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Metal–organic frameworks constructed from three kinds of new Fe-containing secondary building units
Inorganica Chimica Acta 384 (2012) 219–224
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Metal–organic frameworks constructed from three kinds of new Fe-containing secondary building units Tian-zhan Zhang, Ying Lu ⇑, Yang-guang Li, Zhiming Zhang, Wei-lin Chen, Hai Fu, En-bo Wang ⇑ Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal University, Renmin Street No. 5268, Changchun, Jilin 130024, PR China
a r t i c l e
i n f o
Article history: Received 6 June 2011 Received in revised form 30 November 2011 Accepted 2 December 2011 Available online 11 December 2011 Keywords: Metal–organic framework Fe-containing second building unit Magnetic properties Structure elucidation
a b s t r a c t Three new Fe-containing MOFs, H3O[Fe5(BTC)3(OAc)2(DMF)2]H2O 1, [FeNa(m-BDC)2]NH2(CH3)2 2 and [N(CH3)4]2[Fe3(HBTC)(BTC)2(H2O)]5.5H2O 3 (BTC = 1,3,5-benzenetricarboxylate, m-BDC = 1,3-benzenedicarboxylate), have been synthesized from the solvothermal reactions of iron salts and carboxylate ligands. The framework of 1 is constructed from two kinds of secondary building units (SBUs): the sinusoidal chain of iron octahedra SBU [Fe2(COO)6]n2n and paddle-wheel SBU [Fe2(COO)4(DMF)]. Compound 1 represents the first example of Fe-containing MOFs constructed from two kinds of SBUs. The framework of 2 is built from zigzag-chain SBUs [FeNa(COO)6]n3n and m-BDC linkers. The framework of 3 is formed by linear tri-nuclear iron SBUs [Fe3(COO)8(H2O)]2 and triangular BTC linkers. To our knowledge, the Fe-containing sinusoidal chain SBU in 1, zigzag-chain SBU in 2 and linear tri-nuclear iron SBU in 3 are found in Fe-containing MOFs for the first time. Magnetic investigation indicates the present of antiferromagnetic exchange interaction within the iron units of compounds. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Metal–organic frameworks (MOFs) are of great contemporary interest due to their versatile functional properties with important applications in areas such as catalysis, gas separation and storage, luminescence, magnetism and ion exchange [1–7]. Although the research on MOFs has been very fruitful, controlling the design of the frameworks to obtain materials with desired properties is still a challenge [8–12]. Most MOFs are constructed from the interconnection of polynuclear metallic units known as the secondary building units (SBUs) with organic ligands through strong covalent bonds. The current studies on MOFs have indicated that choosing a suitable SBU is very critical for the achievement of MOFs with required properties, not only due to the structural properties of SBUs have significant influence on the topology and stability of MOFs, but also the functional properties (such as magnetism, luminescence) of SBUs can be imparted to the whole framework. Moreover, once a new SBU fitting for the built of MOFs with desired properties is found, a series of frameworks is promising through combining it with a variety of organic linkers using the reticular chemistry concepts, such as the well studied MOF system based on the copper paddle-wheel and the basic zinc acetate SBUs [13–19].
⇑ Corresponding authors. Tel./fax: +86 431 85098787 (E.-b. Wang). E-mail addresses: [email protected] (Y. Lu), [email protected] (E.-b. Wang). 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.12.006
Polynuclear iron units have attracted much attention owing to its biological function, catalytic and magnetic properties [20–26]. Designing MOFs based on Fe-containing SBUs may be a promising way to the achievement of high-efficiency biomimetic catalyst and magnetic porous materials [23,27]. However, compared to the extensive research on copper- and zinc-containing MOFs, the studies on iron-containing MOFs is very limited [28–35]. The main reason is that the high trend of iron ion to undergo hydrolysis into a stable polymeric hydrous iron oxide makes its synthesis more difficult than that of copper- and zinc-containing MOFs. Until now, only several kinds of Fe-containing SBUs have been found in the structure of MOFs, which include paddle-wheel Fe2 unit, triangle Fe3 unit, square-planar and prismatoid Fe4 unit, iron oxide octahedral chain and helical chain of iron octahedra [16,36–38]. On the basis of the above points, we dedicate to the exploration of new Fe-containing SBUs and corresponding MOFs. At the present work, we focus on the iron-BTC system under solvothermal conditions. From this system, we successfully obtained there new Fe-containing MOFs, H3O[Fe5(BTC)3(OAc)2(DMF)2]H2O (1), [FeNa(m-BDC)2]NH2(CH3)2 (2), and [N(CH3)4]2[Fe3(HBTC) (BTC)2(H2O)]5.5H2O (3). The framework of 1 is constructed from two kinds of SBUs: the sinusoidal chain of iron octahedra SBU and paddlewheel SBU. The framework of 2 is built by zigzag-chain SBUs and m-BDC linkers. The framework of 3 is formed by linear tri-nuclear iron SBU [Fe3(COO)8(H2O)]2 and triangular BTC linkers. As far as we know, the three kinds of Fe-containing SBUs presenting in compounds 1–3, sinusoidal chain SBU, zigzag-chain SBU and linear trinuclear iron SBU, are found in Fe-containing MOFs for the first time.
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by slow cooling to room temperature. After being washing with DMF several times, 3 was collected as yellow octahedron crystals with a yield of about 91% (based on Fe). Elem. Anal. Calc.: Fe, 17.1; C, 42.6; H, 4.0; N, 28.5. Found: Fe,16.8; C, 42.7; H, 3.9; N, 28.1%.
