Poly[[tetra­aqua­bis(μ₂-2-nitro­tereph­thalato-κ³O¹:O⁴,O^4′)(μ₂-piperazine-κ³N:N′)dicadmium(II)] dihydrate]

In the title complex, {[Cd₂(C₈H₃NO₆)₂(C₄H₁₀N₂)(H₂O)₄]·2H₂O}_n, the Cd^II atoms show distorted octa­hedral coordination. The two carboxyl­ate groups of the dianionic 2-nitro­terephthalate ligand adopt monodentate and 1,2-bridging modes. The piperazine mol­ecule is in a chair conformation and lies on a crystallographic inversion centre. The Cd^II atoms are connected via three O atoms from two carboxyl­ate groups and two N atoms from piperazine mol­ecules to form a two-dimensional macro-ring layer structure. These layers are further aggregated to form a three-dimensional structure via rich intra- and inter­layer hydrogen-bonding networks. This study illustrates that, by using the labile Cd^II salt and a combination of 2-nitro­terephthalate and piperazine as ligands, it is possible to generate inter­esting metal–organic frameworks with rich intra- and inter­layer O—H...O hydrogen-bonding networks.

S1. Comment

An important objective in crystal engineering is the control and manipulation of either co-coordination bonds or weak intermolecular interactions in order to tune the properties of metal–organic frameworks (Guilera & Steed, 1999; Burrows et al., 2000; Guo et al., 2008). Such studies depend on the coordination habits and geometric preferences of both metal ions and bridging ligands, as well as on the influence of hydrogen bonding, van der Waals interactions etc. (Baca et al., 2003). The vast majority of current work centres around the controlled assembly of donor and acceptor building blocks. Amongst the more labile metal ions are Cu⁺, Cu²⁺, Ag⁺, Cd²⁺, Zn²⁺, Co²⁺ and Ni²⁺ (James, 2003). The terephthalate dianion has often been used as a bridging ligand in such a self-assembly approach, e.g. in poly[[[µ₂-1,3-bis(1H-benzimidazol-2-yl)benzene-κ²N³:N^3'](µ₂-terephthalato-κ²O:O')zinc(II)] ethanol solvate] (Meng et al., 2007), catena-poly[[diaquapyridinecadmium(II)]-µ-terephthalato] (Ni et al., 2006), poly[(µ₂-1,4-benzenedicarboxylato)aquadipyridinecopper(II) 0.25-hydrate] (Wang et al., 2007), catena-poly[[bis(1H-benzimidazole-κN³)cobalt(II)]-µ-terephthalato-κ³O¹,O^1':O⁴] (Pan et al., 2005), [Ni(bdc)(2,2'-bipyridine)(H₂O)₂]_n [bdc = benzenedicarboxylato?] (Go et al., 2004) and so on. However, in spite of this wealth of possibilities, only a few complexes of metal–nitroterephthalate systems have been reported to date. We have used reactions of divalent metal cations with 2-nitroterephthalate dianions, in the expectation of generating some interesting metal–organic frameworks. We have recently reported the crystal structure of catena-poly[[[triaquazinc(II)]-µ-2-nitroterephthalato-κO^1':κ²O⁴,O^4'] monohydrate] (Guo & Guo, 2007) [Non sequitur - text missing?] using piperazine as second ligand, and have obtained the novel title six-coordinated 2-nitroterephthalate–Cd complex, (I). We describe here the structure of this two-dimensional macro-ring metal–nitroterephthalate coordination polymer, with rich intra- and interlayer O—H···O hydrogen-bonding networks.

The asymmetric unit in the structure of (I) comprises one Cd atom, one half of a piperazine molecule, one complete 2-nitroterephthalate dianion and three non-equivalent water molecules, and is shown in Fig. 1 in a symmetry-expanded view which displays the full coordination of the Cd atom. Selected geometric parameters are given in Table 1.

The Cd atom in (I) is surrounded by an O₅N donor set in octahedral geometry. The four equatorial sites are occupied by one N atom of piperazine and three O atoms from a monodentate (O1) and a bidentate carboxylate group [O3ⁱ and O4ⁱ; symmetry code: (i) -x + 2, y - 1/2, -z + 1/2]. Atoms O7 and O8 from two coordinated water molecules occupy the opposing apices of the octahedron. The Cd—O/N distances are in the range 2.190 (5)–2.409 (5) Å and are shorter than the corresponding distances in catena-poly[[diaquabis(2-methoxymethyl-1H-benzimidazole-κN³)cadmium(II)]-µ-terephthalato-κ²O:O'] (Zu et al., 2007) and catena-poly[[diaquapyridinecadmium(II)]-µ-terephthalato] (Ni et al., 2006). The cis O—Cd1—O/N bond angles range from 54.87 (17) to 107.0 (2)° while the trans O—Cd—O/N bond angles span the range 153.37 (19)–172.1 (2)°. Thus, the coordination octahedron around the Cd atom is significantly distorted.

