Crystal structure of a heterometallic coordination polymer: poly[di-aqua-bis-(μ7-benzene-1,3,5-tri-carboxyl-ato)dicalcium(II)copper(II)].

In the title complex, [Ca2Cu(C9H3O6)2(H2O)2] n , the CaII and CuII cations are bridged by the benzene-1,3,5-tri-carboxyl-ate anions (BTC3-) to form the coordination polymer, in which each BTC3- anion bridges two CuII and five CaII cations with a μ7 coordination mode. The CuII cation, located at an inversion centre, is in a nearly square-planar geometry defined by four O atoms from four bridging BTC3- anions, while the CaII cation is in a distorted octa-hedral geometry defined by five O atoms from bridging BTC3- anions and one water mol-ecule. O-H⋯O hydrogen bonds between coordinating water mol-ecules and carboxyl groups further stabilize the structure; π-π stacking is also observed between parallel benzene rings, the centroid-to-centroid distance being 3.357 (2) Å.

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 self-assembly 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-metal-containing compounds is to employ appropriate bridging ligands. As a multifunctional hybrid ligand, H3BTC (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 H3BTC ligand are few in number (Chen et al., 2004 ▸; Li et al., 2010 ▸; Sun et al., 2014 ▸, 2016 ▸; Xu et al., 2014 ▸). As part of our ongoing studies on these compounds, we describe here synthesis and crystal structure of the title compound, [Ca2Cu(BTC)2(H2O)2], (1).

Structural commentary

The asymmetric unit of (1) contains one copper(II) cation (located at an inversion centre), one calcium(II) cation, one BTC3− anion and one coordinating water mol­ecule (Fig. 1 ▸). The Cu—O bond lengths are in the range 1.9435 (19)–1.9800 (19) Å and the Ca—O bond lengths are in the range of 2.280 (2)–2.466 (2) Å (Table 1 ▸). All data are comparable to those reported for other related CuII–BTC and CaII–BTC complexes (Chui et al., 1999 ▸; Yang et al., 2004 ▸) . Each CuII cation is four-coordinated by four oxygen atoms from four different BTC3− anions, forming a nearly square-planar geometry. Each CaII cation is six-coordinated by five carboxyl­ate oxygen atoms from five different BTC3− 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 H3BTC molecule is fully deprotonated and bridges two CuII ions and five CaII ions in a μ7 coordination mode.

Figure 1

The coordination mode and atom-numbering scheme for (1). Displacement ellipsoids for non H-atoms are drawn at the 50% probability level, with H atoms shown as spheres of arbitrary radius. [Symmetry codes: (A) x, y, z + 1; (B) −x, −y + 1, −z + 2; (C) x, y − 1, z; (D) −x, −y + 1, −z + 1; (E) −x + 1, −y + 2, −z + 2; (F) x, y + 1, z; (G) −x + 1, −y + 1, −z + 2; (H) x, y, z − 1.]

Table 1

Selected bond lengths (Å)

Ca1—O1	2.338 (2)	Ca1—O6iv	2.357 (2)
Ca1—O3i	2.280 (2)	Ca1—OW1	2.390 (2)
Ca1—O4ii	2.333 (2)	Cu1—O2	1.9435 (19)
Ca1—O5iii	2.466 (2)	Cu1—O5v	1.9800 (19)

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) .

Supra­molecular features

Each CuO4 quadrilateral shares a vertex (O5) with two CaO6 polyhedra to form a trinuclear unit {CuCa2O14} with Ca–O–Cu–O–Ca connectivity (Fig. 2 ▸). Such units are cross-linked by the μ7-BTC3− anions to create a three-dimensional framework (Fig. 3 ▸). In addition, the terminal water mol­ecule is hydrogen bonded to the carboxyl­ate O atoms (Table 2 ▸), forming a two-dimensional network parallel to (100). π-π stacking inter­actions between (C1–C6) benzene rings [Cg⋯Cg(−x, 1 − y, 2 − z) = 3.357 (2) Å] further stabilize the crystal structure.

Figure 2

The trinuclear unit constructed from a [CaO6] octa­hedron and a [CuO4] quadrilateral.

Figure 3

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

Table 2

Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
OW1—HW1A⋯O4v	0.84 (1)	1.95 (1)	2.793 (3)	173 (3)
OW1—HW1B⋯O2vi	0.84 (1)	2.31 (2)	3.020 (3)	143 (3)

Symmetry codes: (v) ; (vi) .