2. Experimental 2.1. Materials and measurement All reagents were purchased commercially and used without further purification. Elemental analyses (C, H, and N) were performed on a Perkin–Elmer 2400 CHN elemental analyzer and a PLASMA– SPEC (I) ICP atomic emission spectrometer (Fe). The IR spectra were obtained on an Alpha Centaurt FT-IR spectrometer in the 400– 4000 cm1 region with a KBr pellet. The TG for 1 and 2 were performed on a Perkin–Elmer TGA7 instrument under flowing O2 with a heating rate of 10 °C min1. The TG for 3 were performed on a Perkin–Elmer TGA7 instrument under flowing N2 with a heating rate of 10 °C min1. Magnetic susceptibility data were collected over the temperature range of 2–300 K at a magnetic field of 1000 Oe on a Quantum Design MPMS-5 SQUID magnetometer. 2.2. Syntheses 2.2.1. Synthesis of compound 1 The mixture of FeCl34H2O (0.3 mmol, 0.04 g), 1,3,5-H3BTC (0.45 mmol, 0.1 g), HAc (1.5 mL) and DMF (4.5 mL) was sealed in a Teflon-lined autoclave and heated at 150 °C for 1.5 days, followed by slow cooling to room temperature. After being washing with DMF several times, 1 was collected as dark yellow block crystals with a yield of about 62% (based on Fe). Elem. Anal. Calc.: Fe, 24.2; C, 35.3; H, 1.9; N, 1.2. Found: Fe, 23.8; C, 35.6; H, 1.8; N, 1.2%. 2.2.2. Synthesis of compound 2 The mixture of FeCl24H2O (0.2 mmol, 0.04 g), m-H2BDC (0.5 mmol, 0.163 g), NaOH (1 mmol, 0.04 g) and DMF (10 mL) was sealed in a Teflon-lined autoclave and heated at 130 °C for 5 days, followed by slow cooling to room temperature. After being washing with DMF several times, 2 was collected as dark yellow rod crystals with a yield of about 72% (based on Fe). Elem. Anal. Calc.: Fe, 12.9; Na, 6.6; C, 47.1; H, 1.8; N, 3.2. Found: Fe, 12.9; Na, 6.6; C, 46.9; H, 1.8; N, 3.3%. 2.2.3. Synthesis of compound 3 The mixture of FeCl3 (0.2 mmol, 0.032 g), H3BTC (0.5 mmol, 0.2 g), TMAOH (0.7 mmol, 0.132 g) and DMF (10 mL) was sealed in a Teflon-lined autoclave and heated at 150 °C for 4 days, followed
2.3. X-ray crystallography study The crystallographic data were collected at 298 K (for 1 and 2) and 150 K (for 3) on the Rigaku R-axis Rapid IP diffractometer 0 (Mo Ka radiation l = 0.71073 Å A) and IP technique. Mult-scan absorption correction was applied. The crystal data were solved by the direct method and refined by the full-matrix least-squares method on F2, using the SHELXTL-97 crystallographic software package [39]. All non-hydrogen atoms were refined anisotropically. In compound 1, the atom O12 of coordination DMF was disordered over two positions with each occupancy of 0.5. In compound 2, the one carboxyl group of BDC ligand was disordered over two positions with each occupancy of 0.5, respectively. And C9 and C10 of the dimethylamine cation were disordered over two positions, respectively. In compound 3, the disordered component in channels appears to consist primarily of tetramethyl ammonium ions. This is evident from the need for the material to charge balance, but also from the structure visible in the difference map for the void region. From this it is evident that there are at least two N(CH3)4+ present and concerted efforts were made to refine this model [40]. In compound 1, the hydrogen atoms were placed geometrically on the carboxyl ligands and DMF molecules. In compound 2, the hydrogen atoms associated with m-BDC in compound 2 were also placed geometrically and refined in a riding model. H atoms attached to the disordered protonated dimethylamine cations were not add. In compound 3, the hydrogen atoms attached to carbon atoms were fixed in ideal positions, while the hydrogen atoms attached to the water molecules were not located. The detailed crystal data and structure refinement for 1, 2 and 3 were given in Table 1. Selected bond lengths of 1, 2 and 3 were listed in Tables S1–S3, respectively. 3. Result and discussion 3.1. Synthesis Compounds 1–3 were synthesized via self-assembly processes under mid-temperature solvothermal conditions. Compound 1
Table 1 Crystal data and structure refinements for compounds 1, 2 and 3.
a b
Compounds
1
2
3
Formula M T (K) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (g cm3) Measured reflections Independent reflections Data/restraints/parameters Goodness of fit (GOF) S Final R indicts [I > 2r(I)]a R indices (all data)b
C17H11.25Fe2.50N0.50O13.50 578.14 293(2) monoclinic C2/c 33.242(7) 12.792(3) 14.120(3) 90 102.32(3) 90 5866(2) 4 1.385 22 000 5161 5161/18/306 1.07 R1 = 0.0690, wR2 = 0.2135 R1 = 0.0825, wR2 = 0.2256
C68H32Fe4N4Na4O32 1732.34 293(2) monoclinic C2/c 13.395(3) 14.354(3) 11.319(2) 90 115.55(3) 90 2024.2(7) 4 1.461 9769 2327 2327/24/170 1.09 R1 = 0.0362, wR2 = 0.0994 R1 = 0.0428, wR2 = 0.1040
C35H39Fe3N2O20.50 983.23 150(2) monoclinic P21/c 18.1258(15) 14.1372(2) 26.6897(16) 90 131.776(3) 90 5184.8(7) 4 1.256 25 581 9129 9129/32/495 1.126 R1 = 0.0878, wR2 = 0.2282 R1 = 0.1550, wR2 = 0.2583
R1 = R||Fo| |Fc||/R|Fo|. wR2 = {R[w(Fo2 Fc2)2]/R[w(Fo2)2]}1/2.
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was synthesized from the reaction of FeCl3, H3BTC and HAc in DMF at 150 °C for 1.5 days. Parallel experiments showed that the ratio of the HAc and DMF played a significant role in the formation of compound 1. Compound 1 could be gotten in the ratio range from 1:2.5 to 1:3. At ratio 1:3, good block crystals suiting for single crystal determination were gotten. At ratio 1:2.5, small crystals of 1 were formed in pure phase. Compound 2 was synthesized from the reaction of FeCl29H2O, m-BDC and NaOH in DMF at 130 °C for 5 days. In the structure of 2, there exists dimethylamine cations, which is believed to be formed by the decarboxylation of DMF solvent under solvothermal conditions [41–43]. When the reactions were carried out in other solvents such as ethanol, tetrahydrofuran and acetonitrile, the crystals of 2 could not be obtained. However, when the reactions were performed by the addition of dimethylamine in the reaction system and using ethanol to replace DMF as solvent, the crystals of 2 could be also obtained. Compound 3 was synthesized from the reaction of FeCl3, H3BTC and TMAOH (TMAOH = tetramethylammonium hydroxide) in DMF at 150 °C for 4 days. Parallel experiments showed that TMAOH was essential for the formation of 3. When TMAOH was absent or replaced by other organic base (tetraethylammonium hydroxide and tetrabutylammonium hydroxide) in the reaction mixture, compound 3 could not be obtained. TMAOH may play a structure-directing agent role in the formation of 3. Although anhydrous ferric chloride was used for the synthesis of 3, there exits water molecules in the structure of 3. Water molecules contained in the final products of 3 may be derived from the –OH of TMAOH or a little content of water in the solvents. In addition, whatever the oxidation states of Fe in the starting materials are +2 (in 2) or +3 (in 1 and 3), the oxidation state of Fe is +2 in all the formed MOFs. However, no reductant was added in reaction systems during the processes of synthesizing 1–3. So it was speculated that the dimethylamine formed by the decomposition of DMF played the reductant role, which prevented the oxidation of Fe(II) and reduced Fe(III) to Fe(II). It is noteworthy that compounds 1 and 3 could not be gotten when were synthesized directly from ferrous salts. 3.2. Structure description 3.2.1. Crystal structure of compounds 1 The 3D framework of 1 is constructed by two kinds of SBUs: one is the sinusoidal chain of iron octahedra [Fe2(COO)6]n2n, the other is the paddlewheel unit [Fe2(COO)4(DMF)] (Fig. 1). The sinusoidalchain SBU contains two crystallographically independent iron atoms (Fe1 and Fe2), both of which has six-coordinated distorted octahedral geometry (Fig. S1). Each Fe1 atom is coordinated by four oxygen atoms from four different BTC ligands and two oxygen atoms from two different acetates, while each Fe2 atom is coordinated by three oxygen atoms from three different BTC ligands, two oxygen atoms from one acetate and one oxygen atom from the other acetate. In the paddlewheel SBU, the two crystallographically equivalent Fe atoms (Fe3) are five-coordinated in a square-pyramidal geometry with four equatorial carboxyl oxygen atoms from four different BTC ligands and an axial oxygen atom from a DMF molecule (Fig. S2). The bond lengths of Fe–Ocarboxyl are in the range of 2.012–2.352 Å and Fe–Odmf is 2.068 Å. The valence sum calculations on the iron sites give the values 2.2 for Fe1, 2.3 for Fe2 and 2.2 for Fe3, which indicate that all the Fe in 1 are in the +2 oxidation state [44]. As the linkers of the 3D framework of 1, BTC ligands can be divided into two types according their different connecting modes with SBUs. One kind of BTC ligand links with two sinusoidal chain SBUs, which bridges adjacent sinusoidal chains to form the 2D layer (Layer A) (Fig. S3). The other kind of BTC ligand connects with two paddlewheel SBUs and one sinusoidal chain SBU, which bridges adjacent paddlewheel SBUs to generate the 2D (4, 4) layer (Layer B) (Fig. S4), and further bridges the alternating Layer A and Layer B into a 3D framework (Fig. 2). This organization leads to the
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Fig. 1. The polyhedral view of the SBUs in compounds 1–3: (a) the paddle wheel SBU of 1; (b) the sinusoidal chain SBU of 1; (c) the zigzag chain SBU of 2; (d) the trinuclear iron SBU of 3. The H atoms are omitted for clarity. Codes: red octahedron, {FeO6}; yellow octahedron, {NaO6}; red ball, O atom; black ball, C atom. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)
formation of channels in two directions: along the c axis the channels have a sector shape of about 5.9 11.3 Å2 and along the b axis rectangular channels can be observed with dimensions of about 10.0 3.7 Å2. Coordinated DMF molecules, the methyl groups of acetate and water molecule are located in these channels. It is noteworthy that 1 represents a rare example of Fe-containing MOFs constructed by two kinds of SBUs.
