}_n, 4, {[Zn₂(L²)(1,4-NDC)₂]·2CH₃OH}_n, 5, and [Cd(L²)(adipic)(H₂O)]_n (H₂adipic = adipic acid), 6, exhibiting a self-catenated 3D framework with the (4²⁰·6⁸)-8T32 topology, a 2D layer with the (4¹³·6²) − (4,4)IIb topology, and a 2D layer with the (4⁴·6²)-sql topology, respectively. The structural diversity observed in complexes 1–6 highlights the pivotal influence of the metal center on the degree of entanglement in CPs within mixed-ligand systems. The thermal stability and luminescent properties of complexes 1–3, 4, and 6 are also discussed.

1. Introduction

The synthesis of coordination polymers (CPs) has attracted significant attention within the research community, not only due to their potential as novel zeolite-like materials for applications such as separation, ion exchange, and catalysis, but also because of their diverse structural topologies [1,2]. The construction of CPs relies on suitable metal-ligand interactions and supramolecular forces [3], which are influenced by various reaction conditions, including metal-to-ligand ratios, solvent systems, and temperature [4,5,6]. Additionally, the intriguing entanglements observed in CPs, such as interpenetration, polycatenation, and self-catenation, arise from the interweaving of independent motifs in various configurations [7,8]. Despite numerous reports on entangled CPs, controlling their structural dimensionality remains a key objective, and understanding the factors that govern this control continues to pose a significant challenge in the field of crystal engineering.

The flexible bis-pyridyl-bis-amide (bpba) ligands, known for their ability to adopt various conformations, offer significant advantages in the formation of entangled CPs. The reaction between copper sulfate and N,N′-di(4-pyridyl)adipoamide has been demonstrated to yield a twelve-fold interpenetrated diamondoid network [9]. Entangled CPs, constructed using flexible ligands such as N,N′-di(4-pyridyl)suberoamide [10] and N,N′-bis(pyrid-3-ylmethyl)adipoamide [11,12], have highlighted the crucial role that angular dicarboxylate ligands play in determining the degree of interpenetration. In our recent studies, we have shown that the combination of angular dicarboxylate ligands with flexible N,N′-di(4-pyridyl)sebacoamide and N,N′-di(4-pyridyl)adipoamide ligands enable these molecules to adapt to the steric demands required for the formation of entangled Co(II) CPs [13].

To compare the roles of angular and flexible bpba ligands in the formation of entangled CPs, we synthesized and reacted the 4,4′-oxybis(N-(pyridin-3-yl)benzamide) (L¹), shown in Figure 1a, and N,N′-bis(3-methylpyridyl)adipoamide (L²), shown in Figure 1b, with dicarboxylic acid and various divalent metal salts. This resulted in the formation of {[Zn₂(L¹)(1,4-NDC)₂]·MeOH}_n (1,4-H₂NDC = naphthalene-1,4-dicarboxylic acid), 1; {[Cu(L¹)(1,4-NDC)(H₂O)]·3H₂O}_n, 2; {[Cd(L¹)(1,4-NDC)]·2H₂O}_n, 3; {[Co₄(L²)_0.5(1,4-NDC)₃(H₂O)₃(µ₃-OH)₂]·EtOH·2H₂O}_n, 4; {[Zn₂(L²)(1,4-NDC)₂]·2CH₃OH}_n, 5; and [Cd(L²)(adipic)(H₂O)]_n (H₂adipic = adipic acid), 6. Structural comparison of these CPs reveals that the identity of the metal plays a significant role in the formation of entangled CPs when mixed ligands are involved. This report focuses on the synthesis and structural characterization of complexes 1–6, along with an evaluation of their thermal and luminescent properties.

2. Materials and Methods

2.1. General Procedures

Solid-state IR spectra, powder X-ray diffraction (PXRD) patterns, and elemental analyses were carried out using a JASCO FT/IR-460 plus spectrometer (JASCO, Easton, MD, USA), a Bruker D2 PHASER diffractometer (Bruker Corporation, Karlsruhe, Germany), and a PE 2400 series II CHNS/O analyzer (PerkinElmer Instruments, Shelton, CT, USA) respectively. Thermogravimetric analysis (TGA) curves were recorded with an SII Nano Technology TGA/DTA 6200 analyzer (Seiko Instruments Inc., Torrance, CA, USA). Solid-state emission spectroscopy (with emission and excitation slit widths set at 5.0 nm) was conducted using a Hitachi F-4500 spectrometer (Hitachi, Tokyo, Japan).

2.2. Materials

The reagents zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O) and copper acetate dihydrate (Cu(CH₃COO)₂·2H₂O) were purchased from SHOWA Co. (Saitama, Japan). Cobalt acetate tetrahydrate (Co(CH₃COO)₂·4H₂O), cadmium acetate monohydrate (Cd(CH₃COO)₂·H₂O), naphthalene-1,4-dicarboxylic acid (1,4-H₂NDC), and adipic acid (H₂adipic) were sourced from Alfa Aesar (Ward Hill, MA, USA). The ligand 4,4′-oxybis(N-(pyridin-3-yl)benzamide) (L¹) [14,15] and N‚N′-bis(3-methylpyridyl)adipoamide (L²) [11] were synthesized following published procedures.

