Synthesis, crystal structure and magnetic properties of poly[[diaqua{μ6-2-[bis(carboxylatomethyl)amino]terephthalato}dicobalt(II)] 1.6-hydrate]

The title compound, {[Co2(C12H7NO8)(H2O)2]·1.6H2O}n, features tetranuclear CoII units bridged by κ 3 O:O:O′- and κ 3 O:O,O′-carboxylate groups from deprotonated 2-aminodiacetic terephthalic acid, which are joined into CoII ribbons via syn–anti carboxylate bridges. The parallel-aligned adtp4− ligands with an alternately reversed arrangement further link adjacent CoII ribbons into (010) layers, which are assembled into a three-dimensional supramolecular architecture via intermolecular hydrogen bonds.

The asymmetric unit of the polymeric title compound {[Co 2 (C 12 H 7 NO 8 )-(H 2 O) 2 ]Á1.6H 2 O} n comprises two Co II ions, which are coordinated by fully deprotonated 2-aminodiacetic terephthalic acid (adtp 4-) and terminal water molecules in distorted octahedral N 1 O 5 and O 6 coordination environments. The title compound features tetranuclear Co II units bridged by 3 O:O:O 0 -and 3 O:O,O 0 -carboxylate groups, which are joined into ribbons via syn-anti carboxylate bridges. The parallel adtp 4ligands with an alternately reversed arrangement further link adjacent Co II ribbons into (010) layers, which are assembled into a three-dimensional supramolecular network via intermolecular hydrogen bonds. The disordered water solvent molecules are situated in channels parallel to [100]. Magnetic measurements and analyses reveal that the title compound displays antiferromagnetic behaviour. The purity of the title compound was characterized by X-ray powder diffraction.

Chemical context
Over the last two decades, coordination polymers (CPs) have become one of the most attractive fields in chemistry because of their fascinating structures and promising applications as solid functional materials in adsorption and separation (Gan et al., 2020;Yang et al., 2020;Qian et al., 2020;Islamoglu et al., 2020), catalysis (Bavykina et al., 2020), sensing (Allendorf et al., 2020), luminescence (Rice et al., 2020) and magnetism (Thorarinsdottir & Harris, 2020;Wang et al., 2019b). Multicarboxylic acids have been employed to synthesize compounds comprising of various dimensional structures such as chains, layers and three-dimensional frameworks. Immense efforts have been devoted to the construction of CPs for successful predictions and the rational design of definite structures; many significant advances in the construction of CPs have occurred by employing well-defined rigid multicarboxylic acids (Padial et al., 2020;Li et al., 2020b;Wang et al., 2019a;Shen et al., 2017;Pang et al., 2017). However, using semi-rigid or flexible ligands, predictions are still tricky and confusing owing to the diversity of ligand configurations, the formation of various polynuclear metal units and the influence of weak interatomic interactions.
Our previous studies have focused on the construction of CPs based on semi-rigid multicarboxylic acids with the aminodiacetate moiety such as 2-aminodiacetic terephthalic acid (H 4 adtp) (Liu et al., 2009). The ortho-carboxylate group of H 4 adtp can be regarded as three carboxylic arms attached to one amino nitrogen atom. The three arms can chelate and/ ISSN 2056-9890 or bridge metal ions through their carboxylate groups into polynuclear metal units or chains. The residual phenyl carboxylate group can cross-link the polynuclear metal units or chains into layers or three-dimensional frameworks. In previous work, we have reported the supramolecular hydrogen-bonded pillar-layered structure of [Mn(H 2 adtp)(H 2 O)] n where the three arms connect Mn II ions into layers with Mn II chains and H 2 adtp ligands joined by hydrogen bonding act as pillars (Ma et al., 2015). Herein we report the layer structure of the title compound, {[Co 2 (C 12 H 7 NO 8 )-(H 2 O) 2 ]Á1.6H 2 O} n (I), based on fully deprotonated H 4 adtp as one of the ligands. The crystal structure, power X-ray diffraction pattern and magnetism of (I) were also studied in detail.

Structural commentary
The asymmetric unit of (I) comprises two Co II ions, one adtp 4ligand, two terminal water ligands and 1.6 disordered solvent water molecules. Regarding the adtp 4ligand, one carboxylate group (C12, O7, O8) of the aminodiacetate moiety adopts a 3 -O:O:O 0 coordination mode and the other one (C10, O5, O6) employs a syn-anti bidentate bridging fashion, whereas the carboxylate group in the ortho-position (C1, O1, O2) coordinates in a 3 -O:O,O 0 mode and that in the meta-position (C8, O3, O4) binds to one Co II ion in monodentate fashion (see Scheme). As shown in Fig. 1 research communications Table 1 Selected bond lengths (Å ).

