Impact of Isomeric Dicarboxylate Ligands on the Formation of One-Dimensional Coordination Polymers and Metallocycles: A Novel cis→trans Isomerization

Abstract

A series of Co(II), Ni(II) and Cu(II) coordination polymers and dinuclear metallocycles containing 4-aminopyridine (4-ampy) and benzenedicarboxylate ligands, {[M(4-ampy)₂(1,4-BDC)]·H₂O·CH₃CH₂OH}_n (M = Ni, 1a; Co, 1b, 1,4-H₂BDC = benzene-1,4-dicarboxylic acid), {[Ni₂(4-ampy)₄(1,3-BDC)₂]·H₂O·CH₃CH₂OH}_n (1,3-H₂BDC = benzene-1,3-dicarboxylic acid), 2, [M₂(4-ampy)₄(1,2-BDC)₂] (M = Ni, 3a; Co, 3b, 1,2-H₂BDC = benzene-1,2-dicarboxylic acid), [Co(4-ampy)₂(1,3-BDC)]_n, 4, {[Cu(4-ampy)₂(1,4-BDC)] CH₃CH₂OH}_n, 5a, and {[Cu(4-ampy)₂(1,4-BDC)]·H₂O}_n, 5b·H₂O, are reported, which were hydrothermally prepared and structurally characterized by using single crystal X-ray diffraction. Complexes 1a and 1b are isomorphous 1D zigzag chains, while 2 displays a concave–convex chain and 3a and 3b are dinuclear metallocycles that differ in the boding modes of the 1,2-BDC²⁻ ligands, forming a 3D and a 2D supramolecular structures with the pcu and sql topologies, respectively. Complex 4 exhibit a 1D helical chain and complexes 5a and 5b·H₂O are 1D linear and zigzag chains, in which the Cu₂-1,4-BDC²⁻ units adopt the cis and trans configurations, respectively. A novel irreversible structural transformation due to cis→trans isomerization of the Cu₂-1,4-BDC²⁻ units was observed in 5b⋅H₂O and 5a upon water adsorption of the desolvated product of 5b·H₂O.

1. Introduction

Coordination polymers (CPs) that exhibit diverse topologies [1,2,3,4] and potential applications in the fields such as sensing, catalysis, gas storage and separation are of great interest to research society during recent years. The self-assembly process of metal ions and organic ligands in suitable solvent systems may lead to the formation of one- (1D), two- (2D) or three-dimensional (3D) CPs and the structural types are governed by the identity of counterions [5,6], metal-to-ligand ratio [7] and temperature [8,9,10,11] as well. Moreover, weak linking forces such as hydrogen bonds and π–π stacking interactions are also important in determining the structural diversity [12,13]. Although many CPs have been prepared and structurally characterized, the design and synthesis of CPs with predicted structural types and particular physical and chemical properties remain allusive in the crystal engineering of CPs. Only through a great effort to better understand the structure–ligand relationship can this goal be accomplished.

Due to the remarkable bonding ability and possible formation of distinct coordination modes involving bridge and chelation, dicarboxylate ligands have been widely adopted as the anionic auxiliary spacers for the preparation of diverse CPs in a mixed-ligand system [14,15,16,17]. On the other hand, the flexible bis-pyridyl-bis-amide (bpba) ligand N,N’-di(3-pyridyl)suberoamide has been reported to react with Cu(II) salts and isomeric 1,2-, 1,3- and 1,4-phenylenediacetic acids under hydrothermal conditions to afford a 3D framework with the (4²·6⁵·8³)(4²·6)-3,5T1 topology, a 3D framework with the (6⁵·8)-cds topology, showing 5-fold interpenetration, and a 1D self-catenated net, respectively [18]. It has also been shown that the reactions of N,N’-di(3-pyridyl)suberoamide with Cd(II) salt and benzene-1,2-dicarboxylic acid (1,2-H₂BDC), benzene-1,3-dicarboxylic acid (1,3-H₂BDC) and benzene-1,4-dicarboxylic acid (1,4-H₂BDC), Scheme 1, under hydrothermal conditions afforded a 1D loop-like chain, a self-catenated net with point symbol (6⁵·8) and a 2D layer with the sql topology [19], respectively. These results evidently show that by controlling the isomeric dicarboxylate ligands, flexible bpba-based CPs with interesting structural diversity can be achieved. Accordingly, it is interesting to investigate the isomeric effect of the dicarboxylate ligands on the structural diversity of the CPs based on rigid neutral ligands, which, to the best of our knowledge, has not been carried out systematically before. We thus performed and studied the reactions of divalent metal salts and the rigid 4-aminopyridine with the isomeric 1,2-, 1,3- and 1,4-H₂BDC, respectively.

Herein, we report the syntheses and structures of six CPs and two metallocycles. The roles of isomeric dicarboxylate ligands in the structural diversity are discussed. Unprecedented cis→trans isomerization involving the 1,4-BDC²⁻ ligands due to different coordination by the carboxylate oxygen atoms was observed for the 1D zigzag and linear Cu(II) CPs on a solvent-dependent irreversible structural transformation, which was demonstrated by using powder X-ray diffraction (PXRD).