2.3. Preparations

2.3.1. {[Zn₂(L¹)(1,4-NDC)₂]·MeOH}_n, 1

A mixture containing Zn(CH₃COO)₂·2H₂O (0.022 g, 0.10 mmol), L¹ (0.041 g, 0.10 mmol) and 1,4-H₂NDC (0.022 g, 0.10 mmol) was prepared in 10 mL of methanol (MeOH) and placed in a 23 mL Teflon-lined stainless steel vessel. The vessel was then sealed and subjected to heating at 100 °C for 48 h under autogenous pressure. Afterward, it was slowly cooled to room temperature at a rate of 2 °C per hour. Colorless, columnar crystals suitable for single-crystal X-ray diffraction were obtained, washed with MeOH, and dried in a vacuum. The final yield was 0.024 g (49%). Anal. calcd for C₄₉H₃₄N₄O₁₂Zn₂ (MW = 1001.54): C, 58.76; N, 5.60; H, 3.42%. Found: C, 57.77; N, 5.92; H, 2.69%. IR (cm⁻¹): 3649(w), 3527(w), 3075(m), 1588(s), 1540(s), 1497(s), 1426(s), 1364(s), 1238(s), 1170(s).

2.3.2. {[Cu(L¹)(1,4-NDC)(H₂O)]·3H₂O}_n, 2

The preparation of complex 2 followed a similar procedure to that of complex 1, with the key difference being the use of a mixture containing Cu(CH₃COO)₂·2H₂O (0.02 g, 0.10 mmol), L¹ (0.041 g, 0.10 mmol), and 1,4-H₂NDC (0.022 g, 0.10 mmol) in a solvent mixture of 2 mL of MeOH and 8 mL of water (H₂O). Blue crystals were obtained, which were subsequently washed with MeOH. Yield: 0.099 g (57%). Anal. calcd for C₃₆H₃₂CuN₄O₁₁ (MW = 760.19): C, 56.88; N, 7.37; H, 4.24%. Found: C, 56.52; N, 7.61; H, 4.21%. IR (cm⁻¹): 3443(s), 3388(s), 1632(s), 1555(s), 1492(s), 1425(s), 1324(s), 1246(s).

2.3.3. {[Cd(L¹)(1,4-NDC)]·2H₂O}_n, 3

The synthetic procedure of complex 3 is similar to that of complex 1, with the exception that a mixture containing Cd(CH₃COO)₂ H₂O (0.03 g, 0.10 mmol), L¹ (0.041 g, 0.10 mmol), and 1,4-H₂NDC (0.022 g, 0.10 mmol) in 8 mL of ethanol and 2 mL of H₂O was used. Colorless crystals were obtained, which were then washed with ethanol (EtOH). Yield: 0.057 g (74%). Anal. calcd for C₃₆H₂₈CdN₄O₉ (MW = 773.02): C, 55.93; N, 7.25; H, 3.65%. Found: C, 54.80; N, 6.97; H, 3.68%. Anal Calcd for C₃₆H₂₈CdN₄O₉ + H₂O (MW = 791.068): C, 54.66; N, 7.08; H, 3.82%. IR (cm⁻¹): 3554.9(s), 3275.9(s), 1558.8(m), 1550.4(m), 1234.9(s), 1172.9(m), 1071(m), 873.6(s), 744.9(s).

2.3.4. {[Co₄(L²)_0.5(1,4-NDC)₃(H₂O)₃(µ₃-OH)₂]·EtOH·2H₂O}_n, 4

A Teflon-lined steel autoclave with a capacity of 23 mL was sealed after introducing Co(OAc)₂·4H₂O (0.025 g, 0.10 mmol), L² (0.033 g, 0.10 mmol), 1,4-H₂NDC (0.022 g, 0.10 mmol), 2 mL of water, and 8 mL of ethanol. The autoclave was then heated to 120 °C for two days, followed by slow cooling to room temperature. Purple crystals formed and were subsequently collected and purified. Yield: 0.0055 g (18%). Anal. calcd for C₄₇H₄₇Co₄N₂O₂₁ (MW = 1211.58): C, 46.59; N, 2.31; H, 3.91%. Found: C, 45.91; N, 2.14; H, 3.99%. FT-IR (cm⁻¹): 3296(m), 1600(s), 1411(s), 1366(s), 785(m).

2.3.5. {[Zn₂(L²)(1,4-NDC)₂]·2CH₃OH}_n, 5

The preparation of complex 5 followed the same procedure as for complex 4, with the exception that Zn(OAc)₂·2H₂O (0.044 g, 0.20 mmol), L² (0.098 g, 0.30 mmol), 1,4-H₂NDC (0.043 g, 0.20 mmol), and 10 mL methanol were used. This resulted in the formation of colorless crystals. Yield: 0.080 g (84%). Anal. calcd for C₄₄H₄₂N₄O₁₂Zn₂ + 2H₂O (MW = 985.63): C, 53.62; N, 5.68; H 4.70%. Found: C, 53.89; N, 5.94; H, 4.00%. FT-IR (cm⁻¹): 3420(s), 1628(m), 1412(m), 1366(m), 828(w), 794(w), 773(w), 700(w), 669(w).

