A Reversible Octahedral to Trigonal-Bipyramidal Cobalt Coordination Change in an Aquo-Accessible Coordination Network Shu-Juan Fu, Ching-Yuan Cheng, and Kuan-Jiuh Lin* Department of Chemistry, Center of Nanoscience and Nanotechnology, National Chung Hsing UniVersity, Taichung, 402, Taiwan
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 8 1381-1384
ReceiVed April 13, 2007; ReVised Manuscript ReceiVed June 5, 2007
ABSTRACT: An aquo-accessible chromatic change in a crystal was induced by means of a reversible coordination geometry shift from octahedra in [Co(C12H8N2)(HO3P-C2H4-PO3H)(OH2)2] (1) to trigonal-bipyramid in [Co(C12H8N2)(HO3P-C2H4-PO3H)] (2) at the cobalt(II) center; the crystal-to-crystal phase transition was confirmed by powder X-ray diffraction and solid-state UV-vis diffuse reflectance spectra. Crystal-to-crystal studies that involve guest removal and readsorption in coordination networks are a particular interest for creating open metal-organic frameworks,1 which have promising applications in catalysis, gas storage, and molecular separation.2,3 The change in coordination geometry of the metal center, which exhibits tunable physiochemical properties, plays an important role in the energetic crystal-to-crystal phase transition.4,5 However, reversible interchangeable coordination geometries are seldom observed due to the drastic structural changes that occur during the transition process.6 Here we report two cobalt(II) coordination networks, [Co(C12H8N2)(HO3P-C2H4-PO3H)(OH2)2] (1) and [Co(C12H8N2)(HO3P-C2H4-PO3H)] (2), whereby aquo-ligands in 1 can be reversibly coordinated to the cobalt center in a crystal-to-crystal fashion, accompanied by interchangeable coordination geometries from octahedra (CN ) 6) in 1 to trigonal-bipyramid (CN ) 5) in 2 at the cobal(II) center. The hydrothermal reaction of CoCl2, ethylenediphosphonic acid, and 1,10-phenanthroline (phen) in a molar ratio of 2:3:1 at 200 °C for 48 h in water produced pink crystals of 1.7 This hydrothermal method gives a yield of 25% based on CoCl2 and is highly reproducible. The structure of 1 was determined by single-crystal X-ray analysis,8 and the phase purity of the bulk materials of 1 was independently confirmed by powder X-ray diffraction pattern.9 The structure (Figure 1a) reveals infinite zigzag chains, [-CoHO3P-(CH2)2-PO3H-Co-], in which the phen and water ligands are bound to the metal centers. Moreover, the incorporated phen ligands play an important role in the construction of a π-stacked layer structure between adjacent crystallographically symmetryrelated chains (see Figure S1, Supporting Information). The corrugated π-sheet layers parallel to [001] are pillared via axial coordination of water ligands that formed hydrogen-bonded chains (d(O1‚‚‚O4) ) 2.723(4) Å). The cobalt(II) ion displays near octahedral coordination; two water molecules are coordinated to the cobalt(II) center at the apical position and two nitrogen atoms from phen and two oxygen atoms from two crystallographic organophosphonate are at the equatorial position. Moreover, within the zigzag chain, it consists of diphosphonic hydrogen bonds with d(PO(H)‚‚‚OP) ) 2.555(4) Å. The striking feature of 1 rests on the coordinated water molecules. It is obvious that 1 exhibits a typical example of chromatic changes; it undergoes a reversible color change from pink to purple upon coordinated water removal and readsorption. Purple crystals of 2 were grown under solvothermal conditions.10,11 The crystal structure of 2 (Figure 1b) shows that the cobalt(II) ion switches to a distorted trigonal-bipyramidal geometry (CN ) 5); two nitrogen atoms from phen and three oxygen atoms are from three crystallographic organophosphonates.12 The trigonal-bipyramidal {CoN2O3} polyhedrons are corner-sharing with chelating and bridging ethylenediphosphonate ligands in a 3D coordination network structure (see Figure S2, Supporting Information). * To whom correspondence should be addressed. Fax: (+886)-42870515. E-mail:
[email protected];
[email protected].