2. Materials and Methods

2.1. General Orocedures

Elemental analyses (C, H, and N) were performed on a PE 2400 series II CHNS/O (PerkinElmer Instruments, Shelton, CT, USA) or an Elementar Vario EL-III analyzer (Elementar Analysensysteme GmbH, Hanau, German). IR spectra (KBr disk) were measured on a JASCO FT/IR-460 plus spectrometer ((JASCO, Easton, MD, USA). Powder X-ray diffraction (PXRD) patterns were obtained from a Bruker D2 PHASER diffractometer (Bruker Corporation, Karlsruhe, Germany).

2.2. Materials

The reagents Ni(OAc)₂·4H₂O, 4-aminopyridine and 1,3-benzenedicarboxylic acid were purchased from Alfa Aesar (Heysham, UK), Co(OAc)₂·4H₂O from J. T. Baker (Phillipsburg, NJ, USA), Cu(OAc)₂·H₂O from SHOWA (Saitama, Japan), and 1,2-benzenedicarboxylic acid and 1,4- benzenedicarboxylic acid from ACROS (Pittsburgh, PA, USA).

2.3. Preparations

2.3.1. {[Ni(4-ampy)₂(1,4-BDC)]·H₂O·CH₃CH₂OH}_n (1a)

A mixture of Ni(OAc)₂·4H₂O (0.25 g, 1.0 mmol), 4-aminopyridine (0.75 g, 8.0 mmol), 1,4-H₂BDC (0.16 g, 1.0 mmol) and 5 mL EtOH was placed in a 23-mL Teflon-lined stainless container. The contained was then sealed and heated at 120 °C for 48 h and cooled down slowly to room T. Green block crystals were collected, washed by diethyl ether, and dried under vacuum. Yield: 0.43 g (90%). Anal. Calcd. for C₁₈H₁₆N₄NiO₄·H₂O·CH₃CH₂OH (MW = 475.12): C, 50.56; H, 5.09; N, 11.79%. Found: C, 50.13; H, 4.83; N, 12.38%. IR (cm⁻¹): 3419 (w), 3349 (m), 3236 (m), 2969 (w), 1642 (s), 1618 (s), 1565 (s), 1537 (s), 1516 (s), 1437 (m), 1402 (s), 1287 (m), 1214 (m), 1060 (w), 1045 (w), 1020 (m), 880 (w), 847 (m), 822 (m), 747 (m), 528 (m).

2.3.2. {[Co(4-ampy)₂(1,4-BDC)]·H₂O·CH₃CH₂OH}_n (1b)

Complex 1b was prepared by following the same procedures for 1a except Co(OAc)₂·4H₂O (0.25 g, 1.0 mmol) was used. Purple block crystals were collected. Yield: 0.41 g (86%). Anal. Calcd. for C₁₈H₁₆CoN₄O₄·2H₂O (MW = 447.31): C, 48.33; H, 4.51; N, 12.53%. Found: C, 48.37; H, 4.51; N, 12.19%. IR (cm⁻¹): 3457 (s), 3341 (s), 3239 (s), 2563 (w), 2348 (w), 2064 (w), 1939 (w), 1649 (s), 1626 (s), 1563 (s), 1547 (s), 1518 (m), 1505 (m), 1455 (m), 1382 (s), 1312 (m), 1285 (m), 1210 (m), 1145 (w), 1057 (m), 1018 (s), 881 (w), 838 (s), 747 (s), 564 (s), 526 (s).

2.3.3. {[Ni₂(4-ampy)₄(1,3-BDC)₂]·H₂O·CH₃CH₂OH}_n (2)

The reagents Ni(OAc)₂·4H₂O (0.25 g, 1.0 mmol), 4-aminopyridine (0.75 g, 8.0 mmol), 1,3-H₂BDC (0.16 g, 1.0 mmol) in 5 mL EtOH were placed in a 23-mL Teflon-lined stainless container. The solvothermal reaction was then carried out according to the procedures for 1a. Green block crystals were found. Yield: 0.26 g (59%). Anal. Calcd. for C₃₆H₃₂N₈Ni₂O₁₈ ·H₂O·CH₃CH₂OH (MW = 886.20): C, 51.56; H, 4.44; N, 12.65%. Found: C, 51.29; H, 4.44; N, 12.70%. IR (cm⁻¹): 3330 (m), 3211 (m), 1610 (s), 1546 (s), 1517 (s), 1480 (m), 1442 (m), 1377 (s), 1279 (m), 1213 (m), 1060 (w), 1018 (m), 824 (m), 749 (m), 717 (m), 658 (m), 529(m), 434 (m), 410 (m).

2.3.4. [Ni₂(4-ampy)₄(1,2-BDC)₂] (3a)

A mixture of Ni(OAc)₂·4H₂O (0.25 g, 1.0 mmol), 4-aminopyridine (0.75 g, 8.0 mmol), 1,2-H₂BDC (0.16 g, 1.0 mmol) in 5 mL EtOH was prepared and green block crystals were obtained by by following the same procedures for 1a. Yield: 0.26 g (63%). Anal. Calcd. for C₃₆H₃₂N₈Ni₂O₈·1.5 H₂O (MW = 849.10): C, 50.92; H, 4.15; N, 13.20%. Found: C, 50.53; H, 3.42; N, 13.12%. IR (cm⁻¹): 3330 (s), 3216 (s), 1621 (s), 1585 (s), 1563 (s), 1518 (s), 1493 (s), 1447 (m), 1423 (s), 1404 (s), 1391 (s), 1342 (m), 1281 (m), 1214 (m), 1086 (w), 1060 (w), 1019 (s), 882 (w), 853 (m), 828 (m), 785 (w), 763 (m), 743 (m), 708 (m), 697 (m), 652 (m), 530 (m), 444 (m), 412 (w).