2.3.6. [Cd(L²)(adipic)(H₂O)]_n, 6

The synthesis of complex 6 mirrored that of complex 4, with the exception that Cd(OAc)₂·2H₂O (0.027 g, 0.10 mmol), L² (0.033 g, 0.10 mmol), and adipic acid (0.015 g, 0.10 mmol) were used in a solvent mixture of 8 mL H₂O and 2 mL ethanol. This process yielded colorless crystals. Yield: 0.012 g (21%). Anal. calcd for C₂₄H₃₂CdN₄O₇ (MW = 600.93): C, 47.97; N,9.32; H, 5.37%. Found: C, 47.61; N, 9.05; H, 5.49%. FT-IR (cm⁻¹): 3415(s), 3262(s), 1643(s), 1542(s), 2922(m), 1306(w), 647(w).

The phase purity of complexes 1–6 was verified through powder X-ray diffraction (PXRD). As shown in Figures S1–S6, the experimental PXRD results align closely with the simulated patterns, confirming the bulk purity of these complexes.

2.4. X-ray Crystallography

The diffraction measurements for complexes 1–6 were obtained using a Bruker AXS SMART APEX II CCD diffractometer equipped with a graphite-monochromated MoK radiation source (α = 0.71073 Å). The collected data underwent reduction, incorporating Lorentz and polarization adjustments, alongside an empirical absorption correction via the multi-scan method, utilizing established computational procedures [16]. The positions of the heavier atoms were identified using the pattern or direct methods, while the lighter atoms were positioned through sequential difference Fourier maps and least-square refinements. Hydrogen atoms, excluding those in water molecules, were introduced using the HADD command in SHELXTL 6.1012 [17]. The crystal data for complexes 1–6 are summarized in

Table 1. Crystal data for complexes 1–6.

	1	2	3
Formula	C49H34N4O12Zn2	C36H32CuN4O11	C36H28CdN4O9
Formula weight	1001.54	760.19	773.02
crystal system	Monoclinic	Orthorhombic	Monoclinic
space group	C2/c	P212121	C2/c
a, Å	16.4613(4)	10.8657(4)	29.3869(4)
b, Å	14.2488(3)	16.0134(5)	10.0253(1)
c, Å	20.9984(5)	19.2892(7)	21.6458(3)
α, °	90	90	90
β, °	105.7361(13)	90	93.1008(7)
γ, °	90	90	90
V, Å3	4740.66(19)	3356.3(2)	6367.79(14)
Z	4	4	8
dcalc, mg/m3	1.403	1.504	1.613
F(000)	2048	1572	3136
µ(MoKα), mm−1	1.078	0.721	0.752
range(2θ) for data collection, deg	3.84 ≤ 2θ ≤ 52.00	3.30 ≤ 2θ ≤ 56.64	2.77 ≤ 2θ ≤ 56.61
independent reflections	4678 [R(int) = 0.0368]	8348 [R(int) = 0.0342]	7862 [R(int) = 0.0247]
data/restraints/parameters	4678/1/319	8348/0/470	7862/1/451
quality-of-fit indicator c	1.010	1.017	1.060
final R indices [I > 2σ(I)] a,b	R1 = 0.0602, wR2 = 0.1417	R1 = 0.0377, wR2 = 0.0841	R1 = 0.0292, wR2 = 0.0705
R indices (all data)	R1 = 0.0635, wR2 = 0.1429	R1 = 0.0526, wR2 = 0.0904	R1 = 0.0370, wR2 = 0.0746
	4	5	6
Formula	C47H47Co4N2O21	C44H42N4O12Zn2	C24H32CdN4O7
Formula weight	1211.58	949.55	600.93
crystal system	Monoclinic	Monoclinic	Monoclinic
space group	P21/c	P21/c	C2/c
a, Å	11.5475(13)	18.5332(3)	19.7055(7)
b, Å	21.944(2)	15.1797(3)	15.5586(6)
c, Å	20.800(2)	15.6862(3)	8.7536(3)
α, °	90	90	90
β, °	92.035(2)	107.0289(10)	111.408(2)
γ, °	90	90	90
V, Å3	5267.1(10)	4219.50(14)	2498.60(16)
Z	4	4	4
dcalc, mg/m3	1.528	1.495	1.597
F(000)	2476	1960	1232
µ(Mo Kα), mm−1	1.316	1.206	0.926
range(2θ) for data collection, deg	3.53 ≤ 2θ ≤ 52.00	3.53 ≤ 2θ ≤ 52.00	3.43 ≤ 2θ ≤ 56.57
independent reflections	10343 [R(int) = 0.1108]	8290 [R(int) = 0.0528]	3091 [R(int) = 0.0608]
data/restraints/parameters	10343/1101/649	8290/2/549	3091/0/168
quality-of-fit indicator c	1.051	1.013	1.018
final R indices [I > 2σ(I)] a,b	R1 = 0.0766, wR2 = 0.1945	R1 = 0.0475, wR2 = 0.1033	R1 = 0.0401, wR2 = 0.0670
R indices (all data)	R1 = 0.1500, wR2 = 0.2315	R1 = 0.1029, wR2 = 0.1240	R1 = 0.0728, wR2 = 0.0758