Figure 2
Tetranuclear Co II units and a Co II ribbon in (I). Phenyl and metapositioned carboxylate groups and the disordered lattice water molecules have been omitted for clarity. [Symmetry codes:

Figure 1
Coordination environments of the Co II ions in (I) with displacement ellipsoids drawn at the 50% probability level; H atoms and the disordered lattice water molecules have been omitted for clarity. Symmetry codes refer to Table 1. into a ribbon via double syn-anti bridging carboxylate groups from the aminodiacetate moieties. The closest Co1Á Á ÁCo2 and Co1Á Á ÁCo1 distances in the ribbon are 3.7074 (8) and 3.5762 (8) Å , respectively. Parallel-aligned adtp 4ligands with an alternately reversed arrangement bind adjacent Co II ribbons into a layer extending parallel to (010) (Fig. 3).

Magnetic properties
The variable-temperature magnetic susceptibilities ( M ) of (I) were measured in the range 2-300 K under 1000 Oe. The M , M À1 and M T versus T plots are shown in Fig. 5. The value of M T at 300 K is 5.43 cm 3 K mol À1 , which is much larger than the expected spin-only value (3.75 cm 3 K mol À1 ) of two isolated Co II ions with g = 2.0, S = 3/2, which may be due to the contribution of the incompletely quenched orbital magnetic moment. As the temperature decreases, the M T value decreases slowly between 300 and 50 K and then it descends more steeply to the minimum value of 0.51 cm 3 K mol À1 at 2 K. The curve clearly indicates that the dominant antiferromagnetic coupling is operating. The temperature dependence of M À1 follows the Curie-Weiss law, and the linear fit by the equation 1/ M = (T À )/C gives C = 5.76 cm À3 K mol À1 and = À21.99 K, which is consistent with an antiferromagnetic behaviour. 3 of 5 Table 2 Hydrogen-bond geometry (Å , ). Symmetry codes: (v) Àx; Ày; Àz; (vi) x À 1; y À 1; z.

Figure 4
The three-dimensional supramolecular network of (I) constructed via intermolecular hydrogen bonds. The disordered water solvent molecules are located in channels parallel to [100].  CUFDIS). The other two Co II complexes are discrete coordination molecules (Liu et al., 2012). Three other complexes with layer structures based on 2-aminodiacetic terephthalic acid without another organic ligand have also been reported, viz. MUMBON, an Mn II complex (Ma et al., 2015), NEVJIJ, a Cd II complex (Ma et al., 2013), and NEVJUV, a Zn II complex (Ma et al., 2013). NEVJUV has similar cell parameters to the title compound, but similar tetranuclear metal units are not found in NEVJUV because the Zn II atoms have lower coordination numbers and the carboxylate oxygen atoms do not bridge the Zn II atoms as in the title compound. To the best of our knowledge, similar tetranuclear metal units have not been reported so far. Besides, one Co II coordination polymer (CCDC reference: 2063370; Ma et al., 2021), {[Co 2 (adtp)-(H 2 O) 6 ]Á5H 2 O} n , has been synthesized, which consists of parallel stacked zigzag chains in which Co II cations are linked together through 3 -adtp 4À anions.

Synthesis and crystallization
H 4 adtp was prepared using a similar protocol to that reported in the literature (Xu et al., 2006). The other chemicals were purchased from commercial sources and used without further purification. A solution of 0.2 mmol (0.0594 g) H 4 adtp in 5.0 ml of H 2 O was adjusted to a pH of 6.1 by adding a 1.0 M KHCO 3 solution drop by drop. The above solution was mixed with 0.5 mmol (0.1455 g) of Co(NO 3 ) 2 Á6H 2 O and 5.0 ml of CH 3 CN, then transferred into a 25.0 ml Teflon-lined stainless steel autoclave. The autoclave was sealed, heated to 393 K and held at that temperature for 72 h. The autoclave was allowed to cool to 303 K within 24 h. Plate-like pink crystals of (I) were collected in 66% yield based on H 4 adtp. Analysis calculated (%) for C 12 Co 2 N 1 O 11.6 H 14. The phase purity of compound (I) was confirmed by powder X-ray diffraction analysis (PXRD; Fig. S1 in the supporting information). The peak positions of the experimental PXRD patterns are in good agreement with those simulated on basis of the present single-crystal X-ray data, indicating that a pure phase was obtained.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The solvent water molecules (O11, O12, O13 and O14) were found to be disordered and were refined isotropically with site occupancies of 0.5, 0.5, 0.35 and 0.25, respectively. The hydrogen atoms of the non-disordered water molecules (O9, O10) were found in an difference density map 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). H atoms of O11, O12, O13 and O14 are not included in the model but were taken into account in the overall formula.

Poly[[diaqua{µ 6 -2-[bis(carboxylatomethyl)amino]terephthalato}dicobalt(II)] 1.6-hydrate]
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.