Poly[[diaqua­(μ₃-2,2-dimethyl­malon­ato)­cadmium(II)] tetra­hydrate]

In the title complex, {[Cd(C₅H₆O₄)(H₂O)₂]·4H₂O}_n, the dimethyl­malonate–cadmium metal–organic framework co-exists with an extended structure of water mol­ecules, which resembles a sodalite-type framework. In the asymmetric unit, there are five independent solvent water mol­ecules, two of which are in special positions. The Cd atoms are eight-coordinated in a distorted square-anti­prismatic geometry by six O atoms of three different dimethyl­malonate groups and by two water mol­ecules, and form a two-dimensional honeycomb layer parallel to the bc plane. Two such layers sandwich the hydrogen-bonded water layer, which has a sodalite-type structure with truncated sodalite units composed of coordinated and solvent water mol­ecules. This work is the first example of a dimethyl­malonate cadmium complex containing truncated sodalite-type water clusters.

S1. Comment

Malonate and substituted malonate derivatives are often ligands of choice for the design of metal–organic frameworks or molecular assemblies because of their manifold coordination modes and the variety of the resulting architectures (Rodriguez-Martin et al., 2002). Some complexes with Cd have been reported and they contain mainly dianionic malonate, which can be found coordinating to the metal both through two distal carboxylate O atoms to form a six-membered ring, and through the non-chelating O atoms to build up bridged compounds, as in catena-[bis(µ₃-malonato)tetraaquadicadmium(II)] (Fu et al., 2006; Zhao et al., 2007), or by two bidentate ligands to form polymeric catena-[aqua(µ₃-malonato)cadmium(II)] (Post & Trotter, 1974) and rhombohedral catena-[(µ₃-malonato)aquacadmium(II) monohydrate] complexes (Naumov et al., 2001). However, only a few complexes with dimethylmalonate as ligand are known. Recently, we reported a five-coordinated dimethylmalonate zinc complex (Guo & Zhao, 2006) and a novel cocrystallization complex of neutral molecules of dimethylmalonic acid with a dianionic dimethylmalonate–barium metal–organic framework (Guo & Guo, 2008). Using dimethylmalonic acid as ligand, in an unsuccessful attempt to obtain a structure similar to or isotypic with that of poly[tetraaqua-di-µ₄-malonatobarium(II)zinc(II)] (Guo & Guo, 2006), we obtained the title novel eight-coordinated dimethylmalonate–cadmium complex, (I), which exhibits the unexpected coexistence of an extended water–sodalite type structure and a metal–organic framework. We report here the crystal structure of (I).

The asymmetric unit of (I) comprises one Cd^II cation, one complete dimethylmalonate dianion, two coordinated water molecules and five non-coordinated water molecules, of which molecules O10 and O11 are in special positions. Fig. 1 shows the structure of (I) in a symmetry-expanded view which displays the full coordination of the Cd atom. Selected geometric parameters are given in Table 1.

In the dianionic dimethylmalonate ligand, the O—C—O angles for the two carboxylate groups are almost the same and the four C—O bond distances of the two carboxylate groups are in the range 1.251 (4)–1.266 (4) Å. This indicates that both carboxylate groups are delocalized [Please check amended text]. As observed in other dimethylmalonate structures, the two carboxylate groups are non-coplanar (Guo & Guo, 2008). The O1/C1/O2 carboxylate group is rotated by 40.2 (4)° out of the central atom plane (C1/C2/C3), while the other, O3/C3/O4, forms an angle with the same plane of 87.9 (4)°; the dihedral angle between the two carboxylate groups is 82.6 (5)°.

The Cd atoms have distorted square-antiprism geometry, coordinated by two chelating O atoms (O2ⁱⁱ and O3ⁱⁱ; see Fig. 1 for symmetry code) of one dimethylmalonate dianion, four O atoms (O1 and O2, and O3ⁱ and O4ⁱ) from the other two dimethylmalonates and two O atoms from water molecules (O5 and O6). The whole dimethylmalonate molecule chelates atom Cd1ⁱⁱⁱ to form a six-membered ring (see Fig. 1 for symmetry code). The bond angle at atom C2 [C3—C2—C1 = 103.8 (2)°] is smaller than the normal value, suggesting that there is greater strain in the six-membered ring of (I) than in the catena-[bis(µ₃-malonato)tetraaquadicadmium(II)] complex (Fu et al., 2006). Atoms O1 and O2 of the O1/C1/O2 carboxylate group adopt a bidentate mode to coordinate to atom Cd1, and similarly, atoms O3 and O4 coordinate to atom Cd1^iv (see Fig. 1 for symmetry code). In this way, one complete dimethylmalonate dianion links three Cd atoms together. This results in a Cd1···Cd1ⁱⁱⁱ distance of 4.7753 (10) Å and a Cd1ⁱⁱⁱ···Cd1^iv distance of 4.1711 (9) Å. In the bc plane, two Cd atoms [Which two?] are bridged together via two O3 atoms or one O2 atom, respectively. This results in two Cd distorted square-antiprisms sharing an edge or a corner. In this way, each Cd atom is connected to three other Cd atoms, and each group of six Cd atoms is associated into a 12-membered ring in the bc plane. These are further joined into a two-dimensional honeycomb layer structure (Fig. 2).