The aquo-ligands can be reversibly coordinated to the cobalt center in a crystal-to-crystal fashion, as confirmed by powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA). First, the color of crystals of 2 immediately returned to pink upon addition of one drop of water. In situ PXRD patterns in Figure 2 provide unambiguous evidence that the structure of 2 gradually changed into 1 because the positions of the diffraction peaks for crystal 2 exposed to moisture (72 h in 40% moisture) are in good agreement with those of 1. Furthermore, PXRD patterns of the study of reversal transformation of 1 into 2 are shown in Figure 3. It is found that the aquo-phase in 1 was thermally stable at least up to 176 °C due to the same PXRD patterns (Figure 3a,b). When the sample of 1 was heated to 200 °C, two significant diffraction peaks at low 2θ < 10° were generated (Figure 3c), which clearly shows that a structural arrangement takes place in the phase transition. In fact, the PXRD pattern of dehydrated 1 at 200 °C is not consistent with that of crystal 2, implying that the local site symmetry of cobalt ion is not entirely in trigonal bipyramidal geometry. According to EPR analysis (see Figure S3, Supporting Information), the g| > g⊥ result clearly establishes that square-pyramidal geometry is preserved in dehydrated 1.13 To pursue a pathway for crystal-tocrystal transformation of 1 into 2, we attempted to explore a simple solvothermal process to accomplish the dehydration of crystal 1. After crystals of compound 1 were heated in dry n-butyl alcohol (10 mL) at 200 °C in a Teflon-lined stainless autoclave, the color of crystal 1 changed from pink to purple; the PXRD pattern shown in Figure 3d is in good agreement with that calculated from singlecrystal data of 2. Finally, it is obvious that the positions of the diffraction peaks for the reabsorbed aquo-accessible phase (Figure 3e) compared well with those in 1, showing that the returned crystal structure remains essentially unchanged. As a result, we suggest that these sheet-like layers in 1 can slide on each other to give a corner-sharing trigonal-bipyramid running along the a-axis in 2 (see Figure 1 and S1b,c, Supporting Information).14 Moreover, a TGA profile of 1 shows three weight loss steps. Release of all coordinated water molecules takes place in the first step in the temperature range of 200-330 °C. The weight loss of 8.33% is somewhat larger than that of the X-ray structure (7.77%), indicating that the moisture absorbed onto the surface of crystals through hydrogen bonding. It is noteworthy that the TGA profile for the reabsorbed aquo-accessible phase is similar to that of 1 (see Figure S4, Supporting Information). The chromatic change of the crystals is principally ascribed to the energy-level change of the d-orbitals induced by the interchangeable coordination geometries of the cobalt center. Therefore, the interconversion of the trigonal-bipyramidal and octahedral geometries was also observable by in situ UV-vis diffuse reflectance spectroscopy (DRS). The original spectrum of crystal 1 exhibits a major peak at λmax ) 534 nm and a subtly discernible shoulder around 468 nm (Figure 4f, inset). The absorption feature is most likely d-d transitions, typical of an octahedral geometry. The band at 534 nm could be attributed to the 4T1g f 4T1g(P) transition.15 Upon removal of ligand water
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Figure 1. Crystal structure of (a) 1 and (b) 2. Structural analyses suggest that the first step is the destruction of diphosphonate H-bonds, and the second step is the sliding of the layers during the phase transition from 1 to 2. Notably, the crystalline nature was not destroyed during the process. Selected bond lengths [Å] and angles [°]: 1: Co1-O3 2.062(2), Co1-O1 2.149(3), Co1-N1 2.154(3), O3-Co1-O3A 100.5(1), O3A-Co1-O1 84.44(12), O3-Co1-N1 92.83, O1-Co1-O1A 176.6(2), O4C‚‚‚O2B 2.556 (3). 2: Co1-O2 1.979(3), Co1-O5C 1.983(4), Co1-O4 2.022(3), Co1-N2 2.111 (4), Co1-N1 2.186(4), O2-Co1-O5C 121.21(16), O2-Co1-O4 99.84(14), O4-Co1-O5C 100.16(15), O2-Co1-N2 120.7(2), O2-Co1-N1 82.7(2).
