Construction of a 3D array of cadmium(II) using squarate as a building block

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Abstract

A unique three-dimensional coordination polymer of cadmium(II), [Cd(C₄O₄)(H₂O)₂]_n1 (C₄O₄²⁻⊕=⊕squarate dianion), has been synthesized and characterized by single crystal structural analysis. The molecular structure shows that each bridging squarate dianion functions as a four-fold monodentate ligand to form a cage-like channel network. The compound crystallizes in the rhombohedral system, space group R3̄ (no. 148) with a⊕=⊕b⊕=⊕11.765(6) Å and c⊕=⊕14.920(7) Å, γ⊕=⊕120°, Z⊕=⊕9, V⊕=⊕1788.5(15) Å³. Thermal investigation reveals that the dehydrated compound is remarkably stable.

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Introduction

Assembling extended structure compounds by selecting organic spacers (e.g. 4,4′-bipyridine, pyrazine, dicarboxylic acids, etc.) and inorganic metal ion species of suitable chemical structure and coordination geometry, respectively, may yield a series of novel networks with various sizes and shapes of void space or cavity.¹ Such types of materials are being investigated intensively because of their multi-dimensional applications in the field of catalysis,^2a nonlinear optics,^2b electrical conductivity,^2c molecular recognition^2d and molecular sieves.^2e Among the different organic spacers, the squarate dianion (C₄O₄²⁻) is an interesting cyclic compound with aromaticity and it can function as a bridging ligand with possible μ-2, μ-3 and μ-4 between metal ions³ (Scheme 1). The most fully investigated metal squarate is M(C₄O₄)(H₂O)₄ [M⊕=⊕Mn(II), Co(II), Ni(II), Fe(II), Zn(II) and Cu(II)]⁴ with two bridging C₄O₄²⁻ (μ-2) ligands at trans positions, which form an infinite chain structure. Each metal ion is also bonded with four water molecules to form a distorted octahedral geometry. West and Niu⁵ first reported another type of metal squarate with the formula M(C₄O₄)(H₂O)₂ [M(II)⊕=⊕Fe(II), Co(II), Ni(II) and Zn(II)]. Later three-dimensional cubic structures were found to have the formula M(C₄O₄)(H₂O)₂ [M⊕=⊕Ni(II)⁶ and Fe(II)^4b], M(C₄O₄)(H₂O)₂(MeCO₂H·H₂O)_1/3 [M⊕=⊕Zn(II), Ni(II) and Mn(II)]⁷ and Co(C₄O₄)(H₂O)₂·0.33H₂O^4b, where C₄O₄²⁻ functions as a bridging ligand between four metal ions (μ-4). Besides these two types of metal squarate, another type with the formula M(HC₄O₄)₂(H₂O)₄ [M⊕=⊕Fe(II)^4b and Mn(II)⁸] has been found, which is a two-dimensional sheet-like network built from (HC₄O₄)₂ and M(H₂O)₄ units.

Scheme 1 Binding modes of the squarate dianion.

In the course of our studies on polymeric coordination compounds⁹ here we have chosen cadmium(II) as the metal ion due to its d¹⁰ configuration, which permits a wide variety of geometries and coordination numbers, with the squarate dianion as an effective building block in order to synthesize an uncharged polymeric network. Here, we report a bridging μ-4 squarate coordination polymer of cadmium(II), exhibiting a 3D channel network, and solid state thermal studies, which show that the cadmium squarate framework is remarkably stable after dehydration.

Experimental

High purity 3,4-dihydroxy-3-cyclobutene-1,2-dione (squaric acid) was purchased from the Aldrich Chemical Company Inc. and used as-received. All other chemicals used were of AR grade.

