A coordination polymer of CdCl₂ with the novel zwitterionic dicarboxyl­ate ligand 2,2′-(2-methyl­benzimid­azol­ium-1,3-di­yl)diacetate (pda)

The title compound, poly[chlorido[μ₄-2,2′-(2-methyl­benz­imid­azol­ium-1,3-di­yl)diacetato]cadmium(II)], [Cd(C₁₂H₁₁N₂O₄)Cl]_n, is an undulating two-dimensional polymer consisting of a paddlewheel Cd₂(CO₂)₄ cluster which lies on an inversion centre. These paddlewheel clusters act as four-connected square building units inter­linked via bridging zwitterionic dicarboxyl­ate ligands into a corrugated layer which is consolidated by π–π inter­actions between benzene rings of benzimidazole groups. Neighbouring layers are further assembled via inter­layer π–π inter­actions into a three-dimensional supra­molecular structure. The key feature of this study is the synthesis of a paddlewheel-based polymer constructed with a novel multifunctional zwitterionic dicar­boxyl­ate ligand.

S1. Comment

Paddlewheel clusters of the type [M₂(CO₂R)₄], in which a metal dimer is bridged by four carboxylate groups, have been the subject of attention in the field of directed design and construction of metal–organic frameworks (MOFs) with desired topologies and specific properties, because of their known coordination properties and well defined shape (Abourahma et al., 2003; Bourne et al., 2001; Cotton et al., 2002). When coordination occurs at the axial positions, the paddlewheel fragment may act as a ditopic linker to construct one-dimensional polymers, whereas the same cluster can act as a four-connected square unit when the linkage occurs at the equatorial sites through the use of polycarboxylate ligands. Yaghi and co-workers have described the behaviour of four-connected square moieties as building blocks in MOFs (Kim et al., 2001). Some interesting topologies, such as discrete three-dimensional porous molecules (Cotton et al., 2001; Ni et al., 2005), interpenetration networks (Wang et al., 2005), bowl-shaped two-dimensional structures (Bourne et al., 2001; Xue et al., 2007), NbO-, PtS-, or lvt-type [Please define] three-dimensional networks (Chen et al., 2005; Wang et al., 2005; Delgado Friedrichs et al., 2003), have been made, based on four-connected paddlewheel building blocks linked by linear dicarboxylate, angular dicarboxylate, tri-directional tricarboxylate or tetrahedral (square) carboxylate ligands. By controlling the features of the organic ligands - size, shape, functionality, flexibility and symmetry - the topologies and properties of a large variety of coordination polymers can be finely tuned. Furthermore, it must be kept in mind that a variety of weak non-covalent interactions, such as hydrogen-bonding, π–π interactions, van der Waals interactions and the like, also play an important role in the assembly of MOFs.

In studying the topologies and properties of MOFs based on the paddlewheel cluster, our efforts have been directed towards the synthesis of various supramolecular complexes based on multifunctional carboxylate ligands. In the present study, we used a long flexible zwitterionic dicarboxylate ligand, 2,2'-(2-methylbenzimidazolium-1,3-diyl)diacetate (pda^-) (Ni et al., 2007), in which two carboxymethyl groups are attached to the two benzimidazole N atoms, giving the latter a positive charge. For charge balance in the free (mono-protonated) ligand Hpda, only one of the two carboxylate groups is protonated. The deprotonated pda^- ligand in the title compound, (I), contributes a single negative charge to the coordination framework. The flexibility of the ligand as a whole and the extended π conjugated system of benzimidazole are expected to play key roles in the topologies found for its metal complexes.

The asymmetric unit of (I) contains one pda^- ligand, one Cd²⁺ ion and one Cl^- anion. Each pair of Cd²⁺ ions is bridged by four carboxylate groups from four different pda^- to form a paddlewheel cluster, Cd₂(CO₂)₄, which lies on an inversion centre (Fig. 1). The paddlewheels, acting as four-connected square building units, are bridged by pda^- to form a two-dimensional undulating sheet structure in which four adjacent paddlewheel units are linked to form a quadrilateral structural subunit. The void in the subunit is occupied by two phenyl rings from two opposiing ligands and two methyl groups from another two opposing ligands (Fig. 2). The axial positions of the paddlewheel cluster are occupied by Cl^- ions. The bond distances and angles around Cd²⁺ (Table 1) are comparable with those in other Cd paddlewheel units (Li & Mak, 1995). The two carboxylate groups of a given ligand ligand lie on opposite sides of the benzimidazole plane. The dihedral angles between the carboxylate groups at C10 and C12 and the benzimidazole plane are 82.4 (2) and 54.6 (4)°, respectively, allowing neighbouring paddle-wheel units to display pronounced canting relative to one another. The torsion angles C1—N2—C11—C12 and C6—N1—C9—C10 are 117.6 (3) and 97.7 (2)°, respectively. The carboxylate groups are tilted in such a way that one O atom of each is closer to the centroid of the positively charged imidazole ring [Cg···O1 = 3.488 (2) and Cg···O4 = 3.476 (3) Å; Cg represents the centroid of the imidazole ring].