2.3.5. [Co₂(4-ampy)₄(1,2-BDC)₂] (3b)

Complex 3b was obtained by following the same procedures for 3a except Co(OAc)₂·4H₂O (0.25 g, 1.0 mmol) was used. Purple block crystals were collected. Yield: 0.36 g (87%). Anal. Calcd. for C₃₆H₃₂Co₂N₈O₈ (MW = 822.56): C, 52.57; H, 3.92; N, 13.62%. Found: C, 52.40; H, 4.07; N, 13.49%. IR (cm⁻¹): 3454 (s), 3401 (s), 3323 (s), 3219 (s), 2569 (w), 2202 (w), 1647 (s), 1623 (s), 1596 (s), 1584 (s), 1563 (s), 1551 (s), 1518 (s), 1480 (m), 1457 (m), 1446 (m), 1391 (s), 1376 (s), 1288 (m), 1209 (s), 1164 (m), 1147 (m), 1087 (w), 1056 (m), 1022 (s), 957 (w), 862 (m), 837 (s), 827 (s), 786 (m), 770 (m), 703 (s), 657 (s), 598 (m), 567 (m), 530 (s), 460 (m), 411 (m).

2.3.6. [Co(4-ampy)₂(1,3-BDC)]_n (4)

A mixture of Co(OAc)₂·4H₂O (0.25 g, 1.0 mmol), 4-aminopyridine (0.75 g, 8.0 mmol), 1,3-H₂BDC (0.16 g, 1.0 mmol) in 5 mL EtOH was prepared and purple block crystals were obtained by following the same procedures for 1a. Yield: 0.23 g (55%). Anal. Calcd. for C₁₈H₁₆CoN₄O₄ (MW = 411.28): C, 52.57; H, 3.92; N, 13.62%. Found: C, 52.31; H, 3.96; N, 13.49%. IR (cm⁻¹): 3469 (s), 3210 (s), 2561 (w), 2347 (w), 2110 (w), 1644 (s), 1617 (s), 1580 (s), 1561 (s), 1518 (s), 1478 (m), 1439 (m), 1377 (s), 1282 (m), 1211 (m), 1151 (w), 1104 (w), 1069 (m), 1055 (m), 1021 (s), 945 (m), 912 (w), 825 (s), 750 (s), 718 (s), 657 (m), 564 (m), 521 (s), 416 (s).

2.3.7. {[Cu(4-ampy)₂(1,4-BDC)]·CH₃CH₂OH}_n (5a) and {[Cu(4-ampy)₂(1,4-BDC)]·H₂O}_n (5b H₂O)

Into a 23-mL Teflon-lined stainless container, a mixture of Cu(OAc)₂·H₂O (0.20 g, 1.0 mmol), 4-aminopyridine (0.75 g, 8.0 mmol), 1,4-H₂BDC (0.16 g, 1.0 mmol) and 5 mL EtOH was placed, which was sealed and heated at 120 °C for 48 h under autogenous pressure. Cooling the container to roon temperature afforded purple (5a) and blue (5b·H₂O) block crystals that were separated manually, washed by diethyl ether, and dried under vacuum. Yield for 5a: 0.29 g (63%). Anal. Calcd. for C₁₈H₁₆CuN₄O₄·CH₃CH₂OH (MW = 461.96): C, 51.99; H, 4.80; N, 12.13%. Found: C, 51.51; H, 4.50; N, 12.17%. IR (cm⁻¹): 3448 (w), 3333 (m), 3203 (m), 2348 (w), 2282 (w), 1743 (w), 1624 (s), 1562 (s), 1519 (s), 1456 (m), 1409 (s), 1345 (m), 1286 (m), 1214 (s), 1060 (m), 1029 (m), 894 (w), 824 (m), 744 (m), 567(w), 521 (m). Yield for 5b·H₂O: 0.13 g (31%). Anal. Calcd. for C₁₈H₁₆CuN₄O₄·H₂O (MW = 433.91): C, 49.82; H, 4.18; N, 12.91%. Found: C, 50.67; H, 4.68; N, 12.46%. IR (cm⁻¹): 3342 (s), 3235 (m), 2965 (w), 2562 (w), 1932 (w), 1644 (s), 1623 (s), 1589 (s), 1518 (s), 1502 (m), 1455 (m), 1359 (s), 1284 (m), 1212 (s), 1150 (m), 1099 (w), 1060 (m), 1026 (m), 980 (w), 882 (s), 751 (s), 668 (m), 579 (m), 526 (m), 435 (w), 404 (w).

2.4. X-ray Crystallography

The single crystal X-ray structures were performed on a Bruker AXS SMART APEX II CCD diffractometer (MoK_α radiation, λ = 0.71073 Å, graphite monochromator) (Bruker AXS, Madison, WI, USA). Lorentz–polarization and empirical absorption correction based on a “multi-scan” were then applied to reduce and correct the reflections collected for each crystal [20]. While some of the heavier atoms were located by using the direct method or Patterson method, the remaining atoms were found in a series of alternating difference Fourier maps and least-square refinements and the hydrogen atoms were added by using the HADD command in SHELXTL 6.1012 [21]. Because of the serious disorder of the co-crystallized solvents in 5b·H₂O, the SQUEEZE/PLATON technique [22] was applied to remove the solvent contribution, while the elemental analysis indicates the cocrystallization of one water molecule.

