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

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.


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.
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-BDC 2− 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).

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·H 2 O, 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 lists the basic crystal parameters and structure refinement results for 1-5b·H 2 O.

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-BDC 2− ligand. Figure 1a

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-BDC 2− ligand. Figure 1

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-BDC 2ligands. Figure 2

Structure of 2
A single-crystal structural analysis shows that complex 2 crystallizes in the monoclinic space group P2 1 /c. The asymmetric unit contains two Ni(II) cations, four 4-ampy ligands and two 1,3-BDC 2− ligands. Figure 2a

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-BDC 2− ligands in each asymmetric unit, forming dinuclear metallocycles that differ in the bonding modes of the 1,2-BDC 2− ligands. The Ni(II) ions of 3a are five-coordinated by two pyridyl nitrogen atoms from two 4-ampy ligands [Ni (1) Considering the whole dinuclear molecule as a node, this supramolecular structure can be simplified as a 3D net with the pcu topology, Figure 3(b), determined by using the ToposPro program [24]. The

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-BDC 2− ligands in each asymmetric unit, forming dinuclear metallocycles that differ in the bonding modes of the 1,2-BDC 2− ligands. The Ni(II) ions of 3a are five-coordinated by two pyridyl nitrogen atoms from two 4-ampy ligands [Ni (1) 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].

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-BDC 2− ligand. The Co(II) ion is four-coordinated by two pyridyl nitrogen atoms from two 4-ampy ligands [Co-N(1) = 2.046 (7)

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-BDC 2− ligand. The Cu(II) ions are four-coordinated by two pyridyl nitrogen atoms from two 4-ampy ligands [Cu (1)     It is worthwhile to further investigate the coordination modes of 1,4-BDC 2− ligands in 5a and 5b·H 2 O. Although the 1,4-BDC 2− ligands in both complexes adopt the same µ 2 -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-BDC 2− ligands in 5a employ the oxygen atoms on the opposite side, those in 5b·H 2 O employ the oxygen atoms on the same side to coordinate the Cu(II) ions, resulting in trans and a cis configurations for the Cu 2 -1,4-BDC 2− units, and forming a purple and a blue complexes, respectively. It is worthwhile to further investigate the coordination modes of 1,4-BDC 2− ligands in 5a and 5b·H2O. Although the 1,4-BDC 2− 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-BDC 2− 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-BDC 2− units, and forming a purple and a blue complexes, respectively.

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-BDC 2− 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 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-κ 2

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

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

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·H 2 O 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·H 2 O 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·H 2 O, 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·H 2 O 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 Cu 2 -1,4-BDC 2− 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-BDC 2− 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 {[Zn 2 (maleate) 2 (dpa) 2 ]·5H 2 O} 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].
Polymers 2020, 12, x FOR PEER REVIEW 10 of 17 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].

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-BDC 2− 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-BDC 2− 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-

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-BDC 2− 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-BDC 2− 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-

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-BDC 2− 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·H 2 O are 1D linear and zigzag chains, in which the Cu 2 -1,4-BDC 2− units of 5a and 5b·H 2 O 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 Cu 2 -1,4-BDC 2− units that directs the structural transformation from a zigzag chain to a linear chain through a desolvated intermediate was observed in 5a and 5b·H 2 O, which provides an insight into understanding the factors that govern the structural transformations in divalent CPs.