Versatile Framework Solids Constructed from Divalent Transition

Five versatile framework solids including 2D layer, 2D grid, 3D open-framework, and 1D polymeric chain have been fabricated on the basis of the assemb...
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Versatile Framework Solids Constructed from Divalent Transition Metals and Citric Acid: Syntheses, Crystal Structures, and Thermal Behaviors Guoqi Zhang, Guoqiang Yang,* and Jin Shi Ma

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 2 375-381

CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. China ReceiVed July 7, 2005; ReVised Manuscript ReceiVed September 6, 2005

ABSTRACT: The reactions of citric acid (C6H8O7 ) H4cit) with various divalent transition metals (CdII, ZnII, CuII, and CoII) in acid solution or under hydrothermal conditions (pH ) 1-4) have resulted in the isolation of five new coordination polymers with versatile multidimensional frameworks. The five metal citrate complexes, namely, [Cd3(Hcit)2(H2O)5‚H2O]n (1), [Cd3(Hcit)2(H2O)2]n (2), [Zn3(Hcit)2(H2O)2]n (3), [Cu2(cit)(H2O)2]n (4), and [Co(H2cit)(H2O)]n (5), have been characterized by elemental analysis, FT-IR spectra, thermal analysis, and single-crystal X-ray diffraction studies. While complex 1 was obtained by evaporation of an EtOHH2O solution of the reaction mixture, complexes 2-5 were all prepared under hydrothermal conditions. The crystal structures of the five complexes are remarkably distinct from each other because of the different synthetic conditions or coordination nature of the metal ions. Complex 1 features a 2D multilayered framework which contains an infinite 1D ladder-like structure. Complexes 2 and 3 are almost isostructural, and both exhibit 2D rhombus-grid frameworks. The copper(II) citrate complex 4 displays a new alkyloxylbridging coordination mode, different from other divalent metal citrate complexes. Consequently, the complex forms a 3D openframework polymer with large void channels. Complex 5 crystallizes in a chiral space group and forms a 1D infinite helical chain along a 21 axis. The thermal behaviors of the five coordination complexes have been investigated and discussed correlated with the crystal structures in detail. Particularly, the open-framework structure of 4 is responsible for the reversible water desorptionadsorption process. Introduction Crystal engineering based on metal-organic coordination polymers has recently attracted considerable interest from chemists, owing to their appeals for interesting supramolecular compositions and versatile framework topologies as well as their potential applications as functional materials in molecular magnetism, catalysis, gas sorption, fluorescent sensing, and optoelectronic devices.1-4 Although it has been elaborated that rational design and subtle choice of organic building blocks as well as metal coordination sites proved significant in controlling the solid structures of the target products, to establish the general and precise principles of constructing desirable framework topologies is still of great challenge. Some factors such as metal ions with distinct binding natures,5 counteranions,6 solvents,7 metal-ligand ratio,8 pH value,9 and even reaction temperature10 have been found to remarkably influence the structural topologies of the resultant coordination frameworks. The characterization of metal-organic hybrid materials based on metal carboxylates is a domain of special interest because of their high performance in producing intriguing structural diversity and potential functions as porous open-framework solids for molecule adsorption, ion exchange, and heterogeneous catalysis.11 A significant number of efforts have been contributed to the construction of metal-carboxylato complexes with 1D, 2D, or 3D framework structures by the selection of a variety of dicarboxylate and tricarboxylate ligands over the past decade.12,13 It has been previously suggested that citric acid (HOC(CO2H)(CH2CO2H)2; H4cit), which occurs at about 0.1 mM in blood plasma14 and about 0.3% w/w in teeth and bone,15,16 is an important multipotent chemical binding ligand toward some biologically relevant metal ions, such as Fe(III), Al(III), Ga* To whom correspondence should be addressed. E-mail: gqyang@ iccas.ac.cn.