Table 1. Crystal data for complexes 1a–5b·H₂O.

Compound	1a	1b	2	3a	3b	4	5a	5b
Formula	C20H24N4NiO6	C20H24CoN4O6	C38H40N8Ni2O10	C36H32N8Ni2O8	C36H32Co2N8O8	C18H16CoN4O4	C20H22CuN4O5	C18H16CuN4O4
Formula weight	475.14	475.36	886.20	882.12	822.56	411.28	461.96	415.89
crystal system	Monoclinic	Monoclinic	Monoclinic	Triclinic	Triclinic	Monoclinic	Triclinic	Orthorhombic
space group	C2/c	C2/c	P21/c	Pī	Pī	P21/c	Pī	Pbcn
a, Å	19.5010(2)	19.5876(9)	8.7320(2)	8.75460(10)	9.19230(10)	9.1936(3)	5.7643(2)	30.447(3)
b, Å	15.7555(2)	15.7653(7)	17.7599(4)	10.42140(10)	10.04730(10)	9.5947(3)	10.9232(4)	7.1791(6)
c, Å	16.8035(2)	16.9997(12)	30.8251(6)	10.63650(10)	10.64370(10)	20.7529(6)	17.0937(6)	21.1316(18)
α, °	90	90	90	70.8840(10)	75.8250(10)	90	104.361(2)	90
β, °	121.2450(10)	121.2690(10)	93.2980(10)	87.1970(10)	87.5800(10)	93.933(2)	95.714(2)	90
γ, °	90	90	90	71.3430(10)	67.1700(10)	90	105.227(2)	90
V, Å3	4414.01(9)	4487.0(4)	4772.42(18)	867.036(15)	876.898(15)	1826.30(10)	990.32(6)	4619.0(7)
Z	8	8	4	1	1	4	2	8
dcalc, mg/m3	1.430	1.407	1.233	1.575	1.558	1.496	1.549	1.196
F(000)	1984	1976	1840	424	422	844	478	1704
µ(Mo Kα), mm−1	0.922	0.807	0.845	1.152	1.012	0.971	1.144	0.971
range(2θ) for data collection, deg	3.56 to 52.00	3.54 to 52.00	3.50 to 52.00	4.06 to 56.58	3.96 to 56.64	3.94 to 56.58	4.04 to 52.00	3.86 to 52.00
independent reflections	4351 [R(int) = 0.0383]	4426 [R(int) = 0.0510]	9367 [R(int) = 0.0620]	4228 [R(int) = 0.0203]	4058 [R(int) = 0.0182]	4523 [R(int) = 0.0286]	3871 [R(int) = 0.0257]	4519 [R(int) = 0.0737]
data/restraints/parameters	4351/3/260	4426/3/260	9367/3/515	4228/0/244	4058/0/244	4523/0/244	3871/1/273	4519/0/244
quality-of-fit indicator c	1.053	1.043	1.069	1.034	1.056	1.026	1.051	1.051
final R indices[I > 2σ (I)] a,b	R1 = 0.0553, wR2 = 0.1683	R1 = 0.0614, wR2 = 0.1777	R1 = 0.0779 wR2 = 0.2195	R1 = 0.0270, wR2 = 0.0642	R1 = 0.0272, wR2 = 0.0733	R1 = 0.0372, wR2 = 0.0860	R1 = 0.0449, wR2 = 0.1126	R1 = 0.0465 wR2 = 0.1156
R indices (all data)	R1 = 0.0872, wR2 = 0.1904	R1 = 0.1027, wR2 = 0.2065	R1 = 0.1196, wR2 = 0.2455	R1 = 0.0328, wR2 = 0.0669	R1 = 0.0304, wR2 = 0.0750	R1 = 0.0620, wR2 = 0.0962	R1 = 0.0596, wR2 = 0.1220	R1 = 0.0938 wR2 = 0.1424

^aR₁ = Σ‖F_o| − |F_c‖/Σ|F_o|. ^bwR₂ = [Σw(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 = 0.1143, b = 4.0870, 1a; a = 0.1174, b = 4.8162, 1b; a = 0.01241, b = 4.1679, 2; a = 0.0288, b = 0.4387, 3a; a = 0.0344, b = 0.4489, 3b; a = 0.0463, b = 0.3841, 4; a = 0.0523, b = 1.9340, 5a; a = 0.0707, b = 0.0000, 5b. ^cquality-of-fit = [Σw(|F_o²| − |F_c²|)²/N_observed − N_parameters)]^1/2.

lists the basic crystal parameters and structure refinement results for 1–5b·H₂O.

