Crystal structure of a heterometallic coordination polymer: poly[diaquabis(μ7-benzene-1,3,5-tricarboxylato)dicalcium(II)copper(II)]

The CaO6 polyhedron and CuO4 quadrilateral are connected by the benzene-1,3,5-tricarboxylate anions to give a three-dimensional polymeric complex.


Chemical context
In recent years, the rational design and synthesis of heterometallic coordination compounds have attracted much attention due to their potential applications in magnetism, luminescence, adsorption, chemical sensing and catalysis, as well as their aesthetically beautiful architectures and topologies (Cui et al., 2012;Huang et al., 2013;Ma et al., 2014;Wimberg et al., 2012). However, hererometallic organic frameworks are investigated less frequently than single-metal organic frameworks in crystal engineering, mainly because of the competitive complexation of different metal ions in the self-assembly progress. Recently, alkaline-earth metal ions have attracted more and more research interest owing to their unpredictable coordination number and pH-dependent selfassembly in the construction of novel topological coordination compounds (Borah et al., 2011;Chen et al., 2011). However, the larger atomic radii and high enthalpy of hydration make it relatively difficult to design the coordination polymers of alkaline-earth metal ions as well as to synthesize them from aqueous solution (Reger et al., 2013). As alkaline-earth metals and transition metals coordinate to the same ligand, it often gives rise to homometallic coordination compounds rather than heterometallic ones. In this regard, one of the effective synthetic strategies in building the alkaline-earth-metalcontaining compounds is to employ appropriate bridging ligands. As a multifunctional hybrid ligand, H 3 BTC (benzene-1,3,5-tricarboxylic acid) in its partly or fully deprotonated form exhibits versatile coordination modes and can bind to the metal ions by making full use of the carboxylate oxygen atoms. In addition, heterometallic compounds incorporating only the H 3 BTC ligand are few in number (Chen et al., 2004;Li et al., 2010;Sun et al., 2014Sun et al., , 2016Xu et al., 2014). As part of our ongoing studies on these compounds, we describe here synthesis and crystal structure of the title compound, [Ca 2 Cu(BTC) 2 (H 2 O) 2 ] n , (1).

Structural commentary
The asymmetric unit of (1) contains one copper(II) cation (located at an inversion centre), one calcium(II) cation, one BTC 3À anion and one coordinating water molecule (Fig. 1) (Chui et al., 1999;Yang et al., 2004) . Each Cu II cation is four-coordinated by four oxygen atoms from four different BTC 3À anions, forming a nearly square-planar geometry. Each Ca II cation is six-coordinated by five carboxylate oxygen atoms from five different BTC 3À anions and one terminal water molecule, displaying a distorted octahedron (Fig. 1). The mean deviation of the equatorial plane constructed by atoms O1, O4, O6 and OW1 is 0.06 Å . The H 3 BTC molecule is fully deprotonated and bridges two Cu II ions and five Ca II ions in a 7 coordination mode.

. Synthesis and crystallization
The title compound was synthesized using a similar procedure to that for the synthesis of the analogous compound [CuSr 2 (BTC) 2 ]Á10H 2 O (Sun et al., 2016). A mixture of H 3 BTC (210 mg, 1 mmol), CuCl 2 Á6H 2 O (121 mg, 0.5 mmol) and CaCl 2 (110 mg, 1 mmol) in 15 mL of distilled water was stirred for 10 min in air; 0.5 M NaOH was then added dropwise, and then the mixture was turned into a Parr Teflon-lined stainless steel vessel and heated to 443 K for 3 d. Blue block-shaped crystals suitable for X-ray diffraction were obtained in 60% yield (based on benzene-1,3,5-tricarboxylic acid).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The hydrogen atoms of the coordinating water molecule were located from a difference-Fourier map, but refined using a riding model with isotropic displacement parameters U iso (H) = 1.2U eq (O). Hydrogen atoms attached to carbon atoms were positioned geometrically (C-H = 0.93 Å ) and refined with U iso (H) = 1.2U eq (C).

Figure 3
Polyhedral view of the three-dimensional heterometallic coordination framework of (1). All H atoms have been omitted for clarity.

Poly[diaquabis(µ 7 -benzene-1,3,5-tricarboxylato)dicalcium(II)copper(II)]
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. 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.  (14) 0.0047 (9) 0.0166 (10) 0.0116 (10) Geometric parameters (Å, º)