3.2.2. Crystal structure of compounds 2 The 3D framework of 2 is built by zigzag-chain SBUs [FeNa(COO)6]n3n (Fig. 1) and m-BDC linkers. The zigzag-chain SBU
Fig. 2. The polyhedral and ball-and-stick view of the 3D framework of 1. Codes: red octahedron, {FeO6}; red ball, O atom; blue ball, N atom; black ball, C atom. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)
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Fig. 3. The polyhedral and ball-and-stick view of the 3D framework of 2. Codes: red octahedron, {FeO6}; yellow octahedron, {NaO6}; red ball, O atom; black ball, C atom. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)
Fig. 4. The polyhedral and ball-and-stick view of the 3D framework of 3. Codes: red octahedron, {FeO6}; red ball, O atom; black ball, C atom. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)
is composed of alternating iron octahedra and sodium octahedra. Each iron octahedron is defined by two chelate carboxyl groups and two bridging carboxyl groups, respectively from four different BDC ligands, and each sodium octahedron is completed by six carboxylate oxygen atoms from six different BDC ligands (Fig. S6). The bond lengths of Fe–O are in the range of 2.024–2.235 Å and Na–O are from 2.393 to 2.469 Å. The valence sum calculation on the iron site give the value 2.10, which indicates that the Fe atom in 2 is in the +2 oxidation state [44]. In 2, each zigzag-chain SBU is connected to four neighboring zigzag-chain SBUs by m-BDC linkers, resulting in a 3D framework (Fig. 3). This organization leads to the formation of channels in two directions: along the c axis the channels have a sector shape of about 6.22 6.22 Å2 and along the a axis rectangular channels can be observed with dimensions of about 7.03 6.22 Å2. The framework of 2 is similar to that of previously reported compound [ZnNa(m-BDC)2]NH2(CH3)2. However, in compound [ZnNa(m-BDC)2]NH2(CH3)2, the zigzag-chain SBU is composed of alternating Zn tetrahedra and Na octahedra [45].
dimensions along b axis, which are filled by [N(CH3)4]+ cations and water molecules. The structure of 3 is analogue to that of compound Ni3(NMe4)1(1,3,5-BTC)3(DMF)2 except little difference in the coordination environments of metal atoms [40]. To our knowledge, except the paddle-wheel SBU has been found in compound [Fe2(H2O)2(BTC)4/3]Cl4.5(DMF) [27], the other SBUs in compounds 1–3, the sinusoidal chain SBU [Fe2(COO)6]n2n, zigzag-chain SBU [FeNa(COO)6]n3n and linear trinuclear iron SBU [Fe3(COO)8(H2O)]2, are the first time appearing in Fe-containing MOFs.
3.2.3. Crystal structure of compounds 3 The 3D framework of 3 is formed by linear tri-nuclear iron SBU [Fe3(COO)8(H2O)]2 (Fig. 1) and triangular BTC linkers. The linear Fe3 SBU consists of three crystallographically independent iron atoms (Fe1, Fe2 and Fe3) (Fig. S8). The central Fe atom (Fe1) adopts an octahedral coordination with six bridging carboxyl groups from six different BTC linkers. One terminal Fe atom (Fe2) is fivecoordinated with distorted square pyramidal geometry, which is completed by one chelate carboxyl group from one BTC linkers and three bridging carboxyl group from three different BTC linkers. The other terminal Fe atom (Fe3) is analogous to Fe2, but the coordination number is increased from 5 to 6, owing to the coordination of one additional water molecule. The bond lengths of Fe–Ocarboxyl are in the range of 2.008–2.227 Å and the length of the Fe—OH2 O bond is 2.130 Å. The valence sum calculations on the iron sites give the values 2.10 for Fe1, 2.01 for Fe2 and 2.03 for Fe3, indicating that they are all in the +2 oxidation state [44]. In 3, each linear Fe3 SBU connects with seven BTC linkers, and each BTC linker bridges two or three linear Fe3 SBUs, leading to a 3D framework (Fig. 4). The framework of 3 has channels of 23.13 8.54 Å
3.3. FT-IR spectroscopy The infrared spectra of compounds 1–3 are given in Fig. S9. The IR spectrum of compound 1 shows the characteristic bands of carboxyl group at 1623, 1562, 1439 and 1370 cm1 for the asymmetric and symmetric vibrations. The Dm (mas(COO)–ms(COO)) is 184 and 192 cm1, indicating bridge coordination mode of carboxyl groups [38]. This is in agreement with the crystal structure of 1. Moreover, it exhibits a characteristic band at 1504 cm1 for DMF with the oxygen atom coordinated to a metal atom and a sharp band at 1624 cm1 attributable to the presence of water molecule in the metal coordination spheres [46,47]. An additional broad strong band near 3430 cm1 is assigned to the O–H stretching vibration of water molecule. The IR spectrum of compound 2 exhibits the absorption bands of the asymmetric and symmetric vibrations of carboxyl group at 1613, 1558, 1480 and 1396 cm1, respectively. The Dm (mas(COO)–ms(COO)) is 133 and 162 cm1, suggesting bidentate bridge coordination mode of carboxyl group [38]. An additional broad strong band near 3425 cm1 is attributed to the N–H absorption vibration for dimethylamine molecule [48]. The IR spectrum of compound 3 displays the characteristic bands of carboxyl group at 1623, 1572, 1441 and 1372 cm1 for the asymmetric vibration and the symmetric vibration. The Dm (mas(COO)–ms(COO)) are 182 and 200 cm1, indicating bidentate bridge coordination mode and monodentate coordination mode of carboxyl group, respectively [36]. Furthermore, it shows a sharp band at 1624 cm1 assignable to the presence of water molecule in