Figure 2. Powder X-ray diffraction patterns (synchrotron radiation λ ) 1.33443 Å) of (a) 1, (b) 2, and (c) reabsorbed water-accessible phase; exposure of crystal 2 to the 40% moisture for 72 h.
molecules in crystal 1 by the heating treatment at 200 °C for 0.5 h, the DRS spectrum (Figure 4a) of dehydrated 1 is similar to that of crystal 2 (Figure 4e, inset), in which the DRS spectrum (Figure 4a) displays optical bands at λ(sh) ) 468; λmax ) 504, 534, and 624 nm. Specifically, the band at 624 nm could be attributed to the 4A2′(F) f 4E′(F) transition of trigonal-bipyramidal geometry.16 Upon gradually diffusing the vapor of water molecules, the optical characteristic of the five-coordinated band (624 nm) significantly decreases in intensity throughout the water-accessible process with time. After an exposure time of 920 s, not only did the transition band of 624 nm totally vanish, but the comparable DRS spectrum
Figure 3. Temperature-dependence of powder X-ray patterns (PXRD) for 1 at (a) 25 °C, (b) 176 °C, (c) 200 °C, and (d) sample 1 heated at 200 °C in dry n-butyl alcohol. (e) PXRD pattern for reabsorbed water-accessible phase; the sample of 1 was heated at 200 °C for 0.5 h and then cooled to room temperature in a H2O vapor/N2 carrier (synchrotron radiation, λ ) 1.32633 Å).
of the aquo-phase (Figure 4d vs Figure 4f) was investigated again. This result may indicate smooth conversion of trigonal-bipyramid in 2 into an aquo-octahedron in 1. Moreover, FTIR microscopy is suited for parallel analysis by measuring the temperature dependence of molecular vibration. The FTIR spectrum of 1 clearly shows that the characteristic stretching ν(O-H) modes are found at 3460, 3296, and 3187 cm-1, in which relatively sharp features are attributed to the presence of a strong
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Figure 4. Time-dependence of UV-vis diffuse reflectance spectroscopy (DRS) for 1 (a) upon heating at 200 °C, exposure time of (b) ) 420 s, (c) 515 s, and (d) 920 s. The sample was initially heated to 200 °C and then cooled to room temperature in a dry N2 flux. In situ DRS data, taken in a H2O vapor/N2 carrier at room temperature, indicate continuous changes in the F(R) with exposure time. The relative reflectance for a powder, F(R), is given by Kubelka-Munk equation (F(R) ) (1 - R)2/2R, where R is the ratio of reflected intensity of the sample to that of a nonabsorbing standard. The inset shows DRS for (e) crystal 2 and (f) crystal 1.
hydrogen-bonding network involving coordinated water molecules and the diphosphonate H-bonds in the solid. These significant features are totally removed when the sample is heated up to 200 °C, indicating that the diphosphonate H-bonds in the 1D backbone are considered to be simultaneously thermally destroyed upon removal of coordinated water molecules (see Figure S5, Supporting Information). In conclusion, a switch from octahedral to trigonal-bipyramidal structure of a metal center in a controlled and reversible fashion was herein reported in a crystal-to-crystal fashion. The present energetic construction procedure from trigonal-bipyramid (CN ) 5) in 2 to octahedron (CN ) 6) in 1 at the cobalt(II) center is quite effective for accessible water molecules. Owing to the presence of square-pyramidal geometry in the process of thermal dehydration of crystal 1, the phase transformation of crystal 1 to 2 was accomplished in dry n-butyl alcohol solvothermal conditions. Acknowledgment. This work was supported by the National Science Council of Taiwan (NSC 94-2113-M005-019; NSC-942120-M007-002) and Ministry of Education of Taiwan Supporting Information Available: X-ray crystallographic information files (CIF) for 1 (CCDC no. 628330) and 2 (CCDC no. 628331). These materials are available free of charge via Internet at http://pubs.acs.org.