(III), and Bi(III).17-19 It has been elaborated that citrate binding to transition metal ions enhanced their solubility and potential bioavailability. Although citric acid has been less explored in the synthesis of coordination polymers, recent studies have shed some light on its rich solution and solid-state coordination chemistry as well as structural flexibility. Specifically, a novel 3D lanthanum coordination polymer and a high nuclearity nickel cluster have been documented recently.20,21 Other transition metal citrates including heavy metals (CoIII, PbII, CdII) have also been preliminarily investigated on the aspects of structural features and spectroscopy.22-24 However, multidimensional and condensed framework polymers resulting from this tricarboxylate ligand and transition metals have rarely been investigated to date. Compared to the general solution reaction, hydrothermal synthesis is of obvious advantage in that the products generally present relatively compact crystal packing and reduced metalaqua coordination geometry to induce condensed metalcarboxylate frameworks. Some novel framework polymers distinct from the common results reported can form under hydrothermal conditions by the same original materials. Herein, we report the first examples of five framework polymers resulting from divalent transition metals (CdII, ZnII, CuII, and CoII) and citric acid by both solution and hydrothermal reactions. The metal-citrate complexes as obtained display interesting and versatile coordination features with 1D, 2D, and 3D frameworks, remarkably depending on the nature of metal ions, the reaction conditions, or both. Experimental Section General. All chemicals were analytical grade and were used without further purification. Deionized water was used throughout this work. Thermogravimetric analysis (TGA) was obtained on a NETZSCH STA 409PC instrument at a temperature range of 30-500 °C (10 °C/min)

10.1021/cg0503245 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/14/2005

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Table 1. Crystal and Structure Refinement Data for Complexes 1-5

empirical formula formula wt crystal system space group T, K λ, Å a, Å b, Å c, Å β, deg V, Å3 Z F(000) dcalc, g/cm3 µ, mm-1 reflns collected unique reflns variables R1,a wR2,b % goodness of fit a

1

2

3

4

5

C12H22O20Cd3 823.50 monoclinic P21/n 293(2) 0.710 73 12.550(3) 10.257(2) 18.015(4) 109.09(3) 2191.5(8) 4 1592 2.496 2.983 20515 5016 372 2.06, 4.77 1.004

C12H14O16Cd3 751.43 monoclinic P21/c 294(2) 0.710 73 6.098(7) 15.292(2) 9.786(1) 102.774(2) 890.03(17) 2 716 2.804 3.642 5329 2074 154 2.15, 4.99 1.222

C12H14O16Zn3 610.34 monoclinic P21/c 293(2) 0.710 73 6.171(1) 14.546(3) 9.560(2) 102.630(3) 837.3(3) 2 608 2.421 4.359 7568 1899 154 2.32, 6.16 1.070

C6H8O9Cu2 351.20 monoclinic P21/c 294(2) 0.710 73 6.929(1) 9.762(1) 14.537(2) 91.377(2) 982.9(2) 4 696 2.373 4.365 5412 1762 154 3.61, 8.68 1.042

C6H8O8Co 267.05 orthorhombic P212121 294(2) 0.710 73 5.916(9) 10.423(2) 13.453(2) 90.00 829.5(2) 4 540 2.138 2.059 4648 1571 150 2.69, 6.28 1.070

R ) (Fo - Fc)/(Fo). b wR ) w(Fo2 - Fc2)2/(w(Fo2)2)1/2.