Synthesis and crystallization

The title compound was synthesized using a similar procedure to that for the synthesis of the analogous compound [CuSr2(BTC)2]·10H2O (Sun et al., 2016 ▸). A mixture of H3BTC (210 mg, 1 mmol), CuCl2·6H2O (121 mg, 0.5 mmol) and CaCl2 (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).

Table 3

Experimental details

Crystal data
Chemical formula	[Ca2Cu(C9H3O6)2(H2O)2]
M r	593.96
Crystal system, space group	Triclinic, P
Temperature (K)	296
a, b, c (Å)	6.664 (3), 8.754 (4), 8.925 (4)
α, β, γ (°)	103.065 (4), 110.140 (4), 92.776 (5)
V (Å3)	471.6 (4)
Z	1
Radiation type	Mo Kα
μ (mm−1)	1.79
Crystal size (mm)	0.18 × 0.15 × 0.14
Data collection
Diffractometer	Bruker SMART CCD
Absorption correction	Multi-scan (SADABS; Bruker, 2009 ▸)
T min, T max	0.721, 0.766
No. of measured, independent and observed [I > 2σ(I)] reflections	2442, 1635, 1588
R int	0.012
(sin θ/λ)max (Å−1)	0.595
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.029, 0.084, 1.04
No. of reflections	1635
No. of parameters	166
No. of restraints	3
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å−3)	0.35, −0.68

Computer programs: APEX2 and SAINT (Bruker, 2009 ▸), SHELXS97 and SHELXTL (Sheldrick, 2008 ▸), SHELXL2014 (Sheldrick, 2015 ▸) and DIAMOND (Brandenburg, 2006 ▸).

Crystal structure: contains datablock(s) I, global. DOI: 10.1107/S205698901700665X/xu5901sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S205698901700665X/xu5901Isup2.hkl

CCDC reference: 1547715

Additional supporting information: crystallographic information; 3D view; checkCIF report

[Ca2Cu(C9H3O6)2(H2O)2]	Z = 1
Mr = 593.96	F(000) = 299
Triclinic, P1	Dx = 2.091 Mg m−3
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 6.664 (3) Å	Cell parameters from 1990 reflections
b = 8.754 (4) Å	θ = 2.4–27.5°
c = 8.925 (4) Å	µ = 1.79 mm−1
α = 103.065 (4)°	T = 296 K
β = 110.140 (4)°	Block, blue
γ = 92.776 (5)°	0.18 × 0.15 × 0.14 mm
V = 471.6 (4) Å3

Bruker SMART CCD diffractometer	1635 independent reflections
Radiation source: fine-focus sealed tube	1588 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.012
φ and ω scans	θmax = 25.0°, θmin = 2.4°
Absorption correction: multi-scan (SADABS; Bruker, 2009)	h = −7→7
Tmin = 0.721, Tmax = 0.766	k = −10→4
2442 measured reflections	l = −10→10

Refinement on F2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.029	Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.084	H atoms treated by a mixture of independent and constrained refinement
S = 1.04	w = 1/[σ2(Fo2) + (0.0501P)2 + 0.6456P] where P = (Fo2 + 2Fc2)/3
1635 reflections	(Δ/σ)max < 0.001
166 parameters	Δρmax = 0.35 e Å−3
3 restraints	Δρmin = −0.68 e Å−3

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 F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > 2sigma(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

	x	y	z	Uiso*/Ueq
Ca1	0.13337 (8)	0.79200 (6)	0.58531 (6)	0.01430 (16)
Cu1	0.5000	1.0000	1.0000	0.01590 (16)
O1	0.2949 (3)	0.6981 (2)	0.8180 (2)	0.0206 (4)
O2	0.3877 (3)	0.8375 (2)	1.0782 (2)	0.0192 (4)
O3	0.2155 (4)	0.5598 (2)	1.4590 (2)	0.0273 (5)
O4	0.1993 (3)	0.2977 (2)	1.4102 (2)	0.0190 (4)
O5	0.2085 (3)	0.0033 (2)	0.8405 (2)	0.0186 (4)
O6	0.0233 (4)	0.1170 (2)	0.6527 (2)	0.0283 (5)
C1	0.2707 (4)	0.5610 (3)	1.0114 (3)	0.0126 (5)
C2	0.2808 (4)	0.5623 (3)	1.1690 (3)	0.0145 (5)
H2A	0.3166	0.6577	1.2499	0.017*
C3	0.2377 (4)	0.4211 (3)	1.2074 (3)	0.0136 (5)
C4	0.1929 (4)	0.2786 (3)	1.0883 (3)	0.0142 (5)
H4A	0.1703	0.1841	1.1148	0.017*
C5	0.1815 (4)	0.2765 (3)	0.9288 (3)	0.0142 (5)
C6	0.2163 (4)	0.4182 (3)	0.8895 (3)	0.0136 (5)
H6A	0.2034	0.4176	0.7822	0.016*
C7	0.3178 (4)	0.7085 (3)	0.9639 (3)	0.0139 (5)
C8	0.2195 (4)	0.4278 (3)	1.3728 (3)	0.0154 (5)
C9	0.1339 (4)	0.1255 (3)	0.7989 (3)	0.0142 (5)
OW1	0.4854 (3)	0.8960 (3)	0.6111 (3)	0.0315 (5)
HW1A	0.587 (4)	0.844 (3)	0.604 (4)	0.038*
HW1B	0.544 (5)	0.9871 (18)	0.669 (4)	0.038*

