DOI: 10.1021/cg1002868
Solvent-Mediated Central Metals Transformation from a Tetranuclear NiII Cage to a Decanuclear CuII “Pocket”
2010, Vol. 10 3835–3837
Fangfang Pan, Jie Wu,* Hongwei Hou,* and Yaoting Fan Department of Chemistry, Zhengzhou University, Henan 450052, China Received March 3, 2010; Revised Manuscript Received July 14, 2010
ABSTRACT: When a well-designed tetranuclear NiII cage [Ni4(bppdca)2(H2bppdca)4(SO4)2(H2O)6] 3 H2O 3 CH3OH (1; H2bppdca = N,N0 -bis(pyridin-3-yl)-2,6-pyridinedicarbox-amide) was immersed into the methanol solution of CuSO4, a novel decanuclear CuII complex [Cu10(H2bppdca)4(SO4)8(μ3OH)4(CH3OH)(H2O)4] 3 11H2O 3 2CH3OH (2) would be obtained through central metals transformation. We propose that the transformation is a recurrent dissolving-exchange-crystallization process with a solvent-mediated mechanism confirmed by visual evidence. Direct reaction of H2bppdca and CuSO4 in the absence of 1 affords different products.
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distorted octahedral geometry surrounded by one oxygen atom from one H2O and five nitrogen atoms from three ligands. In this structure, every two metal centers are bridged by one or two ligands, except Ni2 and Ni4. The four NiII centers are almost in a plane (with the mean deviation of 0.033 A˚) and are linked by the six ligands to form a cage. Two SO42- anions limited in the cavum are free, which makes the structure out of the ordinary. As for the six ligands, each of two provides five N donors, and each of the other four provides two terminal pyridine N atoms to coordinate to Ni. It is worthy of note that 1 will lose crystallinity, turning opaque in most solvents, and dissolve easily in DMF, while it is infinitesimally soluble in methanol and can remain in a crystalline state well, which is essential for investigation of the recurrent dissolvingexchange-crystallization process. In this way, we immersed complex 1 in the methanol solution of CuSO4 and left it undisturbed at ambient temperature. The macroscopical color variation after about 10 days indicates the occurrence of the reaction. Further observation by polarization microscopy reveals new tiny single crystals emerged covering the surface of 1. This also could be detected by SEM (Figures 3 and 4a). EDS (Figure 4b) and AAS (Figure S1) determinations indicate that all metal ions in the newly formed tiny crystals are CuII ions. This implies that central metal exchange takes place between a minute amount of dissolved 1 and CuII ions. Moreover, we could observe the gradual growth of the newly formed crystals on the surface of 1 with the aid of a simple microscope. Such a phenomenon demonstrates that the initial crystals act as the crystalline supporter to assist the crystallization of newly formed crystals, and alternant dissolving, reaction, as well as crystallization make the initial complex 1 convert to the newly formed CuII complex gradually. The recurrent dissolving-exchange-crystallization mechanism may be more exact for this process. About two months later, the deliberately cultivated newly formed crystals became bigger and suitable for X-ray single crystal diffraction, which reveals that this newly formed crystal presents a decanuclear CuII structure [Cu10(H2bppdca)4(SO4)8(μ3-OH)4(CH3OH)(H2O)4] 3 11H2O 3 2CH3OH (2). Both the reaction process and the great structural transformation indicate that the process from 1 to 2 undergoes a dissolvingexchange-crystallization process with a solvent-mediated mechanism, not a single-crystal to single-crystal cation-exchange transformation, since complex 1 dissolves slightly in the methanol solution of CuSO4, which is an important condition for the solvent-mediated process. The central metal exchanged product 2 is totally different from 1. As shown in Figure 5, ten CuII ions and four ligands constitute a “coordination pocket”, in which eight CuII ions form a [Cu8] cage, and the other two CuII ions lie in the cage and are passed
r 2010 American Chemical Society
Published on Web 07/21/2010
Recent years have witnessed considerable interest in the exchange-induced crystalline to crystalline phase transformation of polymeric complexes due to the theoretical significance and potential practical applications in physical and chemical adsorption, ion exchange, sensor technology, drug delivery, and so on.1-14 So far, reports about the transformation can be classified into two kinds: one is the single-crystal to single-crystal exchange process, which is dominantly claimed as a solid-state diffusion mechanism;15-26 the other is the solvent-mediated process, which was also taken for a “solid-state process”. Several examples involving the solvent-mediated process have been reported, but the exact mechanism remains unclear, and most reports are concerning anions or neutral groups,27-30 while the solventmediated process based on metal centers is relatively rare.31-33 What is more, the majority of the reported solvent-mediated transformations were accompanied by catastrophic failure in obtaining final crystals, thus preventing the identification of the exchanged products and deduction of the dynamic behavior.27-30 As is well-known, when the crystals of a complex dissolve slightly in some certain solution, the subtle dissolved complex may react with the ions in the solution. Further, the reactions break the precipitation-dissolution equilibrium, in turn unleashing the dissolving. The homogeneous phase reactions are regarded as a solvent-mediated mechanism.27,28 Nevertheless, it is quite difficult to clearly demonstrate the integrated transformation course, since the newly formed crystalline solids are always too tiny to be seen with the naked eye. Until now, it has still been an enormous challenge to obtain the crystal structures of the final exchanged products. In this work, we present the entire recurrent dissolving-exchange-crystallization process with a solventmediated mechanism for the first time from a tetranuclear NiII cage to a decanuclear CuII “pocket” (Figure 1), and we provide visual evidence for this mechanism. A tetranuclear complex {[Ni4(bppdca)2(H2bppdca)4(H2O)6] 3 2SO4 3 H2O 3 CH3OH (1)} is prepared by the reaction of N,N0 bis(pyridine-3-yl)-2,6-pyridinedicarboxamide (H2bppdca) and NiSO4 3 6H2O in a mixed solution of methanol and DMF. X-ray diffraction reveals that 1 belongs to the triclinic space group P1. As shown in Figure 2, the monomer consists of four NiII ions, six H2bppdca ligands, and six H2O. NiII ions have two coordination modes: one (Ni1/Ni3) is in a six-coordinated octahedral geometry with four nitrogen atoms (N1, N11, N21, and N26) from four ligands forming the equatorial coordination sphere and two oxygen atoms (O13, O14) from two H2O molecules along the axial direction; the other (Ni2/Ni4) is in a slightly
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Figure 1. Pre- and postexchanged crystal structures. Figure 4. (a) SEM and (b) EDS of the CuSO4-exchanged product of 1.
