hydrothermal synthesis, crystal structure,
and thermal properties of [M(ox)(bpy)]
(M = Fe, Co, Ni, Zn; ox = C2O42- ; bpy = 4, 4'-bipyridine)
and [MCl2(bpy)] (M = Fe, Co, Ni, Co/Ni)
Michael A. Lawandy*, Xiaoying Huang, Jack Y. Lu, Ru-Ji Wang, and Jing Li
Department of Chemistry, Rutgers University, Camden, New Jersey, 08102
*Rutgers Undergraduate Research Fellow
Two-dimensional metal coordination polymers with a general formula[M(ox)(bpy)] (M = Fe, Co, Ni, Zn; ox = C2O42- ; bpy = 4, 4'-bipyridine) and [MCl2(bpy)] (M = Fe, Co, Ni, or Co/Ni; 1:1 molar) bpy = 4, 4'-bipyridine) have been designed and synthesized via the hydrothermal method. The compounds [Fe(ox)(bpy)](I),[Co(ox)(bpy)](II),[Ni(ox)(bpy)](III), and[Zn(ox)(bpy)](IV) represent the first coordination network structure constructed by bridging oxalate and 4,4'-bpy mixed ligands. The compounds[FeCl2(bpy)](V),[CoCl2(bpy)](VI),[NiCl2(bpy)](VII), and[ZnCl2(bpy)](VIII) represent a similar structure type constructed by bridging chlorine atoms and 4,4'-bpy ligands. Compounds I-IV are isostructural and belong to the orthorhombic crystal system, space group Immm (No. 71). Compounds V-VIII are also isostructural and belong to the orthorombic crystal system, space group Cmmm (No. 65). I-IV have a non-interpenetrated structure that contains layers of octahedral metal centers coordinated by two oxalate and two bpy ligands. In like fashion, V-VIII have a non-interpenetrated structure containing metal centers octahedrally coordinated to four bridging chlorine atoms and two bpy ligands at trans position. Thermogravimetric analyses (TGA) indicate that compounds I-IV are thermally stable up to 300oC, while compounds V-VIII are thermally stable up to 400oC.
The hydrothermal method1 has proven to be a very effective way to grow crystals and hence, has been widely used in the synthesis of various solid-state inorganic materials. The purpose of this study is to design and synthesize coordination polymers containing metal centers and rigid ligands via this technique2. Two ligands that have been selected are 4,4'-bipyridine and oxalate. The former ligand (bpy) is unique in its ability to act as a rigid, rod-like organic building block3-9. Oxalate is selected because of its strength, rigidity, and specificity. We describe the preparation, crystal structure analysis, and thermal properties of two new two-dimensional coordination frameworks [M(ox)(bpy)] (M = Fe, Co, Ni, Zn; ox = C2O42- ; bpy = 4, 4'-bipyridine) and [MCl2(bpy)] (M = Fe, Co, Ni, Co/Ni).
All chemicals used are as purchased without purification. FeCl2 (99.5%, Alfa Aear), FeBr2•6H2O (99.5%, Alfa Aesar), FeCl2•4H2O (99+%, ACROS), FeCl3 (98%, Strem), CoCl2 (99.7%, Alfa Aesar), CoCl2•6H2O (98%, ACROS), CoBr2•6H2O (99%, Alfa Aesar), NiCl2 (99%, Alfa Aesar), NiCl2•6H2O (97%, ACROS), NiBr2•6H2O (98%, Alfa Aesar), ZnCl2 (98%, Aldrich), 4,4'-bypyridine (98%, Alfa Aesar), oxalic acid (H2C2O4•2H2O, 97%, Alfa Aesar), and sodium oxalate (Na2C2O4, 99%, Alfa Aesar).
Synthesis of[Fe(ox)(bpy)] (I)
The reactions of FeBr2•6H2O (0.1618 g), bpy (0.0781 g), oxalic acid (0.063 g), and H2O (4 mL) in the mole ratio of 1:1:1:444 in a 23 mL acid digestion bomb at 170oC for a period of seven days resulted in red plate crystals of I. The product was washed with water and acetone and dried in air. Compound I was isolated in quantitative yield.
Synthesis of[Co(ox)(bpy)] (II)
Orange plate crystals were isolated from the reaction of CoBr2•6H2O (0.1638 g), bpy (0.0781 g), oxalate acid (0.0063 g), and H2O (4 mL) in the mole ratio of 1:1:1:444. The reactions were carried out in a 23 mL acid digestion bomb at 170oC for seven days. II in quantitative yield was collected. A single-phase polycrystalline sample of II was obtained in the reaction of CoCl2 (0.1298 g), bpy (0.1562 g), sodium oxalate (0.1340 g) and H2O (8 mL) in the molar ratio of 1:1:1:444 under the conditions described previously.