with flowing N2(g). A BIO-RAD FT-165 IR spectrometer was used to gather FT-IR spectra using KBr pellets. Elemental analysis was done with a Carlo Erba-1106 instrument. Preparation of {[Cd3(Hcit)2(H2O)5]‚H2O}n (1). Cd(NO3)2‚4H2O (1.20 g, 4.0 mmol), citric acid (0.38 g, 2.0 mmol), and deionized water (10 mL) at a pH of about 2.0 were allowed to reflux for 5 h and then cooled to room temperature to give a clear solution. Ethanol (4 mL) was subsequently added to the reaction solution to induce crystallization, and large blocklike colorless crystals appeared from the resultant solution after 2 days; the solution remained intact for an additional week until the crystal stopped growing. The products were isolated by filtration and dried in air for 2 days. Yield 0.30 g (36% based on citric acid). IR (KBr pellet, cm-1): 3385w, 2362m, 1620w, 1427w, 1264s, 1158s, 1107m, 906m, 859s, 776w, 680s, 627s, 561s. Anal. Calcd for C12H22O20Cd3 (827.78): C, 17.50; H, 2.69. Found: C, 17.61; H, 2.69. Preparation of [Cd3(Hcit)2(H2O)2]n (2). Cd(NO3)2‚4H2O (0.60 g, 2.0 mmol), citric acid (0.38 g, 2.0 mmol), and deionized water (6 mL) were mixed, and the pH was carefully adjusted to 4.0 with ammonia (30%). Then the mixture was moved to a Teflon-lined vessel (12 mL), which was sealed and held at 140 °C for 16 h. The reaction vessel was allowed to slowly cool to room temperature. Large amounts of colorless blocklike crystals were obtained as a pure phase. The crystals were then washed with deionized water and ethanol and dried in air. Yield: 0.23 g (45% based on Cd). IR (KBr pellet, cm-1): 3481s, 3381w, 3042w, 2821s, 2720s, 2363m, 1583w, 1426s, 1399s, 1298s, 1270s, 1134s, 1080m, 930s, 860m, 625s. Anal. Calcd for C12H14O16Cd3 (755.74): C, 19.18; H, 1.88. Found: C, 19.07; H, 1.94. Preparation of [Zn3(Hcit)2(H2O)2]n (3). The hydrothermal procedure for the preparation of complex 3 is similar to that for 2 except for the replacement of Cd(NO3)2‚4H2O with Zn(NO3)2‚4H2O; large amounts of colorless blocklike crystals were harvested from the reaction mixture as a pure phase. The crystals were then washed with deionized water and ethanol and dried in air. Yield: 0.20 g (48% based on Zn). IR (KBr pellet, cm-1): 3456w, 3047w, 2833s, 2722s, 2624s, 1616w, 1477s, 1423w, 1259s, 1139s, 1078s, 904m, 633s. Anal. Calcd for C12H14O16Zn3 (605.82): C, 23.61; H, 2.31. Found: C, 23.51; H, 2.34. Preparation of [Cu2(cit)(H2O)2]n (4). The hydrothermal procedure for the preparation of complex 4 is similar to that for 2 except for the replacement of Cd(NO3)2‚4H2O with Cu(NO3)2‚3H2O; light green blocklike crystals were obtained from the reaction as a pure phase. The products were then washed with deionized water three times and dried in air. Yield: 0.15 g (86% based on Cu). IR (KBr pellet, cm-1): 3423w, 2929s, 1595w, 1439s, 1380s, 1258s, 1123s, 1090s, 915s, 857s, 803s, 650m, 579s. Anal. Calcd for C6H8O9Cu2 (349.90): C, 20.52; H, 2.30. Found: C, 20.61; H, 2.67. Preparation of [Co(H2cit)(H2O)]n (5). The hydrothermal procedure for the preparation of complex 5 is similar to that for 2 except for the replacement of Cd(NO3)2‚4H2O with CoCl2‚6H2O; purple blocklike crystals were obtained from the reaction as a pure phase. The products were then washed with deionized water three times and dried in air.

Yield: 0.17 g (31% based Co). IR (KBr pellet, cm-1): 3484s, 3396s, 2979w, 2650w, 1734s, 1616s, 1552s, 1423m, 1321m, 1191s, 1155s, 880w, 787s, 709s, 673s, 575w. Anal. Calcd for C6H8O8Co (266.96): C, 26.98; H, 3.02. Found: C, 27.18; H, 3.14. X-ray Crystallography. Suitable single crystals of compounds 1-5 were selected and mounted in air onto thin glass fibers. Accurate unit cell parameters were determined by a least-squares fit of 2θ values, measured for 200 strong reflections, and intensity data sets were measured on a Bruker Smart 1000 CCD or Rigaku Raxis Rapid IP diffractometer with Mo KR radiation (λ ) 0.710 73 Å) at room temperature. The intensities were corrected for Lorentz and polarization effects, but no corrections for extinction were made. All structures were solved by direct methods. The non-hydrogen atoms were located in successive difference Fourier syntheses. The final refinement was performed by full matrix least-squares methods with anisotropic thermal parameters for non-hydrogen atoms on F2. All hydrogen atoms were unambiguously located by difference maps and refined isotropically. Crystallographic data and experimental details for structure analyses are summarized in Table 1. Selected bond lengths and angles of the complexes are listed in Table 2. Summary for hydrogen bond data is listed in Table 3.

Results and Discussion Syntheses. The syntheses of five metal-citrate complexes were accomplished with different methods from the literature. The preparation of compound 1 was similar to a recent report of CdII-citrate species with modification of metal-ligand ratio and reaction temperature.23 Accordingly, the structure of 1 drastically changed into a 2D layered framework. Complexes 2-5 were obtained by using a hydrothermal method upon keeping the original pH of 4.0 with ammonia. Although complexes 1 and 2 came from the same original materials and molar ratio, the hydrothermal method greatly altered the coordination features of the ligand and the resulting framework solids. For the hydrothermal syntheses of 2-5, replacement of ammonia with sodium hydroxide also resulted in the isolation of the same products. Obviously, the polymers obtained from hydrothermal conditions exhibited more condensed structures than those obtained from the general methods.17-24 To our knowledge, only one example of a lanthanide citrate polymer was obtained under hydrothermal conditions in the literature, which displays a quite compact framework.20 Hence, this work provides an additional method for the synthesis of new solid materials with rigid open-framework and compact structures from a flexible organic ligand.