One interesting feature of the pda^- ligand is the large conjugated π system of benzimidazole, which enables π–π interactions in the formation of MOFs (Hunter & Sanders, 1990; Wang et al., 2008). In the crystal structure of (I), the phenyl rings of the ligands at (x, y, z) and (1 - x, 2 - y, -z), which are strictly parallel because they are related by inversion, have a perpendicular distance of 3.321 (1) Å, a ring-centroid separation of 3.654 (7) Å and a ring-centroid slippage of 1.523 Å. These π–π interactions play an important role in assembling the paddlewheel units into the undulating two-dimensional structure of (I), which is greatly different from those of phenyldicarboxylate-containing paddlewheel polymers. Furthermore, the phenyl rings involved in these intralayer π–π interactions occupy what would otherwise be voids in the polymer, obviating the possibility of molecular interpenetration, although the ligand is likely long enough to form part of an interpenetrated structure. Similarly, a π–π interaction is observed between the phenyl rings of the molecules at (x, y, z) and (-x, 2 - y, -z) [perpendicular distance 3.329 (2) Å, ring-centroid separation 3.785 (3) Å, ring-centroid slippage 1.799 Å].

The interlayer π–π interactions mediate a parallel assembly along the a axis, completing the three-dimensional supramolecular structure (Fig. 3). Neighbouring layers are closely packed, with a nearest interlayer Cd···Cd distance of just 5.829 (4) Å. As a consequence of the efficient packing, the structure possesses a solvent-accessible volume of only 4.54% of the unit cell, as calculated by PLATON (Spek, 2003). No crystalline solvent is encapsulated in this compound.

Isostructural frameworks associated with CuCl₂, ZnCl₂ and MnCl₂ are also constructed by this ligand. Its flexibility, positively charged imidazole and large π conjugated system bestow advantages on this ligand for the formation of extended MOFs.

Related literature

For related literature, see: Abourahma et al. (2003); Bourne et al. (2001); Chen et al. (2005); Cotton et al. (2001, 2002); Delgado Friedrichs, O'Keeffe & Yaghi (2003); Hunter & Sanders (1990); Kim et al. (2001); Li & Mak (1995); Ni et al. (2005, 2007); Spek (2003); Wang et al. (2005, 2008); Xue et al. (2007).

S2. Experimental

The pH of a 4:1 ethanol–water solution (10 ml) containing CdCl₂.2H₂O (0.0658 g, 0.3 mmol) and Hpda (0.0498 g,0.2 mmol) was adjusted to 7 using triethylamine. The solution was sealed in a Teflon-lined steel bomb (25 ml) and heated at 413 K for 2 d. Colourless block crystals were collected (yield 16%). Elemental analysis, calculated for C₁₂H₁₁N₂O₄ClCd: C 36.46, H 2.78, N 7.09%; found: C 36.62, H 2.92, N 7.35%.

S3. Refinement

All H atoms were generated geometrically, with C—H distances of 0.93 Å, and refined with a riding model, with U_iso(H) = ?.?U_eq(C,N) [Please complete]

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: SHELXTL (Sheldrick, 2008), DIAMOND (Brandenburg, 1999); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures

	Fig. 1. The molecular structure of (I), showing the atom-numbering scheme and the paddle-wheel unit. Displacement ellipsoids are drawn at the 30% probability level. H atoms have been omitted.
	Fig. 2. The undulating quadrilateral structural subunit of (I), with the π–π interaction shown as a dashed line.
	Fig. 3. Schematic diagram of the three-dimensional supramolecular structure of (I), stabilized by interlayer π–π interactions. For clarity, the paddle-wheel building block is simplified as a four-connected node.

poly[chlorido[µ₄-2,2'-(2-methylbenzimidazolium-1,3- diyl)diacetato]cadmium(II)]