In the present structure, the O—C—O angle for the O1/C1/O2 carboxylate group is 127.1 (7)°, larger than the value of 121.2 (6)° for the O3/C8/O4 carboxylate group. The mean planes between the O1/C1/O2 group and the benzene ring make a dihedral angle of 60.3 (10)°, and the dihedral angle for the O3/C8/O4 group is 17.3 (9)°. The two C—O bond distances of the O1/C1/O2 group are 1.225 (9) and 1.238 (8) Å, respectively, and those of the O3/C8/O4 group are 1.234 (9) and 1.232 (9) Å, respectively. These indicate that the mesomeric effect for the bidentate 1,2-chelating carboxylate group is somewhat greater than that of the monodentate carboxylate group. It is worth noting that these C—O bond distances are shorter than those in the six-coordinate catena-poly[[[triaquazinc(II)]-µ-2-nitroterephthalato-κO^1':κ²O⁴,O^4'] monohydrate] (Guo & Guo, 2007).

The Cd atoms of (I) are linked together via the dianionic 2-nitroterephthalate ligand into a one-dimensional chain. The piperazine molecules are in a chair conformation and lie on a crystallographic inversion centre. They coordinate two Cd atoms, thus leading to the connection of two chains via their two N atoms. These result in Cd···Cdⁱ and Cd···Cdⁱⁱⁱ separations of 10.793 (3) and 7.113 (2) Å, respectively, and a Cdⁱ···Cd···Cdⁱⁱⁱ angle of 134.38 (1)° (see Fig. 1 for symmetry codes). The other two angles, Cdⁱ···Cd···Cdⁱⁱ and Cdⁱⁱ···Cd···Cdⁱⁱⁱ, are 110.62 (1) and 114.57 (1)°, respectively. In this way, six Cd atoms are associated into a 46-membered macro-ring approximately parallel to (302) via four dianionic 2-nitroterephthalate ligands and two piperazine molecules. Each six Cd atoms build up a polygon, in which the three diagonal distances are 17.952 (7), 20.234 (2) and 19.312 (6) Å, respectively. These macro-rings are further joined into a honeycomb two-dimensional layer structure (Fig. 2). The resulting layers are stacked parallel to the (302) plane.

The H atoms of two of the coordinated water molecules are involved in the formation of some strong intra- and intermolecular hydrogen bonds (Brown, 1976). The other water molecule, the piperazine molecule and the nitro group (O5/N1/O6) are also engaged in distinct hydrogen-bonded interactions (Table 2). Hydrogen bonding plays an important role in manipulation of the two-dimensional macro-ring structure. There are at least the following hydrogen-bonded graph sets (Bernstein et al., 1995): (a) a 22-membered R₄⁴(22) motif via O8–H8A···O9 and O9–H9D···O5^iv; (b) a six-membered ring R₁¹(6) motif via O8–H8B···O2; (c) a 17-membered R₄⁴(17) motif via O8–H8A···O9 and O9–H9C···O3^v; (d) an eight-membered R₂²(8) motif via O7–H7A···O4^vii; (e) an 18-membered R₃³(18) motif via O7—H7B···O2^vi; (f) an eight-membered R₂²(8) motif via N2–H2A···O8^iv; (g) an 11-membered R₂³(11) motif via N2–H2A···O8^iv and the piperazine ring. Neighbouring layers are linked together via these hydrogen-bonded interactions, except for the six-membered R₁¹(6) motif (Fig. 3). Thus, the three-dimensional connection of the structure is achieved.

Related literature

For related literature, see: Baca et al. (2003); Bernstein et al. (1995); Brown (1976); Burrows et al. (2000); Go et al. (2004); Guilera & Steed (1999); Guo & Guo (2007); Guo et al. (2008); James (2003); Meng et al. (2007); Ni et al. (2006); Pan et al. (2005); Wang et al. (2007); Zu et al. (2007).

S2. Experimental

The title complex was prepared under continuous stirring with successive addition of CdSO₄.8H₂O (0.40 g, 0.52 mmol), 2-nitroterephthalic acid (0.50 g, 2.4 mmol) and piperazine (0.20 g, 2.3 mol) to distilled water (25 ml) at room temperature. After filtration, slow evaporation over a period of 2 weeks at room temperature provided colourless needle-like crystals of (I).