^a R₁ = Σ‖F_o| − |F_c‖/Σ|F_o|. ^b wR₂ = [Σ_W(F_o² − F_c²)²/Σw(F_o²)²]^1/2. w = 1/[σ²(F_o²) + (ap)² + (bp)], p = [max(F_o² or 0) + 2(F_c²)]/3. a = 0, b = 75.1500 for 1; a = 0.0427, b = 1.2489 for 2; a = 0.0321, b = 9.5066 for 3; a = 0.1155, b = 6.7541 for 4; a = 0.0436, b = 6.4686 for 5; a = 0.0264, b = 0.0592 for 6. ^c quality-of-fit = [Σ_W(|F_o²| − |F_c²|)²/(N_observed − N_parameters)]^1/2.

.

3. Results and Discussions

3.1. Structure of 1

The X-ray crystallography analysis shows that complex 1 forms in the monoclinic space group C2/c. Each asymmetric unit comprises two Zn(II) ions, one L¹ ligand, two 1,4-NDC²⁻ ligands, and one coordinated MeOH molecule. Figure 2a depicts the coordination environment surrounding the dinuclear Zn(II) metal centers [Zn---Zn = 3.0037(10) Å]. Each of the symmetry-related Zn(II) ions is five-coordinated by one pyridyl nitrogen atom [Zn-N = 2.034(4) Å] from the L¹ ligand and four carboxylate oxygen atoms [Zn-O = 1.993(4)–2.09(5) Å] from four 1,4-NDC²⁻ ligands, resulting in distorted square pyramidal geometries (τ₅ = 0.20; τ₅ = 1 and 0 indicate trigonal bipyramidal and square pyramidal geometries, respectively) [18]. The dinuclear paddlewheel Zn(II) units are further extended through the connection of L¹ and 1,4-NDC²⁻ ligands, resulting in a three-dimensional (3D) network. In terms of topology, when treating the Zn(II) ions as six-connected nodes, 1,4-NDC²⁻ ligands as four-connected nodes, and the L¹ ligands as connectors, the structure of 1 can be represented as a 3D framework with the rare (3²·6²·7²)(3⁴·4⁶·6⁴·7)-sqc493 topology (standard representation) [19], as depicted in Figure 2b, showing 2-fold interpenetration, illustrated in Figure 2c. Furthermore, by considering the dinuclear units as 6-connected nodes where L¹ and 1,4-NDC²⁻ ligands act as linkers, the structure of 1 can be represented as a 6-connected, 2-fold interpenetrating 3D network featuring the (4¹²·6³)-pcu topology, as shown in Figure 2d,e.

3.2. Structure of 2

The crystals of complex 2 adopt the orthorhombic, non-centrosymmetric space group P2₁2₁2₁. Each asymmetric unit comprises a Cu(II) ion, an L¹ ligand, a 1,4-NDC²⁻ ligand, one coordinated H₂O molecule, and three co-crystallized H₂O molecules. As depicted in Figure 3a, the Cu(II) ion exhibits a coordination number of five, involving two pyridyl nitrogen atoms [Cu-N = 2.075(3) and 2.077(2) Å] from two L¹ ligands, two carboxylate oxygen atoms [Cu-O = 1.962(2)–1.973(2) Å] from two 1,4-NDC²⁻ ligands, and one oxygen atom [Cu-O = 2.346(3) Å] from the coordinated water molecule. This configuration forms a square pyramidal geometry (τ₅ = 0.01). Through the linkage provided by the L¹ ligands, the Cu(II) ions are arranged into a one-dimensional (1D) 2-fold [CuL¹]_n helix with a pitch of 17.31 Å. Notably, the angular backbones of the L¹ ligands enable the interleaving of two additional helices, resulting in a fascinating triple-stranded helix within the structure of complex 2, as shown in Figure 3b. The triple-stranded helices are further stabilized by linear chains formed by the 1,4-NDC²⁻ ligands, resulting in the formation of a 1D single-walled nanotubular framework, as depicted in Figure 3c.

The triple-stranded structure of complex 2 stands in marked contrast to the quadruple-stranded structure of [Co(L³)(2,4-PDC)(H₂O)]_n [where L³ = N,N′-di(3-pyridyl)suberoamide and 2,4-H₂PDC = 2,4-pyridinedicarboxylic acid) [20], as well as the quintuple-stranded structure of [Zn(2,4-PDC)(L⁴)(H₂O)]_∞ (where L⁴ = N,N′-di(3-pyridyl)dodecanediamide) [21]. This observation suggests that the number of helices is likely influenced by the combined effects of the metal and ligand identities.