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Crystal Growth & Design, Vol. 6, No. 2, 2006 377

Table 2. Selected Bond Lengths (Å) for 1-5

Table 3. Hydrogen Bond Data for Complexes 1-5

1 Cd(1)-O(4)1 Cd(1)-O(2) Cd(1)-O(6)1 Cd(1)-O(15) Cd(2)-O(1) Cd(2)-O(16) Cd(2)-O(10) Cd(3)-O(9) Cd(3)-O(11) Cd(3)-O(17) Cd(3)-O(19)

Cd(1)-O(5)1 Cd(1)-O(3) Cd(1)-O(7)1 Cd(2)-O(4) Cd(2)-O(2) Cd(2)-O(9) Cd(2)-O(12)2 Cd(3)-O(8) Cd(3)-O(13)2 Cd(3)-O(18)

2.579(2) 2.473(2) 2.404(2) 2.255(2) 2.417(2) 2.276(2) 2.350(2) 2.391(2) 2.299(2) 2.296(2) 2.304(2)

2.269(2) 2.333(2) 2.327(2) 2.407(2) 2.332(2) 2.549(2) 2.273(2) 2.348(2) 2.497(2) 2.281(2)

interaction

H‚‚‚A (Å)

D‚‚‚A (Å)

D-H‚‚‚A (deg)

O1W-H2W‚‚‚O10

1.976

1 2.754

159.37

O1W-H1W‚‚‚O7 O1-H1A‚‚‚O6 O15-H15B‚‚‚O18

2.190 2.000 2.081

2.793 2.729 2.830

154.85 137.50 164.47

O15-H15A‚‚‚O12

2.428

3.069

135.42

O16-H16B‚‚‚O3

2.066

2.737

176.39

O16-H16A‚‚‚O13 O8-H8A‚‚‚O13 O17-H17B‚‚‚O14

2.511 1.990 1.872

3.243 2.605 2.698

152.12 147.62 167.68

O17-H17A‚‚‚O12 O18-H18B‚‚‚O5 O18-H18A‚‚‚O1W O19-H19B‚‚‚O1W

2.214 2.044 1.905 2.054

2.890 2.740 2.678 2.825

156.28 155.43 165.90 177.74

O19-H19A‚‚‚O14

1.762

2.653

165.00

O8-H8A‚‚‚O2 O8-H8A‚‚‚O6 O8-H8B‚‚‚O3 O1-H1A‚‚‚O7

2.33(5) 2.32(5) 1.99(5) 1.93(4)

2 3.037(3) 2.888(3) 2.794(3) 2.660(3)

146(4) 127(4) 174(4) 170(4)

x, -y - 1/2, z - 1/2

O8-H8A‚‚‚O6 O8-H8A‚‚‚O2 O8-H8B‚‚‚O3 O1-H1A‚‚‚O7

2.332 2.397 2.075 1.889

3 2.800 3.070 2.873 2.664

122.38 151.90 173.44 170.29

-x, -y + 1, -z + 1 -x, -y + 1, -z + 1 x - 1, y, z x, -y + 1/2, z + 1

O9-H9A‚‚‚O2 O8-H8B‚‚‚O2 O8-H8A‚‚‚O9

1.91 2.42 2.00

4 2.741(4) 3.220(5) 2.838(5)

167.1 156.8 166.9

-x, y + 1/2, -z + 1/2 -x, y + 1/2, -z + 1/2 -x, -y, -z

O8-H8B‚‚‚O4 O8-H8A‚‚‚O7 O6-H6‚‚‚O5 O1-H1‚‚‚O5

2.12(5) 1.96(5) 1.81(4) 1.72(4)

5 2.876(4) 2.749(3) 2.668(3) 2.602(3)

152(4) 172(4) 171(4) 175(4)

x + 1/2, -y + 1/2, -z x + 1/2, -y + 1/2, -z -x + 1/2, -y, z + 1/2 -x, y + 1/2, -z + 1/2

2 Cd(1)-O(2) Cd(1)-O(1) Cd(2)-O(8) Cd(2)-O(7) Cd(2)-O(2)2

2.255(2) 2.318(2) 2.268(2) 2.354(2) 2.503(2)