Physical measurements

Elemental analyses (carbon and hydrogen) were performed using a Perkin-Elmer 240C elemental analyser, and cadmium(II) contents were estimated gravimetrically. Infrared spectra (4000–400 cm⁻¹) were taken using a Nicolet Magna-IR 750 spectrometer, series-II, where KBr was used as the dispersal medium. Thermal analysis (TGA–DTA) was carried out on a Shimadzu DT-30 thermal analyzer under dinitrogen (flow rate: 30 cm³ min⁻¹). The sample particle size was within 150–200 mesh. X-Ray powder diffraction data were collected using a Seifert XRD-3000P instrument, using Cu-Kα (λ⊕=⊕1.5406 Å) radiation (30 kV/30 mA); the primary slits were 3 mm/soller/2 mm and the secondary slits were soller/0.1 mm.

Preparation of [Cd(C₄O₄)(H₂O)₂]_n1

An aqueous solution (5 cm³) of squaric acid (1 mmol, 0.114 g) was slowly added to an aqueous solution (10 cm³) of Cd(MeCO₂)₂·2H₂O (1 mmol, 0.246 g). Immediately, a white crystalline solid of complex 1 separated out, and this was washed with methanol and dried in a vacuum desiccator. It was dissolved in water at ca. 90 °C and the solution was stored in a CaCl₂ desiccator. Suitable single crystals were separated out after a few days (80% yield). Anal. calc. for C₄H₄O₆Cd: C, 18.43; H, 1.53; Cd, 43.16. Found: C, 18.42; H, 1.54; Cd, 43.12%.

Crystallographic data collection and refinement

Reflection data were measured using an AFC7S diffractometer in ω–2θ scan mode using graphite monochromated Mo-Kα radiation. The intensity data were corrected for Lorentz and polarization effects¹⁰ and an empirical absorption correction based on Ψ scan was also employed. A total of 4561 reflections was measured, of which 1441 were unique (R_int⊕=⊕0.117). The structure was solved by Patterson synthesis and followed by Fourier methods. Complex neutral atom scattering factors¹¹ were used throughout. All calculations were carried out using SHELXS-86,¹² SHELXL-97,¹³ PLATON99¹⁴ and ORTEP-3 ¹⁵ programs. Details of data collection and refinement are given in Table 1.

Table 1 Crystal data and structure refinement for [Cd(C₄O₄)(H₂O)₂]_n1^a

Parameter
a Click b105080j.txt for full crystallographic data (CCDC 155487).
Empirical formula	C4H4CdO6
Crystal dimensions/mm	0.20⊕×⊕0.20⊕×⊕0.20
M	260.48
Crystal system	Trigonal (rhombohedral)
Space group	R3̄ (no. 148)
a/Å	11.765(6)
b/Å	11.765(6)
c/Å	14.920(7)
α/°	90
β/°	90
γ/°	120
V/Å^3	1788.5(15)
Z	9
T/K	293(2)
D c/Mg m^−3	2.177
λ(Mo-Kα)/Å	0.71073
μ(Mo-Kα)/mm^−1	2.729
F(000)	1116
θ Range/°	2.4–32.5
Total data	4561
Unique data	1441 (Rint⊕=⊕0.117)
Observed data [I⊕>⊕2σ(I)]	1435
R 1	0.0419
wR 2, S	0.1046, 0.90

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Results and discussion

The IR spectrum of compound 1 (not shown) reveals a strong and broad absorption in the region 1490–1530 cm⁻¹, which is assigned to a mixture of C–C and C–O stretching vibrations of the C₄O₄²⁻ ligand. In addition, only one sharp band at ca. 1761 cm⁻¹, assigned to C[double bond, length half m-dash]O, indicates that all four oxygen atoms of C₄O₄²⁻ are in the same environment.

Structural description of complex 1

X-Ray structure determination reveals that the polymer has the stoichiometry Cd(C₄O₄)(H₂O)₂, having a 3D infinite network. An ORTEP-3 view with atom numbering scheme is shown in Fig. 1 in which every C₄O₄²⁻ moiety binds to four different cadmium(II) centres, i.e. each C₄O₄²⁻ functions as a four-fold monodentate bridging ligand. Thus the binding geometry of C₄O₄²⁻ corresponds very well to a system of completely delocalized π-electrons.