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the metal coordination spheres [47]. An additional broad strong band near 3430 cm1 is assigned to the O–H stretching vibration of water molecule [48]. 3.4. Thermogravimetric analyses The TG curve of 1 displays two continuous weight loss processes (Fig. S10). The first weight loss of 13.2% covering a temperature range from 30 to 127 °C could be assigned to the loss of DMF molecules (calcd: 13.9%). The continuous weight loss of 54.8% is caused by the release of BTC and acetate ligands (calcd: 54.6%). The decomposition process is completed at 450 °C giving black iron oxide as the final decomposition product, which constituted 32.6% of the initial weight (calcd: 31.5%). The TG curve of 2 shows four weight loss stages occurring at 30–100, 200–274, 337–370 and 393–460 °C, respectively (Fig. S10). The weight loss, 71.1% in total (calcd: 70%), is attributed to the loss of 22% dimethylamine cations at there previous stages (calcd: 20%) and the decomposition of 49.1% m-BDC ligands (calcd: 50%) at last. The decomposition process is completed at 460 °C giving black sodium iron oxide as the final decomposition product, which constituted 28.9% of the initial weight (calcd: 30%). The TG curve of 3 exhibits two continuous weight loss stages occurring at 16–330 and 330–430 °C, respectively (Fig. S10). The weight loss, 77.3% in total (calcd: 75.6%), is attributed to the loss of tetramethylamine cations, water molecules and BTC ligands. The decomposition process is completed at 430 °C giving black iron oxide as the final decomposition product, which constituted 22.69% of the initial weight (calcd: 24%). 3.5. UV analysis The UV–Vis diffuse reflectance spectra (DRS) of the three compounds (Fig. S11) all display a similar rang of absorption bands in the UV region at around 42 372 (236 nm) to 29 154 (343 nm) cm1, which correspond to the intraligand n ? p⁄ and p ? p⁄ transitions [49]. Compounds 1, 2 and 3 also exhibit well-developed bands in the range of 6024 (1660 nm)–11 764 (850 nm) cm1 owing to the d–d transitions of the octahedrally coordinated high-spin Fe(II) ions [50]. One medium strong broad peak at 21 978 cm1 (455 nm) for 1, 21 691 cm1 (461 nm) for 2 and 21 551 cm1 (464 nm) for 3 were assigned to the metal-to-ligand charge-transfer transitions from the occupied d orbitals of the FeII ions to the empty p⁄ orbitals of the carboxylic ligands [51].
Fig. 5. Polt of vmT vs. T for compound 1.
Fig. 6. Polt of vmT vs. T for compound 2.
3.6. Magnetic properties The magnetic susceptibility of 1, 2 and 3 was studied from 300 to 2 K at 1000 Oe. For 1, the vmT value, where vm the molar magnetic susceptibility in terms of the formula, decreases from 19 cm3 mol1 K at 300 K to 1.25 cm3 mol1 K at 2 K (Fig. 5). The vmT value at room temperature (19 cm3 mol1 K) is higher than the expected value (15 cm3 mol1 K, assuming g = 2.0 for Fe2+) for five independent high-spin Fe(II) ions (S = 2). These results suggest the presence of antiferromagnetic interactions between iron(II) spins, probably within the Fe chain [52]. As shown in Fig. 6, the vmT value of 2 at 300 K is 2.3 cm3 K mol1, which is lower than the spin-only value (g = 2.0) of 3 cm3 K mol1 for noninteracting FeII (S = 2) ion in the iron–sodium chain. When the temperature was lowered, the vmT product steadily decreased to reach a minimum of 0.8 cm3 K mol1 at 2 K. The experimental data have been well fitted by the Curie–Weiss law above 2 K with the following Curie and Weiss constants: C = 2.25 cm3 K mol1 and h = 7.1 K, respectively. The negative Weiss constant confirms the presence of dominating antiferromagnetic behaviors between spin carriers [53,54]. The
Fig. 7. Polt of vmT vs. T for compound 3.
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magnetic study of 3 shows that the vmT value decreases from 9.5 at 300 cm3 mol1 K to 0.47 at 2 K, indicating the presence of antiferromagnetic exchange interactions in this compound (Fig. 7). The vmT value at room temperature (9.5 cm3 mol1 K) is close to the expected value (9 cm3 mol1 K, assuming g = 2.0 for Fe2+) for three independent high-spin Fe(II) atoms in each Fe3 unit per formula (S = 2). The linear fitting of the reciprocal of the susceptibility versus temperature curve with the Curie–Weiss law in the range of 150– 300 K gives values of C = 13.4 cm3 mol1 K and h = 173.83 K. 4. Conclusions Three new Fe-containing MOFs have been synthesized under the solvothermal conditions. They present three new Fe-containing SBUs, sinusoidal chain SBU, zigzag-chain SBU and linear trinuclear iron SBU, for the construction of new Fe-containing MOFs. Moreover, the studies about their syntheses and structures provide some insight for the formation of Fe-containing MOFs. In the future, we are going to synthesize new Fe-containing MOFs exhibiting interesting structures and properties through combining the new Fecontaining SBUs found in this work with other carboxylate ligands. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 20901015), the Science and Technology Development Project Foundation of Jilin Province (No. 20080120) and Science Foundation for Young Teachers of Northeast Normal University (No. 20090406), the Fundamental Research Funds for the Central Universities (10QNJJ009/09SSXT120). Appendix A. Supplementary material CCDC 805270, 805271 and 805272 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2011.12.006. References [1] H.K. Chae, D.Y. Siberio-Pérez, J. Kim, Y.B. Go, M. Eddaoudi, A.J. Matzger, M. O’Keeffe, O.M. Yaghi, Nature 427 (2004) 123. [2] G. Férey, M. Latroche, C. Serre, F. Millange, T. Loiseau, A. Percheron-Guégan, Chem. Commun. 24 (2003) 2976. [3] W.G. Lu, C.Y. Su, T.B. Lu, L. Jiang, J.M. Chen, J. Am. Chem. Soc. 128 (2006) 34. [4] J.S. Seo, D. Whang, H. Lee, S.I. Jun, J. Oh, Y.J. Jeon, K. Kim, Nature 404 (2000) 982. [5] D.N. Dybtsev, A.L. Nuzhdin, H. Chun, K.P. Bryliakov, E.P. Talsi, V.P. Fedin, K. Kim, Angew. Chem., Int. Ed. 45 (2006) 916. [6] G.J. Halder, C.J. Kepert, B. Moubaraki, K.S. Murray, J.D. Cashion, Science 298 (2002) 1762. [7] G.F. Qrey, M. Latroche, C. Serre, F. Millange, T. Loiseau, A. Percheron-GuQgan, Chem. Commun. (2003) 2976. [8] S.S.Y. Chui, S.M.F. Lo, J.P.H. Charmant, A.G. Orpen, I.D. Williams, Science 283 (1999) 1148. [9] M. Dinca˘, A. Dailly, Y. Liu, C.M. Brown, D.A. Neumann, J.R. Long, J. Am. Chem. Soc. 128 (2006) 16876. [10] S.Q. Ma, D.F. Sun, X.S. Wang, H.C. Zhou, Angew. Chem., Int. Ed. 46 (2007) 2458.
