Hydrothermal Synthesis of Metal-Organic Frameworks Based on Aromatic Polycarboxylate and Flexible Bis(imidazole) Ligands Yan Qi, Feng Luo, Yunxia Che, and Jimin Zheng* Department of Chemistry, Nankai UniVersity, Tianjin, 300071, P. R. China
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 2 606–611
ReceiVed August 10, 2007; ReVised Manuscript ReceiVed October 25, 2007
ABSTRACT: Via hydrothermal synthesis, the self-assembly of M(II) ions, H3BTC or H2NDC with three structure-related flexible bis(imidazole) ligands, L1, L2, and L3, generated six metal-organic polymers (M ) Co, Ni, Zn; BTC ) 1,3,5-benzenetricarboxylate, NDC ) 1,2-benzenebicarboxylate, L1 ) 1,4-bis(imidazol-1-ylmethyl)benzene, L2 ) 1,1′-(1,4-butanediyl)bis(imidazole), L3 ) 1,1′(1,4-hexanediyl)bis (imidazole)): {Co3(L1)3(BTC)2(µ-H2O)3 · 2H2O}n (1), {Zn2(L2)(HBTC)2 · 2H2O}n (2), {Co(L3)(HBTC)}n (3), {Co(L1) (NDC)}n (4), {Ni(L2)(NDC)}n (5), and {Co(L3) (NDC)}n (6). The structure of 1 is the rare 4-connected self-penetrating metal-organic framework (MOF) with the (63)2(64 · 82)2(62 · 84) topology notation; polymers 2 and 6 are two-dimensional (2D) (3,4)-connected and 4-connected nets, respectively. If the O-H. . .O/2.631 Å hydrogen bonds between HBTC2- dianions are not accounted for, then polymer 3 is a 2D (3,5)-connected net; contrarily, it is a pillar-layered three-dimensional supramolecular framework characterized by (4,5)-connected (42 · 62 · 82)(42 · 68) topology. 4 and 5 show 4-connected MOFs with 65 · 10 and 65 · 8 CdSO4-type topology, respectively. Furthermore, their thermal properties are studied by thermogravimetric analysis. Introduction Crystal engineering based on metal-organic frameworks (MOFs) has recently attracted considerable interest, owing to their elegant framework topologies as well as their potential applications in molecular magnetism, catalysis, gas sorption, fluorescent sensing, and optoelectronic devices.1 Studies in this field have been focused on the design and preparation, as well as the structure–property relationships. Although big progress has been achieved,2 it is still a great challenge to predict the exact structures and composition of the assembly products built by coordination bonds and/or hydrogen bonds in crystal engineering. As we know, there are several factors, such as the coordination nature of ligand structure, counterions, and so on, which may be key for the rational design of MOFs. Therefore, systematic research on this topic is still important for understanding the roles of these factors in the formation of metal-organic coordination frameworks. Lately, a large number of MOFs based on flexible bis(imidazole) ligands have been constructed.3 Such ligands bearing alkyl spacers are a good choice of a N-donor ligand, and to hold the energetic minimum, the flexible nature of spacers allows the ligands to bend and rotate when it coordinates to metal centers, and this often causes the structural diversity. Upon careful inspection of the reported cases, we found that most of them are focused on the metal-bis(imidazole)-inorganic counterion (such as Cl-, ClO4-, SO42-, NO3-) system; by contrast, the exploration of metal-bis(imidazole)-organic counterion (such as BTC3-, NDC2-) system is less developed.4 Hence, in light of this, a series of the metal-bis(imidazole)-organic counterion compounds are prepared by the self-assembly of M(II) ions, flexible bis(imidazole) ligands (L1, L2, and L3), and H3BTC or H2NDC. Experimental Section Materials and Physical Measurements. Commercially available reagents were used as received without further purification. The ligands L1, L2, and L3 were prepared according to reported procedures.4 * Corresponding author. E-mail:
[email protected]; tel: +86-2223508056; fax: +86-22-23508056.