Fig. 1 ORTEP-3 plot of a portion of the 3D polymeric complex [Cd(C₄O₄)(H₂O)₂]_n with atom labelling scheme. Thermal ellipsoids are drawn at the 50% probability level. (Colour code: Cd blue, O red, C black, H green.) (Symmetry codes: a⊕=⊕1/3⊕−⊕x⊕+⊕y, 2/3⊕−⊕x, −1/3⊕+⊕z; b⊕=⊕1/3⊕+⊕x, 2/3⊕+⊕y, −1/3⊕+⊕z; c⊕=⊕1⊕−⊕x⊕+⊕y, 1⊕−⊕x, z; d⊕=⊕1/3⊕−⊕y, −1/3⊕+⊕x⊕−⊕y, −1/3⊕+⊕z; e⊕=⊕1/3⊕+⊕x, −1/3⊕+⊕y, −1/3⊕+⊕z; f⊕=⊕1⊕−⊕y, x⊕−⊕y, z; g⊕=⊕2/3⊕−⊕x⊕+⊕y, 1/3⊕−⊕x, 1/3⊕+⊕z; h⊕=⊕−1/3⊕+⊕x, −2/3⊕+⊕y, 1/3⊕+⊕z; i⊕=⊕−x⊕+⊕y, −x, z; j⊕=⊕2/3⊕−⊕y, 1/3⊕+⊕x⊕−⊕y, 1/3⊕+⊕z; k⊕=⊕2/3⊕+⊕x, 1/3⊕+⊕y, 1/3⊕+⊕z; l⊕=⊕−y, x⊕−⊕y, z; m⊕=⊕2/3⊕−⊕x, 1/3⊕−⊕y, 1/3⊕−⊕z; n⊕=⊕−1/3⊕+⊕y, 1/3⊕−⊕x⊕+⊕y, 1/3⊕−⊕z; p⊕=⊕1⊕−⊕x, 1⊕−⊕y, −z). Click image or 1.htm to access a 3D representation.

In the asymmetric unit, each cadmium(II) centre is attached to four different C₄O₄²⁻ moieties and two water molecules to form an octahedron with the CdO₆ chromophore. Each cadmiun atom occupies an inversion centre. The two oxygen atoms of water molecules (O1W, O1W^m, m⊕=⊕2/3⊕−⊕x, 1/3⊕−⊕y, 1/3⊕−⊕z) and other two oxygen atoms (O1, O1^m) from the two squarato moieties form the equatorial plane, and the oxygen atoms (O2ⁿ and O2^f, n⊕=⊕−1/3⊕+⊕y, 1/3⊕−⊕x⊕+⊕y, 1/3⊕−⊕z; f⊕=⊕1⊕−⊕y, x⊕−⊕y, z) of another two squarato moieties occupy the trans axial positions (selected bond length and angle data are given in Table 2).

Table 2 Selected bond lengths (Å) and angles (°) for [Cd(C₄O₄)(H₂O)₂]_n1

Symmetry codes: m⊕=⊕2/3⊕−⊕x, 1/3⊕−⊕y, 1/3⊕−⊕z; n⊕=⊕−1/3⊕+⊕y, 1/3⊕−⊕x⊕+⊕y, 1/3⊕−⊕z; f⊕=⊕1⊕−⊕y, x⊕−⊕y, z.
Cd–O1	2.276(3)	Cd–O2	2.311(2)
Cd–O1W	2.234(4)	O1–C1	1.253(4)
C1–C2	1.460(4)	O2–C2	1.253(5)
O1–Cd–O1W	95.19(12)	O1–Cd–O2^f	93.25(12)
O1–Cd–O1^m	180.00	O1–Cd–O1W^m	84.82(12)
O1–Cd–O2^n	86.75(12)	O1W–Cd–O2^f	83.21(12)
O1^m–Cd–O1W	84.82(12)	O1W–Cd–O1W^m	180.00
O1W–Cd–O2^n	96.80(13)	O1^m–Cd–O2^f	86.74(12)
O1W^m–Cd–O2^f	96.77(12)	O2^f–Cd–O2^n	180.00
O1^m–Cd–O1W^m	95.17(12)	O1^m–Cd–O2^n	93.26(12)
O1W^m–Cd–O2^n	83.21(13)

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It should be noted that each cadmium(II) sits with equatorial atoms without any deviation from the mean plane. There are four branches emanating from every cadmium(II) centre, and in each branch each cadmium(II) is connected to another cadmium(II) centre via a C₄O₄²⁻ bridge, which in turn produces three other branches forming a robust 3D polymeric network (Fig. 2).