The detailed structure of an infinite water layer in the bc plane is shown in Fig. 3. The (H₂O)₄ subunit comprises water molecules O7, O9, O10 and O11. Along b-axis direction, adjacent tetramers are connected to each other by hydrogen bonds. In this way, the tetramers produce a one-dimensional water tape T4(0) (Infantes & Motherwell, 2002), which contains an O7···O11^vi···O7^vi···O11 tetramer and an O9···O10···O9^viii···O10^viii tetramer (see Fig. 3 for symmetry codes). In the c direction, these tetramers form two different T4(1) water tapes and connect further with each other into an O7···O9···O10···O9^ix···O7^ix···O11^vi hexamer. Adjacent tetramers and hexamers share an edge or a corner, giving rise to an infinite water layer L4(6)6(8). The hydrogen-bonding parameters of the water molecules are summarized in Table 2. As can be seen from Table 2 and Fig. 3, within the water layer, water molecules O10 and O11 display tetrahedral geometries with double hydrogen-bond donors and acceptors. The O···O distances are in the range 2.750 (3)–2.897 (4) Å with an average of 2.845 Å, which is comparable with the range observed in ice II (2.77–2.84 Å; Gregory et al., 1997). The O···O···O angles vary from 83.49 to 123.62°.

One of the most important features of the present structure is the fact that the two-dimensional metal–organic framework formed by Cd^II cations and dimethylmalonate ligands integrates with the two-dimensional hydrogen-bonded water framework, which has a sodalite-type structure with truncated sodalite units. Each truncated sodalite unit comprises 16 solvent water molecules and three coordinated water molecules. To the best of our knowledge, such an extended sodalite-type water structure has not been reported so far. As can be seen in Fig. 4, the structure as a whole consists of two distinct layers that stack alternately in the [100] direction. In addition to the two-dimensional dimethylmalonate–cadmium metal–organic layer, there is the extended two-dimensional hydrogen-bonded water layer, which is based on water molecules O8, O5, O6 and the above-mentioned infinite L4(6)6(8) water cluster and has a sodalite-type structure with truncated sodalite units. The connectivity between neighbouring layers is completed by intra- and interlayer hydrogen-bond interactions.

S2. Experimental

The title complex was prepared by successive addition of dimethylmalonic acid (0.53 g, 4 mmol), Ba(OH)₂.8H₂O (0.63 g, 2 mmol) and 3CdSO₄.8H₂O (0.77 g, 1 mmol) to distilled water (20 ml) at room temperature with continuous stirring. After filtration, slow evaporation over a period of two weeks at room temperature provided colourless needle crystals of (I).

S3. Refinement

One H atom of each water molecule in a special position and the H atoms of other water molecules were found in difference Fourier maps. However, during refinement, they were fixed [Restrained?] at O—H = 0.85 (1) Å, with U_iso(H) = 1.2U_eq(O); then use DFIX instructions, finally use Afix 3 instructions [Please rephrase without software-specific terminology]. The H atoms of the C—H groups were treated as riding, with C—H = 0.96 Å, and U_iso(H) = 1.5U_eq(C).