Crystal data

[Cd(C12H11N2O4)Cl]	F(000) = 776
Mr = 395.08	Dx = 2.074 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 3548 reflections
a = 7.2418 (9) Å	θ = 2.2–27.0°
b = 13.4867 (17) Å	µ = 1.95 mm^−1
c = 12.9531 (16) Å	T = 173 K
β = 90.229 (2)°	Block, colourless
V = 1265.1 (3) Å^3	0.40 × 0.28 × 0.16 mm
Z = 4

Data collection

Bruker SMART CCD area-detector diffractometer	2693 independent reflections
Radiation source: fine-focus sealed tube	2241 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.023
ω scans	θmax = 27.0°, θmin = 2.2°
Absorption correction: multi-scan (SADABS; Bruker, 1998)	h = −9→9
Tmin = 0.555, Tmax = 0.732	k = −9→17
5847 measured reflections	l = −14→16

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)] = 0.032	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.125	H-atom parameters constrained
S = 1.21	w = 1/[σ^2(Fo^2) + (0.0723P)^2 + 1.7823P] where P = (Fo^2 + 2Fc^2)/3
2693 reflections	(Δ/σ)max = 0.001
181 parameters	Δρmax = 1.04 e Å^−3
0 restraints	Δρmin = −0.82 e Å^−3

Special details

Experimental. IR: 1658 (s), 1628 (s), 1473 (m), 1385 (s), 762 (m), 617 (m).

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	0.13675 (5)	0.94640 (2)	0.58680 (3)	0.01299 (15)
Cl1	0.30720 (17)	0.83063 (9)	0.69607 (10)	0.0202 (3)
O1	0.0973 (5)	1.0048 (3)	0.3121 (3)	0.0222 (8)
O2	0.2818 (6)	0.9424 (3)	0.4343 (3)	0.0304 (10)
O3	0.2445 (5)	0.6001 (3)	−0.0988 (3)	0.0233 (8)
O4	0.4336 (5)	0.6679 (2)	0.0185 (3)	0.0188 (8)
N1	0.3229 (6)	0.9224 (3)	0.1645 (3)	0.0145 (8)
N2	0.2095 (5)	0.8331 (3)	0.0387 (3)	0.0124 (8)
C1	0.2241 (7)	0.9300 (3)	0.0005 (4)	0.0144 (10)
C2	0.1762 (7)	0.9722 (4)	−0.0930 (4)	0.0177 (10)
H2A	0.1233	0.9341	−0.1472	0.021*
C3	0.2089 (7)	1.0724 (4)	−0.1039 (4)	0.0178 (10)
H3A	0.1771	1.1040	−0.1671	0.021*
C4	0.2875 (7)	1.1287 (4)	−0.0246 (4)	0.0175 (10)
H4A	0.3101	1.1973	−0.0359	0.021*
C5	0.3335 (7)	1.0875 (4)	0.0701 (4)	0.0180 (10)
H5A	0.3864	1.1258	0.1242	0.022*
C6	0.2971 (6)	0.9858 (4)	0.0814 (4)	0.0120 (9)
C7	0.2671 (7)	0.8312 (3)	0.1366 (4)	0.0160 (10)
C8	0.2710 (7)	0.7465 (4)	0.2067 (4)	0.0200 (11)
H8A	0.2246	0.6876	0.1707	0.030*
H8B	0.1930	0.7603	0.2666	0.030*
H8C	0.3981	0.7346	0.2298	0.030*
C9	0.3958 (7)	0.9478 (3)	0.2650 (4)	0.0150 (10)
H9A	0.4723	1.0083	0.2592	0.018*
H9B	0.4762	0.8933	0.2898	0.018*
C10	0.2408 (7)	0.9659 (4)	0.3448 (4)	0.0165 (10)
C11	0.1472 (7)	0.7475 (4)	−0.0224 (4)	0.0171 (10)
H11A	0.1096	0.7712	−0.0917	0.021*
H11B	0.0365	0.7190	0.0109	0.021*
C12	0.2915 (7)	0.6649 (3)	−0.0356 (4)	0.0142 (9)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cd1	0.0139 (2)	0.0131 (2)	0.0120 (2)	0.00039 (12)	−0.00122 (14)	0.00061 (12)
Cl1	0.0239 (6)	0.0166 (6)	0.0200 (6)	0.0057 (5)	−0.0069 (5)	−0.0007 (5)
O1	0.0193 (19)	0.030 (2)	0.0170 (19)	0.0065 (16)	0.0078 (15)	0.0001 (16)
O2	0.025 (2)	0.056 (3)	0.010 (2)	−0.0004 (18)	0.0018 (16)	0.0012 (17)
O3	0.024 (2)	0.0160 (18)	0.030 (2)	0.0045 (15)	−0.0065 (17)	−0.0074 (15)
O4	0.0184 (18)	0.0169 (17)	0.0211 (19)	0.0027 (14)	−0.0050 (15)	−0.0017 (14)
N1	0.014 (2)	0.0129 (19)	0.017 (2)	0.0014 (15)	−0.0048 (16)	−0.0018 (16)
N2	0.0121 (19)	0.0137 (19)	0.011 (2)	0.0015 (15)	0.0008 (15)	−0.0021 (15)
C1	0.011 (2)	0.010 (2)	0.022 (3)	0.0033 (17)	−0.0010 (19)	−0.0020 (18)
C2	0.012 (2)	0.019 (2)	0.022 (3)	0.0047 (19)	−0.003 (2)	−0.003 (2)
C3	0.019 (2)	0.020 (2)	0.014 (3)	0.007 (2)	0.003 (2)	0.0082 (19)
C4	0.021 (3)	0.014 (2)	0.017 (3)	0.0031 (19)	0.006 (2)	0.0040 (19)
C5	0.018 (2)	0.010 (2)	0.026 (3)	0.0017 (18)	0.001 (2)	−0.004 (2)
C6	0.010 (2)	0.017 (2)	0.009 (2)	0.0013 (17)	0.0041 (17)	0.0002 (18)
C7	0.015 (2)	0.011 (2)	0.022 (3)	0.0021 (18)	−0.0049 (19)	−0.0011 (19)
C8	0.026 (3)	0.019 (2)	0.015 (3)	−0.001 (2)	0.002 (2)	0.002 (2)
C9	0.013 (2)	0.018 (2)	0.014 (3)	−0.0005 (18)	0.0006 (19)	−0.0016 (18)
C10	0.019 (3)	0.017 (2)	0.014 (3)	−0.0044 (19)	0.000 (2)	−0.0054 (19)
C11	0.014 (2)	0.016 (2)	0.021 (3)	0.0014 (19)	−0.004 (2)	−0.007 (2)
C12	0.018 (2)	0.010 (2)	0.015 (2)	−0.0010 (18)	0.0008 (19)	0.0033 (18)