3.3. Structure of 3

The structure of complex 3 was determined in the C2/c monoclinic space group, where the asymmetric unit comprises a Cd(II) ion, one 1,4-NDC²⁻ ligand, one L¹ ligand, and two co-crystallized water molecules. Figure 4a shows the coordination geometry around the dinuclear Cd(II) metal centers. Each symmetry-equivalent Cd(II) ion is coordinated by six atoms: two nitrogen atoms from pyridyl groups [Cd-N = 2.3642(19) and 2.3473(18) Å] of two L¹ ligands and four oxygen atoms from carboxylate groups [Cd-O = 2.3053(14)–2.4408(15) Å] of three 1,4-NDC²⁻ ligands, resulting in a distorted octahedral geometry. Topologically, when considering the Cd(II) ions as four-connection nodes and the 1,4-NDC²⁻ ligands as three-connection nodes, with the L¹ ligands acting as linkers, the structure of complex 3 can be simplified into a 3D network with a (4²·6.10²·12)(4²·6)-coe-3,4-C2/c topology (Figure 4b) exhibiting a 3-fold interpenetration (standard representation), as seen in Figure 4c. Furthermore, when the dinuclear units are considered as 4-connected nodes and the L¹ and 1,4-NDC²⁻ ligands as linkers, the structure of complex 3 can be further simplified into a 3-fold interpenetrated 3D network with a (6⁶)-dia topology (cluster representation), as depicted in Figure 4d,e.

3.4. Structure of 4

Complex 4 crystallizes in the monoclinic space group P2₁/c, with each asymmetric unit comprising four Co(II) ions, a half of an L² ligand positioned at independent inversion centers, three 1,4-NDC²⁻ ligands, two μ₃-OH⁻ groups, three coordinated water molecules, and two co-crystallized water molecules, as shown in Figure 5a. Figure 5b illustrates the coordination environment around the Co(II) metal centers, where Co(1), Co(2), and Co(3) atoms exhibit distorted octahedral geometries, while Co(4) displays a distorted square pyramidal geometry (τ₅ = 0.43). The Co(1) atom is coordinated by six oxygen atoms from four 1,4-NDC²⁻ ligands [Co-O = 2.055(5)–2.161(5) Å], one OH⁻ group [Co-O = 2.110(4) Å] and one coordinated water molecule [Co-O = 2.068(4) Å]. The Co(2) atom is coordinated by six oxygen atoms from three 1,4-NDC²⁻ ligands [Co-O = 2.068(5)–2.134(4) Å], two OH⁻ groups [Co-O = 2.111(4)–2.131(4) Å], and a water molecule with a Co-O bond length of 2.075(4) Å]. On the other hand, the Co(3) ion is bound to one nitrogen atom from the L² ligand [Co-N = 2.095(5) Å] and five oxygen atoms from three 1,4-NDC²⁻ ligands, with Co-O distances spanning from 2.086(5) to 2.189(4) Å, alongside one hydroxyl group [Co-O = 2.092(4) Å] and a coordinated water molecule [Co-O = 2.095(5) Å]. Finally, the Co(4) ion is coordinated by five oxygen atoms from three 1,4-NDC²⁻ ligands [Co-O = 2.025(4)–2.130(5) Å] and two OH⁻ groups [Co-O = 1.984(4)–2.019(4) Å]. Additionally, two symmetry-related Co₄(1,4-NDC)₃(L)_0.5(H₂O)₃(µ₃-OH)₂ units are linked together to form a Co₈ cluster, which are further interconnected by L² and 1,4-NDC²⁻ ligands to create a 3D framework.

Topologically, the Co(II) ions can be regarded as five-connected nodes, the 1,4-NDC²⁻ ligands as four- and five-connected nodes and the µ₃-OH ligands as three-connected nodes, with the L² ligands acting as linkers. This arrangement results in a 3D framework with a (4²·6·8³)(4²·6²·7·8)(4²·6)₂(4³·6·7²·8⁴)(4⁴·6⁴·7·8)(4⁴·7·8⁴·9)(4⁶·6⁴)₂-3,3,4,4,5,5,5,5,5T1 topology, Figure 5c. Furthermore, if the Co₈ clusters are considered as eight-connected nodes, the structure of complex 4 can be simplified into a uninodal self-penetrating network with a (4²⁰·6⁸)-8T32 topology, as shown in Figure 5d. For comparison, a similar coordination polymer with mixed ligands, having the formula {[Co₈(μ₃–OH)₄(1,4-NDC)₆(btp)(H₂O)₆]·H₂O}_n [btp = 4,4′-bis(triazol-1-ylmethyl)biphenyl)] has been reported [22].