Computing details

Data collection: CrystalClear (Rigaku/MSC, 2005); cell refinement: CrystalClear (Rigaku/MSC, 2005); data reduction: CrystalClear (Rigaku/MSC, 2005); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (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 coordination polyhedra for the Cd atoms. Displacement ellipsoids are drawn at the 40% probability level and H atoms are shown as small spheres of arbitrary radii. [Symmetry codes: (i) -x + 1/2, y + 1/2, -z + 1/2; (ii) x, -y + 1, z + 1/2; (iii) x, -y + 1, z - 1/2; (iv) -x + 1/2, y - 1/2, -z + 1/2.]
	Fig. 2. A packing diagram for (I), showing the two-dimensional polymeric layer in the bc plane, viewed down the a axis. The H atoms of the methyl groups have been omitted for clarity.
	Fig. 3. A packing diagram for (I), showing the infinite layered water cluster. [Symmetry codes: (vi) -x + 1, -y + 1, -z + 1; (viii) -x + 1, -y + 2, -z + 1; (ix) -x + 1, y, -z + 3/2.]
	Fig. 4. The packing of (I), showing the two-dimensional polymeric layers in the bc plane and the hydrogen-bonding interactions (dashed lines) that link them in the a-axis direction.

Poly[[diaqua(µ₃-2,2-dimethylmalonato)cadmium(II)] tetrahydrate]

Crystal data

[Cd(C5H6O4)(H2O)2]·4H2O	F(000) = 1408
Mr = 350.59	Dx = 1.850 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 3863 reflections
a = 26.015 (5) Å	θ = 1.9–27.1°
b = 12.094 (2) Å	µ = 1.77 mm^−1
c = 8.4579 (17) Å	T = 133 K
β = 108.91 (3)°	Needle, colourless
V = 2517.5 (9) Å^3	0.18 × 0.10 × 0.08 mm
Z = 8

Data collection

Rigaku Saturn diffractometer	2206 independent reflections
Radiation source: rotating anode	1994 reflections with I > 2σ(I)
Confocal monochromator	Rint = 0.034
Detector resolution: 27.483 pixels mm^-1	θmax = 25.0°, θmin = 2.9°
ω scans	h = −30→30
Absorption correction: multi-scan (CrystalClear; Rigaku/MSC, 2005)	k = −14→14
Tmin = 0.804, Tmax = 0.876	l = −8→10
7015 measured reflections

