Neutral and Cationic Chelidonate Coordination Polymers with N,N′-Bridging Ligands

The biomolecule chelidonic acid (H2chel, 4-oxo-4H-pyran-2,6-dicarboxylic acid) has been used to build new coordination polymers with the bridging N,N′-ligands 4,4′-bipyridine (4,4-bipy) and 1,2-bis(4-pyridyl)ethane (bpe). Four compounds have been obtained as single crystals: 1D cationic coordination polymers [M(4,4-bipy)(OH2)4]2+ with chelidonate anions and water molecules in the second coordination sphere in 1∞[Zn(4,4-bipy)(H2O)4]chel·3H2O (2) and in the two pseudopolymorphic 1∞[Cu(4,4-bipy)(H2O)4]chel·nH2O (n = 3, 4a; n = 6, 4b), and the 2D neutral coordination polymers 2∞[Zn(chel)(4,4-bipy)(H2O)]·2H2O (1) and 2∞[Zn(chel)(bpe)(H2O)]·H2O (3) where the chelidonate anion acts as a bridging ligand. The effects of the hydrogen bonds on the crystal packing were analyzed. The role of the water molecules hosted within the crystals lattices was also studied.


Introduction
Chelidonic acid (H2chel, 4-oxo-4H-pyran-2,6-dicarboxylic acid) is a biomolecule found in several plants and is considered to be an active component of some medicinal herbs [1]. The compound exhibits some pharmacological effects such as mild analgesic, antimicrobial, oncostatic and sedative [2] effects and its therapeutic potential in allergic disorders [1], intestinal inflammation [2] and regulation of depression associated with inflammation [3] have been investigated. From the point of view of coordination chemistry, H2chel after deprotonation is an angular dicarboxylate that can act as a linker between metal ions to yield coordination polymers. From the supramolecular point of view, chel 2-presents several oxygen atoms that can act as hydrogen acceptors towards biologically important hydrogen donors such as water molecules. In this way, the dianion chelidonate can be considered a biologically and environmentally interesting building block for the construction of new metallosupramolecular compounds. This kind of building blocks offers advantages such as its natural availability, the presence of various metal-binding sites which allow the display of different coordination modes and consequently a structural diversity and the possibility of establishing different hydrogen bonds patterns in aqueous media.
Several structural studies on metal chelidonates that are coordination polymers of different dimensionality can be found in the literature: 1D compounds with Zn(II) [4], Cu(II) [5,6], Dy(III)-Ba(II) [7] or Lu(III) [8], 2D systems with several lanthanide cations [8] and also with Cu(II) [9] and 3D polymers with Cd(II) [5], Cu(II) and Ag(I) [9], Pr(III), Nd(III), Sm(III), Lu-Ba and Sm-Ba [7] and with Ba(II) [10]. In all these compounds the chelidonate is the only ligand used. However, the structural information available on mixed ligand chelidonate coordination polymers is scarce. To the best of our knowledge only a few examples have been reported: 1D coordination polymers with monodentate ligands such as pyridine [11] or dmso [12] and with the bidentate chelating ligand 2,2′bipyridine [13], which also stabilised the 2D copper(II) coordination polymer 2 ∞[Cu(chel)(bipy)] [13]. In all of these coordination polymers the chelidonate dianion behaves as a bridging ligand between two, three or four metal centres with the coordination modes shown in Scheme 1.

Scheme 1. Coordination modes of the chelidonate ligand in coordination polymers.
In view of these previous results on the role of the chelidonate anion in coordination polymers, we decided to study its coordinative possibilities and supramolecular role in aqueous media in systems that contain the bridging N,N'-ligands 4,4′-bipyridine (4,4bipy) and 1,2-bis(4-pyridyl)ethane (bpe), which can facilitate the extension of the metalligand interaction in at least one direction. The aim of this work was to prepare crystalline solids of coordination polymers formed with chelidonate anions to analyse their coordinative behaviour, searching for new coordination modes, and to study their participation in the supramolecular arrangements, searching for new hydrogen bond patterns in aqueous media.
The IR spectra of all of the compounds exhibit broad bands of medium intensity at around 3400 cm -1 corresponding to the OH stretching vibrations of the coordinated and crystallisation water molecules. The strong IR bands between 1600 and 1640 cm -1 can be assigned to the νasym(OCO) vibration mode and the medium intensity bands at around 1360 cm -1 are attributable to the νsym(OCO) vibration mode.

