Synthesis and crystal structure of catena-poly[[hexaaqua{μ3-2-[bis(carboxylatomethyl)amino]terephthalato}dicobalt(II)] pentahydrate] containing water tapes and pentamers

The title compound consists of parallel stacked chains of composition {[Co2(C12H7NO8)(H2O)6]}n in which CoII cations are linked together through 2-aminodiacetic terephthalate anions. The chains are held together by networks of hydrogen bonds involving both coordinated and free water molecules in water tapes and pentamers.

The title coordination polymer, {[Co 2 (C 12 H 7 NO 8 )(H 2 O) 6 ]Á5H 2 O} n , was crystallized at room temperature from an aqueous solution of 2-aminodiacetic terephthalic acid (H 4 adtp) and cobalt(II) nitrate. The asymmetric unit consists of one adtp 4À ligand, one and two half Co II ions, six water ligands coordinated to Co II ions and five uncoordinated water molecules. Two of the cobalt cations lie on centres of inversion and are coordinated in octahedral O 2 (OH 2 ) 4 environments, whereas the other adopts a slightly distorted octahedral NO 3 (OH 2 ) 2 environment. The crystal structure contains parallel stacked, onedimensional zigzag chains, {[Co 2 (C 12 H 7 NO 8 )(H 2 O) 6 ]} n , which assemble into a three-dimensional supramolecular architecture via networks of hydrogen bonds involving the coordinated and free water molecules. One-dimensional 'water tapes' are formed, containing alternating six-membered and twelve-membered rings of water molecules, together with water pentamers, in which a central uncoordinated water molecule is hydrogen bonded to two coordinated and two free water molecules in a tetrahedral arrangement.

Chemical context
Water clusters, which are aggregations of water molecules assembled via hydrogen bonding, are often observed in organic and organic-inorganic hybrid crystal structures. To date, a number of discrete water clusters of different sizes and conformations have been identified, including tetramers (Thakur et al., 2021;Ahmed et al., 2018), pentamers (Ghosh & Bharadwaj, 2006), hexamers (Zhao et al., 2015;, heptamers (He et al., 2012;Hedayetullah Mir & Vittal, 2008), octamers (Hao et al., 2013;Wei et al., 2009;Ghosh & Bharadwaj, 2006), decamers (Mukhopadhyay & Bernal, 2006), and other higher member clusters (Liu et al., 2018;Chen et al., 2020). In addition, examples of infinite water clusters consisting of one-dimensional water chains or 'tapes' (Gacki et al., 2020;Zhao et al., 2019;Saraei et al., 2019;Han et al., 2019;Liu et al., 2020;Saraei et al., 2018), two-dimensional water layers (Mei et al., 2016) and three-dimensional water frameworks (Huang et al., 2007Wu et al., 2013) have also been reported recently. Water clusters are often held in the cavities of the host structures as guest molecules, which can enhance the stability of the structure. Water clusters, when hydrogen bonded to the host structures, play a vital role in assembling organic and organic-inorganic complex molecules into threedimensional architectures (Thakur et al., 2021;Zia et al., 2020;ISSN 2056-9890 Huang et al., 2019Liu et al., 2018). Our work focuses on the construction of metal complexes using semi-rigid multicarboxylic acids containing aminodiacetate moieties, and analysing the affects of weak hydrogen-bonding interactions on their supramolecular assemblies (Ma et al., 2015a). We have previously reported the synthesis of two Cu II complexes based on 2-(carboxyphenyl)-iminodiacetic acid (H 3 cpida) and 1,10phenanthroline (phen), and discussed the influence of hydrogen bonding on the resulting structures (Ma et al., 2015b). Herein we report the synthesis and structural characterization of a Co II coordination polymer, {[Co 2 (C 12 H 7-NO 8 )(H 2 O) 6 ]Á5H 2 O} n (I), based on 2-aminodiacetic terephthalic acid (H 4 adtp). The hydrogen-bonding interactions in (I), which result in the formation of one-dimensional water tapes and isolated water pentamers, are discussed in detail.

