A two-dimensional coordination polymer containing a two-dimensional zinc–carboxyl­ate layer constructed from helical chains: poly[(μ₄-benzene-1,2-dicarboxyl­ato)zinc(II)]

The title compound, [Zn(C₈H₄O₄)]_n, consists of one Zn^II cation and one benzene-1,2-dicarboxyl­ate dianion (BDC²⁻) as the building unit. The Zn^II cation is four-coordinated by four carboxyl­ate O atoms from four dianionic BDC²⁻ ligands in a distorted tetra­hedral geometry. The Zn^II cations are linked by the BDC²⁻ ligands to generate a structure featuring two-dimensional zinc–carboxyl­ate layers containing left- and right-handed helical chains. The two-dimensional layers are stacked along the a direction. The thermal stability of the title com­pound has been studied.

S1. Comment

The syntheses of coordination polymers or metal–organic frameworks based on metal cations and organic ligands are currently of great interest because these materials exhibit potential applications in catalysis, molecular magnets, photoluminescence, adsorption and phase separation; they also have an aesthetically appealing structural topology (Ma et al., 2009; Kitagawa et al., 2004; Eddaoudi et al., 2001; Cui et al., 2012). The conventional synthesis methods for coordination polymers are hydrothermal and solvothermal synthesis methods. Ionothermal synthesis, the use of an ionic liquid as solvent in the preparation of crystalline solids, is a new synthetic methodology developed recently (Reichert et al., 2006; Parnham & Morris, 2007). Ionic liquids have many intriguing characteristic physicochemical properties such as high ionic conductivity, nonflammability and negligible vapour pressure, which gives ionothermal synthesis an advantage over traditional hydrothermal and solvothermal materials synthesis methods. An ionic liqulid comprising only completely dissociated molecular cations and anions with a strong ionic atmosphere participates as both solvent and structure-directing agent in the ionthermal reaction (Morris, 2009; Chen et al., 2008). Compared with the traditional hydrothermal or solvothermal methods, the change from molecular to ionic reaction media leads to new types of material being accessible, with structural properties that may be traced directly to the chemistry of the ionic liqulid (Lin et al., 2007; Xu et al., 2007; Zhang et al., 2010). We are interested in the functional compounds prepared recently under ionothermal reactions (Liu et al., 2011; Wei et al., 2012). We report herein the ionothermal synthesis and crystal structure of poly[(µ₄-benzene-1,2-dicarboxylato)zinc(II)], (I), incorporating the benzene-1,2-dicarboxylate (BDC^2-) ligand. According to a literature search, only two zinc(II) compounds based on this ligand have been reported (Wen et al., 2007; Padmanabhan et al., 2007).

The asymmetric unit of (I) consists of one Zn^II cation and one BDC^2- dianion. As depicted in Fig. 1, the coordination of the Zn1 cation is composed of four carboxylate O atoms [O1, O4ⁱ, O3ⁱⁱ and O2ⁱⁱⁱ; symmetry codes: (i) x, -y + 1/2, z + 1/2; (ii) -x + 1, y + 1/2, -z + 3/2; (iii) -x + 1, y - 1/2, -z + 3/2] from four BDC^2- ligands in a distorted tetrahedral geometry. The [ZnO₄] tetrahedron is distorted, with O—Zn—O angles varying from 96.56 (7) to 127.67 (7)° and Zn—O bond lengths ranging from 1.9380 (16) to 2.0144 (16) Å (Table 1). The bond lengths involving the Zn^II atom are comparable to the values in related Zn^II complexes (Baca et al., 2001; Chen et al., 2004). The BDC^2- ligand adopts a tetradentate bridging coordination mode through its four monodentate carboxylate O atoms. The BDC^2- ligands bridge the Zn^II atoms to form a two-dimensional layer in the bc plane (Fig. 2). The two-dimensional layer contains a two-dimensional zinc–carboxylate laminar layer formed from Zn^II atoms and carboxylate groups. As depicted in Fig. 3, the O1—C7—O2 carboxylate bridges link the Zn^II atoms to form a one-dimensional helical chain along the b direction. Neighbouring helices are bridged by the O3—C8—O4 carboxylate groups through Zn1—O bonds to give a two-dimensional zinc–carboxylate layer (Fig. 3). Neighbouring helices have opposite chirality and the parallel left- and right-handed helices alternate along the c axis. The whole structure is therefore achiral. In the whole two-dimensional layered structure, all the benzene rings are located on both sides of the zinc–carboxylate layer. These two-dimensional layers are stacked along the a direction with no observed interaction between the layers (Fig. 4).