Structural Studies
All of the compounds described here were isolated as single crystals and their structures were elucidated by X-ray diffraction. The significant structural parameters for compounds 1-4 are listed in Tables S1 and S2, and crystal structure and refinement data are listed in Table S5.
Structural analysis revealed that all of the compounds are hydrates and that 2, 4a and 4b are formed by cationic chains that are stabilised by chelidonate anions and water molecules. When the chelidonate ligand is coordinated to the metal centre and forms part of the structural backbone, the resulting compounds are 2D coordination polymers, as revealed by X-ray analysis of 1 and 3.

Crystal Structures of the Cationic Polymers 2, 4a and 4b
The coordination environment of the metal ions and the 1D polymeric nature of compounds 2, 4a and 4b are represented in Figure 1. A summary of the bond lengths and angles in the first coordination sphere is provided in Table 1.
The three structures are polymeric chains based on the cationic complexes [M(4,4bipy)(OH2)4] 2+ (M = Cu(II) and Zn(II)), where the N-donor ligands act as a bridge between the metal centres. The second coordination spheres are formed by dianionic chelidonate anions and three or six water molecules. Complexes 4a and 4b crystallise in the orthorhombic chiral P21212 and triclinic P-1 space groups, respectively, and they can be considered pseudopolymorphs [29] since they have the same crystalline form, albeit with different numbers of water molecules trapped within the crystal networks. All M II metal centres are {N2O4}-hexacoordinated through the nitrogen bipy atoms in trans positions and four oxygen atoms belonging to four water molecules in the equatorial plane. The copper atoms in 4a and 4b are in an elongated octahedral environment and they exhibit the expected Jahn-Teller distortion, with four short metal-ligand bonds (Cu-Owater and Cu-N with distances of around 2 Å) and two longer bonds with water molecules (2.417(2) Å in 4a, 2.291(2) and 2.173(3) Å in 4b). Clearly the Jahn-Teller distortion is more pronounced in compound 4a, where the pyridine rings of the 4,4′-bipyridine ligand are further from coplanarity, as shown in Figure 1.
In compound 2 the Zn-O and Zn-N distances are in the range 2.08-2.19 Å (Table 1) and the values for the orthogonal angles in the octahedral polyhedron are between 87.15 and 93.49°. Comparison of the orthogonality of the coordination octahedra in the three compounds shows that the smallest deviation is observed in the copper(II) compound 4b, with values between 88.26 and 91.70°.
The intermetallic distance through the 4,4-bipy bridging ligand is shorter in copper polymers (11.176 Å in 4a and 11.290 Å in 4b) than in 2 (11.487 Å). The aromatic rings in 4,4-bipy are not coplanar and the angles between the planes are 23.02° and 13.72° in 4a and 4b, respectively, and the disposition is closer to coplanarity in 2 (around 6°) ( Figure  1).
In all structures the second coordination sphere plays a crucial role in the supramolecular organisation. The cationic chains are connected by hydrogen bonds to the chelidonate anions and different numbers of water molecules to form different arrangements in each structure. The main hydrogen-bonding interactions responsible for the 3D crystal packing of compounds 2, 4a and 4b are listed in Table S3. In all cases, the resulting supramolecular networks are formed and propagated through Ochelidonate•••Owater hydrogen bonds, with the formation of different association patterns such as rings, discrete associations or infinite layers.
In compound 2 the three uncoordinated water molecules are not associated with each other, so the notation W1 has been selected to describe their isolated participation in the supramolecular organisation. One of the water molecules, O2w, and two chelidonate anions form a discrete dimer through H-bonding interactions to yield a four-membered ring, with O•••O distances of 2.781 and 2.932 Å (Figure 2, Table S3). Another crystallisation water molecule, O1w, establishes H-bonding interactions with the two carboxylate groups of a chelidonate to form an eight-membered ring and the third water molecule is Hbonded with the ketonic oxygen of the chelidonate. In this way, all of the oxygen atoms of the chelidonate are involved as acceptors in hydrogen-bonding interactions. The cationic chains of 2 are anchored to these water-chelidonate dimers through Owcoord…Ocarboxy and Owcoord…Owcrystallisation interactions (Table S3). Cationic chains and the anionic units are arranged in an alternating manner to form the final supramolecular organisation ( Figure 2).
In the structures of 4a and 4b chelidonate anions and water molecules are associated through different H-bonding interactions to form different anionic arrangements ( Figures  3 and 4) that are extended in two dimensions, thus leading to anionic [(H2O)nchel] 2-sheets. In this way, the layers act as the 'glue' between the cationic units to form the final metallosupramolecular arrangements [30]. This kind of 2D anion-water association has been observed in other cases, such as [Ni(4,4-bipy)(H2O)4]•0.5(btc)•H2O (H4btc, 1,2,4,5benzenetetracarboxylic acid) [25] In 4a the crystallisation water molecules are also not interconnected by hydrogen bonds (W1 motif) and only one of them (O2w) is involved in the formation of the supramolecular [(H2O)3chel)] 2− layer (  (Table S3).

