Crystal structure of a heterometallic coordination polymer: catena-poly[[[tetraaquacobalt(II)]-μ-pyridine-2,6-dicarboxylato-calcium(II)-μ-pyridine-2,6-dicarboxylato] dihydrate]

The pyridine-2,6-dicarboxylate anions bridge the CaII and CoII cations to form a polymeric complex chain propagating along the b-axis direction.


Chemical context
The controllable synthesis of heterometallic polymers, with their fascinating structures and outstanding properties, is still a challenge in crystal engineering (Cai et al., 2012;Ma et al., 2014;Sun et al., 2014;Ward, 2007). The influencing factors include the coordination geometry of the metal centre, reaction of solvent, temperature, metal-to-ligand ratio, pH value, the nature of ligand, and so on (Chen et al., 2012;Guo & Cao, 2009;Ni et al., 2009;Yamada et al., 2011). According to our earlier study (Sun et al., 2016), heterometallic complexes containing both alkaline earth metals and d-block transition metals are available because the former are structurally malleable and they have a strong affinity to O atoms rather than N atoms (Cao et al., 2015;Yu et al., 2013), and the latter have a strong tendency to coordinate to both N-and O-atom donors (Hu et al., 2013;Zhang et al., 2013). Meanwhile, pyridinedicarboxylic acid (H 2 pdc) is widely used in the construction of various metal-organic frameworks for two main reasons. Firstly, the O and N atoms in these ligands made them easy to chelate or bridge metal ions. Secondly, they can be completely or partially deprotonated to generate Hpdc À or pyc 2À , displaying a variety of coordination modes. As a part of our ongoing studies on heterometallic frameworks, we describe here the synthesis and crystal structure of the title complex,1

Structural commentary
The asymmetric unit of 1 contains one cobalt centre, one calcium centre, two pdc 2À anions, four coordinated water molecules and two lattice water molecules (Fig. 1). The Co-O(N) bond lengths are in the range 2.0172 (13)-2.2018 (12) Å and the Ca-O bond lengths are in the range 2.3358 (12)-2.3727 (12) Å (Table 1). All the data are comparable to those reported for other related Co II -pdc and Ca II -pdc complexes (Jung et al., 2008;Shi et al., 2012). Each Co II centre is chelated by four O and two N atoms from two pdc 2À anions, forming a distorted octahedral geometry. The mean deviation of the equatorial plane constructed by atoms N1, N2, O5 and O7 is 0.02 Å . Each Ca II centre is six-coordinated by two carboxylate O atoms from two pdc 2À anions and four water molecules, displaying a distorted octahedron (Fig. 1). The mean deviation of the equatorial plane constructed by atoms O4, OW1, OW3 and OW4 is 0.08 Å . The CoN 2 O 4 and CaO 6 polyhedra are linked by pdc 2À anions to form polymeric chains along the baxis direction (Fig. 2).

Database survey
A search of the Cambridge Structural Database (Version 5.39, last update February 2018; Groom et al., 2016) for cobalt complexes of the ligand pyridine-2,6-dicarboxylic acid gave 180 hits, of which 58 are polymeric complexes. They include a number of alkali metal heterometallic coordination polymes, four involving K + and seven Na + , but no alkali earth metal heterometallic coordination polymers. Hence, the title compound 1 is the first reported heterometallic coordination polymer involving the ligand pyridine-2,6-dicarboxylic acid, Co II and an alkali earth metal (Ca II ). Symmetry code: (i) x; y À 1; z.

Figure 2
The chain formed by pdc 2À anions, and Co II and Ca II centres, propagating along the b-axis direction. Table 2 Hydrogen-bond geometry (Å , ).

Figure 1
The coordination mode and atom-numbering scheme for the asymmetric unit of 1. Displacement ellipsoids are drawn at the 50% probability level [symmetry codes:

Synthesis and crystallization
A mixture of H 2 pdc (167 mg, 1 mmol), Co(CH 3 COO) 2 Á4H 2 O (125 mg, 0.5 mmol) and CaCl 2 (110 mg, 1 mmol) in 15 ml of distilled H 2 O was stirred for 10 min in air. 0.5 M NaOH was added dropwise and the mixture was turned into a Parr Teflonlined stainless steel vessel and heated at 423 K for 3 d. Blue [purple in CIF?] block-shaped crystals of 1 were obtained in a yield of 70% (based on pyridine-2,6-dicarboxylic acid).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The H atoms of the water molecules were located from difference-Fourier maps and refined with distance restraints: O-H = 0.85 (1) Å , HÁ Á ÁH = 1.34 (1) Å with U iso (H) = 1.5U eq (O). C-bound H atoms atoms were included in calculated positions and refined as riding: C-H = 0.93 Å with U iso (H) = 1.2U eq (C).

Funding information
This work was supported financially by the National Natural Science Foundation of China (No. 21271189).

Figure 3
A view along the c axis of the crystal packing of 1. The hydrogen bonds are shown as dashed lines (see Table 2). For clarity, only the H atoms involved in these interactions have been included. Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT (Bruker, 2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: SHELXTL97 (Sheldrick, 2008) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and publCIF (Westrip, 2010). Hydrogen site location: mixed H atoms treated by a mixture of independent and constrained refinement w = 1/[σ 2 (F o 2 ) + (0.0385P) 2 + 0.4728P] where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.44 e Å −3 Δρ min = −0.49 e Å −3 Extinction correction: SHELXL2014 (Sheldrick, 2015), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0300 (14) 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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.