Zn(1)-O(1) Zn(1)-O(4) Zn(2)-O(5)2 Zn(2)-O(7)

2.119(2) 2.087(2) 1.970 (2) 2.071(2)

Cu(1)-O(1) Cu(1)-O(3)3 Cu(1)-O(6)3 Cu(2)-O(4) Cu(2)-O(8)

1.946(2) 2.289(3) 1.939(3) 1.924(3) 2.098(3)

Co(1)-O(1) Co(1)-O(3) Co(1)-O(8)

2.036(2) 2.095(3) 2.089(3)

Cd(1)-O(4) Cd(2)-O(5)2 Cd(2)-O(3)2 Cd(2)-O(6) Cd(2)-O(4)2

2.269(2) 2.226(2) 2.321(2) 2.359(2) 2.601(2)

Zn(1)-O(2) Zn(2)-O(3)2 Zn(2)-O(6) Zn(2)-O(8)

2.039(2) 2.014 (2) 2.318(2) 2.028(2)

Cu(1)-O(2) Cu(1)-O(5)4 Cu(2)-O(1) Cu(2)-O(7)3 Cu(2)-O(9)

1.937 (3) 1.952(3) 1.926(3) 1.928(2) 2.293(3)

Co(1)-O(2)5 Co(1)-O(4) Co(1)-O(2)

2.072(2) 2.109(2) 2.217(2)

3

4

5

a

Symmetry transformations used to generate equivalent atoms for: (1) -x + 1/2, y + 1/2, -z + 1/2; (2) -x, -y, -z; (3) -x, y + 1/2, -z + 1/2; (4) x, -y - 1/2, z - 1/2; (5) x + 1/2, -y + 1/2, -z.

Description of the Crystal Structures. Complex 1 crystallized with a monoclinic space group P21/n. The structure featured a 2D polymeric pattern with a repeating unit [Cd3(C6H5O7)2(H2O)5‚H2O]. Figure 1 illustrates the ORTEP structure of 1, which shows an asymmetric trinuclear motif. Each Cd(II) cation is surrounded by an O7 environment, while the three cadmium(II) centers all adopt distinct coordination modes. Five water molecules coordinate to cadmium centers in each unit, one water molecule for Cd1 and Cd2 and three for Cd3. The general Cd-O bond distances are in the range of 2.255(2)2.579(2) Å, which are very similar to other reported cadmium(II) complexes.23,25 There are two types of citrate ligands in the asymmetric unit; the oxygen atoms from R-hydroxyl, R-carboxylate, and β-carboxylate groups all participate in the coordination with metal centers. This is remarkably distinct from the case of the reported cadmium(II)-citrate species,23 which contained only one type of ligand and the citrate ligand was just doubly deprotonated; the complex adopted a molecular composition of [Cd(C6H6O7)(H2O)]n. Particularly, in 1, some of R-carboxylate and β-carboxylate groups bridge the discrete metal centers with an edge-sharing mode and serve to extend the structure to a 2D polymer with a novel topology. Figure 2a shows the projection of the 2D layered structure. The structure features a layered structure in an ABAB... fashion, in which the adjacent ribbons are connected with bridging ligand moieties. It is more interesting that ribbon A, comprised of Cd1 and one type of ligand, adopts a ladder-like molecular architecture (Figure 3a), which includes many small square grids. Ribbon B contains two CdII centers (Cd2 and Cd3) and another type of ligand. Furthermore, the coordinated water distributed outside

symmetry operation x - 1/2, -y + 1/2, z - 1/ 2 -x, -y, -z -x + 1/2, y + 1/2, -z + 1/2 x - 1/2, -y + 1/2, z + 1/ 2 -x + 1/2, y + 1/2, -z + 1/2 x, y + 1, z x - 1/2, -y + 1/2, z - 1/ 2 -x + 1, -y + 1, -z -x, -y + 1, -z -x + 1/2, y + 1/2, -z - 1/2 -x + 1, -y, -z -x, -y, -z -x, -y, -z

the layer planes and the crystalline water between the adjacent layer planes contribute to bridge the discrete layers to a 3D network with hydrogen bonding interactions (see Supporting Information). Complex 2 crystallized in the monoclinic space group P21/c with a formula [Cd3(C6H5O7)2(H2O)‚H2O]n. Unlike the complex 1, the asymmetric unit of 2 contains two discrete cadmium(II) centers (Figure 4). Two cadmium(II) centers exist in two different coordination environments. One is coordinated with

Figure 1. The ORTEP structure of 1 with 30% thermal ellipsoid probability showing the asymmetric unit of the bridging cadmiumcitrate cluster. Hydrogen atoms have been omitted for clarity.

