ARTICLE pubs.acs.org/crystal
Microporous, Homochiral Structures Containing Iron Oxo-Clusters Supported by Antimony(III) Tartrate Scaffolds Qiang Gao, Xiqu Wang, Marlon T. Conato, Tatyana Makarenko, and Allan J. Jacobson* Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States
bS Supporting Information ABSTRACT: A chiral cluster compound, dipotassium bis-(μtartrato)-diantimony(III), K2Sb2L2 (H4L = L-tartaric acid), was used as a secondary building unit to react with Fe(III) perchlorate and Fe(II) sulfate in water and DMF (DMF = dimethylformamide) in the presence of NaN(CN)2 or 4,40 -bipyridine. Three distinct homochiral structures have been obtained: H3Na7[Fe4Sb6(μ4-O)6(μ3-O)2(L-tartrate)6] 3 14H2O (1), H5K3[(CH3)2NH2]2[Fe7Sb6(μ4-O)6(μ3-O)2(L-tartrate)6]2 3 28H2O (2), and Na3K5[(CH3)2NH2]4[Fe6Sb6(μ4-O)6(μ3-O)2(L-tartrate)6]2 3 30H2O (3). All of these compounds contain iron oxo-clusters sandwiched by Sb3(μ3-O)(L-tartrate)3 SBUs. 1 consists of isolated [Fe4Sb6(μ4-O)6(μ3-O)2(L-tartrate)6]10 clusters, which are interconnected by Na+ ions into 2D homochiral layers. The structure of 2 features [Fe7Sb6(μ4-O)6(μ3-O)2(L-tartrate)6]5 clusters; the clusters are further linked into negatively charged honeycomb layers. Both types of clusters are found in 3 and are also assembled into negatively charged honeycomb layers. The accessibility of the microporosity of 2 was demonstrated by adsorption of 2-butanol. The magnetic properties of the compounds were measured from 5 to 300 K and indicate predominantly antiferromagnetic exchange interactions.
’ INTRODUCTION Chirality plays an important role in functional materials. Materials with chirality find applications, for example, in enantioselective catalysis and enantiomeric separations and are of fundamental interest in the study of magnetochirality.1 In the past decades, significant effort has been devoted to the designed synthesis of homochiral frameworks, and several strategies have been developed. The use of achiral ligands normally leads to racemic mixtures on a micro- or macroscale. Occasionally, one enantiomer prevails when crystal growth proceeds from a single nucleus.2 A more predictable approach is to use a single enantiomeric form of one of the reactants. These structural components can be chiral-bridging ligands, templates, or secondary building units (SBUs).3 High nuclearity metal oxo clusters are currently of interest because of their fascinating physical properties.48 Among them, iron clusters with oxygen atoms as bridging ligands have been extensively studied in both materials chemistry9 and bioinorganic chemistry.10 Iron clusters with nuclearities of up to 22 iron atoms have been synthesized.11 In the iron oxo-clusters, carboxylate and alkoxide groups act both as bridging and as terminal ligands. Clusters may also be terminated by coordination to lone pair cations such as Pb2+ and Sb3+ where the lone pairs prevent further extension of the structures.12 Compounds containing lone pair cations are often noncentric and may show second-harmonic generation, piezoelectricity, pyroelectricity, or ferroelectricity.13 In our previous work, we have used SBUs of chiral Ni aspartate helices cross-linked by achiral bridges to construct a microporous homochiral framework.14 Following this approach and r 2011 American Chemical Society