	U11	U22	U33	U12	U13	U23
Ca1	0.0188 (3)	0.0142 (3)	0.0099 (3)	0.0011 (2)	0.0048 (2)	0.0040 (2)
Cu1	0.0204 (3)	0.0110 (2)	0.0147 (3)	0.00010 (17)	0.00383 (19)	0.00479 (18)
O1	0.0344 (11)	0.0158 (9)	0.0143 (9)	0.0058 (8)	0.0090 (8)	0.0084 (7)
O2	0.0282 (10)	0.0125 (9)	0.0145 (9)	−0.0024 (7)	0.0056 (8)	0.0035 (7)
O3	0.0480 (13)	0.0191 (10)	0.0181 (10)	0.0061 (9)	0.0181 (10)	0.0016 (8)
O4	0.0236 (10)	0.0189 (10)	0.0181 (9)	0.0014 (7)	0.0094 (8)	0.0092 (8)
O5	0.0228 (10)	0.0107 (9)	0.0180 (9)	0.0032 (7)	0.0027 (8)	0.0030 (7)
O6	0.0452 (13)	0.0206 (10)	0.0118 (10)	0.0027 (9)	0.0010 (9)	0.0051 (8)
C1	0.0118 (12)	0.0128 (12)	0.0131 (12)	0.0020 (9)	0.0033 (10)	0.0049 (10)
C2	0.0160 (12)	0.0128 (12)	0.0141 (12)	0.0015 (9)	0.0051 (10)	0.0032 (10)
C3	0.0148 (12)	0.0140 (12)	0.0124 (12)	0.0025 (9)	0.0049 (10)	0.0043 (10)
C4	0.0169 (12)	0.0119 (12)	0.0155 (12)	0.0035 (9)	0.0057 (10)	0.0065 (10)
C5	0.0143 (12)	0.0135 (12)	0.0131 (12)	0.0032 (10)	0.0030 (10)	0.0033 (10)
C6	0.0159 (12)	0.0130 (12)	0.0125 (12)	0.0044 (10)	0.0040 (10)	0.0057 (10)
C7	0.0145 (12)	0.0135 (12)	0.0167 (13)	0.0051 (9)	0.0063 (10)	0.0078 (10)
C8	0.0154 (12)	0.0174 (13)	0.0130 (12)	0.0024 (10)	0.0047 (10)	0.0036 (10)
C9	0.0177 (12)	0.0129 (12)	0.0129 (13)	0.0017 (10)	0.0056 (10)	0.0050 (10)
OW1	0.0262 (11)	0.0264 (11)	0.0459 (14)	0.0047 (9)	0.0166 (10)	0.0116 (10)