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Metal–organic frameworks constructed from three kinds of new Fe-containing secondary building units Tian-zhan Zhang, Ying Lu ⇑, Yang-guang Li, Zhiming Zhang, Wei-lin Chen, Hai Fu, En-bo Wang ⇑ Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal University, Renmin Street No. 5268, Changchun, Jilin 130024, PR China
a r t i c l e
i n f o
Article history: Received 6 June 2011 Received in revised form 30 November 2011 Accepted 2 December 2011 Available online 11 December 2011 Keywords: Metal–organic framework Fe-containing second building unit Magnetic properties Structure elucidation
a b s t r a c t Three new Fe-containing MOFs, H3O[Fe5(BTC)3(OAc)2(DMF)2]H2O 1, [FeNa(m-BDC)2]NH2(CH3)2 2 and [N(CH3)4]2[Fe3(HBTC)(BTC)2(H2O)]5.5H2O 3 (BTC = 1,3,5-benzenetricarboxylate, m-BDC = 1,3-benzenedicarboxylate), have been synthesized from the solvothermal reactions of iron salts and carboxylate ligands. The framework of 1 is constructed from two kinds of secondary building units (SBUs): the sinusoidal chain of iron octahedra SBU [Fe2(COO)6]n2n and paddle-wheel SBU [Fe2(COO)4(DMF)]. Compound 1 represents the first example of Fe-containing MOFs constructed from two kinds of SBUs. The framework of 2 is built from zigzag-chain SBUs [FeNa(COO)6]n3n and m-BDC linkers. The framework of 3 is formed by linear tri-nuclear iron SBUs [Fe3(COO)8(H2O)]2 and triangular BTC linkers. To our knowledge, the Fe-containing sinusoidal chain SBU in 1, zigzag-chain SBU in 2 and linear tri-nuclear iron SBU in 3 are found in Fe-containing MOFs for the first time. Magnetic investigation indicates the present of antiferromagnetic exchange interaction within the iron units of compounds. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Metal–organic frameworks (MOFs) are of great contemporary interest due to their versatile functional properties with important applications in areas such as catalysis, gas separation and storage, luminescence, magnetism and ion exchange [1–7]. Although the research on MOFs has been very fruitful, controlling the design of the frameworks to obtain materials with desired properties is still a challenge [8–12]. Most MOFs are constructed from the interconnection of polynuclear metallic units known as the secondary building units (SBUs) with organic ligands through strong covalent bonds. The current studies on MOFs have indicated that choosing a suitable SBU is very critical for the achievement of MOFs with required properties, not only due to the structural properties of SBUs have significant influence on the topology and stability of MOFs, but also the functional properties (such as magnetism, luminescence) of SBUs can be imparted to the whole framework. Moreover, once a new SBU fitting for the built of MOFs with desired properties is found, a series of frameworks is promising through combining it with a variety of organic linkers using the reticular chemistry concepts, such as the well studied MOF system based on the copper paddle-wheel and the basic zinc acetate SBUs [13–19].
⇑ Corresponding authors. Tel./fax: +86 431 85098787 (E.-b. Wang). E-mail addresses: [email protected] (Y. Lu), [email protected] (E.-b. Wang). 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.12.006
Polynuclear iron units have attracted much attention owing to its biological function, catalytic and magnetic properties [20–26]. Designing MOFs based on Fe-containing SBUs may be a promising way to the achievement of high-efficiency biomimetic catalyst and magnetic porous materials [23,27]. However, compared to the extensive research on copper- and zinc-containing MOFs, the studies on iron-containing MOFs is very limited [28–35]. The main reason is that the high trend of iron ion to undergo hydrolysis into a stable polymeric hydrous iron oxide makes its synthesis more difficult than that of copper- and zinc-containing MOFs. Until now, only several kinds of Fe-containing SBUs have been found in the structure of MOFs, which include paddle-wheel Fe2 unit, triangle Fe3 unit, square-planar and prismatoid Fe4 unit, iron oxide octahedral chain and helical chain of iron octahedra [16,36–38]. On the basis of the above points, we dedicate to the exploration of new Fe-containing SBUs and corresponding MOFs. At the present work, we focus on the iron-BTC system under solvothermal conditions. From this system, we successfully obtained there new Fe-containing MOFs, H3O[Fe5(BTC)3(OAc)2(DMF)2]H2O (1), [FeNa(m-BDC)2]NH2(CH3)2 (2), and [N(CH3)4]2[Fe3(HBTC) (BTC)2(H2O)]5.5H2O (3). The framework of 1 is constructed from two kinds of SBUs: the sinusoidal chain of iron octahedra SBU and paddlewheel SBU. The framework of 2 is built by zigzag-chain SBUs and m-BDC linkers. The framework of 3 is formed by linear tri-nuclear iron SBU [Fe3(COO)8(H2O)]2 and triangular BTC linkers. As far as we know, the three kinds of Fe-containing SBUs presenting in compounds 1–3, sinusoidal chain SBU, zigzag-chain SBU and linear trinuclear iron SBU, are found in Fe-containing MOFs for the first time.
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by slow cooling to room temperature. After being washing with DMF several times, 3 was collected as yellow octahedron crystals with a yield of about 91% (based on Fe). Elem. Anal. Calc.: Fe, 17.1; C, 42.6; H, 4.0; N, 28.5. Found: Fe,16.8; C, 42.7; H, 3.9; N, 28.1%.