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Figure 4. The ORTEP structure of 2 with 30% thermal ellipsoid probability showing the repeating asymmetric unit.

Figure 5. The ORTEP structure of 3 with 30% thermal ellipsoid probability showing the repeating asymmetric unit. Figure 2. Projections of (a) 1 showing the 2D layered network in an ABAB... fashion and (b) 2 showing the 2D rhombus-grid framework with a dimension of ca. 6.094(1) × 9.750(4) Å2. Carbon, oxygen, and cadmium atoms are represented in black, red, and pink balls, respectively. Hydrogen atoms have been omitted for clarity.

Figure 3. (a) A view of ribbon A in the 2D layer structure of 1 showing a novel 1D ladder-like structure bridged with citrate in the lattice and (b) the infinite 1D polymeric chain existing in 2. Carbon, oxygen, and cadmium atoms are represented in black, red, and pink balls, respectively. Hydrogen atoms have been omitted for clarity.

two distinct citrate ligands by six oxygen atoms belonging to R-hydroxyl, R-carboxylate, and one of the β-carboxylate groups. The other is seven-coordinate with six oxygen atoms from one R-carboxylate group and two β-carboxylate groups of three symmetrically related citrate ligands in a chelating pattern and one oxygen atom from the coordinated water molecule. The Cd-O bond distances are in the range of 2.226(2)-2.601(2) Å, slightly different from those in complex 1. Two oxygen atoms (O2 and O3) from the R-carboxylate of the citrate ligand further coordinate to a symmetrically related cadmium center with an edge-sharing mode. The citrate ligand thus bridges the trinuclear units to an infinite 1D polymer, which is apparently different from the ladder-like structure consisting of small square grids

in 1 (Figure 2b). It is worth mentioning that the hydrothermal reaction for the synthesis of complex 2 greatly reduce the metal-aqua coordination as expected. The coordinated water number decreases from five in 1 to one in 2. Hence the extended structure of 2 shows a more uniform pattern. The calculated crystal density remarkably increases from 2.496 to 2.804 g/cm3. Significantly, the 1D polymeric chain is further expanded along a nearly perpendicular direction to form a 2D rhombus-grid framework (Figure 3b). The large grid has a dimension of ca. 6.094(1) × 9.750(4) Å2. Some square or rectangular grid architectures based on metal-organic coordination compounds have recently reported,26,27 which mainly utilized the rigid organic dicarboxylate ligands such as terephthalic acid or fumaric acid. However, the peculiar rhombus-grid framework resulting from the flexible citric acid is indeed unprecedented. The structure of complex 3 is almost identical to that of 2. The cell parameters listed in Table 1 display the similarity of two kinds of crystals. In the structure of 3 (Figure 5), the coordination environments of zinc(II) centers are slightly different from those of cadmium(II) in complex 2. One of the metal centers is six-coordinated, similar to the Cd(1) center of 2, but the other is five-coordinated with three oxygen atoms belonging to β-carboxylate groups and one belonging to an R-carboxylate group. One water molecule holds the fifth coordination site of this zinc(II) center. Unlike the case of complex 2, only one oxygen atom (O3) from the R-carboxylate of the citrate ligand further coordinates to a symmetrically related zinc center in complex 3. The general Zn-O bond separations are in the range of 1.970(2)-2.318(2) Å, falling in the rational Zn-O bond lengths. Finally, complex 3 is a 2D polymeric structure as well, which features a rhombus-grid framework. The grid has a dimension of ca. 6.156(1) × 9.500(2) Å2, just slightly different from the grid framework in 2.

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Figure 7. Projection of the 3D open-framework structure in complex 4. Carbon, oxygen, and copper atoms are represented in black, red, and pink balls, respectively. Hydrogen atoms have been omitted for clarity. Figure 6. The ORTEP structure of complex 4 with 30% thermal ellipsoid probability showing the dinuclear repeating asymmetric unit.