3. Results and Discussion

3.1. Structures of 1a and 1b

Green and purple crystals of complexes 1a and 1b, respectively, are isomorphous and conform to the monoclinic space group C2/c, with each asymmetric unit consisting of one M(II) (M = Ni, 1a; Co, 1b) cation, two 4-ampy ligands and one 1,4-BDC²⁻ ligand. Figure 1a depicts a representative drawing showing the coordination environment of the M(II) ion (M = Ni, 1a; Co, 1b).The M(II) ion is coordinated by six atoms involving two pyridyl nitrogen atoms from two 4-ampy ligands [Ni-N(1) = 2.032(3) Å, Ni-N(3) = 2.040(3) Å; Co-N(1) = 2.059(4) Å, Co(1)-N(3) = 2.073(4) Å] and four oxygen atoms from two 1,4-BDC²⁻ ligands [Ni-O(1) = 2.078(3) Å, Ni-O(2) = 2.189(3) Å, Ni-O(3A) = 2.128(3) Å, Ni-O(4A) = 2.114(3) Å; Co-O(1) = 2.092(3) Å, Co-O(2) = 2.276(3) Å, Co-O(3A) = 2.158(3) Å, Co-O(4A) = 2.189(4) Å], displaying a distorted octahedral geometry [N(1)-M-N(3) = 94.34(14) and 96.98(15); N(1)-M-O(1) = 97.23(13) and 100.36(14), N(3)-M-O(1) = 100.97(12) and 103.09(14), N(1)-M-O(4A) = 100.93(13) and 99.36(15), N(3)-M-O(4A) = 94.92(12) and 95.90(15), O(1)-M-O(4A) = 154.81(13) and 150.60(15), N(1)-M-O(3A) = 162.64(12) and 158.71(15), N(3)-M-O(3A) = 90.24(12) and 90.78(14), O(1)-M-O(3A) = 98.32(12) and 97.11(14), O(4A)-M-O(3A) = 61.94(11) and 60.00(14), N(1)-M-O(2) = 91.76(12) and 89.75(13), N(3)-M-O(2) = 162.13(12) and 162.59(14), O(1)-M-O(2) = 61.55(10) and 59.78(11), O(4A)-M-O(2) = 100.39(11) and 98.84(13), O(3A)-M-O(2) = 88.89(11) and 88.60(12)° for 1a and 1b, respectively. The M(II) ions are linked by 1,4-BDC²⁻ ligands to afford 1D zigzag chains, Figure 1b, which are further linked by the π–π stacking interactions (3.92 Å for 1a and 3.89 Å for 1b), Figure 1c.

3.2. Structure of 2

A single-crystal structural analysis shows that complex 2 crystallizes in the monoclinic space group P2₁/c. The asymmetric unit contains two Ni(II) cations, four 4-ampy ligands and two 1,3-BDC²⁻ ligands. Figure 2a depicts a drawing showing the coordination environments of the Ni(II) ions. Both of the two independent Ni(II) ions are six-coordinated by two pyridyl nitrogen atoms from two 4-ampy ligands [Ni(1)-N(1) = 2.021(4) Å, Ni(1)-N(3) = 2.029(4) Å, Ni(2)-N(5) = 2.020(4) Å, Ni(2)-N(7) = 2.025(5) Å] and four oxygen atoms from two 1,3-BDC²⁻ ligands [Ni(1)-O(1) = 2.252(5) Å, Ni(1)-O(2) = 2.031(2) Å, Ni(1)-O(7A) = 2.031(3) Å, Ni(1)-O(8A) = 2.253(4) Å, Ni(2)-O(3) = 2.089(3) Å, Ni(2)-O(4) = 2.167(4) Å, Ni(2)-O(5) = 2.273(4) Å, Ni(2)-O(6) = 2.029(3) Å], resulting in distorted octahedral geometries [N(1)-Ni(1)-N(3) = 94.83(18), N(1)-Ni(1)-O(7A) = 96.87(16), N(3)-Ni(1)-O(7A) = 95.18(15), N(1)-Ni(1)-O(2) = 97.14(15), N(3)-Ni(1)-O(2) = 98.77(16), O(7A)-Ni(1)-O(2) = 159.28(16), N(1)-Ni(1)-O(1) = 158.22(15), N(3)-Ni(1)-O(1) = 92.16(15), O(7A)-Ni(1)-O(1) = 103.02(14), O(2)-Ni(1)-O(1) = 61.36(12), N(1)-Ni(1)-O(8A) = 90.91(16), N(3)-Ni(1)-O(8A) = 156.09(14), O(7A)-Ni(1)-O(8A) = 61.06(13), O(2)-Ni(1)-O(8A) = 103.52(14), O(1)-Ni(1)-O(8A) = 91.00(13), N(5)-Ni(2)-N(7) = 95.43(18), N(5)-Ni(2)-O(6) = 97.10(16), N(7)-Ni(2)-O(6) = 94.74(16), N(5)-Ni(2)-O(3) = 97.59(15), N(7)-Ni(2)-O(3), = 97.76(17), O(6)-Ni(2)-O(3) = 159.68(16), N(5)-Ni(2)-O(4) = 158.89(15), N(7)-Ni(2)-O(4) = 91.53(16), O(6)-Ni(2)-O(4) = 102.17(14), O(3)-Ni(2)-O(4) = 61.66(12), N(5)-Ni(2)-O(5) = 91.08(16), N(7)-Ni(2)-O(5) = 155.07(15), O(6)-Ni(2)-O(5) = 60.53(13), O(3)-Ni(2)-O(5) = 105.21(14), O(4)-Ni(2)-O(5) = 90.93(14)°]. The 1,3-BDC²⁻ ligands link the Ni(II) ions to afford 1D concave–convex chains, Figure 2b, which are further connected by the N-H---O (H---O = 2.16 Å, $\angle$N-H---O = 151.7°; H---O = 2.17 Å, $\angle$N-H---O = 150.4°) hydrogen bonds originating from the amine hydrogen atoms of the 4-ampy ligands to the carboxylate oxygen atoms, Figure 2c.