Crystal Structures of Neutral Polymers 1 and 3
The Zn(II) polymeric compounds 1 and 3 crystallise in the P-1 and P21/c space groups, respectively. The significant structural parameters are listed in Table S2 and a summary of bond lengths/Å and angles/° in the first coordination sphere is provided in Table 2. In both compounds, the two carboxylate groups of the chelidonate ligand are coordinated to the metal centre to produce monodimensional chains. These chains are bridged by the rigid 4,4-bipyridine or the more flexible 1,2-bis(4-pyridyl)ethane N-donor ligands to yield the final 2D coordination compounds (Figures 5 and 6).
Chelidonate acts in both structures as a bridging ligand in a μ-1κ 2 O III ,O IV :2κO V coordination mode (Scheme 3), which has not been previously observed in other coordination polymers with this ligand. Each N-donor ligand acts as a bis-monodentate system to bridge zinc atoms with Zn•••Zn distances of 11.346 Å and 13.434 Å in 1 and 3, respectively. Chelidonate ligands also connect two zinc metal centres to produce similar Zn•••Zn distances of 9.928 Å in 1 and 9.611 Å in 3. In this way rhomboidal grid layer structures are formed.  in 3), the other has a longer distance of around 2.5 Å and the other carboxylate group is monodentate.   The layers in 1 are stacked along the b axis to yield a final 3D arrangement with an ABAB stacking mode ( Figure 5). These layers are connected by means of hydrogen bonds involving the coordinated water molecule and oxygen atoms from carboxylate groups of neighbouring layers (d(O1W…Ochel) = 2.699(4) and 2.772(5) Å; Table S4, detail shown in Figure 5). The two-crystallisation independent free water molecules (W1) are located in the voids of the rhomboidal grids ( Figure 5) and are weakly attached to the 2D layers.
The supramolecular arrangement in the corrugated layers of 3 is achieved by means of the same supramolecular synthon described above for compound 1: hydrogen bonds involving the coordinated water molecule and oxygen atoms from carboxylate groups of neighbouring layers (d(O1W…Ochel) = 2.762(3) and 2.825(3) Å, Table S4 and detail in Figure 7). As in compound 1, the only crystallisation water molecule (W1) is weakly attached to the metal organic framework through a hydrogen-bonding interaction with the ketonic group of a chelidonate ligand (d(Ow…O) = 3.125(5) Å).
In both structures the layers are packed efficiently, as evidenced by the Kitaigorodskii [31] packing indexes of 64.9% in 1 and 69.7% in 3.