Synthesis of[Ni(ox)(bpy)] (III)
The reaction of NiBr2•3H2O (0.2665 g), bpy (0.1562 g), oxalate acid (0.1260 g), and H2O (8 mL) in the mole ratio of 1:1:1:444 under the same conditions as described for crystal growth of I and II produced light blue plate crystals of III in quantitative yield. Similar reactions of NiCl2 (0.1296 g), bpy (0.1562 g), sodium oxalate (0.1340 g), and H2O (8 or16 mL) in the mole ratio of 1:1:1:444 or 888 yielded single phased powder samples of III.
Synthesis of[Zn(ox)(bpy)] (IV)
Colorless transparent thin plate crystals of IV were grown from reactions of ZnCl2 (0.0682 g), bpy (0.0781 g), oxalate acid (0.0630 g), and H2O (8 mL) in the mole ratio of 1:1:1:888. The same reaction conditions were used as in the synthesis of compounds I and II. The yield for this reaction was approximately 60%. Subsequent reactions using ZnCl2 (0.1363 g), bpy (0.1562 g), sodium oxalate (0.1340 g), and H2O (8 mL) in the mole ratio of 1:1:1:444 under identical conditions yielded a polycrystalline, pure-phase samples of IV.
Synthesis of[FeCl2(bpy)] (V)
The reactions of FeCl3 (0.1622 g), H2C2O4•2H2O (0.1261 g), bpy (0.1562 g), and H2O (5 mL) in the mole ratio of 1:1:1:278 under the previous conditions yielded brownish column-like crystals of V. The yield of this reaction was calculated to be 76%. Powder XRD analysis revealed that the mixing of FeCl2•4H2O (0.1988 g) and bpy (0.1562 g) gave rise to Fe2O3. Only when FeCl3 was used as the reagent in the presence of oxalic acid did single-phased V form.
Synthesis of[CoCl2(bpy)] (VI)
Pale pink-purple crystals were isolated from the reaction of CoCl21•6H2O (0.1189 g), H2C2O4•2H2O (0.063 g), bpy (0.0781 g), and H2O (8 mL) in the mole ratio of 1:1:1:889 in a 23 mL acid digestion bomb at 170oC for a period of seven days. The secondary product of this reaction was [Co(ox)(bpy)]. A single phased pale pink-purplish polycrystalline sample of VI was produced by subsequent reactions using CoCl2 (0.1298 g) and bpy (0.1562g) in H2O (8 mL) at 170oC for 3 days. The yield for this reaction was 70%.
Single-phased pale greenish-yellow polycrystalline samples of VII was produced from the the reaction of NiCl2 (0.1296 g), bpy (0.1562 g), and H2O (4 mL) in the mole ratio of 1:1:1:444 in a 23 mL acid digestion bomb at 170oC for 3 days. The yield for this reaction was 72 %.
Synthesis of[(Co/Ni)Cl2(bpy)] (VIII)
The reaction of CoCl2•6H2O (0.1190 g), NiCl2•6H2O (0.1198 g), bpy (0.1562 g), and H2O (8 mL) in the mole ratio of 1:1:1:444 in a 23 mL acid digestion bomb at 170oC for 7 days yielded single-phased polycrystalline pale grayish samples of VIII. The yield for this reaction was 70%. The same product was obtained with anhydrous metal chlorides as reagents.
A red plate crystal of I (0.2 x 0.3 x 0.5 mm), an orange plate crystal of II (0.1 x 0.2 x 0.5 mm), a light blue plate crystal of III (0.2 x 0.4 x 0.5 mm), a colorless plate crystal of IV (0.02 x 0.02 x 0.1 mm), a brownish blocklike crystal of V (0.06 x 0.08 x 0.13 mm), and a pale pink-purplish column-like crystal of VI were selected for the crystal structure analysis. Each crystal was mounted on a glass fiber in air on an Enraf-Nonius CAD4 automated diffractometer. Twenty-five reflections were centered in each case using graphite-monochromated Mo-Ka radiation. The data were collected at room temperature with w-scan method within the limits 7o£2Q£50o (I-IV), 5o£2Q£52o (V) and 7o£2Q£50o (VI). Raw data were corrected for Lorentz and polarization effects, and an empirical absorption correction10 was applied in each case. The structures were solved using the SHELX-97 program11. The non-hydrogen atoms were located by direct phase determination and subjected to anisotropic refinement. The hydrogens were generated theoretically. The full-matrix least-square calculations on F2 were applied on the final refinements. The unit cell parameters, along with data collection and refinement details, are given in Table 1 and Table 6. Selected bond lengths and angles are reported in Tables 2-5 and Table 7. Crystal drawings were generated by SCHAKAL 9212.