2. Experimental 2.1. Materials and measurement All reagents were purchased commercially and used without further purification. Elemental analyses (C, H, and N) were performed on a Perkin–Elmer 2400 CHN elemental analyzer and a PLASMA– SPEC (I) ICP atomic emission spectrometer (Fe). The IR spectra were obtained on an Alpha Centaurt FT-IR spectrometer in the 400– 4000 cm1 region with a KBr pellet. The TG for 1 and 2 were performed on a Perkin–Elmer TGA7 instrument under flowing O2 with a heating rate of 10 °C min1. The TG for 3 were performed on a Perkin–Elmer TGA7 instrument under flowing N2 with a heating rate of 10 °C min1. Magnetic susceptibility data were collected over the temperature range of 2–300 K at a magnetic field of 1000 Oe on a Quantum Design MPMS-5 SQUID magnetometer. 2.2. Syntheses 2.2.1. Synthesis of compound 1 The mixture of FeCl34H2O (0.3 mmol, 0.04 g), 1,3,5-H3BTC (0.45 mmol, 0.1 g), HAc (1.5 mL) and DMF (4.5 mL) was sealed in a Teflon-lined autoclave and heated at 150 °C for 1.5 days, followed by slow cooling to room temperature. After being washing with DMF several times, 1 was collected as dark yellow block crystals with a yield of about 62% (based on Fe). Elem. Anal. Calc.: Fe, 24.2; C, 35.3; H, 1.9; N, 1.2. Found: Fe, 23.8; C, 35.6; H, 1.8; N, 1.2%. 2.2.2. Synthesis of compound 2 The mixture of FeCl24H2O (0.2 mmol, 0.04 g), m-H2BDC (0.5 mmol, 0.163 g), NaOH (1 mmol, 0.04 g) and DMF (10 mL) was sealed in a Teflon-lined autoclave and heated at 130 °C for 5 days, followed by slow cooling to room temperature. After being washing with DMF several times, 2 was collected as dark yellow rod crystals with a yield of about 72% (based on Fe). Elem. Anal. Calc.: Fe, 12.9; Na, 6.6; C, 47.1; H, 1.8; N, 3.2. Found: Fe, 12.9; Na, 6.6; C, 46.9; H, 1.8; N, 3.3%. 2.2.3. Synthesis of compound 3 The mixture of FeCl3 (0.2 mmol, 0.032 g), H3BTC (0.5 mmol, 0.2 g), TMAOH (0.7 mmol, 0.132 g) and DMF (10 mL) was sealed in a Teflon-lined autoclave and heated at 150 °C for 4 days, followed
2.3. X-ray crystallography study The crystallographic data were collected at 298 K (for 1 and 2) and 150 K (for 3) on the Rigaku R-axis Rapid IP diffractometer 0 (Mo Ka radiation l = 0.71073 Å A) and IP technique. Mult-scan absorption correction was applied. The crystal data were solved by the direct method and refined by the full-matrix least-squares method on F2, using the SHELXTL-97 crystallographic software package [39]. All non-hydrogen atoms were refined anisotropically. In compound 1, the atom O12 of coordination DMF was disordered over two positions with each occupancy of 0.5. In compound 2, the one carboxyl group of BDC ligand was disordered over two positions with each occupancy of 0.5, respectively. And C9 and C10 of the dimethylamine cation were disordered over two positions, respectively. In compound 3, the disordered component in channels appears to consist primarily of tetramethyl ammonium ions. This is evident from the need for the material to charge balance, but also from the structure visible in the difference map for the void region. From this it is evident that there are at least two N(CH3)4+ present and concerted efforts were made to refine this model [40]. In compound 1, the hydrogen atoms were placed geometrically on the carboxyl ligands and DMF molecules. In compound 2, the hydrogen atoms associated with m-BDC in compound 2 were also placed geometrically and refined in a riding model. H atoms attached to the disordered protonated dimethylamine cations were not add. In compound 3, the hydrogen atoms attached to carbon atoms were fixed in ideal positions, while the hydrogen atoms attached to the water molecules were not located. The detailed crystal data and structure refinement for 1, 2 and 3 were given in Table 1. Selected bond lengths of 1, 2 and 3 were listed in Tables S1–S3, respectively. 3. Result and discussion 3.1. Synthesis Compounds 1–3 were synthesized via self-assembly processes under mid-temperature solvothermal conditions. Compound 1
Table 1 Crystal data and structure refinements for compounds 1, 2 and 3.
a b
Compounds
1
2
3
Formula M T (K) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (g cm3) Measured reflections Independent reflections Data/restraints/parameters Goodness of fit (GOF) S Final R indicts [I > 2r(I)]a R indices (all data)b
C17H11.25Fe2.50N0.50O13.50 578.14 293(2) monoclinic C2/c 33.242(7) 12.792(3) 14.120(3) 90 102.32(3) 90 5866(2) 4 1.385 22 000 5161 5161/18/306 1.07 R1 = 0.0690, wR2 = 0.2135 R1 = 0.0825, wR2 = 0.2256
C68H32Fe4N4Na4O32 1732.34 293(2) monoclinic C2/c 13.395(3) 14.354(3) 11.319(2) 90 115.55(3) 90 2024.2(7) 4 1.461 9769 2327 2327/24/170 1.09 R1 = 0.0362, wR2 = 0.0994 R1 = 0.0428, wR2 = 0.1040
C35H39Fe3N2O20.50 983.23 150(2) monoclinic P21/c 18.1258(15) 14.1372(2) 26.6897(16) 90 131.776(3) 90 5184.8(7) 4 1.256 25 581 9129 9129/32/495 1.126 R1 = 0.0878, wR2 = 0.2282 R1 = 0.1550, wR2 = 0.2583
R1 = R||Fo| |Fc||/R|Fo|. wR2 = {R[w(Fo2 Fc2)2]/R[w(Fo2)2]}1/2.
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was synthesized from the reaction of FeCl3, H3BTC and HAc in DMF at 150 °C for 1.5 days. Parallel experiments showed that the ratio of the HAc and DMF played a significant role in the formation of compound 1. Compound 1 could be gotten in the ratio range from 1:2.5 to 1:3. At ratio 1:3, good block crystals suiting for single crystal determination were gotten. At ratio 1:2.5, small crystals of 1 were formed in pure phase. Compound 2 was synthesized from the reaction of FeCl29H2O, m-BDC and NaOH in DMF at 130 °C for 5 days. In the structure of 2, there exists dimethylamine cations, which is believed to be formed by the decarboxylation of DMF solvent under solvothermal conditions [41–43]. When the reactions were carried out in other solvents such as ethanol, tetrahydrofuran and acetonitrile, the crystals of 2 could not be obtained. However, when the reactions were performed by the addition of dimethylamine in the reaction system and using ethanol to replace DMF as solvent, the crystals of 2 could be also obtained. Compound 3 was synthesized from the reaction of FeCl3, H3BTC and TMAOH (TMAOH = tetramethylammonium hydroxide) in DMF at 150 °C for 4 days. Parallel experiments showed that TMAOH was essential for the formation of 3. When TMAOH was absent or replaced by other organic base (tetraethylammonium hydroxide and tetrabutylammonium hydroxide) in the reaction mixture, compound 3 could not be obtained. TMAOH may play a structure-directing agent role in the formation of 3. Although anhydrous ferric chloride was used for the synthesis of 3, there exits water molecules in the structure of 3. Water molecules contained in the final products of 3 may be derived from the –OH of TMAOH or a little content of water in the solvents. In addition, whatever the oxidation states of Fe in the starting materials are +2 (in 2) or +3 (in 1 and 3), the oxidation state of Fe is +2 in all the formed MOFs. However, no reductant was added in reaction systems during the processes of synthesizing 1–3. So it was speculated that the dimethylamine formed by the decomposition of DMF played the reductant role, which prevented the oxidation of Fe(II) and reduced Fe(III) to Fe(II). It is noteworthy that compounds 1 and 3 could not be gotten when were synthesized directly from ferrous salts. 3.2. Structure description 3.2.1. Crystal structure of compounds 1 The 3D framework of 1 is constructed by two kinds of SBUs: one is the sinusoidal chain of iron octahedra [Fe2(COO)6]n2n, the other is the paddlewheel unit [Fe2(COO)4(DMF)] (Fig. 1). The sinusoidalchain SBU contains two crystallographically independent iron atoms (Fe1 and Fe2), both of which has six-coordinated distorted octahedral geometry (Fig. S1). Each Fe1 atom is coordinated by four oxygen atoms from four different BTC ligands and two oxygen atoms from two different acetates, while each Fe2 atom is coordinated by three oxygen atoms from three different BTC ligands, two oxygen atoms from one acetate and one oxygen atom from the other acetate. In the paddlewheel SBU, the two crystallographically equivalent Fe atoms (Fe3) are five-coordinated in a square-pyramidal geometry with four equatorial carboxyl oxygen atoms from four different BTC ligands and an axial oxygen atom from a DMF molecule (Fig. S2). The bond lengths of Fe–Ocarboxyl are in the range of 2.012–2.352 Å and Fe–Odmf is 2.068 Å. The valence sum calculations on the iron sites give the values 2.2 for Fe1, 2.3 for Fe2 and 2.2 for Fe3, which indicate that all the Fe in 1 are in the +2 oxidation state [44]. As the linkers of the 3D framework of 1, BTC ligands can be divided into two types according their different connecting modes with SBUs. One kind of BTC ligand links with two sinusoidal chain SBUs, which bridges adjacent sinusoidal chains to form the 2D layer (Layer A) (Fig. S3). The other kind of BTC ligand connects with two paddlewheel SBUs and one sinusoidal chain SBU, which bridges adjacent paddlewheel SBUs to generate the 2D (4, 4) layer (Layer B) (Fig. S4), and further bridges the alternating Layer A and Layer B into a 3D framework (Fig. 2). This organization leads to the
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Fig. 1. The polyhedral view of the SBUs in compounds 1–3: (a) the paddle wheel SBU of 1; (b) the sinusoidal chain SBU of 1; (c) the zigzag chain SBU of 2; (d) the trinuclear iron SBU of 3. The H atoms are omitted for clarity. Codes: red octahedron, {FeO6}; yellow octahedron, {NaO6}; red ball, O atom; black ball, C atom. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)
formation of channels in two directions: along the c axis the channels have a sector shape of about 5.9 11.3 Å2 and along the b axis rectangular channels can be observed with dimensions of about 10.0 3.7 Å2. Coordinated DMF molecules, the methyl groups of acetate and water molecule are located in these channels. It is noteworthy that 1 represents a rare example of Fe-containing MOFs constructed by two kinds of SBUs.