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)] = 0.026	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.070	H-atom parameters constrained
S = 1.11	w = 1/[σ^2(Fo^2) + (0.0383P)^2 + 2.7223P] where P = (Fo^2 + 2Fc^2)/3
2206 reflections	(Δ/σ)max = 0.002
148 parameters	Δρmax = 0.63 e Å^−3
0 restraints	Δρmin = −0.97 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	0.275830 (9)	0.591693 (16)	0.46320 (2)	0.01566 (11)
C1	0.32171 (13)	0.3964 (2)	0.3804 (4)	0.0167 (6)
C2	0.35519 (12)	0.2947 (2)	0.3593 (4)	0.0174 (6)
C3	0.31333 (12)	0.2223 (2)	0.2304 (4)	0.0172 (6)
C4	0.38115 (13)	0.2342 (2)	0.5231 (4)	0.0211 (7)
H4A	0.3997	0.1697	0.5034	0.032*
H4B	0.3535	0.2123	0.5692	0.032*
H4C	0.4066	0.2822	0.6003	0.032*
C5	0.39935 (13)	0.3375 (3)	0.2915 (4)	0.0217 (7)
H5D	0.4249	0.3813	0.3755	0.033*
H5E	0.3830	0.3821	0.1942	0.033*
H5C	0.4178	0.2760	0.2622	0.033*
O1	0.33072 (9)	0.43750 (17)	0.5232 (3)	0.0197 (5)
O2	0.28736 (9)	0.43678 (17)	0.2509 (3)	0.0189 (5)
O3	0.30655 (9)	0.23509 (16)	0.0781 (2)	0.0199 (5)
O4	0.28541 (8)	0.15553 (16)	0.2827 (2)	0.0189 (5)
O5	0.34119 (8)	0.65266 (17)	0.3355 (3)	0.0194 (5)
H5A	0.3314	0.6213	0.2413	0.023*
H5B	0.3366	0.7216	0.3397	0.023*
O6	0.20148 (9)	0.48531 (17)	0.4594 (3)	0.0196 (5)
H6A	0.1881	0.4624	0.3584	0.023*
H6B	0.2028	0.4374	0.5346	0.023*
O7	0.45421 (9)	0.62994 (19)	0.4614 (3)	0.0269 (5)
H7A	0.4203	0.6386	0.4356	0.032*
H7B	0.4692	0.5812	0.5344	0.032*
O8	0.34666 (9)	0.88549 (18)	0.3442 (3)	0.0227 (5)
H8A	0.3805	0.8975	0.3640	0.027*
H8B	0.3343	0.9000	0.4241	0.027*
O9	0.45810 (10)	0.8693 (2)	0.4634 (3)	0.0328 (6)
H9A	0.4634	0.8009	0.4668	0.039*
H9B	0.4706	0.9074	0.3996	0.039*
O10	0.5000	1.0046 (3)	0.7500	0.0341 (8)
H10A	0.4766	0.9583	0.6936	0.041*
O11	0.5000	0.5084 (3)	0.2500	0.0287 (8)
H11A	0.4830	0.5516	0.2959	0.034*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cd1	0.01856 (16)	0.01327 (15)	0.01579 (16)	0.00017 (7)	0.00643 (11)	0.00029 (7)
C1	0.0185 (16)	0.0145 (14)	0.0184 (16)	−0.0029 (11)	0.0077 (13)	0.0019 (11)
C2	0.0188 (15)	0.0155 (15)	0.0184 (15)	0.0001 (12)	0.0065 (12)	0.0000 (11)
C3	0.0187 (15)	0.0128 (14)	0.0203 (16)	0.0042 (12)	0.0067 (12)	−0.0001 (12)
C4	0.0243 (16)	0.0171 (15)	0.0203 (15)	0.0012 (12)	0.0051 (13)	−0.0010 (12)
C5	0.0182 (15)	0.0240 (16)	0.0259 (17)	−0.0001 (12)	0.0111 (13)	0.0008 (13)
O1	0.0282 (12)	0.0155 (10)	0.0173 (11)	0.0006 (9)	0.0098 (9)	−0.0004 (8)
O2	0.0211 (11)	0.0157 (10)	0.0204 (11)	0.0025 (9)	0.0073 (9)	0.0014 (8)
O3	0.0279 (12)	0.0162 (11)	0.0163 (11)	0.0010 (9)	0.0082 (9)	−0.0002 (8)
O4	0.0210 (11)	0.0175 (11)	0.0194 (11)	−0.0033 (8)	0.0081 (9)	−0.0003 (8)
O5	0.0248 (12)	0.0156 (10)	0.0200 (11)	−0.0026 (8)	0.0105 (9)	−0.0037 (8)
O6	0.0248 (12)	0.0175 (11)	0.0170 (11)	−0.0013 (8)	0.0075 (9)	0.0021 (8)
O7	0.0210 (12)	0.0300 (12)	0.0290 (13)	0.0042 (10)	0.0072 (10)	0.0015 (10)
O8	0.0232 (12)	0.0250 (11)	0.0232 (12)	0.0013 (9)	0.0119 (10)	0.0001 (9)
O9	0.0331 (14)	0.0281 (13)	0.0385 (14)	−0.0030 (10)	0.0133 (11)	0.0003 (11)
O10	0.034 (2)	0.0314 (19)	0.0325 (19)	0.000	0.0050 (16)	0.000
O11	0.0295 (19)	0.0290 (18)	0.0316 (19)	0.000	0.0154 (15)	0.000

Geometric parameters (Å, º)