References (1) (a) Rao, C. N. R.; Natarajan, S.; Vaidhyanathan, R. Angew. Chem., Int. Ed. 2004, 43, 1466-1496. (b) Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem., Int. Ed. 2004, 43, 2334-2375. (c) Li, H.; Eddaoudi, M.; O’Keeffe, M.; Yaghi, O. M. Nature 1999, 402, 276-279. (d) Davis, M. E. Nature 2002, 417, 813-821. (2) (a) Fujita, M.; Kwon, Y. J.; Washizu, S.; Ogura, K. J. Am. Chem. Soc. 1994, 116, 1151-1152. (b) Seo, J. S.; Wand, D.; Lee, H.; Jun, S. I.; Oh, J.; Jeon, Y.; Kim, K. Nature 2000, 404, 982-986. (3) (a) Min, K. S.; Suh, M. P. Chem. Eur. J. 2001, 7, 303-313. (b) Bradshaw, D.; Prior, T. J.; Cussen, E. J.; Claridge, J. B.; Rosseinsky, M. J. J. Am. Chem. Soc. 2004, 126, 6106-6114. (4) (a) Chui, S. S. Y.; Lo, S. M. F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D. Science 1999, 283, 1148-1150. (b) Kepert, C. J.; Rosseinsky, M. J. Chem. Commun. 1999, 375-376
(5) (a) Biradha, K.; Fujita, M. Angew. Chem., Int. Ed. 2002, 41, 33923395. (b) Suh, M. P.; Ko, W. J.; Choi, H. J. J. Am. Chem. Soc. 2002, 124, 10976-10977. (c) Takamizawa, S.; Nakata, E.; Saito, T. Angew. Chem., Int. Ed. 2004, 43, 1368-1371. (d) Abrahams, B. F.; Moylan, M.; Orchard, S. D.; Robson, R. Angew. Chem., Int. Ed. 2003, 42, 1848-1851. (6) (a) Rather, B.; Zaworotko, M. J. Chem. Commun. 2003, 830-831. (b) Lee, E. Y.; Suh, M. P. Angew. Chem., Int. Ed. 2004, 43, 27982801. (c) Takamizawa, S.; Nakata, E.; Yokoyama, H.; Mochizuki, K.; Mori, W. Angew. Chem., Int. Ed. 2003, 42, 4331-4334. (d) Hanson, K.; Calin, N.; Bugaris, D.; Scancella, M.; Sevov, S. C. J. Am. Chem. Soc. 2004, 126, 10502-10503. (e) Takaoka, K.; Kawana, M.; Tominaga, M.; Fujita, M. Angew. Chem., Int. Ed. 2005, 44, 2151-2154. (f) Lin, K. J.; Fu, S. J.; Cheng, C. Y.; Chen, W. H.; Kao, H. M. Angew. Chem., Int. Ed. 2004, 43, 4186-4189. (g) Uemura, K.; Kitagawa, S.; Kondo, M.; Fukui, K.; Kitaura, R.; Chang, H. C.; Mizutani, T. Chem. Eur. J. 2002, 8, 3586-3600. (7) Synthesis of C14H18CoN2O8P2 (1): The reaction was carried out in a 3-mL Teflon-lined acid digestion bomb (Parr), heated in a programmable electric furnace (Lindbergy/Blue). A reaction mixture of CoCl2‚6H2O (0.1189 g, 0.5 mmol), ethylenediphosphonoic acid (0.1421 g, 0.75 mmol), 1,10-phenanthroline monohydrate (0.0495 g, 0.24 mmol), CsOH (99%, 50 wt % solution in water, 0.1 mL), and H2O (8 mL) was placed in a 23 mL Teflon-lined stainless autoclave, which was sealed and heated at 100 °C for 2 h at 120 °C h-1, heated at 200 °C for 48 h, cooled to 70 °C at 9 °C h-1, and then allowed to cool to room temperature (final pH < 1). The resulting pink crystals were collected by filtration and washed with deionized water. (8) Crystal data for C14H18CoN2O8P2 (1) (pink): monoclinic, T ) 298 K, space group C2/c, a ) 12.237(2) Å, b ) 15.295(3) Å, c ) 9.6444(19) Å; β ) 108.791(4)°; V ) 1708.9(6) Å3, Z ) 4, F(000) ) 948, Mo KR radiation (λ ) 0.71069 Å), σ ) 1.80 Mg m-3, 2θmax ) 55°; R1 ) 0.0469, wR2(F2) ) 0.1091, and GOF ) 1.025. (9) It is noteworthy that the simulated diffraction patterns based on the analysis of a single-crystal X-ray structure are in good agreement with the PXRD patterns obtained for 1, which indicates that the sample of 1 is a pure phase. (10) Crystals