S3. Refinement

All water H atoms were found in difference Fourier maps. However, during refinement, they were fixed at O—H distances of 0.85 Å, with U_iso(H) = 1.2U_eq(O). N- and C-bound H atoms were treated as riding, with N—H = 0.91 Å and C—H = 0.93–0.97 Å, and with U_iso (H) = 1.2U_eq(N or C).

Computing details

Data collection: SMART (Bruker, 1997); cell refinement: SMART (Bruker, 1997); data reduction: SAINT (Bruker, 1997); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures

	Fig. 1. A view of the structure of (I), showing the atom-numbering scheme and the coordination polyhedra for the Cd atoms. Displacement ellipsoids are drawn at the 30% probability level and H atoms are shown as small spheres of arbitrary radii. [Symmetry codes: (i) -x + 2, y - 1/2, -z + 1/2; (ii) -x + 2, y + 1/2, -z + 1/2; (iii) -x + 1, -y, -z - 1.]
	Fig. 2. A packing diagram for (I), viewed along the nearest [302] direction, showing the two-dimensional macro-ring layer of Cd atoms and ligands.
	Fig. 3. A packing diagram of (I). Hydrogen-bonding interactions between neighbouring layers are shown as dashed lines.

Poly[µ₂-Piperazine-N,N'-tetraaquabis[(µ₂-2-nitroterephthalato-O:O',O'')dicadmium(II)] dihydrate]

Crystal data

[Cd2(C8H3NO6)2(H2O)4(C4H10N2)]·2H2O	F(000) = 832
Mr = 837.26	Dx = 2.079 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 4289 reflections
a = 10.572 (3) Å	θ = 2.3–26.4°
b = 17.755 (5) Å	µ = 1.69 mm^−1
c = 7.390 (2) Å	T = 294 K
β = 105.374 (5)°	Needle, colourless
V = 1337.5 (6) Å^3	0.20 × 0.16 × 0.12 mm
Z = 2