3.3. Structures of 3a and 3b

Single-crystal structural analyses show that green and purple crystals of 3a and 3b, respectively, crystallize in the triclinic space group Pī with two M(II) (M = Ni, 3a; Co, 3b) cations, two 4-ampy ligands and one 1,2-BDC²⁻ ligands in each asymmetric unit, forming dinuclear metallocycles that differ in the bonding modes of the 1,2-BDC²⁻ ligands. The Ni(II) ions of 3a are five-coordinated by two pyridyl nitrogen atoms from two 4-ampy ligands [Ni(1)-N(1) = 2.022(7) Å, Ni(1)-N(3) = 2.023(8) Å] and three oxygen atoms from two 1,2-BDC²⁻ ligands [Ni(1)-O(1) = 1.980(2) Å, Ni(1)-O(3A) = 2.183(2) Å, Ni(1)-O(4A) = 2.087(5) Å], resulting in distorted square pyramidal geometries [O(1)-Ni(1)-N(1) = 101.90(5), O(1)-Ni(1)-N(3) = 92.36(5), N(1)-Ni(1)-N(3) = 99.22(5), O(1)-Ni(1)-O(4A) = 103.03(4), N(1)-Ni(1)-O(4A) = 100.47(5), N(3)-Ni(1)-O(4A) = 151.81(5), O(1)-Ni(1)-O(3A) = 160.22(5), N(1)-Ni(1)-O(3A) = 93.44(5), N(3)-Ni(1)-O(3A) = 97.47(5)°], Figure 3a. The Ni---O(2) distance is 3.2845(14) Å, which is significantly longer than the sum of the van der Waal’s radius of Ni (1.63 Å) and O (1.52 Å) [23] and can be regarded as nonbonding. The dinuclear metallocycles of 3a are supported by the π–π interactions (3.40 Å) and N-H---O (H---O = 2.20 Å, $\angle$N-H---O = 157.9°; H---O = 2.08 Å, $\angle$N-H---O = 150.2°) hydrogen bonds to the carboxylate oxygen atoms to form a 3D supramolecular structure. Considering the whole dinuclear molecule as a node, this supramolecular structure can be simplified as a 3D net with the pcu topology, Figure 3b, determined by using the ToposPro program [24].

The Co(II) ions of 3b are four-coordinated by two pyridyl nitrogen atoms from two 4-ampy ligands [Co(1)-N(1) = 2.037(8) Å, Co(1)-N(3) = 2.019(0) Å] and two oxygen atoms from two 1,2-BDC²⁻ ligands [Co(1)-O(1) = 1.967(4) Å, Co(1)-O(3) = 1.964(2) Å], exhibiting a distorted tetrahedral geometries [O(1)-Co-O(3A) = 108.23(5), O(1)-Co-N(1) = 112.86(6), O(3A)-Co-N(1) = 115.05(6), O(1)-Co-N(3) = 98.13(5), O(3A)-Co-N(3) = 114.21(5), N(1)-Co-N(3) = 107.20(5)°], Figure 3c. The Co---O distances to O(2) and O(2A) and O(4) and O(4A) are 2.713(2) and 3.0973(2) Å, respectively, which are significantly shorter than the sum of van der Waal’s radius of Co (2.0 Å) and O (1.52 Å) and indicate weak interactions. The dinuclear metallocycles are further linked by the π–π interactions (3.64 Å) and N-H---O (H---O = 2.03 Å, $\angle$N-H---O = 168.8°; H---O = 2.12 Å, $\angle$N-H---O = 162.5°) hydrogen bonds to the carboxylate oxygen atoms to form a 2D supramolecular structure with the sql topology, Figure 3d. The different topologies of the supramolecular structures for 3a and 3b are presumably due to the different orientations of the hydrogen bonds.

3.4. Structure of 4

A single-crystal structural analysis reveals that complex 4 crystallizes in the monoclinic space group P2₁/c with each asymmetric unit containing one Co(II) cation, two 4-ampy ligands and one 1,3-BDC²⁻ ligand. The Co(II) ion is four-coordinated by two pyridyl nitrogen atoms from two 4-ampy ligands [Co-N(1) = 2.046(7) Å, Co-N(3) = 2.024(6) Å] and two oxygen atoms from two 1,3-BDC²⁻ ligands [Co-O(1) = 1.931(8) Å, Co-O(4A) = 1.993(9) Å], showing a distorted tetrahedral geometry [O(1)-Co-O(4A) = 109.64(7), O(1)-Co-N(3) = 113.91(7), O(4A)-Co-N(3) = 118.06(7), O(1)-Co-N(1) = 104.26(7), O(4A)-Co-N(1) = 104.23(6), N(3)-Co-N(1) = 105.19(7)°], Figure 4a. The Co(II) ions are coordinated by the 1,3-BDC²⁻ ligands to afford 1D helical chains, Figure 4b, which are further linked by the N-H---O (H---O = 2.09 Å, $\angle$N-H---O = 164.9°) hydrogen bonds from the amine hydrogen atoms to the carboxylate oxygen atoms, Figure 4c.

