A novel two-dimensional coordination polymer: poly[di­aqua­tetra-[mu]₂-chlorido-[[mu]₂-2,2'-(piperazine-1,4-di­ium-1,4-di­yl)di­acetate]­di­cadmium(II)]

In the crystal structure of the title two-dimensional metal-organic polymeric complex, [Cd₂Cl₄(C₈H₁₄N₂O₄)(H₂O)₂]_n, the asymmetric unit contains a crystallographically independent Cd^II cation, two chloride ligands, an aqua ligand and half a 2,2'-(piperazine-1,4-diium-1,4-di­yl)di­acetate (H₂PDA) ligand, the piperazine ring centroid of which is located on a crystallographic inversion centre. Each Cd^II centre is six-coordinated in an octa­hedral environment by an O atom from an H₂PDA ligand and an O atom from an aqua ligand in a trans disposition, and by four chloride ligands arranged in the plane perpendicular to the O-Cd-O axis. The complex forms a two-dimensional layer polymer containing [CdCl₂]_n chains, which are inter­connected into an extensive three-dimensional hydrogen-bonded network by C-H...O, C-H...Cl and O-H...O hydrogen bonds.

S1. Comment

In recent years, the synthesis and characterization of hybrid organic–inorganic framework solids has flourished, not only because of their intriguing architectures and topologies, but also because of their specific applications in catalysis, gas absorption, chiral separation, luminescence, nonlinear optics and magnetism (Kitagawa et al., 2004; Ferey et al., 2005; Roy et al., 2009; Zhang et al., 2009). It is well known that the construction of metal–organic complexes is strongly dependant on a number of factors, including the solvent system, temperature, concentration, organic ligands and metal atoms (Kan et al., 2012; Liu et al., 2012). Among these factors, it is the judicious selection of the organic ligands which plays a key role in directing the ultimate complex architectures. A popular method for the construction of coordination polymers is to utilize polycarboxylate ligands, due to their variety of coordination modes and structural features.

2,2'-(Piperazine-1,4-diyl)diacetic acid (H₂PDA) was chosen to construct novel coordination polymers in view of the following characteristics: (a) the dicarboxylic ligand has excellent coordination capability and flexible coordination patterns; (b) H₂PDA may show a variety of coordination modes and conformations, owing to the flexibility of its two carboxylate groups; and (c) as an artificial amino acid, proton transfer from the carboxy groups to the piperazine N atoms may be useful for the formation of hydrogen bonds and the stabilization of supramolecular assemblies. However, to the best of our knowledge, only a few coordination polymers of H₂PDA have yet been reported (Yang et al., 2008; Cheng et al., 2010). Here, we have selected H₂PDA as the organic ligand and have generated the title Cd^II coordination polymer, [Cd₂Cl₂(H₂PDA)(H₂O)₂]_n, (I), the crystal structure of which we now report.

Complex (I) crystallizes in the monoclinic space group P2₁/c and its asymmetric unit contains a crystallographically independent Cd^II cation, two chloride ligands, an aqua ligand and one-half of a H₂PDA ligand, the piperazine ring centroid of which is located on a crystallographic inversion centre. The coordination environment at the Cd^II cation is a {CdCl₄O₂} octahedron, consisting of two O atoms, one from an H₂PDA ligand and one from an aqua ligand, in the apical positions and four chloride ligands arranged in the basal plane (Table 1 and Fig. 1). The average Cd—O and Cd—Cl distances in (I) are comparable with those in other Cd-based complexes (Wang et al., 2012; Chen et al., 2012).

In (I), each Cd^II cation is surrounded by four bridging chloride ligands, giving rise to [CdCl₂]_n polymeric chains along the a axis. The bond angles of the Cd—Cl—Cd bridges range from 88.58 (3) to 90.55 (4)°, which are larger than those reported for other one-dimensional cadmium polymers bridged by chloride ligands (Huang et al., 1998; Jian et al., 2006; Darling et al., 2012). The closest Cd···Cd distance of 3.6381 (7) Å is much larger than the interatomic distance in bulk Cd (2.98 Å; [Standard reference?]). In the [CdCl₂]_n polymeric chains, the Cd^II cations are essentially collinear, with Cd···Cd···Cd angles of 174.785 (11)°. In the H₂PDA ligand, the dihedral angle between the carboxylate groups and the mean plane of the piperazine ring is 64.8 (2)°. Moreover, the six-membered piperazine ring is in a standard chair form, and the two acetates are mutually trans and diequatorial. H₂PDA exists as a zwitterionic ligand in (I), as shown in the scheme, and connects adjacent [CdCl₂]_n chains into a two-dimensional hybrid organic–inorganic layer that is oriented along the a crystal direction (Fig. 3). The Cd···Cd distance through the H₂PDA ligand is 10.6788 (9) Å. If each Cd^II cation is treated as a connecting node, this layer motif can be rendered as a simple (6,3) rhomboid grid (Fig. 4).