Attempts to grow crystals of VII and VIII suitable for structure analysis by single-crystal X-ray diffraction were not successful. The unit cells of these compounds were refined using powder XRD data of single-phased samples. The refinement was also performed on V and VI as a comparison. The refinement was performed using JADE (Windows) software package13. The refinement results, including those of V and VI, are listed in Table 8.
X-ray powder diffraction analyses were performed on a Rigaku D/M-2200T automated diffraction system (Ultima+). All measurements were made between a 2Q range of 5o and 80o at the operating power of 40 kV/40 mA. All polycrystalline samples were examined for the purpose of phase identification.
Thermogravimetric (TGA) analyses of the title compounds were performed on a computer controlled TA Instrument 2050TGA analyzer. Single phased powder samples of I (9.4430 mg), II (5.4460 mg), III (11.0880), IV (28.1680 mg), V (23.424 mg), VI (20.832 mg), VII (13.369 mg), and VIII (19.686 mg) were loaded into alumina pans and heated with a ramp rate of 10oC/min from room temperature to 500-550oC for I-IV and to 800oC for V-VIII.
Results and Discussion
Compounds I-IV crystallize in the orthorhombic crystal system, space group Immm (No. 71), while compounds V-VIII crystallize in the orthorombic crystal system, space group Cmmm (No. 65). The crystal structure of compounds I-IV consists of two-dimensional [M(ox)(bpy] networks (Figure 1) and the crystal structure of compounds V-VIII is similar to that of I-IV, also composed of two-dimensional networks (Figure 2). For compounds I-IV, the metal atoms are divalent and have an octahedral coordination with four oxygen atoms from oxalate ions and two nitrogen atoms from bipyridine molecules. Similarly, the metals are also divalent and have an octahedral coordination with four chloride atoms and by two nitrogen from bipyridine molecules in V-VIII. In all the compounds, the 2D sheets are formed in the ac plane by connecting metal centers via bridging chlorine (or oxalate) and 4,4'-bpy ligands. Hence, these compounds have rectangular-shaped voids that are enclosed by four M2+ located at the four corners of the rectangle, either two oxalates or four chlorine atoms, and two bpy's, each bridging to two metal centers (Figure 1.2). The approximate cross-section of these voids is 5 x 11 Å for I-IV and 3.6 x 11 Å for V-VIII. The 2D nets stack on top of each other at a distance of ½b to complete the structure in the third-dimension.
For compounds I-IV, all O-M-N and N-M-N angles are 90o and 180o, respectively. Likewise, for compounds V-VIII, all N-M-Cl and N-M-M angles are 90o and 180o, correspondingly. In compounds I-IV, the M-M distances within the 2D net are a (5.3-5.5Å) and c (11.3-11.5Å), respectively. Similarly, the M-M distances within the 2D net are c (3.58-3.62 Å) and a (11.33-11.41 Å). All bpy rings are perpendicular to the ac planes so that they are parallel to each other. The distance between any two adjacent rings is 3.3-3.5Å for compounds I-IV and 5.3-5.5Å for compounds V-VIII. These distances allows for weak p-p interactions, which often stabilizes the structures.
Shown in Figures 3-8 are the results from thermogravimetric analyses (TGA) performed on I, II, III, IV, V, and VI. The curves of compounds I-IV indicate that these compounds underwent a single-step weight loss process and that they are thermally stable up to at least 300oC. In addition, the curve for compound V indicates that this compound underwent a three-step weight loss process and that it is thermally stable to at least 400oC. Also, the compounds VI-VIII underwent a two-step weight loss process and are at least stable to 400oC. The decomposition process completed at approximately 450oC for I, 475oC for II, 525oC for III, 425oC for IV, and 800oC for V-VIII. The thermal stability of these compounds may be attributed to the p-p interactions between the adjacent parallel bpy rings.
Two new types of two-dimensional coordination networks have been designed and synthesized via the hydrothermal method. Relatively high yields have been obtained for all compounds. The crystal structures have been analyzed by single crystal and powder XRD methods. Both [M(ox)(bpy)] and [MCl2(bpy)] are quite stable due to the weak p-p interactions between the adjacent parallel bipyradine rings. Compounds I-IV are thermally stable up to 300oC and compounds V-VIII are thermally stable up to 400oC.
Financial support from the National Science Foundation (DMR-9553066) is greatly appreciated. The FT-IR/TGA/DSC apparatus was purchased through a NSF ARI grant (CHE 9601710-ARI).
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