3.2.2. Crystal structure of compounds 2 The 3D framework of 2 is built by zigzag-chain SBUs [FeNa(COO)6]n3n (Fig. 1) and m-BDC linkers. The zigzag-chain SBU
Fig. 2. The polyhedral and ball-and-stick view of the 3D framework of 1. Codes: red octahedron, {FeO6}; red ball, O atom; blue ball, N atom; black ball, C atom. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)
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Fig. 3. The polyhedral and ball-and-stick view of the 3D framework of 2. Codes: red octahedron, {FeO6}; yellow octahedron, {NaO6}; red ball, O atom; black ball, C atom. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)
Fig. 4. The polyhedral and ball-and-stick view of the 3D framework of 3. Codes: red octahedron, {FeO6}; red ball, O atom; black ball, C atom. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)
is composed of alternating iron octahedra and sodium octahedra. Each iron octahedron is defined by two chelate carboxyl groups and two bridging carboxyl groups, respectively from four different BDC ligands, and each sodium octahedron is completed by six carboxylate oxygen atoms from six different BDC ligands (Fig. S6). The bond lengths of Fe–O are in the range of 2.024–2.235 Å and Na–O are from 2.393 to 2.469 Å. The valence sum calculation on the iron site give the value 2.10, which indicates that the Fe atom in 2 is in the +2 oxidation state [44]. In 2, each zigzag-chain SBU is connected to four neighboring zigzag-chain SBUs by m-BDC linkers, resulting in a 3D framework (Fig. 3). This organization leads to the formation of channels in two directions: along the c axis the channels have a sector shape of about 6.22 6.22 Å2 and along the a axis rectangular channels can be observed with dimensions of about 7.03 6.22 Å2. The framework of 2 is similar to that of previously reported compound [ZnNa(m-BDC)2]NH2(CH3)2. However, in compound [ZnNa(m-BDC)2]NH2(CH3)2, the zigzag-chain SBU is composed of alternating Zn tetrahedra and Na octahedra [45].
dimensions along b axis, which are filled by [N(CH3)4]+ cations and water molecules. The structure of 3 is analogue to that of compound Ni3(NMe4)1(1,3,5-BTC)3(DMF)2 except little difference in the coordination environments of metal atoms [40]. To our knowledge, except the paddle-wheel SBU has been found in compound [Fe2(H2O)2(BTC)4/3]Cl4.5(DMF) [27], the other SBUs in compounds 1–3, the sinusoidal chain SBU [Fe2(COO)6]n2n, zigzag-chain SBU [FeNa(COO)6]n3n and linear trinuclear iron SBU [Fe3(COO)8(H2O)]2, are the first time appearing in Fe-containing MOFs.
3.2.3. Crystal structure of compounds 3 The 3D framework of 3 is formed by linear tri-nuclear iron SBU [Fe3(COO)8(H2O)]2 (Fig. 1) and triangular BTC linkers. The linear Fe3 SBU consists of three crystallographically independent iron atoms (Fe1, Fe2 and Fe3) (Fig. S8). The central Fe atom (Fe1) adopts an octahedral coordination with six bridging carboxyl groups from six different BTC linkers. One terminal Fe atom (Fe2) is fivecoordinated with distorted square pyramidal geometry, which is completed by one chelate carboxyl group from one BTC linkers and three bridging carboxyl group from three different BTC linkers. The other terminal Fe atom (Fe3) is analogous to Fe2, but the coordination number is increased from 5 to 6, owing to the coordination of one additional water molecule. The bond lengths of Fe–Ocarboxyl are in the range of 2.008–2.227 Å and the length of the Fe—OH2 O bond is 2.130 Å. The valence sum calculations on the iron sites give the values 2.10 for Fe1, 2.01 for Fe2 and 2.03 for Fe3, indicating that they are all in the +2 oxidation state [44]. In 3, each linear Fe3 SBU connects with seven BTC linkers, and each BTC linker bridges two or three linear Fe3 SBUs, leading to a 3D framework (Fig. 4). The framework of 3 has channels of 23.13 8.54 Å
3.3. FT-IR spectroscopy The infrared spectra of compounds 1–3 are given in Fig. S9. The IR spectrum of compound 1 shows the characteristic bands of carboxyl group at 1623, 1562, 1439 and 1370 cm1 for the asymmetric and symmetric vibrations. The Dm (mas(COO)–ms(COO)) is 184 and 192 cm1, indicating bridge coordination mode of carboxyl groups [38]. This is in agreement with the crystal structure of 1. Moreover, it exhibits a characteristic band at 1504 cm1 for DMF with the oxygen atom coordinated to a metal atom and a sharp band at 1624 cm1 attributable to the presence of water molecule in the metal coordination spheres [46,47]. An additional broad strong band near 3430 cm1 is assigned to the O–H stretching vibration of water molecule. The IR spectrum of compound 2 exhibits the absorption bands of the asymmetric and symmetric vibrations of carboxyl group at 1613, 1558, 1480 and 1396 cm1, respectively. The Dm (mas(COO)–ms(COO)) is 133 and 162 cm1, suggesting bidentate bridge coordination mode of carboxyl group [38]. An additional broad strong band near 3425 cm1 is attributed to the N–H absorption vibration for dimethylamine molecule [48]. The IR spectrum of compound 3 displays the characteristic bands of carboxyl group at 1623, 1572, 1441 and 1372 cm1 for the asymmetric vibration and the symmetric vibration. The Dm (mas(COO)–ms(COO)) are 182 and 200 cm1, indicating bidentate bridge coordination mode and monodentate coordination mode of carboxyl group, respectively [36]. Furthermore, it shows a sharp band at 1624 cm1 assignable to the presence of water molecule in