There are extensive inter- and intramolecular hydrogen bonds of N—H···O, O—H···O, C—H···O and C—H···Cl types connecting the two-dimensional layers of (I). There are hydrogen bonds between atoms N1 and O1/O1W, with donor–acceptor distances of 2.631 (4) and 2.873 (4) Å, respectively. Carbonyl atom O2 acts as an acceptor in the hydrogen-bonding interaction involving aqua atom O1W, with an O···O distance of 2.760 (3) Å. Furthermore, neighbouring two-dimensional layers are interconnected by C3—H3B···O2ⁱⁱⁱ, C4—H4A···Cl1ⁱⁱ, C4—H4B···Cl2ⁱ and O1W—H1WB···O2^iv hydrogen bonds (see Table 2 for symmetry codes), generating an extensive three-dimensional hydrogen-bonded network (Fig. 5).

In conclusion, we have synthesized a two-dimensional layer polymer containing [CdCl₂]_n chains. More interestingly, neighbouring two-dimensional layers are interconnected by C—H···O, C—H···Cl and O—H···O hydrogen bonds, generating an extensive three-dimensional hydrogen-bonded network.

Related literature

For related literature, see: Chen et al. (2012); Cheng et al. (2010); Darling et al. (2012); Ferey et al. (2005); Huang et al. (1998); Jian et al. (2006); Kan et al. (2012); Kitagawa et al. (2004); Liu et al. (2012); Roy et al. (2009); Wang et al. (2012); Yang et al. (2008); Zhang et al. (2009).

S2. Experimental

A mixture of CdCl₂.H₂O (20.1 mg, 0.1 mmol) and H₂PDA (20.2 mg, 0.1 mmol) in H₂O (10 ml) was sealed in a 16 ml Teflon-lined stainless steel container and heated at 383 K for 72 h. After cooling to room temperature, white block-shaped crystals of (I) were collected by filtration and washed several times with water and ethanol (yield 12.1%, based on H₂PDA). Elemental analysis, calculated for C₄H₉CdCl₂NO₃: C 15.89, N 4.63, H 3.00%; found: C 15.93, N 4.64, H 3.01%.

S3. Refinement

H atoms bonded to C atoms were placed in calculated positions and treated using a riding-model approximation, with C—H = 0.99 Å (methylene) and N—H = 0.93 Å, and with U_iso(H) = 1.2U_eq(C,N). Water H atoms were located in a difference Fourier map, and were refined with O—H = 0.85 Å and with U_iso(H) = 1.5U_eq(O).

Computing details

Data collection: SMART (Bruker 2000); cell refinement: SAINT (Bruker 2000); data reduction: SAINT (Bruker 2000); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures

	Fig. 1. A view of the local coordination of the Cd^II cation in (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. [Symmetry code: (v) x, -y + 1/2, z + 1/2.]
	Fig. 2. A view of the [CdCl2]n chain in (I).
	Fig. 3. A view of the two-dimensional layer in (I).
	Fig. 4. A representation of the (3,6) grid in the layer shown in Fig. 3, with the Cd^II cations shown as small spheres.
	Fig. 5. A perspective view of the three-dimensional supramolecular structure of (I), showing the N—H···O, O—H···O, C—H···O and C—H···Cl hydrogen bonds (dashed lines).

Poly[diaquatetra-µ₂-chlorido-[µ₂-2,2'-(piperazine-1,4-diium-1,4-diyl)diacetate]dicadmium(II)]

Crystal data

[Cd2Cl4(C8H14N2O4)(H2O)2]	F(000) = 584
Mr = 604.86	Dx = 2.357 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 2721 reflections
a = 8.7426 (11) Å	θ = 2.3–22.4°
b = 14.3464 (19) Å	µ = 3.15 mm^−1
c = 7.2686 (10) Å	T = 153 K
β = 110.808 (2)°	Block, colourless
V = 852.2 (2) Å^3	0.20 × 0.19 × 0.18 mm
Z = 2

Data collection

Bruker SMART CCD area-detector diffractometer	1664 independent reflections
Radiation source: fine-focus sealed tube	1582 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.029
ϕ and ω scans	θmax = 26.0°, θmin = 2.5°
Absorption correction: multi-scan (SADABS; Bruker, 2000)	h = −10→5
Tmin = 0.538, Tmax = 0.568	k = −17→17
4469 measured reflections	l = −8→8