3.5. Structure of 5

The crystal structure of complex 5 was resolved in the P2₁/c monoclinic space group. The asymmetric unit consists of two Zn(II) ions, one L² ligand, two 1,4-NDC²⁻ ligands, and two methanol molecules co-crystallized within the structure. Figure 6a displays the coordination sphere surrounding the Zn(II) centers, showing that both Zn(1) (τ₅ = 0.11) and Zn(2) (τ₅ = 0.11) exhibit distorted square pyramidal configurations. Each The Zn(1) and Zn(2) ion is bonded to four oxygen atoms from the four 1,4-NDC²⁻ ligands [Zn(1)-O = 2.039(3)–2.064(3) Å; Zn(2)-O = 2.026(3)–2.072(3) Å] and one nitrogen atom from the L² ligand [Zn(1)-N(1) = 2.018(3) Å; Zn(2)-N(4C) = 2.026(3) Å]. The two Zn(II) ions are bridged by four 1,4-NDC²⁻ ligands in a paddlewheel fashion, which are further axially linked by the L² ligands to form two-dimensional (2D) layers. Topologically, when considering the Zn(II) ions as five-connected nodes, the 1,4-NDC²⁻ ligands as four-connected nodes, and the L² ligands as linkers, the structure of complex 5 can be reduced to a 4,5-coordinated 2D network with a new (4²·6³·8)(4⁶·6⁴) topology (standard representation), as illustrated in Figure 6b. Alternatively, when the dinuclear units are considered as four-connected nodes and the organic ligands are viewed as linkers, the structure of complex 5 can be further reduced to a six-connected 2D net with a (4¹³·6²) − (4,4)IIb topology (cluster representation), as depicted in Figure 6c.

3.6. Structure of 6

Crystals of complex 6 conform to the monoclinic space group C2/c, with each asymmetric unit containing half of a Cd(II) ion, half of an L² ligand, half of an adipic²⁻ ligand, and half of a coordinated water molecule. Figure 7a illustrates the coordination environment surrounding the Cd(II) metal center. The Cd(II) ion is bonded to four oxygen atoms from two adipate ligands [Cd-O = 2.384(2) and 2.457(2) Å], two nitrogen atoms from two L ligands [Cd-N = 2.344(2)], and one oxygen atom from a water molecule [Cd-O(4) = 2.320(4) Å], forming a distorted pentagonal bipyramidal structure. The Cd(II) ions are connected through the L² and adipic²⁻ ligands, forming pairs of 2D layers, Figure 7b, that interdigitate with each other through water molecules, as shown in Figure 7c,d. Topologically, if the Cd ions are considered four-connected nodes, and the organic ligands as linkers, the structure of complex 6 can be further simplified into a four-coordinated 2D network with a (4⁴·6²)-sql topology, as depicted in Figure 7e.

The combination of the angular L¹ or flexible L² with the rigid, linear dicarboxylate 1,4-NDC²⁻ resulted in the formation of interpenetrated CPs 1 and 3, as well as a self-catenated 3D framework in complex 4. In a previous study, we proposed that Co(II) CPs, stabilized by the angular dicarboxylate ligand and a flexible bpba ligand, could be tailored to accommodate the steric demands required for the development of the entangled networks [14]. The fact that complexes 1, 3, and 4 exhibit entangled structures, while the other two do not, suggests that the identity of the metal ion plays a crucial role. When supported by 1,4-NDC²⁻, the angular L¹ may form the entangled networks if the appropriate metal ion is used. Conversely, the combination of 1,4-NDC²⁻ with flexible bpba ligands led to the formation of non-entangled Cd(II) CPs, including {[Cd(L²)(1,4-NDC)(H₂O)]·H₂O}_n, {[Cd(L^A)(1,4-NDC)]·2H₂O}_n [L^A = bis(N-pyrid-3-ylmethyl)suberoamide], and {[Cd₂(L^A)(1,4-NDC)₂]·3H₂O}_n. These structures exhibit a 2D layer characterized by the (4⁴·6²)-sql topology, alongside 3D frameworks featuring the (3²·6²·7²)(3²·6⁵·7³)₂-4,5T61 and (3²·5⁴)(3⁴·4⁶·5⁸·6⁶·7⁴)-sqc776 topologies, respectively [23]. Interestingly, a structural comparison of complex 6 with {[Cd(L²)(1,4-NDC)(H₂O)]·H₂O}_n, which shares the same structural topology, indicates that the Cd(II) ion is a key factor in determining the structural type.

3.7. Ligand Conformation and Bonding Mode

The conformations of the bpba ligands are defined as follows: If the torsional angle (θ) of the bridging carbon atoms is within the range of 0 ≤ θ ≤ 90°, the conformation is classified as gauche (G); if 90 < θ ≤ 180°, it is classified as anti (A). Additionally, the terms cis and trans are used to describe whether the two C = O groups are oriented in the same direction (cis) or in the opposite direction (trans). Due to varying orientations of the pyridyl nitrogen atoms and amide oxygen atoms, bpba ligands can exhibit three additional conformations: syn-syn, syn-anti and anti-anti. In this context, the L¹ ligands in complex 1 adopt a cis syn-syn conformation, while in complexes 2 and 3, they adopt the trans syn-anti conformation. The L² ligands in complexes 4, 5, and 6 exhibit the following conformations, respectively: GAG trans anti-anti, GAG cis anti-anti, and AAA trans syn-syn (see

Table 2. Ligand conformations of L¹ and L² and coordination modes of dicarboxylate ligands in 1–6.