Almost 60 zinc(II) compounds involving the BDC^2- ligand have been reported previously; however, all but two of these exclusively contain a second N-containing ligand. For example, Zn^II cations are linked by tri- or bidentate BDC^2- ligands to give the one-dimensional chain structures of [Zn(BDC)(bpy)(H₂O)]_n (bpy = 2,2'-bipyridyl; Yao et al., 2002) and [Zn(BDC)(im)₂]_n (im = imidazole; Liu et al., 2002), respectively. Both contain terminal N-containing coordinating ligands. When bridging N-containing ligands are employed, two-dimensional layers and three-dimensional coordination networks can be obtained (Zhang et al., 2008; Bai et al., 2010; Su et al., 2009). As mentioned above, only two zinc–BDC compounds without a second ligand, namely [Zn(HBDC)₂(H₂O)₂] (Wen et al., 2007) and {(H₂pda)[Zn(BDC)₂]}_n (H₂pda = propane-1,3-diammonium; Padmanabhan et al., 2007), have been reported. [Zn(HBDC)₂(H₂O)₂], with monoanionic HBDC^- ligands, is a discrete mononuclear molecule, which is further extended into a three-dimensional supramolecular framework through extensive hydrogen bonds, whereas {(H₂pda)[Zn(BDC)₂]}_n possesses an anionic one-dimensional chain charge balanced by propane-1,3-diammonium dications, with the BDC^2- ligand binding to two metal ions through two monodentate carboxylate groups. Both compounds obtained under hydrothermal conditions are different from the title compound obtained from ionothermal reaction, which indicates that the solvent has a significant influence on the structure of the final product and the ionothermal reaction can produce novel structure much different from those from conventional conditions.

Ionic liquids can play multiple roles in ionothermal syntheses and crystallization of compounds. Neither cation nor anion of the 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid is incorporated in compound (I), indicating that 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid only serves as a solvent. While [However] there are some compounds synthesized under ionothermal reactions with the anion and/or cation of the ionic liquid occluded (Liu et al., 2011; Xu et al., 2007; Zhang et al., 2010).

The thermogravimetric analysis (TGA) of the title compound was undertaken and the TGA curve is shown in Fig. S1 in the Supplementary materials. Compound (I) displays a small, gradual weight loss starting at 413 K, and a sharp, continual weight loss then occurs at 523 K, which is attributed to the decomposition of the organic ligand.

Related literature

For related literature, see: Baca et al. (2001); Bai et al. (2010); Chen et al. (2004, 2008); Cui et al. (2012); Eddaoudi et al. (2001); Kitagawa et al. (2004); Lin et al. (2007); Liu et al. (2002, 2011); Ma et al. (2009); Morris (2009); Padmanabhan et al. (2007); Parnham & Morris (2007); Reichert et al. (2006); Su et al. (2009); Wei et al. (2012); Wen et al. (2007); Xu et al. (2007); Yao et al. (2002); Zhang et al. (2008, 2010).

S2. Experimental

Zn(NO₃)₂.6H₂O (89.2 mg, 0.3 mmol) and benzene-1,2-dicarboxylic acid (49.8 mg, 0.3 mmol) were mixed with 1-ethyl-3-methylimidazolium tetrafluoroborate (1 g) in a 25 ml Parr Teflon-lined stainless steel vessel. The vessel was sealed and heated to 433 K. The temperature was maintained for 6 d and then the mixture was allowed to cool naturally to obtain colourless block-shaped crystals [yield 23% based on Zn(NO₃)₃.6H₂O].

S3. Refinement

H atoms bonded to C atoms were placed in calculated positions and treated using a riding-model approximation, with C—H = 0.93 Å and U_iso(H) = 1.2U_eq(C).

