A double-layered zinc(II) coordination polymer with the ligand 3,5-bis­(car­boxyl­atomethoxy)benzoate

In the title compound, poly[hexa­aqua­bis[μ₄-3,5-bis­(carboxyl­atomethoxy)benzoato]trizinc(II)], [Zn₃(C₁₁H₇O₈)₂(H₂O)₆]_n, there are two crystallographically distinct Zn^II cations which are bridged by polycarboxyl­ate ligands in a μ₄-bridging mode. A pair of ligands bridges adjacent Zn atoms to give centrosymmetric dimetal building blocks which act as four-connected nodes to be further inter­linked into a two-dimensional double-layered framework with (4,4) topology. Other Zn atoms, lying on inversion centres, occupy the cavities of this topological structure. This submission shows a versatile polycarboxyl­ate ligand with rigid and flexible functional groups, the co-operation and complementarity of which would meet the coordination requirements of a variety of topological structures.

S1. Comment

Carboxylate-containing ligands have been intensively investigated to construct metal–organic frameworks with an intriguing variety of topologies and potential applications in gas sorption, separation and/or catalysis (Eddaoudi et al., 2001; Kitagawa et al., 2004; Rao et al., 2004). A wide variety of metal–carboxylate architectures are obtained by varying the nature of the reactants or the synthetic conditions. Polycarboxylate ligands with suitable spacers are good choices for such architectures because the topological structures can be adjusted not only by the carboxylate groups but also by organic spacers. Rigid spacer-based ligands such as phenyldicarboxylate have commonly been used to design functional metal–organic frameworks since these ligands allow for a certain level of control of steric effects in the assembly process and result in predictable motifs (Bourne et al., 2001; Kim et al., 2001). Nevertheless, it is also known that flexible ligands have good conformational freedom, which manifests itself in the various connecting modes that offer opportunities to create structurally diverse molecular architectures (Chen et al., 2007). In addition, compounds that are capable of rotational motion in response to outside driving forces such as light, heat or electric fields are intriguing for the development of molecular devices (Horike et al., 2006). Therefore, efforts recently focusing on manipulating flexible ligands for metal–organic frameworks have been increasing. Considering the respective advantages of rigid and flexible ligands, their cooperation has been proved to be successful in creating metal–organic architectures with intriguing topological structures and/or potential applications (Wang et al., 2007; Wei et al., 2008).

As part of our ongoing investigation of metal–organic frameworks involving semi-rigid organic ligands (Wang et al., 2008), we have developed a versatile polycarboxylate ligand, 3,5-di(carboxymethoxy)benzoate acid (H₃L), which contains non-equivalent carboxylate groups from a rigid benzoate group and two flexible carboxymethoxy groups. The complementarity of the coordination function and geometric conformation from these different carboxylate groups matches the requirement for structural variety in metal–organic frameworks. Here, we report the title compound, (I), a two-dimensional zinc-containing double-layered framework structure, prepared by reacting H₃L with ZnAc₂.2H₂O under hydrothermal conditions.

Compound (I) is a two-dimensional double-layer polymer in which the asymmetric unit contains one ligand, two crystallographically independent Zn^II ions and three coordinated water molecules (Fig. 1). The coordination geometry of atom Zn1 can be described as a distorted tetrahedron consisting of three carboxylate O atoms from three different ligands and one aqua O atom [O7, O9, O5ⁱ and O2ⁱⁱ; symmetry codes: (i) -x+2, -y+1, -z; (ii) x-1/2, -y+3/2, z+1/2]. Atom Zn2 is located on an inversion centre at (1/2, 1/2, 1/2) and is coordinated by two carboxylate O atoms from two different ligands and four aqua O atoms [O8, O10, O11 and their symmetry-related atoms at 1-x, 1-y, 1-z)], giving a slightly distorted octahedral geometry. The whole ligand is completely deprotonated in this complex and acts in a µ₄-bridging mode. One carboxymethoxy group is approximately coplanar with the aromatic ring, with a C5—O6—C10—C11 torsion angle of 177.2 (2)°, while another deviates from this ring, with a C3—O3—C8—C9 torsion angle of 67.0 (3)° and a dihedral angle between the carboxylate group and the aromatic ring plane of 102.8 (2)°. The carboxylate group of the coplanar carboxymethoxy group behaves as a syn–anti bidentate bridging ligand to coordinate atoms Zn1 and Zn2, while each of the other two carboxylate groups adopts a monodentate coordination mode to bridge different Zn1 atoms. The Zn1—O6 distance of 2.706 (2) Å shows some weak interaction between Zn1 and an uncoordinated ether O atom from the coplanar carboxymethoxy group, which may be viewed as a semi-chelating coordination mode. Such a weak interaction could lead to the result of this carboxymethoxy group being coplanar with the phenyl ring. Therefore, the environment of atom Zn1 can also be described as a significantly distorted trigonal bipyramid, with atoms O6 and O2ⁱⁱ located at the apical positions and atoms O7, O9 and O5ⁱ in the equatorial positions.