	Ligand Conformation	Coordination Mode
1
2
3
4
5
6

for details). Furthermore, the dicarboxylate ligands in complexes 1–6 bridge between two to five metal ions, resulting in diverse coordination modes. Notably the 1,4-NDC²⁻ ligands in complex 4 adopt three different coordination modes, leading to the formation of a self-catenated 3D framework.

The structural differences in L¹ ligands of complexes 1–3 can be further analyzed. As illustrated in Figure 8, distances d1 and d2 indicate the separation between the bridging oxygen atom and the two metal ions, whereas d3 measures the distance between the two metal ions connected by the L¹ ligand. Additionally, θ1, θ2_, and θ3 refer to the dihedral angles between the adjacent six-membered rings, specifically the angles between rings I and II, II and III, and III and IV, respectively. The C-O-C angle represents the bond angle around the bridging oxygen atom.

Table 3. Comparison of angles and distances for Complexes 1–3.

Complex	d1 (Å)	d2 (Å)	d3 (Å)	θ1 (°)	θ2 (°)	θ3 (°)	C-O-C
1	11.24	11.24	20.29	41.67	51.07	41.67	123.18
2	11.17	11.39	17.31	52.23	59.04	13.89	121.16
3	11.10	11.43	21.55	77.93	27.74	16.66	121.09

shows that the distances d1, d2, and d3, as well as the C-O-C angles, are similar across complexes 1–3. However, the θ1, θ2_, and θ3 angles exhibit significant variations, highlighting the influence of the metal ions on the structural configuration of the L¹ ligand. These differences contribute to the distinct structural types observed in complexes 1–3.

3.8. Structural Comparisons

The reactions of divalent metal salts with 4,4′-oxybis(N-(pyridine-4-yl)-benzamide L³, an isomer of L¹, and 1,4-H₂NDC in different solvents led to the synthesis of [Zn(L³)(1,4-NDC)·H₂O]_n, [Cd(L³)(1,4-NDC)(H₂O)·MeOH]_n, and [Co(L³)(1,4-NDC)(H₂O)0.5·MeOH]_n. These compounds all feature eight-fold interpenetrating frameworks adopting the dia topology [24]. Structural comparisons between these three L³-based CPs and the L¹-based CPs 1–3 reveal that the isomerism of L¹ and L³ plays distinct roles in determining structural diversity. While L³ primarily governs the structural diversity of the resulting CPs, the structures of CPs containing L¹ are more influenced by the identity of the metal ion.

Table 4. The structures of metal complexes containing L¹ or L³.

Complex	Structure	Reference
[{Cu2(L1)(adman)4}·3DMF]2	Dinuclear complex	[15]
{[Cd(L1)(5-NIP)(H2O)]·H2O}n	2D, sql	[25]
{[Cd(L1)(2,5-TPD)]·DMA}n	2D, (42·67·8)(42·6)	[25]
{[Cd(L1)(2,5-TPD)]·2H2O}n	2D, (42·67·8)(42·6)	[25]
{[Zn(L1)(2,5-TPD)]·2H2O}n	2D, {42·62}	[25]
{[Zn(L1)(1,3-BDC)]·H2O}n	2D, {42·62}	[25]
{[Zn2(L1)(5-AIP)2]·2H2O}n	2D, {63}{66}	[25]
[{Cd(L1)(IPA)}(H2O)2]n	2D, {42·67·8}{42·6}	[14]
[{Cd(L1)(TPA)}(H2O)]n	2D, sql	[14]
[{Cd(L1)2(1,4-PDA)2(H2O)2}(H2O)6 ]n	1D, {42·6}	[14]
[{Cd2(L1)2(1,2-CTA)2}(H2O)4]n	2D, {33·410·5·6}	[14]
{[Co(L1)(chdc)]}n	1D wave-like double chain	[26]
{[Zn(L1)(hip)]·2.8H2O}n	2D, sql	[26]
{[Zn2(L1)(chdc)2]·2H2O}n	2D, (42·63·8)(42·6)	[27]
{[Zn(L1)(mip)]·3H2O}n	2D, sql	[27]
{[Zn(L1)(1,3-BDC)]·H2O}n	2D, sql	[27]
{[Cd2(L1)(chdc)2]·2H2O}n	2D, (42·63·8)(42·6)	[27]
{[Cd(L1)(mip)]·2H2O}n	2D, (42·67·8)(42·6)	[27]
{[Cd(L1)(1,3-BDC)]·2H2O}n	2D, (42·67·8)(42·6)	[27]
{[Zn2(L1)(1,4-NDC)2]·MeOH}n, 1	2-fold interpenetrated 3D, sqc493 (pcu)	this work
{[Cu(L1)(1,4-NDC)(H2O)]·3H2O}n, 2	1D triple-strained helical chain	this work
{[Cd(L1)(1,4-NDC)]·2H2O}n, 3	3-fold interpenetrated 3D, coe (dia)	this work
[{Co2(L3)2(1,4-PDA)2}(H2O)2]n	2D, {412·52·6}	[14]
[{Co(L3)(1,3-PDA)}(H2O)2]n	2D, sql	[14]
[{Co(L3)(IPA)}(DMF)2(H2O)]n	2D, sql	[28]
[{Co2(L3)2(TPA)2}(DMF)4]n	2-fold interpenetrated 3D, pcu	[28]
{[Ni(L3)(1,4-BDC)]·2DMF}n	2-fold interpenetrated 3D, pcu	[29]
{[Co(L3)(TDC)]·2DMF}n	2-fold interpenetrated 3D	[29]