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the metal coordination spheres [47]. An additional broad strong band near 3430 cm1 is assigned to the O–H stretching vibration of water molecule [48]. 3.4. Thermogravimetric analyses The TG curve of 1 displays two continuous weight loss processes (Fig. S10). The first weight loss of 13.2% covering a temperature range from 30 to 127 °C could be assigned to the loss of DMF molecules (calcd: 13.9%). The continuous weight loss of 54.8% is caused by the release of BTC and acetate ligands (calcd: 54.6%). The decomposition process is completed at 450 °C giving black iron oxide as the final decomposition product, which constituted 32.6% of the initial weight (calcd: 31.5%). The TG curve of 2 shows four weight loss stages occurring at 30–100, 200–274, 337–370 and 393–460 °C, respectively (Fig. S10). The weight loss, 71.1% in total (calcd: 70%), is attributed to the loss of 22% dimethylamine cations at there previous stages (calcd: 20%) and the decomposition of 49.1% m-BDC ligands (calcd: 50%) at last. The decomposition process is completed at 460 °C giving black sodium iron oxide as the final decomposition product, which constituted 28.9% of the initial weight (calcd: 30%). The TG curve of 3 exhibits two continuous weight loss stages occurring at 16–330 and 330–430 °C, respectively (Fig. S10). The weight loss, 77.3% in total (calcd: 75.6%), is attributed to the loss of tetramethylamine cations, water molecules and BTC ligands. The decomposition process is completed at 430 °C giving black iron oxide as the final decomposition product, which constituted 22.69% of the initial weight (calcd: 24%). 3.5. UV analysis The UV–Vis diffuse reflectance spectra (DRS) of the three compounds (Fig. S11) all display a similar rang of absorption bands in the UV region at around 42 372 (236 nm) to 29 154 (343 nm) cm1, which correspond to the intraligand n ? p⁄ and p ? p⁄ transitions [49]. Compounds 1, 2 and 3 also exhibit well-developed bands in the range of 6024 (1660 nm)–11 764 (850 nm) cm1 owing to the d–d transitions of the octahedrally coordinated high-spin Fe(II) ions [50]. One medium strong broad peak at 21 978 cm1 (455 nm) for 1, 21 691 cm1 (461 nm) for 2 and 21 551 cm1 (464 nm) for 3 were assigned to the metal-to-ligand charge-transfer transitions from the occupied d orbitals of the FeII ions to the empty p⁄ orbitals of the carboxylic ligands [51].
Fig. 5. Polt of vmT vs. T for compound 1.
Fig. 6. Polt of vmT vs. T for compound 2.
3.6. Magnetic properties The magnetic susceptibility of 1, 2 and 3 was studied from 300 to 2 K at 1000 Oe. For 1, the vmT value, where vm the molar magnetic susceptibility in terms of the formula, decreases from 19 cm3 mol1 K at 300 K to 1.25 cm3 mol1 K at 2 K (Fig. 5). The vmT value at room temperature (19 cm3 mol1 K) is higher than the expected value (15 cm3 mol1 K, assuming g = 2.0 for Fe2+) for five independent high-spin Fe(II) ions (S = 2). These results suggest the presence of antiferromagnetic interactions between iron(II) spins, probably within the Fe chain [52]. As shown in Fig. 6, the vmT value of 2 at 300 K is 2.3 cm3 K mol1, which is lower than the spin-only value (g = 2.0) of 3 cm3 K mol1 for noninteracting FeII (S = 2) ion in the iron–sodium chain. When the temperature was lowered, the vmT product steadily decreased to reach a minimum of 0.8 cm3 K mol1 at 2 K. The experimental data have been well fitted by the Curie–Weiss law above 2 K with the following Curie and Weiss constants: C = 2.25 cm3 K mol1 and h = 7.1 K, respectively. The negative Weiss constant confirms the presence of dominating antiferromagnetic behaviors between spin carriers [53,54]. The
Fig. 7. Polt of vmT vs. T for compound 3.
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magnetic study of 3 shows that the vmT value decreases from 9.5 at 300 cm3 mol1 K to 0.47 at 2 K, indicating the presence of antiferromagnetic exchange interactions in this compound (Fig. 7). The vmT value at room temperature (9.5 cm3 mol1 K) is close to the expected value (9 cm3 mol1 K, assuming g = 2.0 for Fe2+) for three independent high-spin Fe(II) atoms in each Fe3 unit per formula (S = 2). The linear fitting of the reciprocal of the susceptibility versus temperature curve with the Curie–Weiss law in the range of 150– 300 K gives values of C = 13.4 cm3 mol1 K and h = 173.83 K. 4. Conclusions Three new Fe-containing MOFs have been synthesized under the solvothermal conditions. They present three new Fe-containing SBUs, sinusoidal chain SBU, zigzag-chain SBU and linear trinuclear iron SBU, for the construction of new Fe-containing MOFs. Moreover, the studies about their syntheses and structures provide some insight for the formation of Fe-containing MOFs. In the future, we are going to synthesize new Fe-containing MOFs exhibiting interesting structures and properties through combining the new Fecontaining SBUs found in this work with other carboxylate ligands. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 20901015), the Science and Technology Development Project Foundation of Jilin Province (No. 20080120) and Science Foundation for Young Teachers of Northeast Normal University (No. 20090406), the Fundamental Research Funds for the Central Universities (10QNJJ009/09SSXT120). Appendix A. Supplementary material CCDC 805270, 805271 and 805272 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2011.12.006. References [1] H.K. Chae, D.Y. Siberio-Pérez, J. Kim, Y.B. Go, M. Eddaoudi, A.J. Matzger, M. O’Keeffe, O.M. Yaghi, Nature 427 (2004) 123. [2] G. Férey, M. Latroche, C. Serre, F. Millange, T. Loiseau, A. Percheron-Guégan, Chem. Commun. 24 (2003) 2976. [3] W.G. Lu, C.Y. Su, T.B. Lu, L. Jiang, J.M. Chen, J. Am. Chem. Soc. 128 (2006) 34. [4] J.S. Seo, D. Whang, H. Lee, S.I. Jun, J. Oh, Y.J. Jeon, K. Kim, Nature 404 (2000) 982. [5] D.N. Dybtsev, A.L. Nuzhdin, H. Chun, K.P. Bryliakov, E.P. Talsi, V.P. Fedin, K. Kim, Angew. Chem., Int. Ed. 45 (2006) 916. [6] G.J. Halder, C.J. Kepert, B. Moubaraki, K.S. Murray, J.D. Cashion, Science 298 (2002) 1762. [7] G.F. Qrey, M. Latroche, C. Serre, F. Millange, T. Loiseau, A. Percheron-GuQgan, Chem. Commun. (2003) 2976. [8] S.S.Y. Chui, S.M.F. Lo, J.P.H. Charmant, A.G. Orpen, I.D. Williams, Science 283 (1999) 1148. [9] M. Dinca˘, A. Dailly, Y. Liu, C.M. Brown, D.A. Neumann, J.R. Long, J. Am. Chem. Soc. 128 (2006) 16876. [10] S.Q. Ma, D.F. Sun, X.S. Wang, H.C. Zhou, Angew. Chem., Int. Ed. 46 (2007) 2458.
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