Computing details

Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008) and DIAMOND (Brandenburg, 2005); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. The structure of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. [Symmetry codes: (i) x, -y + 1/2, z + 1/2; (ii) -x + 1, y + 1/2, -z + 3/2; (iii) -x + 1, y - 1/2, -z + 3/2.]
	Fig. 2. A perspective view of the two-dimensional layered structure, viewed along (a) the a and (b) the b axis. [Symmetry codes: (ii) -x + 1, y + 1/2, -z + 3/2; (iii) -x + 1, y - 1/2, -z + 3/2; (iv) x, -y + 1/2, z - 1/2.]
	Fig. 3. A perspective view of the two-dimensional zinc–carboxylate layer showing the left- (L) and right-handed (R) helical chains. [Symmetry code: (i) x, -y + 1/2, z +1/2.]
	Fig. 4. A view of the packing of (I), viewed along the b axis.

poly[(µ₄-benzene-1,2-dicarboxylato)zinc(II)]

Crystal data

[Zn(C8H4O4)]	F(000) = 456
Mr = 229.48	Dx = 2.070 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 10.5752 (2) Å	Cell parameters from 4148 reflections
b = 5.2925 (1) Å	θ = 3.1–27.6°
c = 13.9229 (2) Å	µ = 3.31 mm^−1
β = 109.078 (1)°	T = 296 K
V = 736.45 (2) Å^3	Block, colorless
Z = 4	0.28 × 0.26 × 0.10 mm

Data collection

Bruker APEXII area-detector diffractometer	1684 independent reflections
Radiation source: fine-focus sealed tube	1555 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.024
ω scans	θmax = 27.6°, θmin = 2.0°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −13→13
Tmin = 0.458, Tmax = 0.734	k = −6→6
6662 measured reflections	l = −18→18

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)] = 0.024	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.065	H-atom parameters constrained
S = 1.01	w = 1/[σ^2(Fo^2) + (0.0326P)^2 + 0.5655P] where P = (Fo^2 + 2Fc^2)/3
1684 reflections	(Δ/σ)max = 0.001
118 parameters	Δρmax = 0.40 e Å^−3
0 restraints	Δρmin = −0.38 e Å^−3

Special details

Experimental. IR (KBr pellet, cm^-1): 3442, 3131, 1642, 1554, 1502, 1439, 1407, 1263, 1122, 1071, 998, 956, 891, 864, 846, 764, 711, 701, 660, 620, 562, 538, 517, 474.

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
Zn1	0.55366 (2)	0.52595 (5)	0.874175 (18)	0.02687 (10)
O1	0.70705 (17)	0.5263 (3)	0.81840 (12)	0.0346 (4)
O2	0.56183 (16)	0.7929 (3)	0.72531 (12)	0.0363 (4)
O3	0.54984 (16)	0.3136 (3)	0.60715 (12)	0.0341 (4)
O4	0.64254 (17)	0.1684 (3)	0.49549 (12)	0.0404 (4)
C1	0.7497 (2)	0.6876 (4)	0.67280 (15)	0.0250 (4)
C2	0.7350 (2)	0.5308 (4)	0.58969 (16)	0.0250 (4)
C3	0.8181 (2)	0.5658 (5)	0.53047 (18)	0.0351 (5)
H3	0.8078	0.4638	0.4740	0.042*
C4	0.9153 (3)	0.7508 (5)	0.5555 (2)	0.0423 (6)
H4	0.9708	0.7721	0.5164	0.051*
C5	0.9302 (2)	0.9044 (5)	0.6386 (2)	0.0408 (6)
H5	0.9962	1.0285	0.6554	0.049*
C6	0.8476 (2)	0.8746 (4)	0.69665 (17)	0.0343 (5)
H6	0.8574	0.9801	0.7521	0.041*
C7	0.6665 (2)	0.6631 (4)	0.74154 (15)	0.0258 (4)
C8	0.6342 (2)	0.3219 (4)	0.56088 (15)	0.0257 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Zn1	0.03287 (16)	0.02642 (15)	0.02661 (15)	0.00415 (9)	0.01694 (12)	0.00333 (9)
O1	0.0330 (8)	0.0486 (10)	0.0259 (8)	0.0020 (7)	0.0148 (7)	0.0087 (6)
O2	0.0401 (9)	0.0344 (8)	0.0422 (9)	0.0079 (7)	0.0243 (7)	0.0029 (7)
O3	0.0427 (9)	0.0303 (8)	0.0372 (8)	−0.0105 (7)	0.0239 (7)	−0.0074 (6)
O4	0.0472 (10)	0.0416 (9)	0.0345 (8)	−0.0036 (8)	0.0162 (7)	−0.0166 (7)
C1	0.0272 (9)	0.0268 (10)	0.0230 (9)	0.0007 (7)	0.0109 (8)	0.0045 (7)
C2	0.0278 (10)	0.0254 (10)	0.0255 (10)	0.0008 (7)	0.0138 (8)	0.0028 (7)
C3	0.0414 (13)	0.0370 (12)	0.0357 (12)	0.0009 (10)	0.0248 (11)	0.0010 (9)
C4	0.0429 (13)	0.0460 (14)	0.0502 (14)	−0.0018 (11)	0.0319 (11)	0.0068 (11)
C5	0.0327 (12)	0.0417 (13)	0.0499 (14)	−0.0109 (10)	0.0161 (11)	0.0051 (11)
C6	0.0381 (12)	0.0324 (11)	0.0340 (11)	−0.0077 (9)	0.0138 (10)	0.0000 (9)
C7	0.0309 (10)	0.0268 (10)	0.0222 (9)	−0.0062 (8)	0.0120 (8)	−0.0062 (7)
C8	0.0312 (10)	0.0240 (9)	0.0227 (9)	0.0015 (8)	0.0099 (8)	0.0017 (7)