Geometric parameters (Å, º)

Cd1—O2	2.241 (4)	C2—C3	1.379 (7)
Cd1—O1^i	2.245 (3)	C2—H2A	0.9500
Cd1—O3^ii	2.249 (4)	C3—C4	1.398 (8)
Cd1—O4^iii	2.305 (3)	C3—H3A	0.9500
Cd1—Cl1	2.4395 (12)	C4—C5	1.386 (7)
Cd1—Cd1^i	3.3217 (7)	C4—H4A	0.9500
O1—C10	1.238 (6)	C5—C6	1.405 (7)
O2—C10	1.237 (7)	C5—H5A	0.9500
O3—C12	1.245 (6)	C7—C8	1.460 (7)
O4—C12	1.243 (6)	C8—H8A	0.9800
N1—C7	1.344 (6)	C8—H8B	0.9800
N1—C6	1.386 (6)	C8—H8C	0.9800
N1—C9	1.444 (6)	C9—C10	1.548 (7)
N2—C7	1.334 (6)	C9—H9A	0.9900
N2—C1	1.402 (6)	C9—H9B	0.9900
N2—C11	1.470 (6)	C11—C12	1.537 (6)
C1—C2	1.382 (7)	C11—H11A	0.9900
C1—C6	1.393 (7)	C11—H11B	0.9900
O2—Cd1—O1^i	151.50 (15)	C5—C4—H4A	119.0
O2—Cd1—O3^ii	84.42 (15)	C3—C4—H4A	119.0
O1^i—Cd1—O3^ii	88.81 (14)	C4—C5—C6	116.1 (5)
O2—Cd1—O4^iii	86.92 (14)	C4—C5—H5A	122.0
O1^i—Cd1—O4^iii	86.42 (14)	C6—C5—H5A	122.0
O3^ii—Cd1—O4^iii	152.39 (13)	N1—C6—C1	107.6 (4)
O2—Cd1—Cl1	104.95 (11)	N1—C6—C5	131.1 (4)
O1^i—Cd1—Cl1	103.33 (10)	C1—C6—C5	121.3 (4)
O3^ii—Cd1—Cl1	110.93 (10)	N2—C7—N1	109.3 (4)
O4^iii—Cd1—Cl1	96.63 (9)	N2—C7—C8	127.7 (4)
O2—Cd1—Cd1^i	72.20 (11)	N1—C7—C8	123.0 (5)
O1^i—Cd1—Cd1^i	79.50 (10)	C7—C8—H8A	109.5
O3^ii—Cd1—Cd1^i	82.68 (9)	C7—C8—H8B	109.5
O4^iii—Cd1—Cd1^i	69.71 (9)	H8A—C8—H8B	109.5
Cl1—Cd1—Cd1^i	166.00 (3)	C7—C8—H8C	109.5
C10—O1—Cd1^i	124.0 (3)	H8A—C8—H8C	109.5
C10—O2—Cd1	135.1 (4)	H8B—C8—H8C	109.5
C12—O3—Cd1^iv	119.9 (3)	N1—C9—C10	112.1 (4)
C12—O4—Cd1^v	136.1 (3)	N1—C9—H9A	109.2
C7—N1—C6	108.4 (4)	C10—C9—H9A	109.2
C7—N1—C9	124.6 (4)	N1—C9—H9B	109.2
C6—N1—C9	127.0 (4)	C10—C9—H9B	109.2
C7—N2—C1	109.2 (4)	H9A—C9—H9B	107.9
C7—N2—C11	126.3 (4)	O2—C10—O1	128.9 (5)