Structural commentary
Compound (I) crystallizes in the triclinic space group Pı ". The asymmetric unit comprises three crystallographically distinct Co II ions, one adtp 4À ligand, six coordinated water ligands and five free water molecules. Regarding the adtp 4À ligand, the carboxylate groups of the aminodiacetate moiety and that in the meta-position adopt monodentate coordination modes on bonding to cobalt, whereas the carboxylate group in the orthoposition coordinates in a syn-anti bidentate bridging fashion (see Scheme). As shown in Fig. 1, Co2 is located in a distorted octahedral N 1 O 5 environment. The adip 4À ion chelates to Co2 via the amino nitrogen atom (N1), two acetate oxygen atoms (O6 and O8) and the ortho-position carboxylate oxygen atom (O4). The remaining two cis-related sites around Co2 are ligated by oxygen atoms (O9 and O14) of terminal water molecules. Co1 and Co3 both lie on inversion centres and are located in octahedral O 6 environments. In each case, a pair of trans-related coordination sites are bonded to equivalent carboxylate oxygen atoms (O2, O2 i for Co1; O3, O3 ii for Co3). The remaining trans-related sites of Co1 and Co3 are ligated by two pairs of equivalent oxygen atoms from terminal water molecules (O12, O12 i and O13, O13 i for Co1; O10, O10 ii and O11, O11 ii for Co3, respectively). The length of the Co2-N1 bond is 2.1712 (18) Å and the Co-O distances lie in the range 2.0128 (15)-2.1330 (15) Å , all of which are reasonable values. The adtp 4À ligand links the Co1 and Co3 atoms via the orthoand meta-position carboxylate groups and a zigzag chain is formed by inversion operations with the closest Co1Á Á ÁCo3 and Co2Á Á ÁCo3 distances being 10.657 (1) and 5.194 (1) Å , respectively (Fig. 2).
The yellow chain also connects with the red and blue chains, assembling into two-dimensional supramolecular layers via the intermolecular hydrogen bonds of groups II (Fig. 5b) and III (Fig. 6b), respectively. In addition, there are a number of other hydrogen-bonding interactions within the structure. The free water molecule H 2 O19 forms four hydrogen bonds, two with coordinated water molecules H 2 O11 and H 2 O14, and two with free water research communications The one-dimensional zigzag coordination chain found in (I).

Figure 3
The three-dimensional supramolecular architecture of (I) composed of zigzag coordination chains linked via intermolecular hydrogen bonds shown as dashed red lines. One zigzag chain, highlighted in yellow, associates directly with three pairs of neighbouring chains, which are highlighted in green, red and blue.

Database survey
including one two-dimensional coordination polymer (refcode CUFDIS; Ma et al., 2021) and two discrete coordination complexes (RAXJUX and RAXKEI; Liu et al., 2012b). No structures containing H 4 adtp with similar cell parameters to those of the title compound have been reported.

Synthesis and crystallization
H 4 adtp was synthesized using a method based on that described in the literature (Xu et al., 2006). The other chemicals were purchased from commercial sources and used without further purification. A mixture of Co(NO 3 ) 2 Á6H 2 O (0.2910 g, 1 mmol), H 4 adtp (0.0594 g, 0.2 mmol) and hexamethylenetetramine (0.0701 g, 0.5 mmol) was dissolved in 6 mL of water. The phase purity of (I) was demonstrated by powder X-ray diffraction analysis (PXRD; Fig. S1 in the supporting information). The peak positions of the experimental PXRD pattern match well with those simulated from the singlecrystal X-ray data, indicating that the pure phase was synthesized.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. During the refinement of (I), O16 was found to be disordered over two sites (O16 and O16A) with occupancies of 0.704 (5) and 0.296 (5). The hydrogen atoms of the water molecules were found in electron-density maps and refined as riding, with U iso (H) = 1.5 U eq (O). Other hydrogen atoms were placed at geometrically calculated positions and treated as riding, with Csp 2 -H = 0.93 Å , Csp 3 -H = 0.97 Å and U iso (H) = 1.2 U eq (C).