3.5. Structure of 5a

A single-crystal structural analysis shows that purple 5a crystallizes in the triclinic space group Pī with each asymmetric unit contains two halves of a Cu(II) cation, two 4-ampy ligands and two halves of a 1,4-BDC²⁻ ligand. The Cu(II) ions are four-coordinated by two pyridyl nitrogen atoms from two 4-ampy ligands [Cu(1)-N(1) = 1.993(3) Å, Cu(2)-N(3) = 1.979(3) Å] and two oxygen atoms from two 1,4-BDC²⁻ ligands [Cu(1)-O(1) = 1.9581(19) Å, Cu(2)-O(3A) = 1.981(2) Å], resulting in a square planar geometry [O(1A)-Cu(1)-O(1) = 180.00(9), O(1A)-Cu(1)-N(1A) = 90.61(9), O(1)-Cu(1)-N(1A) = 89.39(9), O(1A)-Cu(1)-N(1) = 89.39(9), O(1)-Cu(1)-N(1) = 90.61(9), N(1A)-Cu(1)-N(1) = 180.0, N(3)-Cu(2)-N(3B) = 180.0, N(3)-Cu(2)-O(3B) = 90.02(10), N(3B)-Cu(2)-O(3B) = 89.98(10), N(3)-Cu(2)-O(3) = 89.98(10), N(3B)-Cu(2)-O(3) = 90.02(10), O(3B)-Cu(2)-O(3) = 180.0°], Figure 5a. The Cu(II) ions are linked by 1,4-BDC²⁻ ligands to afford a 1D linear chain, Figure 5b, which are further linked by the N-H---O (H---O = 2.11 Å, $\angle$N-H---O = 155.9°; H---O = 2.20 Å, $\angle$N-H---O = 136.0°) hydrogen bonds to the carboxylate oxygen atoms, Figure 5c.

3.6. Structure of 5b·H₂O

A single-crystal structural analysis shows that 5b·H₂O crystallizes in the orthorhombic space group Pbcn, and each asymmetric unit contains one Cu(II) cation, two 4-ampy ligands and one 1,4-BDC²⁻ ligand. The Cu(II) ion is four-coordinated by two pyridyl nitrogen atoms from two 4-ampy ligands [Cu-N(1) = 2.008(3) Å; Cu-N(3) = 2.001(2) Å] and two oxygen atoms from two 1,4-BDC²⁻ ligands [Cu-O(1) = 1.942(5) Å; Cu-O(4A) = 1.963(1) Å], resulting in a distorted square planar geometry [O(1)-Cu-O(4A) = 169.65(12), O(1)-Cu-N(1) = 92.29(10), O(4A)-Cu-N(1) = 89.26(10), O(1)-Cu-N(3) = 89.90(10), O(4A)-Cu-N(3) = 88.78(10), N(1)-Cu-N(3) = 177.56(12)°], Figure 6a. The 1,4-BDC²⁻ ligands link the Cu(II) ions to afford a 1D zigzag chain, Figure 6b. These chains are further linked by the N-H---O (H---O = 2.26 Å, $\angle$N-H---O = 131.7°; H---O = 2.02 Å, $\angle$N-H---O = 164.7°) hydrogen bonds to the carboxylate oxygen atoms to form a 3D supramolecular structure, Figure 6c.

It is worthwhile to further investigate the coordination modes of 1,4-BDC²⁻ ligands in 5a and 5b·H₂O. Although the 1,4-BDC²⁻ ligands in both complexes adopt the same μ₂-κO:κO’ bonding mode, vide infra, they differ in the orientations of the carboxylate oxygen atoms that link the Cu(II) ions to form a 1D linear chain and a 1D zigzag, respectively. As shown in Figure 7, while the 1,4-BDC²⁻ ligands in 5a employ the oxygen atoms on the opposite side, those in 5b·H₂O employ the oxygen atoms on the same side to coordinate the Cu(II) ions, resulting in trans and a cis configurations for the Cu₂-1,4-BDC²⁻ units, and forming a purple and a blue complexes, respectively.

3.7. Ligand Isomerism and Metal Atom Effect

Structural comparisons of the Ni(II) complexes, 1a, 2 and 3a, supported by the 1,4-, 1,3- and 1,2-BDC²⁻ dicarboxylate ligands reveal that donor-atom orientations of the dicarboxylate ligands play important roles in determining the structural diversity, showing a zigzag chain, a concave–convex chain and a dinuclear metallocycle, respectively. A similar role can also be observed for the Co(II) complexes 1b, 4, and 3b, and a zigzag chain, a helical chain and a dinuclear metallocycle, respectively, were prepared.

Table 2. Structural parameters of 1–5.

Complex	Metal	CN	Dicarboxylate	Coordination Mode	Structure
1a	Ni(II)	6	1,4-BDC2−	Scheme 2a	Zigzag chain
1b	Co(II)	6	1,4-BDC2−	Scheme 2a	Zigzag chain
2	Ni(II)	6	1,3-BDC2−	Scheme 2b	Concave–convex chain
3a	Ni(II)	5	1,2-BDC2−	Scheme 2c	Dinuclear metallocycle
3b	Co(II)	4	1,2-BDC2−	Scheme 2d	Dinuclear metallocycle
4	Co(II)	4	1,3-BDC2−	Scheme 2e	Helical chain
5a	Cu(II)	4	1,4-BDC2−	Scheme 2f	Linear chain
5b·H2O	Cu(II)	4	1,4-BDC2−	Scheme 2g	Zigzag chain

lists some structural parameters and Scheme 2 shows the various coordination modes found for all of the complexes.