As illustrated in Figure 6, complex 4 comprises a dinuclear [Cu2(cit)(H2O)2] repeating unit. The Cu(II) centers are surrounded by two different types of coordination environments. Cu(1) is five-coordinate with two oxygen atoms from R-carboxylate groups, two oxygen atoms from β-carboxylate groups, and one from the R-hydroxyl, while Cu(2) is coordinated with two oxygen atoms of β-carboxylate groups and one bridging R-hydroxyl. Additionally, two water molecules occupy the two remaining coordination sites to meet the five-coordinated environment. The generally short Cu-O bond distances are in the range of 1.924(3)-2.098(3) Å, two relatively long Cu-O separations are 2.289(3) and 2.293(3) Å, respectively, but all belong to the common Cu-O bond lengths reported.28 Interestingly, the repeating dinuclear motif adopts a dimeric structure containing a six-member ring, which is composed of one β-carboxylate group, one bridging R-hydroxyl, and two copper ions. The Cu-Cu nonbonding distance is short as 3.254(3) Å. The coordination mode of bridging R-hydroxyl from the tetraionized citrate ion has been not observed in the common zinc(II), cadmium(II), cobalt(II), or lanthanide(III) citrate complexes20,22-24,29 but has been reported in limited polynuclear iron(III) or aluminum(III) citrate complexes.17 Hence, the synthesis of complex 4 provides a pathway for the preparation of additional high nuclearity Cu(II) clusters with the citrate ligand, which deserves further investigation. It is worth noting that though the Zn(II) or Cd(II) complexes depicted above usually form 2D extended frameworks, Cu(II)citrate 4 exhibits a 3D open-framework structure. A ball and stick representation of the open framework is displayed in Figure 7. The large 1D void channels can be clearly observed when it is viewed down the crystallographic c axis. The channels have a dimension of ca. 9.925(1) × 6.248(4) Å2 and are not occupied by any solvent molecules. Complex 5 crystallizes in a chiral space group P212121 with the Flack parameter refined to be -0.004(19), indicating that the complex crystallizes in an absolute configuration. The asymmetric unit contains one Co(II) ion, one citrate ligand, and one water molecule (Figure 8a). The Co(II) ion is surrounded by an O8 environment and adopts an octahedral coordination mode. One citrate ligand contributes its R-hydroxyl, R-carboxylate, and one β-carboxylate groups to bind to the Co(II) ion, two oxygen atoms from R-carboxylate groups of two adjacent

Figure 8. (a) The ORTEP structure of complex 5 with 30% thermal ellipsoid probability and (b) a polyhedral representation showing the extended 1D helical chain in 5 along the crystallographic a axis. Carbon, oxygen, and cobalt atoms are represented in black, red, and pink balls, respectively. Hydrogen atoms have been omitted for clarity.

citrate ligands occupy another two apexes, and one water molecule saturates the octahedral environment. The other uncoordinated β-carboxylate group of the citrate ligand is pronated and participates in the hydrogen bonding interaction. Furthermore, the bonded oxygen atom O2 is further coordinated to the adjacent Co(II) ion in an edge-sharing fashion, and the O3 of the R-carboxylate group is also coordinated to another Co(II) ion. As a consequence, the adjacent molecules are extended by a screw axis to form a 1D helical chainlike structure along the a axis (Figure 8b). This coordination and bridging mode is obviously distinct from the other metal citrate complexes described above. In complexes 1-4, the citrate ligand participates in complexation with all of its R-hydroxyl, R-carboxylate, and β-carboxylate groups, resulting in the 2D or 3D network polymers, while in complex 5, one β-carboxylate group of the citrate ligand is uncoordinated, and hence, the extended structure is only one-dimensional. The Co-O bond distances

380 Crystal Growth & Design, Vol. 6, No. 2, 2006

Figure 9. TGA curves of complexes 1-5 under N2 atmosphere.