of 2 were grown in the process of solvothermal treatment: (a) Purple crystals of C14H12CoN2O6P2 (2) were directly obtained after the solvothermal process in which crystals of 1 were heated at 200 °C under dry n-butyl alcohol (10 mL). PXRD patterns show that the structure of 1 was entirely changed into compound 2. (b) Purple 2 was also obtained by the following solvothermal synthesis. A reaction mixture of CoCl2‚6H2O (0.1189 g, 0.5 mmol), ethylenediphosphonoic acid (0.1421 g, 0.75 mmol), 1,10-phenanthroline monohydrate (0.0495 g, 0.24 mmol), KOH/MeOH (0.3 mL, 1.5 M), and dry ethanol (8 mL) was placed in a 23 mL Teflon-lined stainless autoclave, which was sealed and heated at 100 °C for 2 h at 120 °C h-1, heated at 200 °C for 48 h, cooled to 70 °C at 9 °C h-1, and then allowed to cool to room temperature (final pH < 7). The resulting purple crystals were collected by filtration and washed with deionized water. The yield was 67% based on CoCl2‚6H2O. Anal. Found: H, 3.79; C, 38.87; N, 6.21. Calcd : H, 3.22; C, 38.70; N, 6.45. (11) Crystal data for C14H12CoN2O6P2 (2) (purple): triclinic, space group P1h, a ) 5.5422(6) Å, b ) 10.9953(12) Å, c ) 12.9331(31) Å; R ) 86.572 (2), β ) 108.791(4), γ ) 84.112(2); V ) 775.71(14) Å3, Z ) 2, F(000) ) 434, Mo KR radiation (λ ) 0.71069 Å), σ ) 1.829 mg m-3, 2θmax ) 55°; R1 ) 0.0547, wR2(F2) ) 0.1399, and GOF ) 1.025. (12) Crystal structure of 2 shows that the cobalt(II) ion is distorted trigonal-bipyramidal geometry (CN ) 5); the axial positions contain one nitrogen atom of phen and one oxygen atom of a organophosphonate ligand with a trans N1-Co1-O4 angle of 171.17(14)°, close to the ideal value of 180°. The equatorial Co1-N2 bonds of Co1 are somewhat shorter at 2.111(4) Å than the axial Co1-N1 bonds (2.186(4) Å). The N2-Co-O5C angle (111.21(19)) of the equatorial plane is compressed from the ideal trigonal value of 120°, and the O2-Co-O5C (121.21(16)) angle of this plane is slightly expended. Confirmed by the τ value, τ ) (β- R)/60, where β and R are the two largest coordination angles, the structure index has a value of 0.84 (τ ) 0 for square pyramidal and τ ) 1 for trigonal bipyramidal geometry); see Higgs, T. C.; Spartalian, K.; O’Connor, C. J.; Matzanke, B. F.; Carrano, C. J. Inorg. Chem. 1998, 37, 2263-2272.
1384 Crystal Growth & Design, Vol. 7, No. 8, 2007 (13) (a) Jacobsen, F. E.; Breece, R. M.; Myers, W. K.; Tierney, D. L.; Cohen, S. M. Inorg. Chem. 2006, 45, 7306-7315. (b) Jenkins, D. M.; Bilio, A. J. D.; Allen, M. J.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2002, 124, 15336-15350. (c) Solomon, E. I.; Lever, A. B. P. Inorganic Electronic Structure and Spectroscopy; WileyInterscience: New York, 2006; Vol. I, Chapter, pp 93-159. (14) Lu, J.; Paliwala, T.; Lim, S. C.; Yu, C.; Niu, T.; Jacobson, A. J. Inorg. Chem. 1997, 36, 923-929.
Communications (15) Jankovics, H.; Daskalakis, M.; Raptopoulous, C. P.; Terzis, A.; Tangoulis, V.; Giapintzakis, J.; Kiss, T.; Salifoglou, A. Inorg. Chem. 2002, 41, 336-3374. (16) Makowska-Grzyska, M. M.; Szajna, E.; Shipley, C.; Arif, A. M.; Mitchell, M. H.; Halfen, J. A.; Berreau, L. M. Inorg. Chem. 2003, 42, 7472-7488.
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