Data collection

Bruker SMART CCD area-detector diffractometer	2331 independent reflections
Radiation source: fine-focus sealed tube	2019 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.059
ϕ and ω scans	θmax = 25.0°, θmin = 2.0°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −7→12
Tmin = 0.734, Tmax = 0.819	k = −21→18
6702 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)] = 0.048	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.129	H-atom parameters constrained
S = 1.15	w = 1/[σ^2(Fo^2) + (0.0411P)^2 + 9.0605P] where P = (Fo^2 + 2Fc^2)/3
2331 reflections	(Δ/σ)max < 0.001
199 parameters	Δρmax = 1.36 e Å^−3
0 restraints	Δρmin = −1.06 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
Cd1	0.75601 (5)	−0.03455 (3)	−0.08662 (7)	0.0298 (2)
O1	0.7942 (6)	0.0683 (3)	0.0828 (7)	0.0389 (12)
O2	0.8063 (6)	0.0185 (3)	0.3603 (7)	0.0419 (13)
O3	1.2094 (5)	0.3347 (3)	0.6460 (8)	0.0423 (13)
O4	1.0683 (5)	0.3974 (3)	0.4379 (7)	0.0392 (12)
O5	0.6601 (6)	0.2721 (3)	0.1028 (10)	0.0633 (18)
O6	0.6206 (5)	0.1656 (3)	0.2073 (9)	0.0497 (14)
O7	0.8739 (5)	0.0186 (3)	−0.2664 (8)	0.0460 (14)
H7A	0.9396	0.0442	−0.2057	0.055*
H7B	0.8531	0.0229	−0.3854	0.055*
O8	0.6421 (5)	−0.0742 (3)	0.1225 (8)	0.0431 (13)
H8A	0.6448	−0.1218	0.1349	0.052*
H8B	0.6773	−0.0480	0.2184	0.052*
N1	0.6941 (6)	0.2162 (3)	0.1969 (9)	0.0362 (14)
N2	0.5704 (6)	−0.0161 (3)	−0.3095 (8)	0.0296 (12)
H2A	0.5025	−0.0219	−0.2565	0.035*
C1	0.8212 (7)	0.0699 (4)	0.2545 (10)	0.0311 (15)
C2	0.8838 (7)	0.1406 (4)	0.3424 (9)	0.0290 (14)
C3	0.8306 (7)	0.2108 (4)	0.3020 (10)	0.0303 (15)
C4	0.8975 (7)	0.2750 (4)	0.3593 (10)	0.0314 (15)
H4	0.8583	0.3217	0.3269	0.038*
C5	1.0250 (7)	0.2697 (4)	0.4666 (9)	0.0298 (15)
C6	1.0786 (7)	0.2008 (4)	0.5156 (10)	0.0352 (16)
H6	1.1639	0.1971	0.5917	0.042*
C7	1.0090 (7)	0.1370 (4)	0.4546 (10)	0.0350 (16)
H7	1.0473	0.0902	0.4900	0.042*
C8	1.1057 (7)	0.3380 (4)	0.5209 (10)	0.0313 (15)
C9	0.5560 (7)	−0.0722 (4)	−0.4567 (10)	0.0330 (15)
H9A	0.5515	−0.1218	−0.4037	0.040*
H9B	0.6327	−0.0709	−0.5052	0.040*
C10	0.5633 (7)	0.0598 (4)	−0.3862 (11)	0.0350 (16)
H10A	0.6405	0.0690	−0.4306	0.042*
H10B	0.5640	0.0958	−0.2872	0.042*
O9	0.6116 (8)	−0.2219 (4)	0.1801 (13)	0.093 (3)
H9C	0.6709	−0.2511	0.1617	0.111*
H9D	0.5333	−0.2383	0.1494	0.111*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cd1	0.0316 (3)	0.0310 (3)	0.0237 (3)	0.0040 (2)	0.0019 (2)	0.00011 (19)
O1	0.058 (3)	0.031 (3)	0.028 (3)	−0.008 (2)	0.012 (2)	−0.004 (2)
O2	0.060 (4)	0.033 (3)	0.028 (3)	−0.010 (2)	0.004 (3)	0.001 (2)
O3	0.040 (3)	0.037 (3)	0.040 (3)	−0.008 (2)	−0.007 (2)	0.000 (2)
O4	0.036 (3)	0.030 (3)	0.044 (3)	−0.006 (2)	−0.004 (2)	0.000 (2)
O5	0.049 (4)	0.049 (4)	0.080 (5)	0.003 (3)	−0.004 (3)	0.018 (3)
O6	0.032 (3)	0.048 (3)	0.066 (4)	−0.011 (2)	0.009 (3)	−0.007 (3)
O7	0.039 (3)	0.067 (4)	0.032 (3)	−0.017 (3)	0.009 (2)	0.000 (2)
O8	0.043 (3)	0.043 (3)	0.045 (3)	−0.006 (2)	0.015 (3)	0.000 (2)
N1	0.027 (3)	0.040 (3)	0.037 (3)	−0.001 (3)	0.000 (3)	−0.003 (3)
N2	0.027 (3)	0.037 (3)	0.021 (3)	0.003 (2)	0.001 (2)	0.004 (2)
C1	0.030 (4)	0.030 (3)	0.034 (4)	0.001 (3)	0.010 (3)	−0.001 (3)
C2	0.032 (4)	0.032 (3)	0.026 (3)	−0.004 (3)	0.013 (3)	−0.001 (3)
C3	0.028 (4)	0.037 (4)	0.027 (4)	−0.002 (3)	0.010 (3)	−0.001 (3)
C4	0.032 (4)	0.028 (3)	0.033 (4)	−0.002 (3)	0.008 (3)	−0.001 (3)
C5	0.039 (4)	0.029 (3)	0.023 (3)	−0.003 (3)	0.010 (3)	−0.002 (3)
C6	0.031 (4)	0.036 (4)	0.035 (4)	−0.001 (3)	0.001 (3)	0.000 (3)
C7	0.038 (4)	0.031 (4)	0.033 (4)	0.000 (3)	0.003 (3)	0.000 (3)
C8	0.034 (4)	0.035 (4)	0.029 (4)	−0.004 (3)	0.016 (3)	−0.005 (3)
C9	0.038 (4)	0.033 (4)	0.023 (4)	0.004 (3)	0.001 (3)	0.002 (3)
C10	0.034 (4)	0.032 (3)	0.037 (4)	0.001 (3)	0.005 (3)	−0.002 (3)
O9	0.068 (5)	0.046 (4)	0.134 (8)	−0.013 (3)	−0.027 (5)	0.014 (4)

Geometric parameters (Å, º)