also motivated by recent reports of chiral {Fe28} wheels composed of chiral tartrate ligands connected Fe7 clusters,15 we developed a related strategy that uses chiral antimony tartrate SBUs in combination with iron oxide clusters to synthesize microporous homochiral magnetic solids. A few examples of homochiral MOFs containing tartrate ligands have been reported in the literature. Cheetham et al. studied the formation of magnesium tartrate using the meso D- and racemic D,L-forms of the ligand.16 Later, Rood et al. investigated compounds formed by L-tartaric acid with Mg, Zn, and Cu.17 We chose to react a water-soluble chiral dimer, dipotassium bis-(μ-L-tartrato)-diantimony(III) (“tartar emetic”, K2Sb2(L-tartrate)2 (Figure S1)),18 with Fe(III) perchlorate and Fe(II) sulfate in water and DMF (DMF = dimethylformamide) in the presence of NaN(CN)2 or 4,40 -bipyridine. Three distinct homochiral structures have been obtained that contain iron oxo-clusters sandwiched by Sb3(μ3-O)(L-tartrate)3 SBUs: H3Na7[Fe4Sb6(μ4-O)6(μ3-O)2(L-tartrate)6] 3 14H2O (1), H5K3[(CH3)2NH2]2[Fe7Sb6(μ4-O)6(μ3-O)2(L-tartrate)6]2 3 28H2O (2), and Na3K5[(CH3)2NH2]4[Fe6Sb6(μ4-O)6(μ3-O)2(L-tartrate)6]2 3 30H2O (3). All of these compounds contain iron oxo-clusters sandwiched by Sb3(μ3-O)(L-tartrate)3 SBUs. 1 consists of isolated [Fe4Sb6(μ4-O)6(μ3-O)2(L-tartrate)6]10 clusters, which are interconnected by Na+ ions into 2D homochiral layers. The structure of 2 features [Fe7Sb6(μ4-O)6(μ3-O)2(L-tartrate)6]5 clusters; the clusters are Received: July 14, 2011 Revised: August 27, 2011 Published: August 29, 2011 4632
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Table 1. Crystallographic Data for 13 1
2
3 C54H116Fe12K5N4Na3O118Sb12
empirical formula
C24H53Fe4Na7O58Sb6
C54H101Fe14K3N2O116Sb12
formula weight
2384.49
4994.57
5105.18
crystal system
monoclinic
hexagonal
hexagonal
space group
C2
P6322
P6122
a [Å]
16.8704(7)
19.0892(9)
18.3733(5)
b [Å]
21.2935(9)
19.0892(9)
18.3733(5)
c [Å]
9.5632(4)
24.5573(12)
73.386(2)
α [deg] β [deg]
90.00 120.0110(10)
90.00 90.00
90.00 90.00
γ [deg]
90.00
120.00
120.00
V [Å3]
2974.8(2)
7749.7(6)
21 454.5(10)
Z
2
2
6
Dc [g/cm3]
2.662
2.140
2.371
μ [mm1]
3.811
3.508
3.678
Flack x
0.004(18)
0.01(4)
0.10(10)
GOF on F2 final R indices [I > 2σ(I)]
1.019 R1 = 0.0301, wR2 = 0.0836
1.215 R1 = 0.0356, wR2 = 0.1298
1.117 R1 = 0.1082, wR2 = 0.2750
R indices (all data)
R1 = 0.0303, wR2 = 0.0838
R1 = 0.0457, wR2 = 0.1371
R1 = 0.1196, wR2 = 0.2839
further linked into negatively charged honeycomb layers. Both types of clusters are found in 3 and are also assembled into negatively charged honeycomb layers. The magnetic properties were studied. The accessibility of the microporosity of 2 was demonstrated by adsorption of 2-butanol.
’ EXPERIMENTAL SECTION Materials and Methods. All of the reactants were reagent grade and were used as purchased without further purification. The IR spectra were measured on a Galaxy Series FTIR 5000 spectrometer with pressed KBr pellets. Thermogravimetric analysis (TGA) measurements were carried out using a TA Instruments Hi-Res 2950 system in N2 or air flow, with a heating rate of 3 °C min1. Elemental analyses were performed by Galbraith Laboratories (Knoxville, TN). The powder X-ray diffraction (PXRD) patterns were collected at room temperature on a Phillps X’pert Pro diffractometer. Magnetic susceptibility measurements were made using a Quantum Design Physical Property Measurement System (QD-PPMS) in the temperature range 5300 K with an applied field of 1000 Oe. Synthesis of H3Na7[Fe4Sb6(μ4-O)6(μ3-O)2(L-tartrate)6] 3 14 H2O (1). 1 was synthesized by hydrothermal reaction from a mixture of Fe(ClO4)3, K2Sb2(L-tartrate)2, NaN(CN)2, H2O, and DMF (molar ratio = 4:3:8:4440:516). The mixture was heated at 100 °C in a sealed Teflon vessel for 3 d. By using vacuum filtration and drying in air, yellow block crystals of 1 were recovered as pure phase. Yield: 56% based on Fe. Anal. Calcd for Na7Fe4Sb6C24O58H43: H, 1.82; C, 12.14; Na, 6.78; Fe, 9.40; Sb, 30.77. Found: H, 1.80; C, 11.91; Na, 6.72; Fe, 9.49; Sb, 29.4. IR (KBr): 3417 (br s), 2868 (w), 1613 (s), 1357 (s), 1121 (s), 1081 (m), 1000 (w), 913 (m), 846 (w), 815 (w), 743 (m), 587 (s).