Analysis of this polymeric network suggests a functional centrosymmetric dimetallic building block unit, generated by a pair of symmetry-related ligands bridging adjacent Zn1 atoms (Fig. 2a) with its centre located on an inversion centre. The pair of ligands adopts an antiparallel mode, with a phenyl ring-centroid distance of 3.895 (4) Å, a interplanar distance of 3.513 (2) Å and a ring-centroid slippage of 1.683 Å, indicating that the two phenyl rings do not overlap. Such building blocks can be regarded as double-layered motifs and they adopt a four-connected node (each pair of connections associated with one layer) to be further interlinked through Zn1—O_benzoate bonds into two-dimensional double-layered framework with (4,4) topology. Four adjacent building blocks are interlinked to form a cavity which is occupied by one octahedral Zn2 atom that links two opposite building blocks through O4 atoms at (x, y, z) and (1 - x, 1 - y, 1 - z), stabilizing the cavity (Fig. 2b). Therefore, the double-layered framework is formed by assembling the building blocks, whereas the ZnO₆ fragments occupy the cavities. Further analysis of this structure shows that each layer consists of independent one-dimensional zigzag metal–organic chains constructed by consecutive linking between atom Zn1 and the ligands through carboxylate O atoms from the benzoate and coplanar carboxymethoxy groups, respectively (Fig. 3). The combination of cross-linking and linking through Zn2—O bonds between chains belonging to different layers forms the double-layered framework.

All coordinated water molecules in complex (I) act as double-proton donors to participate in typical hydrogen bonding, with carboxylate O atoms acting as the acceptors (Table 2). The ether O atoms are not involved in hydrogen bonding. The four atoms H9A, H10A, H10B and H11A participate in the hydrogen bonding to consolidate the two-dimensional double-layered structure, whereas atoms H9B and H11A form hydrogen bonds with carboxylate O atoms from neighbouring double-layered structures to form a three-dimensional supramolecular structure. Adjacent double-layered structures stack antiparallel in an ABAB··· pattern.

S2. Experimental

A mixture of ZnAc₂.2H₂O (0.0438 g, 0.2 mmol) and H₃L (0.1224 g, 0.4 mmol) in H₂O (4 ml) and tetrahydrofuran (4 ml) was sealed in a Teflon-lined steel bomb (15 ml) and then heated at 353 K for 3 d. Colourless block crystals of (I) were collected (yield: 20%). Elemental analysis, calculated for C₂₂H₂₆O₂₂Zn₃: C 31.50, H 3.10%; found: C 31.32, H 2.92%. Spectroscopic analysis, IR (Medium?, ν, cm^-1): 3494 (m), 1638 (s), 1604 (s), 1546 (s), 1425 (s), 1404 (s), 1359 (m), 1324 (m), 1171 (s), 1097 (m), 1079 (m), 751 (m), 792 (m).

S3. Refinement

C-bound H atoms were fixed geometrically and treated as riding, with C—H = 0.97 (methylene) or 0.93 Å (aromatic) and with U_iso(H) = 1.2U_eq(C). The H atoms of the water molecules were located in a difference Fourier map and included in the subsequent refinement using restraints O—H = 0.85 (1) Å and H···H = 1.39 (2) Å. In the last cycles of refinement, they were treated as riding on the O atoms to which they are attached, with U_iso(H) = 1.5U_eq(O).

Computing details

Data collection: SMART (Bruker, 1998); cell refinement: SAINT (Bruker, 1998); data reduction: SAINT (Bruker, 1998); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2003), DIAMOND (Brandenburg, 1999) and CAMERON (Watkin et al., 1993); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures

	Fig. 1. The local coordination of Zn^II cations in the title compound, showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. H atoms have been omitted for clarity. [Symmetry codes: (i) -x+2, -y+1, -z; (ii) x-1/2, -y+3/2, z+1/2; (iii) -x+1, -y+1, -z+1.].
	Fig. 2. (a) The centrosymmetric dimetal building block [symmetry code: (i) 1-x, 1 - y,1 -z]. (b) The two-dimensional double-layered structure consisting of 4-connected building blocks. The layer formed is parallel to the (101) plane. H atoms have been omitted for clarity. In the online version of the journal, green indicates the building block and red the ZnO6 fragement.
	Fig. 3. The independent one-dimensional zigzag metal–organic chain in each layer, constructed by Zn1 atoms and ligands.[Symmetry codes: (i) -x+2, -y+1, -z; (ii) x-1/2, -y+3/2, z+1/2; (iii) x-3/2, -y+1/2, z-1/2.].

poly[hexaaquabis[µ₄-3,5-bis(carboxylatomethoxy)benzoato]trizinc(II)]

Crystal data

[Zn3(C11H7O8)2(H2O)6]	F(000) = 848
Mr = 838.54	Dx = 2.002 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 4284 reflections
a = 8.0458 (9) Å	θ = 2.9–27.0°
b = 14.2074 (16) Å	µ = 2.67 mm^−1
c = 12.1692 (13) Å	T = 173 K
β = 90.261 (2)°	Block, colourless
V = 1391.0 (3) Å^3	0.42 × 0.37 × 0.19 mm
Z = 2

Data collection

Bruker SMART CCD area-detector diffractometer	3006 independent reflections
Radiation source: fine-focus sealed tube	2581 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.021
ω scans	θmax = 27.0°, θmin = 2.2°
Absorption correction: multi-scan (SADABS; Bruker, 1998)	h = −5→10
Tmin = 0.342, Tmax = 0.602	k = −18→17
7056 measured reflections	l = −15→15

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) = 0.086	H-atom parameters constrained
S = 1.05	w = 1/[σ^2(Fo^2) + (0.047P)^2 + 1.5975P] where P = (Fo^2 + 2Fc^2)/3
3006 reflections	(Δ/σ)max < 0.001
214 parameters	Δρmax = 0.96 e Å^−3
0 restraints	Δρmin = −0.54 e Å^−3