Ca1—O1	2.338 (2)	O5—Ca1viii	2.466 (2)
Ca1—O3i	2.280 (2)	O6—C9	1.241 (3)
Ca1—O4ii	2.333 (2)	O6—Ca1iv	2.357 (2)
Ca1—O5iii	2.466 (2)	O6—Ca1viii	2.954 (2)
Ca1—O6iv	2.357 (2)	C1—C2	1.382 (4)
Ca1—OW1	2.390 (2)	C1—C6	1.395 (4)
Ca1—Cu1	3.6439 (13)	C1—C7	1.499 (3)
Cu1—O2v	1.9435 (19)	C2—C3	1.398 (4)
Cu1—O2	1.9435 (19)	C2—H2A	0.9300
Cu1—O5vi	1.9800 (19)	C3—C4	1.386 (4)
Cu1—O5iii	1.9800 (19)	C3—C8	1.511 (3)
Cu1—Ca1v	3.6439 (13)	C4—C5	1.395 (4)
O1—C7	1.239 (3)	C4—H4A	0.9300
O2—C7	1.278 (3)	C5—C6	1.393 (4)
O3—C8	1.242 (3)	C5—C9	1.485 (3)
O3—Ca1vii	2.280 (2)	C6—H6A	0.9300
O4—C8	1.271 (3)	C8—Ca1ii	3.141 (3)
O4—Ca1ii	2.333 (2)	C9—Ca1viii	3.104 (3)
O5—C9	1.278 (3)	OW1—HW1A	0.844 (10)
O5—Cu1viii	1.9800 (19)	OW1—HW1B	0.836 (10)
O3i—Ca1—O4ii	99.86 (8)	C7—O2—Cu1	110.48 (16)
O3i—Ca1—O1	81.17 (8)	C8—O3—Ca1vii	168.1 (2)
O4ii—Ca1—O1	87.46 (7)	C8—O4—Ca1ii	118.28 (16)
O3i—Ca1—O6iv	97.47 (8)	C9—O5—Cu1viii	125.84 (16)
O4ii—Ca1—O6iv	93.57 (8)	C9—O5—Ca1viii	107.73 (15)
O1—Ca1—O6iv	178.43 (7)	Cu1viii—O5—Ca1viii	109.61 (8)
O3i—Ca1—OW1	83.82 (8)	C9—O6—Ca1iv	157.26 (18)
O4ii—Ca1—OW1	173.99 (8)	C9—O6—Ca1viii	85.06 (15)
O1—Ca1—OW1	88.42 (8)	Ca1iv—O6—Ca1viii	114.30 (8)
O6iv—Ca1—OW1	90.64 (8)	C2—C1—C6	120.0 (2)
O3i—Ca1—O5iii	148.03 (7)	C2—C1—C7	122.6 (2)
O4ii—Ca1—O5iii	91.33 (7)	C6—C1—C7	117.4 (2)
O1—Ca1—O5iii	69.43 (7)	C1—C2—C3	120.3 (2)
O6iv—Ca1—O5iii	111.71 (7)	C1—C2—H2A	119.8
OW1—Ca1—O5iii	83.12 (7)	C3—C2—H2A	119.8
O3i—Ca1—Cu1	118.21 (6)	C4—C3—C2	119.6 (2)
O4ii—Ca1—Cu1	110.50 (5)	C4—C3—C8	120.9 (2)
O1—Ca1—Cu1	49.46 (5)	C2—C3—C8	119.2 (2)
O6iv—Ca1—Cu1	131.04 (6)	C3—C4—C5	120.3 (2)
OW1—Ca1—Cu1	63.49 (6)	C3—C4—H4A	119.9
O5iii—Ca1—Cu1	30.79 (4)	C5—C4—H4A	119.9
O6iii—Ca1—Cu1	73.03 (4)	C6—C5—C4	119.8 (2)
C9iii—Ca1—Cu1	50.47 (5)	C6—C5—C9	118.8 (2)
C8ii—Ca1—Cu1	106.72 (6)	C4—C5—C9	121.4 (2)
O2v—Cu1—O2	180.000 (1)	C5—C6—C1	119.9 (2)
O2v—Cu1—O5vi	91.13 (8)	C5—C6—H6A	120.1
O2—Cu1—O5vi	88.87 (8)	C1—C6—H6A	120.1
O2v—Cu1—O5iii	88.87 (8)	O1—C7—O2	123.6 (2)
O2—Cu1—O5iii	91.13 (8)	O1—C7—C1	118.5 (2)
O5vi—Cu1—O5iii	180.000 (1)	O2—C7—C1	117.8 (2)
O2v—Cu1—Ca1v	87.31 (6)	O3—C8—O4	124.8 (2)
O2—Cu1—Ca1v	92.69 (6)	O3—C8—C3	117.4 (2)
O5vi—Cu1—Ca1v	39.61 (5)	O4—C8—C3	117.7 (2)
O5iii—Cu1—Ca1v	140.39 (5)	O6—C9—O5	120.3 (2)
O2v—Cu1—Ca1	92.69 (6)	O6—C9—C5	121.0 (2)
O2—Cu1—Ca1	87.31 (6)	O5—C9—C5	118.7 (2)
O5vi—Cu1—Ca1	140.39 (5)	Ca1—OW1—HW1A	127 (2)
O5iii—Cu1—Ca1	39.61 (5)	Ca1—OW1—HW1B	122 (2)
Ca1v—Cu1—Ca1	180.0	HW1A—OW1—HW1B	105.9 (16)
C7—O1—Ca1	146.05 (17)

D—H···A	D—H	H···A	D···A	D—H···A
OW1—HW1A···O4vi	0.84 (1)	1.95 (1)	2.793 (3)	173 (3)
OW1—HW1B···O2v	0.84 (1)	2.31 (2)	3.020 (3)	143 (3)