Geometric parameters (Å, º)

Zn1—O4^i	1.9380 (16)	C1—C6	1.392 (3)
Zn1—O3^ii	1.9414 (15)	C1—C7	1.503 (3)
Zn1—O2^iii	1.9560 (16)	C2—C3	1.401 (3)
Zn1—O1	2.0144 (16)	C2—C8	1.496 (3)
O1—C7	1.246 (3)	C3—C4	1.379 (3)
O2—C7	1.258 (3)	C3—H3	0.9300
O2—Zn1^ii	1.9560 (16)	C4—C5	1.380 (4)
O3—C8	1.261 (2)	C4—H4	0.9300
O3—Zn1^iii	1.9414 (15)	C5—C6	1.380 (3)
O4—C8	1.245 (2)	C5—H5	0.9300
O4—Zn1^iv	1.9380 (16)	C6—H6	0.9300
C1—C2	1.391 (3)
O4^i—Zn1—O3^ii	115.69 (7)	C4—C3—H3	119.9
O4^i—Zn1—O2^iii	107.17 (8)	C2—C3—H3	119.9
O3^ii—Zn1—O2^iii	109.91 (7)	C3—C4—C5	120.2 (2)
O4^i—Zn1—O1	96.56 (7)	C3—C4—H4	119.9
O3^ii—Zn1—O1	127.67 (7)	C5—C4—H4	119.9
O2^iii—Zn1—O1	97.05 (7)	C4—C5—C6	120.2 (2)
C7—O1—Zn1	103.70 (14)	C4—C5—H5	119.9
C7—O2—Zn1^ii	141.28 (14)	C6—C5—H5	119.9
C8—O3—Zn1^iii	128.32 (14)	C5—C6—C1	120.4 (2)
C8—O4—Zn1^iv	148.63 (16)	C5—C6—H6	119.8
C2—C1—C6	119.70 (18)	C1—C6—H6	119.8
C2—C1—C7	123.26 (18)	O1—C7—O2	120.47 (18)
C6—C1—C7	117.03 (18)	O1—C7—C1	119.20 (18)
C1—C2—C3	119.3 (2)	O2—C7—C1	120.15 (18)
C1—C2—C8	122.72 (18)	O4—C8—O3	125.6 (2)
C3—C2—C8	117.96 (19)	O4—C8—C2	117.66 (18)
C4—C3—C2	120.3 (2)	O3—C8—C2	116.73 (18)

Symmetry codes: (i) x, −y+1/2, z+1/2; (ii) −x+1, y+1/2, −z+3/2; (iii) −x+1, y−1/2, −z+3/2; (iv) x, −y+1/2, z−1/2.