Cd1—O1	2.190 (5)	C1—C2	1.485 (9)
Cd1—N2	2.226 (5)	C2—C7	1.365 (10)
Cd1—O3^i	2.409 (5)	C2—C3	1.368 (10)
O4—Cd1^ii	2.244 (5)	C3—C4	1.349 (9)
Cd1—O7	2.256 (5)	C4—C5	1.374 (10)
Cd1—O8	2.307 (5)	C4—H4	0.9300
O1—C1	1.225 (9)	C5—C6	1.356 (10)
O2—C1	1.238 (8)	C5—C8	1.476 (9)
O3—C8	1.234 (9)	C6—C7	1.361 (10)
O4—C8	1.232 (9)	C6—H6	0.9300
O5—N1	1.210 (8)	C7—H7	0.9300
O6—N1	1.203 (8)	C9—C10^iii	1.487 (10)
O7—H7A	0.8513	C9—H9A	0.9700
O7—H7B	0.8509	C9—H9B	0.9700
O8—H8A	0.8494	C10—C9^iii	1.487 (10)
O8—H8B	0.8478	C10—H10A	0.9700
N1—C3	1.449 (9)	C10—H10B	0.9700
N2—C9	1.452 (9)	O9—H9C	0.8511
N2—C10	1.457 (9)	O9—H9D	0.8498
N2—H2A	0.9100
O1—Cd1—N2	107.0 (2)	O1—C1—O2	127.1 (7)
O1—Cd1—O4^i	99.5 (2)	O1—C1—C2	115.3 (6)
N2—Cd1—O4^i	153.4 (2)	O2—C1—C2	117.6 (6)
O1—Cd1—O7	86.6 (2)	C7—C2—C3	116.9 (6)
N2—Cd1—O7	91.1 (2)	C7—C2—C1	118.3 (6)
O4^i—Cd1—O7	89.9 (2)	C3—C2—C1	124.4 (6)
O1—Cd1—O8	85.49 (19)	C4—C3—C2	123.4 (7)
N2—Cd1—O8	91.5 (2)	C4—C3—N1	118.5 (6)
O4^i—Cd1—O8	91.1 (2)	C2—C3—N1	118.1 (6)
O7—Cd1—O8	172.1 (2)	C3—C4—C5	118.4 (6)
O1—Cd1—O3^i	153.37 (19)	C3—C4—H4	120.8
N2—Cd1—O3^i	98.83 (19)	C5—C4—H4	120.8
O4^i—Cd1—O3^i	54.87 (17)	C6—C5—C4	119.5 (6)
O7—Cd1—O3^i	99.3 (2)	C6—C5—C8	119.9 (6)
O8—Cd1—O3^i	87.7 (2)	C4—C5—C8	120.5 (6)
O1—Cd1—C8^i	126.4 (2)	C5—C6—C7	120.8 (7)
N2—Cd1—C8^i	126.3 (2)	C5—C6—H6	119.6
O4^i—Cd1—C8^i	27.4 (2)	C7—C6—H6	119.6
O7—Cd1—C8^i	95.8 (2)	C6—C7—C2	120.9 (7)
O8—Cd1—C8^i	88.7 (2)	C6—C7—H7	119.5
O3^i—Cd1—C8^i	27.5 (2)	C2—C7—H7	119.5
C1—O1—Cd1	124.7 (4)	O4—C8—O3	121.2 (6)
C8—O3—Cd1^ii	88.0 (4)	O4—C8—C5	118.9 (7)
C8—O4—Cd1^ii	95.9 (4)	O3—C8—C5	119.9 (6)
Cd1—O7—H7A	114.4	O4—C8—Cd1^ii	56.8 (3)
Cd1—O7—H7B	127.8	O3—C8—Cd1^ii	64.5 (4)
H7A—O7—H7B	116.7	C5—C8—Cd1^ii	174.8 (5)
Cd1—O8—H8A	111.6	N2—C9—H9A	109.1
Cd1—O8—H8B	101.6	C10^iii—C9—H9A	109.1
H8A—O8—H8B	117.5	N2—C9—H9B	109.1
O6—N1—O5	123.4 (7)	C10^iii—C9—H9B	109.1
O6—N1—C3	119.0 (6)	H9A—C9—H9B	107.8
O5—N1—C3	117.7 (6)	N2—C10—H10A	109.0
C9—N2—Cd1	110.5 (4)	C9^iii—C10—H10A	109.0
C10—N2—Cd1	111.6 (4)	N2—C10—H10B	109.0
C9—N2—H2A	107.8	C9^iii—C10—H10B	109.0
C10—N2—H2A	107.8	H10A—C10—H10B	107.8
Cd1—N2—H2A	107.8	H9C—O9—H9D	117.1

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O9—H9D···O5^iv	0.85	2.45	3.198 (10)	148
O9—H9C···O3^v	0.85	2.21	2.820 (9)	129
N2—H2A···O8^iv	0.91	2.65	3.344 (8)	133
O8—H8B···O2	0.85	1.90	2.683 (7)	154
O8—H8A···O9	0.85	1.86	2.690 (8)	165
O7—H7B···O2^vi	0.85	1.81	2.660 (8)	173
O7—H7A···O4^vii	0.85	1.81	2.660 (7)	174

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