adman = 1-adamantanecarboxylic acid; 5-H₂NIP = 5-nitroisophthalic acid; 2,5-H₂TPD = 2,5-thiophenedicarboxylic acid; 1,3-H₂BDC = 1,3-benzenedicarboxylic acid; 5-H₂AIP = 5-aminoisophthalic acid; H₂IPA = isophthalate; H₂TPA = terephthalate; 1,2-H₂CTA = 1,2-carboxytranscinamate; 1,4-H₂PDA = 1,4-phenylene dicarboxylate; 1,3-H₂PDA = 1,3-phenylene dicarboxylate; 1,4-H₂BDC = 1,4-benzenedicarboxylic acid; H₂TDC = thiopehenedicarboxylic acid; H₂chdc = trans-1,4-cyclohexanedicarboxylic acid; H₂mip = 5-methylisophthalic acid; H₂hip = 5-hydroxyisophthalic acid.

summarizes the structures of complexes containing L¹ or L³ ligands, highlighting the importance of the combined effects of metal and ligand identities, as well as the natures of the solvent molecules, in determining structural diversity. Notably, while entangled CPs are commonly found in L³-based CPs, complexes 1 and 3 are unique examples of entangled CPs among those based on L¹.

3.9. Thermal Properties

Thermogravimetric analyses (TGA) were conducted to investigate the thermal decomposition of complexes 1–6, as summarized in

Table 5. Thermal properties of 1–6.

Complex	Weight Loss of Solvent, T, °C (Observed/Calcd),%	Weight Loss of Ligand, T, °C (Observed/Calcd),%
1	MeOH, 30–120 (3.20/2.36)	L1 + 1,4-NDC2−, 300–800 (62.36/60.22)
2	3H2O, 30–120 (7.11/7.90)	L1 + 1,4-NDC2 + H2O, 240–900 (84.53/81.72)
3	2H2O, 30–150 (4.66/5.37)	L1 + 1,4-NDC2−, 270–800 (80.80/81.54)
4	20–330 (15.5/15.1)	330–700 (65.2/60.7)
5	30–120 (4.3/6.7)	120–800 (70.0/79.9)
6	100–230 (4.6/3.0)	230–800 (79.5/78.6)

and illustrated in Figures S7–S12. Complexes 1–6 exhibit two-step weight loss patterns corresponding to the removal of the solvents followed by the decomposition of the ligands. Notably, complex 4 displays the highest onset temperature for the removal of organic ligands, which leads to the collapse of the framework. This suggests that the self-catenated 3D framework in complex 4 may possess enhanced structural stability compared to the other complexes.

3.10. Luminescent Properties

The solid-state excitation and emission spectra of L¹, L², 1,4-H₂NDC, adipic acid, and complexes 1, 3, 5, and 6 were investigated at room temperature, as shown in Figures S13–S17. The corresponding excitation and emission wavelengths are listed in

Table 6. The excitation and emission wavelengths of L¹, 1,4-H₂NDC, L² and adipic acid and complexes 1, 3, 5, and 6 in solid state.

Compound	λex (nm)	λem (nm)	Complex	λex (nm)	λem (nm)
L1	318	410	1	340	386
1,4-H2NDC	280, 360sh	480	3	334	381
L2	300	460	5	342	385
Adipic acid	235	360	6	272	297

. The free ligands L¹, L², 1,4-H₂NDC, and adipic acid exhibit emissions at 410, 480, 460, and 360 nm, respectively. These emissions may be ascribed to intraligand (IL) transitions involving n → π* or π → π* shifts. However, emissions from complexes 1, 3, 5, and 6 occur at 386, 381, 385, and 297 nm, respectively. Due to the d¹⁰ electronic configuration of Zn(II) and Cd(II) ions, which prevents notable oxidation or reduction, these emissions are most likely caused by ligand-to-ligand charge transfer (LLCT), potentially with some metal-to-ligand charge transfer (MLCT) contributions [30,31]. The red and blue shifts observed with reference to the free ligands likely result from variations in ligand conformations, coordination modes, and the formation of different structural arrangements.

4. Conclusions

Six new CPs have been successfully synthesized using either the angular ligand L¹ or flexible ligand L² in combination with dicarboxylic acids and metal salts. The results underscore the propensity of both angular L¹ and flexible L² ligands to form entangled CPs. The isomerism exhibited by ligands L¹ and L² significantly influences the structural entanglement observed in the resulting CPs. For instant, complexes 1 and 3 stand out as prominent examples of entangled CPs derived from the L¹ ligand. This study offers valuable insights into the factors that govern the formation of entangled CPs. It highlights that the judicious selection of metal ions is crucial in designing CPs based on angular L¹ or flexible L² ligands, especially when supported by 1,4-NDC²⁻ ligand.