Elemental analysis (EA) for C, H, and N was performed on a PerkinElmer 240 analyzer. Thermogravimetric analyses were performed with a Shimadzu TGA-50H TG analyzer in the range of 30–800 °C under a nitrogen flow at a heating rate of 5 °C/min for all measurements. Synthesis of 1. A mixture of CoCl2 · 6H2O (0.24 g, 1.0 mmol), H3BTC (0.21 g, 1.0 mmol), and L1 (0.24 g, 1.0 mmol) was placed in a Teflon-lined stainless steel vessel, and then the pH was adjusted to 6.0 by addition of dilute NaOH solution (1.0 mol/L). The mixture was sealed and heated at 150 °C for 3 days, and then the reaction system was cooled to room temperature. Purple crystals were obtained in yield (based on Co): 52%. Elemental analysis (%): Calcd. for (C120H116Co6N24O34): C 51.62, H 4.19, N 12.04. Found: C 51.71, H 4.16, N 11.95. Synthesis of 2. A mixture of Zn(NO3)2 · 6H2O (0.30 g, 1.0 mmol), H3BTC (0.21 g, 1.0 mmol), and L2 (0.19 g, 1.0 mmol) was placed in a Teflon-lined stainless steel vessel, and then the pH was adjusted to 5.5 by addition of dilute NaOH solution (1.0 mol/L). The mixture was sealed and heated at 160 °C for 3 days, and then the reaction system was cooled to room temperature. Colorless crystals of 2 were obtained in yield (based on Zn): 58%. Elemental analysis (%): Calcd. for (C14H13N2O7Zn): C 43.49, H 3.39, N 7.24. Found: C 43.55, H 3.31, N 7.29. Synthesis of 3. A mixture of CoCl2 · 6H2O (0.24 g, 1.0 mmol), H3BTC (0.21 g, 1.0 mmol), and L3 (0.22 g, 1.0 mmol) was placed in a Teflon-lined stainless steel vessel, and then the pH was adjusted to 5.0 by addition of dilute NaOH solution (1.0 mol/L). The mixture was sealed and heated at 150 °C for 3 days, and then the reaction system was cooled to room temperature. Purple crystals of 3 were obtained in yield (based on Co): 43%. Elemental analysis (%): Calcd. for (C21H22CoN4O6): C 51.97, H 4.57, N 11.54. Found: C 51.90, H 4.63, N 11.59. Synthesis of 4. The reaction conditions are similar as described in 1 except that equal H2NDC was used instead of H3BTC. Purple crystals of 4 were obtained in yield (based on Co): 41%. Elemental analysis (%): Calcd. for (C22H18CoN4O4): C 57.28, H 3.93, N 12.14. Found: C 57.19, H 3.98, N 12.21. Synthesis of 5. The reaction conditions are similar as described in 2 except that equal H2NDC and NiCl2 were used instead of H3BTC and Zn(NO3)2. Green crystals of 5 were obtained in yield (based on Ni): 56%. Elemental analysis (%): Calcd. for (C36H36N8Ni2O8): C 52.34, H 4.39, N 13.56. Found: C 52.28, H 4.45, N 13.62. Synthesis of 6. The reaction conditions are similar as described in 3 except that equal H2NDC was used instead of H3BTC. Purple crystals of 6 were obtained in yield (based on Ni): 51%. Elemental analysis (%): Calcd. for (C20H22CoN4O4): C 54.43, H 5.02, N 12.69. Found: C 54.50, H 5.10, N 12.64. X-ray Structural Studies. Suitable single crystals of 1–6 were selected and mounted in air onto thin glass fibers. Accurate unit cell
10.1021/cg700758c CCC: $40.75 2008 American Chemical Society Published on Web 12/13/2007
MOFs Based on Bis(imidazole) Ligands
Crystal Growth & Design, Vol. 8, No. 2, 2008 607
Table 1. Crystallographic Data and Structure Refinement Details for 1–6
formula fw T (K) space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) Z Dc (g cm-3) V (Å-3) S R1 [I > 2 σ(I)], wR2 (all data) Flack factor
1
2
3
4
5
6
C60H58Co3N12O17 1395.97 293(2) monoclinic, P2(1)/c 14.178(3) 10.118(2) 20.936(4) 90 99.79(3) 90 2 1.566 2959.6(10) 1.024 0.0533, 0.1139 -0.008(10)
C28H26N4O14Zn2 386.63 298(2) triclinic, P1j 8.7582(18) 8.8731(18) 10.210(2) 95.59(3) 100.37(3) 102.60(3) 1 1.703 753.9(3) 1.105 0.0271, 0.0788
C21H22CoN4O6 485.36 293(2) triclinic, P1j 8.2544(17) 10.044(2) 12.819(3) 96.61(3) 102.07(3) 90.54(3) 2 1.562 1031.8(4) 1.031 0.0515, 0.1291
C22H18CoN4O4 461.33 293(2) monoclinic, Cc 10.452(2) 18.951(4) 10.550(2) 90 103.78(3) 90 4 1.510 2029.5(7) 1.042 0.0282, 0.0688
C18H18N4NiO4 413.08 298(2) monoclinic, C2/c 15.389(3) 9.4636(19) 12.728(3) 90 104.98(3) 90 4 1.532 1790.7(6) 1.036 0.0263, 0.0657
C20H22CoN4O4 441.35 293(2) monoclinic, P2(1)/c 11.409(2) 10.998(2) 17.116(6) 90 112.34(2) 90 4 1.476 1986.5(9) 1.038 0.0483, 0.1178
Scheme 1
parameters were determined by a least-squares fit of 2θ values, and intensity data were measured on a Rigaku R-AXIS RAPID IP area diffractometer with Mo KR radiation (λ ) 0.71073 Å) at room temperature. The intensities were corrected for Lorentz and polarization effects as well as for empirical absorption based on multiscan technique; all structures were solved by direct methods and refined by full-matrix least-squares fitting on F2 by SHELX-97. All non-hydrogen atoms were refined with anisotropic thermal parameters. Crystallographic data for the three compounds are summarized in Table 1.
Results and Discussion Synthesis. In this system, the formation of them is sensitive to the pH value of solution. For all of them, if pH value was >6 or