Cd1—O1	2.303 (2)	C4—H4B	0.9600
Cd1—O2	2.678 (2)	C4—H4C	0.9600
Cd1—O5	2.407 (2)	C5—H5D	0.9600
Cd1—O6	2.315 (2)	C5—H5E	0.9600
Cd1—O3^i	2.690 (2)	C5—H5C	0.9600
Cd1—O3^ii	2.338 (2)	O5—H5A	0.8437
O2—Cd1^iii	2.379 (2)	O5—H5B	0.8446
O4—Cd1^iv	2.305 (2)	O6—H6A	0.8570
C1—O1	1.256 (4)	O6—H6B	0.8531
C1—O2	1.266 (4)	O7—H7A	0.8448
C1—C2	1.550 (4)	O7—H7B	0.8512
C2—C4	1.519 (4)	O8—H8A	0.8531
C2—C5	1.531 (4)	O8—H8B	0.8546
C2—C3	1.540 (4)	O9—H9A	0.8373
C3—O3	1.251 (4)	O9—H9B	0.8502
C3—O4	1.258 (4)	O10—H10A	0.8510
C4—H4A	0.9600	O11—H11A	0.8553
O1—Cd1—O4^i	131.92 (7)	O4—C3—C2	118.2 (3)
O1—Cd1—O6	90.50 (8)	C2—C4—H4A	109.5
O4^i—Cd1—O6	80.99 (8)	C2—C4—H4B	109.5
O1—Cd1—O3^ii	121.98 (8)	H4A—C4—H4B	109.5
O4^i—Cd1—O3^ii	96.78 (7)	C2—C4—H4C	109.5
O6—Cd1—O3^ii	132.48 (8)	H4A—C4—H4C	109.5
O1—Cd1—O2^ii	78.09 (8)	H4B—C4—H4C	109.5
O4^i—Cd1—O2^ii	142.44 (8)	C2—C5—H5D	109.5
O6—Cd1—O2^ii	76.21 (8)	C2—C5—H5E	109.5
O3^ii—Cd1—O2^ii	77.98 (7)	H5D—C5—H5E	109.5
O1—Cd1—O5	82.49 (7)	C2—C5—H5C	109.5
O4^i—Cd1—O5	82.81 (7)	H5D—C5—H5C	109.5
O6—Cd1—O5	150.79 (7)	H5E—C5—H5C	109.5
O3^ii—Cd1—O5	73.50 (7)	C1—O1—Cd1	101.49 (18)
O2^ii—Cd1—O5	129.08 (7)	C1—O2—Cd1^iii	130.5 (2)
O1—C1—O2	122.6 (3)	C3—O3—Cd1^iii	116.98 (18)
O1—C1—C2	119.2 (3)	C3—O4—Cd1^iv	101.93 (17)
O2—C1—C2	118.1 (3)	Cd1—O5—H5A	104.1
C4—C2—C5	109.8 (3)	Cd1—O5—H5B	98.8
C4—C2—C3	112.0 (2)	H5A—O5—H5B	118.7
C5—C2—C3	111.8 (2)	Cd1—O6—H6A	105.2
C4—C2—C1	112.2 (2)	Cd1—O6—H6B	122.4
C5—C2—C1	107.1 (2)	H6A—O6—H6B	115.3
C3—C2—C1	103.8 (2)	H7A—O7—H7B	117.9
O3—C3—O4	122.4 (3)	H8A—O8—H8B	115.9
O3—C3—C2	119.3 (3)	H9A—O9—H9B	117.7
O1—C1—C2—C4	19.9 (4)	C2—C1—O1—Cd1	174.1 (2)
O2—C1—C2—C4	−162.3 (3)	O4^i—Cd1—O1—C1	4.1 (2)
O1—C1—C2—C5	−100.7 (3)	O6—Cd1—O1—C1	82.38 (19)
O2—C1—C2—C5	77.1 (3)	O3^ii—Cd1—O1—C1	−134.34 (18)
O1—C1—C2—C3	141.0 (3)	O2^ii—Cd1—O1—C1	158.2 (2)
O2—C1—C2—C3	−41.2 (3)	O5—Cd1—O1—C1	−69.17 (19)
C4—C2—C3—O3	−148.3 (3)	O1—C1—O2—Cd1^iii	163.3 (2)
C5—C2—C3—O3	−24.6 (4)	C2—C1—O2—Cd1^iii	−14.4 (4)
C1—C2—C3—O3	90.5 (3)	O4—C3—O3—Cd1^iii	115.1 (3)
C4—C2—C3—O4	34.8 (4)	C2—C3—O3—Cd1^iii	−61.6 (3)
C5—C2—C3—O4	158.6 (3)	O3—C3—O4—Cd1^iv	−4.2 (3)
C1—C2—C3—O4	−86.4 (3)	C2—C3—O4—Cd1^iv	172.5 (2)
O2—C1—O1—Cd1	−3.5 (3)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O5—H5A···O1^iii	0.84	1.97	2.788 (3)	162
O5—H5B···O8	0.84	2.00	2.819 (3)	164
O6—H6A···O8^iv	0.86	1.90	2.745 (3)	168
O6—H6B···O4^v	0.85	1.85	2.701 (3)	172
O7—H7A···O5	0.84	1.96	2.797 (3)	170
O7—H7B···O11^vi	0.85	2.05	2.879 (3)	165
O8—H8A···O9	0.85	1.95	2.750 (3)	156
O8—H8B···O6^vii	0.85	2.09	2.849 (3)	148
O8—H8B···O3^ii	0.85	2.35	2.910 (3)	124
O9—H9A···O7	0.84	2.08	2.897 (4)	165
O9—H9B···O10^viii	0.85	1.99	2.837 (3)	180
O10—H10A···O9	0.85	2.14	2.833 (3)	139
O11—H11A···O7	0.86	2.02	2.856 (3)	164

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