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.031	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.082	H-atom parameters constrained
S = 1.15	w = 1/[σ^2(Fo^2) + (0.0437P)^2 + 0.2938P] where P = (Fo^2 + 2Fc^2)/3
1664 reflections	(Δ/σ)max < 0.001
100 parameters	Δρmax = 0.59 e Å^−3
0 restraints	Δρmin = −1.14 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
C1	0.1873 (4)	0.0176 (2)	−0.0298 (5)	0.0220 (7)
C2	0.2752 (5)	−0.0569 (2)	0.1224 (5)	0.0288 (8)
H2A	0.1949	−0.1035	0.1321	0.035*
H2B	0.3568	−0.0895	0.0797	0.035*
C3	0.5082 (4)	0.0405 (3)	0.3230 (5)	0.0260 (8)
H3A	0.4757	0.0897	0.2209	0.031*
H3B	0.5849	−0.0022	0.2925	0.031*
C4	0.4077 (4)	−0.0842 (2)	0.4784 (5)	0.0281 (8)
H4A	0.4822	−0.1300	0.4520	0.034*
H4B	0.3091	−0.1180	0.4788	0.034*
Cd1	0.13987 (3)	0.244236 (16)	−0.03240 (4)	0.02446 (14)
Cl1	0.36262 (11)	0.22178 (7)	−0.18887 (13)	0.0280 (2)
Cl2	−0.08013 (13)	0.24235 (6)	−0.37172 (15)	0.0291 (2)
N1	0.3592 (3)	−0.01283 (19)	0.3193 (4)	0.0202 (6)
H1	0.2868	0.0286	0.3438	0.024*
O1	0.1524 (3)	0.09003 (17)	0.0417 (4)	0.0298 (6)
O2	0.1574 (3)	−0.00200 (18)	−0.2045 (3)	0.0320 (6)
O1W	0.1325 (3)	0.41230 (18)	−0.0496 (3)	0.0255 (6)
H1WA	0.1428	0.4348	0.0623	0.038*
H1WB	0.0417	0.4305	−0.1325	0.038*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0169 (16)	0.0264 (18)	0.0198 (17)	−0.0018 (13)	0.0027 (13)	0.0008 (13)
C2	0.032 (2)	0.0239 (17)	0.0216 (18)	−0.0006 (15)	−0.0012 (15)	−0.0032 (14)
C3	0.0219 (18)	0.0321 (19)	0.0204 (17)	−0.0030 (14)	0.0031 (14)	0.0052 (14)
C4	0.0275 (19)	0.0258 (18)	0.0231 (18)	−0.0049 (15)	−0.0006 (15)	0.0062 (14)
Cd1	0.0285 (2)	0.02603 (19)	0.0187 (2)	0.00147 (9)	0.00829 (15)	0.00091 (9)
Cl1	0.0253 (5)	0.0373 (5)	0.0201 (4)	0.0007 (4)	0.0065 (4)	−0.0020 (4)
Cl2	0.0257 (5)	0.0388 (5)	0.0214 (5)	−0.0036 (3)	0.0069 (4)	0.0025 (3)
N1	0.0177 (14)	0.0219 (14)	0.0168 (13)	0.0024 (11)	0.0010 (11)	0.0009 (11)
O1	0.0360 (15)	0.0273 (14)	0.0254 (14)	0.0053 (11)	0.0100 (11)	0.0038 (11)
O2	0.0386 (15)	0.0375 (14)	0.0162 (13)	−0.0066 (12)	0.0051 (11)	−0.0013 (10)
O1W	0.0224 (13)	0.0316 (14)	0.0201 (13)	0.0012 (10)	0.0045 (10)	−0.0014 (10)

Geometric parameters (Å, º)