Synthesis of H5K3[(CH3)2NH2]2[Fe7Sb6(μ4-O)6(μ3-O)2(L-tartrate)6]2 3 28H2O (2). 2 was synthesized by hydrothermal reac-
tion from a mixture of Fe(ClO4)3, K2Sb2(L-tartrate)2, 4,40 -bpy, H2O, and DMF (molar ratio = 8:3:4:4440:516). The mixture was heated at 120 °C in a sealed Teflon vessel for 3 d. By using vacuum filtration and drying in air, dark red hexagonal crystals of 2 were recovered as the major phase together with a minor yellow impurity. If 4,40 -bipyridine is left out of the reaction, clear solutions are formed with no formation of 2. Some other reagents such as pyrazine also give compound 2, but with a lower yield and purity. Yield: 42% based on Fe. Anal. Calcd for
K3Fe14Sb12C54O116H101N2: H, 2.04; C, 12.99; N, 0.56; K, 2.35; Fe, 15.66; Sb, 29.26. Found: H, 2.50; C, 13.12; N, 0.69; K, 3.33; Fe, 15.4; Sb, 26.1. IR (KBr): 3418 (br s), 2867 (w), 1614 (s), 1470 (w), 1358 (s), 1119 (s), 1081 (m), 998 (w), 910 (m), 846 (w), 815 (w), 743 (m), 589 (s).
Synthesis of Na3K5[(CH3)2NH2]4[Fe6Sb6(μ4-O)6(μ3-O)2(Ltartrate)6]2 3 30H2O (3). 3 was synthesized by hydrothermal reaction
from a mixture of Fe(ClO4)3, FeSO4, K2Sb2(L-tartrate)2, NaN(CN)2, H2O, and DMF (molar ratio = 4:2:3:8:4440:516). The mixture was heated at 100 °C in a sealed Teflon vessel for 3 d. By using vacuum filtration and drying in air, brown hexagonal crystals of 3 were recovered as major phase together with a few yellow impurities. Yield: 40% based on Fe. Anal. Calcd for Na3K5Fe12Sb12C54O118H116N4: H, 2.29; C, 12.70; N, 1.09; Na, 1.35; K, 3.83; Fe, 13.13; Sb, 28.62. Found: H, 2.30; C, 12.65; N, 0.96; Na, 3.06; K, 3.68; Fe, 11.2; Sb, 29.6. IR (KBr): 3422 (br s), 2856 (w), 1621 (s), 1353 (s), 1123 (s), 1075 (m), 1000 (w), 905 (m), 847 (w), 809 (w), 734 (m), 583 (s). Crystallography. Single-crystal X-ray analyses were performed at room temperature on a Siemens SMART platform diffractometer outfitted with an Apex II area detector and monochromatized graphite Mo Kα radiation (λ = 0.71073 Å). The structures were solved by direct methods and refined using the SHELXTL software package.19 Crystallographic data and structural refinements for compounds 13 are summarized in Table 1. More details on the crystallographic studies as well as atomic displacement parameters are given in CIF files. In 1 and 2, carbon-bonded hydrogen atoms were placed in geometrically calculated positions; hydrogen atoms on water molecules were not assigned or directly included in the molecule formula. In 3, no hydrogen atoms were assigned due to the limited quality of the data. In 2 and 3, only partial lattice water molecules were assigned, while the remaining water molecules, as well as [(CH3)2NH2]+ cations and protons, were based on elemental analysis, TG analysis, and the need for charge balance. CCDC 809396809398 (13, respectively) contain the supplementary crystallographic data for this Article. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
’ RESULTS AND DISCUSSION Description of the Crystal Structures. The coordination modes of L-tartrate ligand are listed in Figure S2. The following 4633
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Figure 1. Ball-and-stick representation of Na3Fe4Sb6O8(L-tartrate)6 cluster in 1. (a) Side and (b) top views. Fe3+ cations are highlighted as yellow octahedra in (b). Fe, Sb, Na, O, H2O (O+0 in legend), and C are shown as yellow, blue, green, red, light blue, and black spheres, respectively.