in 5 are all in the range of 2.036(2)-2.217(2) Å, comparable to other cobalt(II) hydroxycarboxylate complexes.30 Additionally, the nearest Co‚‚‚Co separation of two adjacent Co(II) ions is 3.679(2) Å. With the exception of the different coordination features in the five framework solids, many O-H‚‚‚O hydrogen bonding interactions were observed in the crystal structures of these complexes. Some important hydrogen bonds data are listed in Table 3. Generally, the coordinated or crystalline water molecules and the R-hydroxyl groups are the main part of hydrogen bonded networks. The hydrogen bonds of complex 1 are relatively much greater in number and more complicated than the others, due to a large number of water molecules existing in the lattice. Particularly, in complex 1, the interlayer water molecules contribute to bridge the discrete layers to a 3D network with hydrogen bonds, while in complex 2, the R-hydroxyl groups and coordinated water molecules are responsible for the formation of hydrogen bonds and link the 2D grid layer to a 3D network. In complex 5, many hydrogen bonds formed from the protonated β-carboxylate groups and coordinated water molecules also construct the 1D chain to a 3D network (see Supporting Information). Overall these strong O-H‚‚‚O hydrogen bonds play an important role in contributing to the stability of the 1D, 2D, or 3D framework solids. Thermal Behavior. TGAs were carried out in the interest of studying the thermal behaviors of five polymer materials (Figure 9). The experiment was performed on samples consisting of numerous single crystals of 1-5 under a N2 atmosphere with a heating rate of 10 °C/min. For complex 1, the first major weight loss of ca. 13.0% corresponding to the departure of both the lattice and coordinated water molecules (six water molecules per asymmetric unit, calculated 13.1%) occurred in stages starting at 91 °C and completing at 281 °C. Then the anhydrous complex suffered from serious mass loss upon heating to 400 °C, attributable to decomposition of the organic chains. For complexes 2 and 3, the TGA curves are very similar, due to their almost identical structures. Both compounds are thermostable up to 283 °C, beyond that the samples begin to decompose. In contrast, complex 5 also suffers no water departure before 220 °C, then the compound breaks down seriously above that. However, as seen from the curve of complex 4, the thermal behavior is remarkably different from those of the other complexes. Coordinated water molecules in the structure of 4 are much easier to remove mostly even at ambient temperature. Indeed, it was found that the gray-green

Zhang et al.

Figure 10. Simulated (a) and experimental (b) powder X-ray diffraction patterns for the sample of 4 as prepared and experimental pattern for the dehydrated sample of 4 (c).

crystals became opaque when exposed to air for a short time, and the water molecules can be completely removed upon heating below 96 °C. As a consequence, an obvious color transformation from gray-green to light blue could be observed during the water loss, whereas the light blue powder recovered to gray-green color immediately upon immersion into water. Microanalysis data,31 FT-IR spectrum, and TGA analysis of the rehydrated samples confirmed that the gray-green samples were identical to the original (see Supporting Information). Powder X-ray diffraction analysis patterns of the sample as-prepared and the dehydrated sample showed no obvious change, indicating that the open-framework structure was stable and had not collapsed even when coordinated water molecules were completely removed from the channels (Figure 10). Furthermore, the dehydrated sample would slowly absorb water in the air at room temperature and about half of water molecules were imbibed over 1 week on the basis of the microanalysis data and IR spectrum of the partially rehydrated samples, which indicated that the 1D void channels could partially remain under usual conditions. The remarkable thermal behavior of complex 4 distinct from other complexes can be interpreted based on the following two points: First from the crystal structure, the two Cu-O(H2O) bond lengths are 2.098(3) and 2.293(3) Å, respectively, obviously longer than other Cu-O bonds (generally in the range of 1.924(3)-1.952(3) Å). This situation is not the same for other four complexes. Hence the very weakly bonded water molecules will be more easily removed. The second factor should be attributed to the large void channels present in the 3D open-framework of 4. We have mentioned above that the channels are unoccupied with a dimension of ca. 9.925(1) × 6.248(4) Å2 and the weakly bonded water molecules exist in the edge of channels. This significant open structure makes the desorption and adsorption process of water molecules easier. Conclusion In summary, we have successfully isolated five new coordination polymers resulting from citric acid and transition metals. All complexes have been characterized by elemental analysis, FT-IR spectra, thermal analysis, and single-crystal X-ray diffraction studies. Complex 1 is a 2D multilayered framework which contains an infinite 1D ladder-like structure. Complexes 2 and 3 are almost isostructural, and both exhibit 2D rhombusgrid frameworks. Complex 4 adopts a new alkyloxyl-bridging

Versatile Framework Solids

coordination mode, remarkably different from other divalent metal citrate complexes, and forms a 3D open-framework polymer with large void channels. Complex 5 forms a 1D infinite helical chain along a 21 axis. The results of thermal analysis are important for understanding the relationship of crystal structures and their thermostability. Moreover, the reversible water desorption-adsorption process for complex 4 reveals a interesting phenomenon whose potential applications deserve further investigation. Acknowledgment. Financial support from NSFC (Grants 50221201, 20173066, and 20333080) and the major state basic research development program (Grant G2000078100) of China is gratefully acknowledged. Supporting Information Available: X-ray crystallographic data for complexes 1-5 in CIF format, more crystal packing patterns, and IR spectra and TGA curves of 4. This material is available free of charge via the Internet at http://pubs.acs.org.

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