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 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	0.64316 (4)	0.72251 (2)	0.17782 (2)	0.01618 (11)
Zn2	0.5000	0.5000	0.5000	0.01854 (12)
O1	1.0033 (3)	0.57204 (14)	−0.35796 (16)	0.0229 (4)
O2	1.0332 (2)	0.68060 (13)	−0.22726 (15)	0.0184 (4)
O3	0.7867 (2)	0.28827 (13)	−0.14645 (16)	0.0199 (4)
O4	1.0984 (3)	0.24290 (15)	−0.34770 (16)	0.0222 (4)
O5	1.1177 (2)	0.25985 (14)	−0.16619 (15)	0.0181 (4)
O6	0.6960 (2)	0.55177 (13)	0.08566 (15)	0.0186 (4)
O7	0.5646 (3)	0.62160 (13)	0.27068 (15)	0.0220 (4)
O8	0.5101 (3)	0.47830 (14)	0.33331 (16)	0.0217 (4)
O9	0.5752 (3)	0.76831 (14)	0.03126 (16)	0.0247 (4)
H9A	0.5485	0.8239	0.0493	0.037*
H9B	0.5289	0.7549	−0.0298	0.037*
O10	0.5310 (4)	0.35424 (17)	0.51517 (19)	0.0478 (7)
H10A	0.5326	0.3166	0.4608	0.072*
H10B	0.4596	0.3334	0.5603	0.072*
O11	0.7595 (3)	0.5174 (2)	0.4939 (2)	0.0514 (8)
H11A	0.8146	0.5433	0.5459	0.077*
H11B	0.7732	0.4576	0.5017	0.077*
C1	0.8994 (3)	0.53836 (19)	−0.1798 (2)	0.0150 (5)
C2	0.8819 (3)	0.44336 (19)	−0.2071 (2)	0.0174 (5)
H2	0.9189	0.4208	−0.2744	0.021*
C3	0.8079 (3)	0.38322 (18)	−0.1316 (2)	0.0160 (5)
C4	0.7476 (3)	0.41724 (18)	−0.0324 (2)	0.0160 (5)
H4	0.6983	0.3766	0.0176	0.019*
C5	0.7616 (3)	0.51305 (18)	−0.0087 (2)	0.0154 (5)
C6	0.8408 (3)	0.57380 (18)	−0.0809 (2)	0.0161 (5)
H6	0.8543	0.6371	−0.0634	0.019*
C7	0.9851 (3)	0.60155 (18)	−0.2611 (2)	0.0161 (5)
C8	0.8479 (3)	0.2495 (2)	−0.2458 (2)	0.0196 (6)
H8A	0.7999	0.2840	−0.3070	0.023*
H8B	0.8107	0.1848	−0.2515	0.023*
C9	1.0352 (3)	0.25173 (18)	−0.2559 (2)	0.0167 (5)
C10	0.6224 (3)	0.48551 (18)	0.1594 (2)	0.0159 (5)
H10C	0.7036	0.4378	0.1789	0.019*
H10D	0.5298	0.4545	0.1231	0.019*
C11	0.5621 (3)	0.53333 (18)	0.2618 (2)	0.0166 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Zn1	0.01836 (17)	0.01714 (17)	0.01307 (17)	−0.00019 (11)	0.00296 (11)	0.00069 (11)
Zn2	0.0232 (2)	0.0190 (2)	0.0135 (2)	−0.00064 (17)	0.00466 (17)	−0.00293 (16)
O1	0.0308 (11)	0.0223 (10)	0.0155 (10)	−0.0027 (8)	0.0078 (8)	−0.0010 (8)
O2	0.0224 (10)	0.0170 (9)	0.0157 (9)	−0.0043 (8)	0.0027 (7)	0.0008 (7)
O3	0.0245 (10)	0.0148 (9)	0.0204 (10)	−0.0035 (8)	0.0085 (8)	−0.0039 (7)
O4	0.0247 (10)	0.0273 (10)	0.0147 (10)	0.0045 (8)	0.0011 (8)	−0.0024 (8)
O5	0.0180 (9)	0.0219 (10)	0.0144 (9)	−0.0022 (7)	−0.0009 (7)	0.0010 (7)
O6	0.0282 (10)	0.0136 (9)	0.0141 (9)	−0.0015 (8)	0.0084 (7)	−0.0016 (7)
O7	0.0344 (11)	0.0159 (9)	0.0157 (9)	−0.0014 (8)	0.0089 (8)	−0.0016 (7)
O8	0.0332 (11)	0.0187 (9)	0.0132 (9)	−0.0028 (8)	0.0055 (8)	−0.0008 (7)
O9	0.0346 (12)	0.0239 (11)	0.0157 (10)	0.0006 (9)	−0.0050 (8)	−0.0015 (8)
O10	0.095 (2)	0.0257 (12)	0.0226 (12)	0.0065 (13)	0.0228 (13)	−0.0015 (9)
O11	0.0246 (12)	0.092 (2)	0.0381 (15)	−0.0032 (13)	0.0022 (10)	−0.0332 (14)
C1	0.0129 (11)	0.0182 (13)	0.0141 (12)	0.0017 (10)	0.0011 (9)	0.0009 (10)
C2	0.0186 (12)	0.0193 (13)	0.0142 (12)	0.0010 (10)	0.0030 (10)	−0.0011 (10)
C3	0.0144 (12)	0.0136 (12)	0.0198 (13)	0.0008 (9)	0.0013 (10)	−0.0008 (10)
C4	0.0179 (12)	0.0139 (12)	0.0164 (13)	−0.0020 (10)	0.0030 (10)	0.0024 (10)
C5	0.0182 (12)	0.0162 (12)	0.0117 (12)	0.0023 (10)	0.0015 (10)	−0.0009 (9)
C6	0.0161 (12)	0.0148 (12)	0.0173 (13)	−0.0001 (10)	0.0006 (10)	−0.0005 (10)
C7	0.0131 (12)	0.0185 (12)	0.0165 (13)	0.0018 (10)	0.0017 (9)	0.0027 (10)
C8	0.0202 (13)	0.0180 (12)	0.0205 (14)	−0.0014 (10)	0.0018 (10)	−0.0055 (10)
C9	0.0217 (13)	0.0110 (11)	0.0174 (13)	0.0005 (10)	−0.0015 (10)	0.0000 (10)
C10	0.0192 (13)	0.0149 (12)	0.0136 (12)	−0.0008 (10)	0.0025 (10)	0.0001 (10)
C11	0.0182 (13)	0.0175 (13)	0.0140 (12)	0.0003 (10)	−0.0014 (10)	−0.0004 (10)

Geometric parameters (Å, º)