C1—O2	1.235 (4)	C4—H4B	0.9900
C1—O1	1.247 (4)	Cd1—O1	2.270 (3)
C1—C2	1.534 (4)	Cd1—O1W	2.414 (3)
C2—N1	1.497 (4)	Cd1—Cl2	2.5324 (11)
C2—H2A	0.9900	Cd1—Cl2^ii	2.5878 (12)
C2—H2B	0.9900	Cd1—Cl1	2.6006 (10)
C3—N1	1.502 (4)	Cd1—Cl1^ii	2.6096 (9)
C3—C4^i	1.505 (5)	Cl1—Cd1^iii	2.6096 (9)
C3—H3A	0.9900	Cl2—Cd1^iii	2.5878 (12)
C3—H3B	0.9900	N1—H1	0.9300
C4—N1	1.489 (4)	O1W—H1WA	0.8500
C4—C3^i	1.505 (5)	O1W—H1WB	0.8500
C4—H4A	0.9900
O2—C1—O1	128.7 (3)	O1—Cd1—Cl2^ii	86.93 (7)
O2—C1—C2	116.8 (3)	O1W—Cd1—Cl2^ii	86.49 (6)
O1—C1—C2	114.5 (3)	Cl2—Cd1—Cl2^ii	90.64 (4)
N1—C2—C1	110.2 (3)	O1—Cd1—Cl1	90.14 (7)
N1—C2—H2A	109.6	O1W—Cd1—Cl1	96.39 (6)
C1—C2—H2A	109.6	Cl2—Cd1—Cl1	90.02 (3)
N1—C2—H2B	109.6	Cl2^ii—Cd1—Cl1	177.07 (3)
C1—C2—H2B	109.6	O1—Cd1—Cl1^ii	89.91 (7)
H2A—C2—H2B	108.1	O1W—Cd1—Cl1^ii	81.96 (6)
N1—C3—C4^i	110.8 (3)	Cl2—Cd1—Cl1^ii	169.85 (3)
N1—C3—H3A	109.5	Cl2^ii—Cd1—Cl1^ii	88.62 (3)
C4^i—C3—H3A	109.5	Cl1—Cd1—Cl1^ii	91.23 (3)
N1—C3—H3B	109.5	Cd1—Cl1—Cd1^iii	88.57 (3)
C4^i—C3—H3B	109.5	Cd1—Cl2—Cd1^iii	90.55 (4)
H3A—C3—H3B	108.1	C4—N1—C2	111.1 (3)
N1—C4—C3^i	111.4 (3)	C4—N1—C3	109.3 (2)
N1—C4—H4A	109.3	C2—N1—C3	111.1 (3)
C3^i—C4—H4A	109.3	C4—N1—H1	108.4
N1—C4—H4B	109.3	C2—N1—H1	108.4
C3^i—C4—H4B	109.3	C3—N1—H1	108.4
H4A—C4—H4B	108.0	C1—O1—Cd1	135.3 (2)
O1—Cd1—O1W	169.65 (8)	Cd1—O1W—H1WA	109.8
O1—Cd1—Cl2	100.17 (7)	Cd1—O1W—H1WB	110.2
O1W—Cd1—Cl2	87.89 (6)	H1WA—O1W—H1WB	108.4
O2—C1—C2—N1	158.0 (3)	C3^i—C4—N1—C3	57.3 (4)
O1—C1—C2—N1	−23.4 (4)	C1—C2—N1—C4	165.2 (3)
O1—Cd1—Cl1—Cd1^iii	111.46 (7)	C1—C2—N1—C3	−73.0 (3)
O1W—Cd1—Cl1—Cd1^iii	−76.58 (6)	C4^i—C3—N1—C4	−56.9 (4)
Cl2—Cd1—Cl1—Cd1^iii	11.30 (3)	C4^i—C3—N1—C2	−179.8 (3)
Cl1^ii—Cd1—Cl1—Cd1^iii	−158.63 (5)	O2—C1—O1—Cd1	−36.5 (6)
O1—Cd1—Cl2—Cd1^iii	−101.53 (7)	C2—C1—O1—Cd1	145.2 (3)
O1W—Cd1—Cl2—Cd1^iii	85.00 (6)	O1W—Cd1—O1—C1	−157.5 (4)
Cl2^ii—Cd1—Cl2—Cd1^iii	171.47 (4)	Cl2—Cd1—O1—C1	61.8 (3)
Cl1—Cd1—Cl2—Cd1^iii	−11.39 (3)	Cl2^ii—Cd1—O1—C1	151.9 (3)
Cl1^ii—Cd1—Cl2—Cd1^iii	85.71 (17)	Cl1—Cd1—O1—C1	−28.2 (3)
C3^i—C4—N1—C2	−179.9 (3)	Cl1^ii—Cd1—O1—C1	−119.4 (3)

Symmetry codes: (i) −x+1, −y, −z+1; (ii) x, −y+1/2, z+1/2; (iii) x, −y+1/2, z−1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C4—H4B···Cl2^iv	0.99	2.59	3.517 (4)	157
C4—H4A···Cl1^v	0.99	2.77	3.455 (4)	127
C3—H3B···O2^vi	0.99	2.55	3.374 (5)	140
O1W—H1WB···O2^vii	0.85	1.98	2.813 (3)	168
O1W—H1WA···O2^ii	0.85	1.91	2.758 (3)	172
N1—H1···O1W^ii	0.93	1.97	2.873 (4)	163
N1—H1···O1	0.93	2.27	2.632 (4)	103

Symmetry codes: (ii) x, −y+1/2, z+1/2; (iv) −x, −y, −z; (v) −x+1, y−1/2, −z+1/2; (vi) −x+1, −y, −z; (vii) −x, y+1/2, −z−1/2.