Figure 2. View of the structure of the tetranuclear complex [Ni4(bppdca)2(H2bppdca)4(H2O)6] 3 2SO4 3 H2O 3 CH3OH from different directions. Hydrogen atoms and solvent molecules are omitted for clarity.
Figure 5. (a) Molecular structure of the CuSO4-exchanged product [Cu10(H2bppdca)4(SO4)8(μ3-OH)4(CH3OH)(H2O)4] 3 11H2O 3 2CH3OH (2). (b) Perspective drawing of 2 showing the [Cu10] aggregate and the local coordination environment around CuII centers. Parts of the schemes are omitted for clarity.
Figure 6. View of the 1D cage-based chain structure of complex 3 along the a-axis direction.
Figure 3. Photographs (above), polarization microscopy images (middle), and SEM images (below) of 1 (left) and 2 (right).
through by the crystallographic 2-fold axis. The four H2bppdca groups are outside the [Cu8] cage and link the eight CuII ions by the terminal pyridine N donors. The ten CuII ions display four-, five-, or six-coordination modes, respectively. Eight SO42- anions and four μ3-OH groups in the “pocket” bridge the ten CuII ions. Two of the eight SO42- anions are in the η1:η1:η2:μ4 mode, bridging five CuII ions, and the rest six, linking ten CuII ions, are in the η1:η1:η1:μ3 mode. The η1:η1:η2:μ4 mode of SO42- is
quite rare.34-36 Each μ3-OH group connects three CuII ions. The structure of 2 is uncommon, and the bridging SO42- groups and μ3-OH groups generate unusual cooperativity or electron transfer between multimetal centers, further making the skeleton more stable. Direct reaction of H2bppdca and CuSO4 in the absence of 1 affords different products. {[Cu2(H2bppdca)4(SO4)2] 3 5CH3OH 3 3H2O}n (3) and [Cu10(H2bppdca)5(SO4)8(μ3-OH)4(H2O)2] 3 4EtOH 3 7H2O (4) were obtained by varying the ratios of H2bppdca to CuSO4 and solvents. Both of the two structures are different from those of the CuSO4-exchanged crystal 2. The structure of 3 (Figure 6) is a binuclear cage-based one-dimensional chain. Four ligands bridge two CuII centers and form the cages, which are linked by SO42- groups to lead to the onedimensional chain. In each cage, there is one SO42- anion trapped in by weak interactions. 4 is a reported complex, which shows also a decanuclear “coordination pocket”.37 The decanuclear structure is very similar to that of 2. The difference is that
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Crystal Growth & Design, Vol. 10, No. 9, 2010
in 4 five ligands are coordinated to the [Cu8] cage. Both complexes 3 and 4 were obtained by the reaction of CuSO4 and the ligand H2bppdca in the methanol-DMF mixed solution. The only difference was the M-ligand ratio. For 3, the M-ligand ratio was 1:2, and for 4, it was 2:1. Obviously, the M-ligand ratio influenced the crystal structures greatly. We deduced that when more metal ions existed in the system, multinuclear structures were always synthesized. What is worth mentioning is that the ligand H2bppdca only dissolves well in DMF, so DMF is essential to the direct reaction of H2bppdca and CuSO4 but unnecessary in a central metals transformation system, which creates a new environment for noble complexes being acquired by such a recurrent dissolving-exchange-crystallization process. In conclusion, in the present work, through recurrent dissolvingexchange-crystallization with solvent-mediated central metals transformation, we get a decanuclear “pocket” from a tetranuclear cage. The transformation is an inviting property of polymeric complexes, and we provide visual evidence for its mechanism. Acknowledgment. This work was funded by the National Natural Science Foundation (Nos. 20671082 and 20971110), the Program for New Century Excellent Talents of the Ministry of Education of China (NCET), The Ministry of Science and Technology of China for the International Science Linkages Program (2009DFA50620), and the Outstanding Talented Persons Foundation of Henan Province. Supporting Information Available: Experimental section, crystallographic data, and SEM and EDS images for 1-3. This material is available free of charge via the Internet at http://pubs.acs.org.
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