C1—N2—C11	124.4 (4)	O2—C10—C9	114.4 (5)
C2—C1—C6	122.1 (4)	O1—C10—C9	116.6 (4)
C2—C1—N2	132.4 (5)	N2—C11—C12	114.9 (4)
C6—C1—N2	105.4 (4)	N2—C11—H11A	108.5
C3—C2—C1	116.8 (5)	C12—C11—H11A	108.5
C3—C2—H2A	121.6	N2—C11—H11B	108.5
C1—C2—H2A	121.6	C12—C11—H11B	108.5
C2—C3—C4	121.8 (5)	H11A—C11—H11B	107.5
C2—C3—H3A	119.1	O4—C12—O3	128.1 (5)
C4—C3—H3A	119.1	O4—C12—C11	118.4 (4)
C5—C4—C3	121.9 (5)	O3—C12—C11	113.4 (4)
O1^i—Cd1—O2—C10	−11.9 (7)	C1—N2—C7—N1	−1.2 (6)
O3^ii—Cd1—O2—C10	−88.9 (5)	C11—N2—C7—N1	176.5 (4)
O4^iii—Cd1—O2—C10	64.8 (5)	C1—N2—C7—C8	178.3 (5)
Cl1—Cd1—O2—C10	160.9 (5)	C11—N2—C7—C8	−4.0 (8)
Cd1^i—Cd1—O2—C10	−4.9 (5)	C6—N1—C7—N2	1.0 (6)
C7—N2—C1—C2	−176.8 (5)	C9—N1—C7—N2	−179.3 (4)
C11—N2—C1—C2	5.5 (8)	C6—N1—C7—C8	−178.5 (5)
C7—N2—C1—C6	0.9 (5)	C9—N1—C7—C8	1.1 (8)
C11—N2—C1—C6	−176.8 (4)	C7—N1—C9—C10	−81.9 (6)
C6—C1—C2—C3	1.8 (7)	C6—N1—C9—C10	97.7 (5)
N2—C1—C2—C3	179.2 (5)	Cd1—O2—C10—O1	2.8 (9)
C1—C2—C3—C4	0.3 (8)	Cd1—O2—C10—C9	179.9 (3)
C2—C3—C4—C5	−1.3 (8)	Cd1^i—O1—C10—O2	3.5 (8)
C3—C4—C5—C6	0.3 (7)	Cd1^i—O1—C10—C9	−173.5 (3)
C7—N1—C6—C1	−0.4 (5)	N1—C9—C10—O2	147.8 (4)
C9—N1—C6—C1	179.9 (4)	N1—C9—C10—O1	−34.7 (6)
C7—N1—C6—C5	−179.7 (5)	C7—N2—C11—C12	−59.8 (6)
C9—N1—C6—C5	0.6 (8)	C1—N2—C11—C12	117.6 (5)
C2—C1—C6—N1	177.7 (4)	Cd1^v—O4—C12—O3	−26.3 (8)
N2—C1—C6—N1	−0.3 (5)	Cd1^v—O4—C12—C11	150.8 (4)
C2—C1—C6—C5	−2.9 (7)	Cd1^iv—O3—C12—O4	22.0 (7)
N2—C1—C6—C5	179.1 (4)	Cd1^iv—O3—C12—C11	−155.1 (3)
C4—C5—C6—N1	−179.0 (5)	N2—C11—C12—O4	11.7 (7)
C4—C5—C6—C1	1.8 (7)	N2—C11—C12—O3	−170.8 (4)

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