Materials and Physical Measurements
All reagents and solvents were obtained commercially and were used as supplied. Elemental analysis (C, H, N) was carried out on a Fisons EA-1108 microanalyser. IR spectra were recorded from KBr discs (4000-400 cm −1 ) with a Bruker IFS28FT spectrophotometer.

Synthesis of the Precursor 1 ∞[Zn(chel)(H2O)2]
1 ∞[Zn(chel)(H2O)2] [4] was obtained by reaction of ZnCO3 and chelidonic acid (1:1 molar ratio) in methanol. The resulting suspension was heated under reflux for 2 h and stirred at room temperature for 1 day. The colourless solid was filtered off, washed with MeOH and vacuum dried. A solution of 4,4′-bipyridine (0.156 g, 1 mmol) in methanol (10 mL) was slowly added to a solution of Cu(NO3)2•2.5H2O (0.233 g, 1 mmol) in water (10 mL). The mixture was heated under reflux for 1 h and was left to cool down to room temperature. A solution of chelidonic acid (0.184 g, 1 mmol) and KOH (0.112 g, 2 mmol) in 2:1 MeOH/H2O was added to the mixture, which was then heated under reflux for 1 h and was left to cool down to room temperature. The blue precipitate was filtered off and dissolved in water. Blue crystals of 4a were obtained by slow evaporation of the aqueous solution. After separation of the crystals of 4a the slow evaporation of the remaining solution gave rise to some brown crystals of 4b.

Synthesis of the Complexes
Data

Crystallography
Crystallographic data were collected on a Bruker Smart 1000 CCD diffractometer using graphite monochromated Mo-Kα radiation (λ = 0.71073 Å). All data were corrected for Lorentz and polarisation effects. The frames were integrated with the Bruker Saint software package [32] and the data were corrected for absorption using the program SADABS. [33] The structures were solved by direct methods using the program SHELXS97 [34]. All non-hydrogen atoms were refined with anisotropic thermal parameters by full-matrix least-squares calculations on F 2 using the programme SHELXL97 [34] or the programme SHELXL [35] with OLEX2 [36]. Hydrogen atoms were inserted at calculated positions and constrained with isotropic thermal parameters. The hydrogen atoms of the water molecules were located from a difference Fourier map and refined with isotropic parameters but the hydrogen atoms for the crystallisation water molecules in 1 could not be located. Compound 4a crystallised in the chiral space group P21212 and its absolute configuration was confirmed by a value of 0.010(6) for the Flack parameter [37]. Drawings were produced with MERCURY [38]. Crystal data and structure refinement parameters are reported in Table S5.

Conclusions
The study described here concerned the crystalline species resulting from chelidonic acid/4,4′-bipyridine or 1,2-bis(4-pyridyl)ethane/Zn(II) or Cu(II) systems in aqueous solutions. The process led to the isolation of single crystals of two kinds of coordination polymers; the 1D Zn(II) and Cu(II) cationic polymers (2,  The five coordination polymers crystallise as hydrates but the crystallisation water molecules play different roles in the neutral polymers and in the solids containing the cationic polymers. In the neutral compounds the water molecules are weakly hydrogen bonded to the metal-organic framework but in the compounds based on the cationic polymers, the water molecules play a key role in the formation of the resulting supramolecular networks. These latter compounds are hydrogen-bonded networks through the second coordination sphere, which is organised in [(H2O)nchel] 2− (n = 3 or 6) layers.
The presence of non-associated water molecules (W1 motif) is common in the networks for all of the compounds reported here. Furthermore, in all compounds a recurrent supramolecular synthon based on the interaction between a water molecule (coordinated in the neutral compounds and free in the cationic species) and the two carboxylate groups of a chelidonate anion (coordinated or not) is observed. This synthon leads to the formation of an eight-membered ring in all cases.
From the point of view of crystal engineering the biological anion chelidonate, in combination with N,N'-bridging ligands, is an interesting tool to obtain coordination polymers and/or hydrogen-bonded networks.