On the other hand, other things being equal, the identity of the metal ion may affect the structural type. The different metal centers of Ni(II) and Co(II) result in μ₂-κ²O,O’:κ²O’’,O’’’, Scheme 2b, and μ₂-κO:κO’, Scheme 2e, coordination modes for the 1,3-BDC²⁻ ligands, leading to the formation of a zigzag chain and a helical chain, respectively. Moreover, complexes 3a and 3b are supported by the 1,2-BDC²⁻ ligands, and the different metal identity results in the μ₂-κ²O,O’:κO’’, Scheme 2c and μ₂-κO:κO’, Scheme 2d, coordination modes for the 1,2-BDC²⁻ ligands, forming a 3D and a 2D supramolecular structures with the pcu and sql topologies, respectively. The different topologies of the supramolecular structures of 3a and 3b, respectively, indicate that subtle change in the bonding modes of the dicarboxylate ligands may affect the supramolecular structures significantly, which are presumably due to the different orientations of the hydrogen bonds. However, the structural types of complexes 1a and 1b that are supported by the 1,4-BDC²⁻ ligands adopting the μ₂-κ²O,O’:κ²O’’,O’’’ coordination mode, Scheme 2a, are not subject to the change of the metal identity, indicating further the significant role of the ligand isomerism in determining the structural diversity.

3.8. Structural Transformation

Structural transformations in CPs initiated by various methods, such as the removal and uptake of solvents, and the exchange of solvents and guest molecules, are intriguing due to their potential applications as switches and sensors [25,26,27,28]. Such changes are not common for the CPs because of the rearrangement of the coordinate and/or covalent bonds [25,26] that require significant energy adjustment and the factors that govern the structural change remain scarcely investigated. Moreover, while the cis-trans isomerization of organic molecules and biomolecules has been known for quite a long time [29], the structural transformations of CPs that involve the cis-trans isomerization of the dicarboxylate ligands are rare [30]. Complexes 5a and 5b·H₂O that differ in metal–ligand configurations and cocrystallized solvent molecules thus provide a good opportunity to investigate the structural transformations due to solvent removal and adsorption.

To investigate the structural transformation upon solvent removal, we first heated the crystals of 5b·H₂O at 150 °C to remove the solvents under vacuum to obtain 5b’, which was then immersed into various solvents. Figure S7 shows that the PXRD patterns of 5b’ and those in the various solvents are significant different from that of 5b·H₂O, indicating the possible structural transformations upon solvent removal and adsorption. Noticeably, immersion of 5b’ into the ethanol solvent afforded a PXRD pattern that is comparable to that of the simulation of 5a, Figure 8, showing the possible irreversible structural transformation of 5b·H₂O to 5a through the desolvated product 5b’, while some extra peaks may indicate partial transformation. This structural transformation can be most probably ascribed to the cis→trans isomerization of the Cu₂-1,4-BDC²⁻ units, Figure 9, which represents a unique example demonstrating that internal cis→trans change due to the different coordination of the carboxylate oxygen atoms of the 1,4-BDC²⁻ ligands may govern the structural transformation, subject to the breaking and formation of the Cu-O bonds to the dicarboxylate ligands as well as the changes in the weak interactions such as the N-H---O hydrogen bonds. Such a cis→trans isomerization is in marked contrast to that observed in {[Zn₂(maleate)₂(dpa)₂]·5H₂O}_n} (dpa = 4,4′-dipyridylamine), in which the cis-trans isomerization from maleate to fumarate—based on the double bonds of the dicarboxylate ligands—is proposed by comparing the ligand isomerism [30].

4. Conclusions

Eight Co(II), Ni(II) and Cu(II) complexes containing the 4-aminopyridine and isomeric benzenedicarboxylate ligands have been successfully synthesized under hydrothermal conditions. Complexes 1a and 1b are isomorphous 1D zigzag chains, while 2 displays a concave–convex chain. Complexes 3a and 3b are dinuclear metallocycles which differ in the boding modes of the 1,2-BDC²⁻ ligands, resulting in a 3D and a 2D supramolecular structures with a pcu and sql topologies, respectively. The different supramolecular structures in 3a and 3b indicate that subtle change in the bonding modes of the dicarboxylate ligands may affect the supramolecular structures significantly. Complex 4 exhibit a 1D helical chain and complexes 5a and 5b·H₂O are 1D linear and zigzag chains, in which the Cu₂-1,4-BDC²⁻ units of 5a and 5b·H₂O adopt the cis and trans configurations, respectively. By the manipulation of the donor–atom positions of the dicarboxylate ligands and the identity of the metal atoms, interesting structural diversity for divalent CPs supported by the rigid 4-aminopyridine can be shown. Furthermore, a novel cis→trans isomerization based on the Cu₂-1,4-BDC²⁻ units that directs the structural transformation from a zigzag chain to a linear chain through a desolvated intermediate was observed in 5a and 5b·H₂O, which provides an insight into understanding the factors that govern the structural transformations in divalent CPs.