Zn1—O7	1.9335 (19)	O10—H10A	0.8510
Zn1—O5^i	1.9457 (19)	O10—H10B	0.8501
Zn1—O9	1.973 (2)	O11—H11A	0.8539
Zn1—O2^ii	2.0051 (18)	O11—H11B	0.8615
Zn2—O8	2.0537 (19)	C1—C6	1.389 (4)
Zn2—O10	2.094 (2)	C1—C2	1.397 (4)
Zn2—O11	2.104 (2)	C1—C7	1.505 (4)
O1—C7	1.260 (3)	C2—C3	1.390 (4)
O2—C7	1.257 (3)	C2—H2	0.9300
O2—Zn1^iii	2.0051 (18)	C3—C4	1.390 (4)
O3—C3	1.372 (3)	C4—C5	1.396 (3)
O3—C8	1.419 (3)	C4—H4	0.9300
O4—C9	1.235 (3)	C5—C6	1.389 (4)
O5—C9	1.281 (3)	C6—H6	0.9300
O5—Zn1^i	1.9457 (19)	C8—C9	1.513 (4)
O6—C5	1.380 (3)	C8—H8A	0.9700
O6—C10	1.431 (3)	C8—H8B	0.9700
O7—C11	1.259 (3)	C10—C11	1.501 (4)
O8—C11	1.244 (3)	C10—H10C	0.9700
O9—H9A	0.8485	C10—H10D	0.9700
O9—H9B	0.8511
O7—Zn1—O5^i	117.63 (9)	O3—C3—C4	114.4 (2)
O7—Zn1—O9	133.07 (9)	O3—C3—C2	124.8 (2)
O5^i—Zn1—O9	99.31 (8)	C4—C3—C2	120.8 (2)
O7—Zn1—O2^ii	91.50 (8)	C3—C4—C5	119.4 (2)
O5^i—Zn1—O2^ii	113.10 (8)	C3—C4—H4	120.3
O9—Zn1—O2^ii	99.97 (8)	C5—C4—H4	120.3
O8^iv—Zn2—O10	93.82 (8)	O6—C5—C6	117.2 (2)
O8—Zn2—O10	86.18 (8)	O6—C5—C4	122.0 (2)
O8—Zn2—O11^iv	93.48 (9)	C6—C5—C4	120.8 (2)
O8—Zn2—O11	86.52 (9)	C5—C6—C1	118.8 (2)
O10—Zn2—O11	90.07 (12)	C5—C6—H6	120.6
O10^iv—Zn2—O11	89.93 (12)	C1—C6—H6	120.6
C7—O2—Zn1^iii	124.09 (17)	O2—C7—O1	124.5 (2)
C3—O3—C8	116.7 (2)	O2—C7—C1	117.3 (2)
C9—O5—Zn1^i	117.29 (17)	O1—C7—C1	118.2 (2)
C5—O6—C10	114.86 (19)	O3—C8—C9	114.3 (2)
C11—O7—Zn1	133.91 (17)	O3—C8—H8A	108.7
C11—O8—Zn2	127.72 (18)	C9—C8—H8A	108.7
Zn1—O9—H9A	98.2	O3—C8—H8B	108.7
Zn1—O9—H9B	146.5	C9—C8—H8B	108.7
H9A—O9—H9B	108.9	H8A—C8—H8B	107.6
Zn2—O10—H10A	123.7	O4—C9—O5	124.5 (3)
Zn2—O10—H10B	108.7	O4—C9—C8	119.0 (2)
H10A—O10—H10B	107.3	O5—C9—C8	116.4 (2)
Zn2—O11—H11A	122.4	O6—C10—C11	111.0 (2)
Zn2—O11—H11B	90.4	O6—C10—H10C	109.4
H11A—O11—H11B	106.1	C11—C10—H10C	109.4
C6—C1—C2	121.4 (2)	O6—C10—H10D	109.4
C6—C1—C7	120.8 (2)	C11—C10—H10D	109.4
C2—C1—C7	117.8 (2)	H10C—C10—H10D	108.0
C3—C2—C1	118.7 (2)	O8—C11—O7	124.8 (2)
C3—C2—H2	120.7	O8—C11—C10	114.0 (2)
C1—C2—H2	120.7	O7—C11—C10	121.1 (2)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O9—H9A···O1^ii	0.85	1.90	2.703 (3)	159
O9—H9B···O4^v	0.85	1.81	2.654 (3)	169
O10—H10A···O5^vi	0.85	2.01	2.828 (3)	161
O10—H10B···O7^iv	0.85	2.16	2.742 (3)	125
O11—H11A···O1^vii	0.85	1.96	2.769 (3)	159
O11—H11B···O10	0.86	2.45	2.970 (4)	120

Symmetry codes: (ii) x−1/2, −y+3/2, z+1/2; (iv) −x+1, −y+1, −z+1; (v) −x+3/2, y+1/2, −z−1/2; (vi) x−1/2, −y+1/2, z+1/2; (vii) x, y, z+1.