Fig. 2 View of the polymeric array formed by [Cd(C₄O₄)(H₂O)₂]_n. (Colour code: Cd blue, O red, C black, H green.)

A void calculation using the PLATON99 software shows that the network contains a channel of 386.5 Å³ per unit cell (21.6%) when viewed down the c-axis (CPK model, Fig. 3). Formation of this 3D network may be due to the unique bridging mode of the squarate dianion. The Cd–O bond distances are in the range 2.234(3)–2.311(2) Å. The water molecules form relatively strong O–H⋯O hydrogen bonds (O⋯O 2.76 and 2.74 Å; O–H⋯O 176 and 154°) with squarato oxygens (Table 3).

Fig. 3 CPK view of [Cd(C₄O₄)(H₂O)₂]_n along the c-axis illustrating the channels. (Colour code: Cd blue, O red, C black, H green.)

Table 3 Hydrogen bond lengths (Å) and angles (°) for [Cd(C₄O₄)(H₂O)₂]_n1

D–H⋯A	D–H	H⋯A	D⋯A	D–H⋯A
Symmetry codes: n⊕=⊕−1/3⊕+⊕y, 1/3⊕−⊕x⊕+⊕y, 1/3⊕−⊕z; p⊕=⊕1⊕−⊕x, 1⊕−⊕y, −z.
O1W–H2⋯O1^n	0.91(6)	1.90(5)	2.744(5)	154(5)
O1W–H1⋯O2^p	0.92(4)	1.84(4)	2.759(4)	176(4)

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Thermal properties

The multidentate functionality of C₄O₄²⁻ and its ability to generate such tightly held bulky building units have a significant role on the stability of this material, while the possibility of the elimination of water molecules presents an opportunity for achieving a porous material with coordinatively unsaturated cadmium(II) centres. Thermogravimetric analysis (TGA) on this material shows a stepwise loss of two water molecules at ca. 220 °C, corroborating the presence of strong H-bonding interactions for the O_squarate⋯H–O–H moiety (see Fig. 1). The dehydrated compound remains stable up to 365 °C without any weight loss, indicating a robust cadmium squarate framework. Interestingly, we have observed that the dehydrated compound becomes rehydrated in the presence of water. Comparison of the X-ray powder diffraction pattern of the starting material [T⊕=⊕298 K; data collection range (2θ)⊕=⊕10–40°] with that of the dehydrated solid shows that the diffraction lines in the latter remain almost similar but only different positions and line widths. This may indicate a slight deformation of the pore structure due to the empty space created by loss of coordinated water molecules. It is worth noting that the XRD pattern of the rehydrated compound shows coincidence of the peak positions and intensities with those observed for the original solid (Fig. 4).

Fig. 4 XRD patterns of the (a) as-synthesized material [Cd(C₄O₄)(H₂O)₂]_n, (b) dehydrated solid [Cd(C₄O₄)]_n, and (c) rehydrated solid resulting from the reintroduction of water molecules into the coordination sphere of the dehydrated solid.

This indicates that the internal structure of the material remains intact and that heating and water removal do not cause much deformation in the Cd–squarate–Cd framework. The proposed dehydrated structure (CPK model, Fig. 5) is predicted to have a channel volume of 486.2 Å³ per unit cell (27.2%). The thermal and structural stability of the dehydrated solid and the presence of the coordinated unsaturated metal centres may enable the material to act as a solvent inclusion compound.

Fig. 5 Perspective CPK view of the proposed structure of the dehydrated complex [Cd(C₄O₄)]_n along the c-axis illustrating the channels. (Colour code: Cd blue, O red, C black).

Acknowledgements

The authors wish to thank the Council of Scientific and Industrial Research, New Delhi, for financial support (granted to N. R. C.).

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