structure discussion is based on CIF files for 13. The formulas of complexes 13 are further confirmed by elemental analysis (EA) and TG studies. H3Na7[Fe4Sb6(μ4-O)6(μ3-O)2( L-tartrate)6] 3 14H2O (1). The structure of 1 contains an isolated Fe4Sb6(μ4-O)6(μ3-O)2(L-tartrate)6 cluster. One Fe4 cluster is sandwiched between two identical Sb3(μ3-O) clusters on each side of the Fe4 plane. In the Fe4 cluster, all Fe3+ cations lie nearly in the plane of an equilateral triangle (Figure 1). Three iron atoms (Fe2, 2 Fe3) are located at the triangle vertices, while the fourth iron cation (Fe1) lies at the center and is connected by six μ4-O atoms to the surrounding three Fe3+ cations. Each μ4-O atom is bonded to one Sb3+, one Na+, and two Fe3+ cations (Figure 1a). Distances between the Fe1 center and the surrounding Fe3+ cations are 3.1182(1) and 3.1492(1) Å, and the angles are 119.45(1)° and 121.11(1)°. All of the Fe3+ cations are six-coordinated in slightly distorted octahedra, with bond lengths in the range of 1.970(4)2.069(4) Å, typical for Fe3+ in octahedral coordination by oxygen atoms. Bond valence sum (BVS) caculations indicate that all of the iron cations are in trivalent state (Fe1, 2.79; Fe2, 2.93; Fe3, 2.93).20 In the Sb3(μ3-O) unit, Sb3+ cations and the μ3-O4 are arranged in a similar way to the iron cations in Fe4 cluster, with the μ3-O4 atom at the center of a triangle of Sb3+ cations. All Sb3+ cations display the typical one-sided coordination environment expected for lone-pair cations. Each Sb3+ cation is coordinated by five oxygen atoms in a distorted tetragonal pyramidal arrangement. Three out of the five SbO bond lengths are short, varying from 1.965(4) to 2.087(4) Å, and the fourth is much longer (2.368(4)2.400(5) Å). Each Sb3+ cation also has a fifth very weak SbO bond with a bond length that varies from 2.576(4) to 2.605(4) Å. All six L-tartrate ligands adopt similar coordination modes (Figure S2). Each uses four oxygen atoms from the four functional groups to chelate one Sb3+ and one Fe3+ cation. The oxygen atom from the hydroxyl group coordinated to the Sb3+ cation is also weakly bonded to a second Sb3+ atom. Two Sb3O units are connected to the Fe4 cluster from both above and below, each by three L-tartrate ligands, leading to a chiral heterometallic sandwich with a thickness of 6.562 Å (distance between the two μ3O4 atoms on the opposite sides, Figure S3). Three of the seven Na+ cations (Na1 and 2 Na4) are located in the same plane as the Fe4 cluster and alternate with the outer Fe3+ cations (Figure 1b). They are each bonded to seven oxygen
Figure 2. View of the packing arrangement of 1 along the c axis. Fe, Sb, Na, O, H2O (O+0 in legend), and C are shown as yellow, blue, green, red, light blue, and black spheres, respectively.
atoms, with two from bridging μ4-O atoms, four from two distinct L-tartrate ligands, and one oxygen atom from a coordinated water molecule (shown as light blue spheres in Figure 1) to form a distorted pentagonal bipyramid. The two apical NaO bonds are short with bond lengths of 2.280(4) Å (2 Na1), 2.223(5) Å, and 2.250(5) Å (Na4), while the other five are much longer, ranging from 2.474(5) to 2.714(8) Å. The other Na+ cations connect each cluster with four neighboring clusters to form a two-dimensional layer in the ab plane (Figure 2). The layers are stacked along the c-axis in an AA packing mode, giving one-dimensional chiral rectangular channels with dimensions of 6.959 Å 9.133 Å (distances between Na2 and Na3) along the c-axis, containing disordered free water molecules. Projections along the a and b axes are show in Figure S4. H5K3[(CH3)2NH2]2[Fe7Sb6 (μ4-O)6(μ3-O)2( L-tartrate)6 ]2 3 28 H2O (2). 2 contains a homochiral heterometallic sandwich similar to that found in 1 (Figures 3, S5). In 2, three crystallographically equivalent Fe1 cations substitute for the three Na+ cations (Na1 and 2 Na4) found in 1 and are coordinated by five oxygen atoms, with bond lengths in the range of 2.005(5)2.281(5) Å. 4634
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Figure 3. Ball-and-stick representation of Fe7Sb6O8(L-tartrate)6 cluster in 2. (a) Side and (b) top views. Fe cations are highlighted as yellow octahedra in (b). C, Fe, K, O, H2O (O+0 in legend), and Sb are shown as black, yellow, pink, red, light blue, and blue spheres, respectively.
Two additional oxygen atoms from two different L-tartrate ligands are also bonded to the Fe1 cation, with larger distances of 2.667(8) and 2.681(7) Å. BVS calculations indicate that Fe1 and the central Fe3 cations are divalent, presumably generated by reduction of Fe3+ cations by the DMF solvent, while Fe2 cation is trivalent (Fe1, 1.90; Fe2, 3.03; Fe3, 2.14). The magnetic measurements (see below) confirm this oxidation state assignment. The distance between the central Fe3 cation and the surrounding Fe2 cation is 3.1025(2) Å, a little shorter than the distances found in 1; the distance between Fe3 and Fe1 is 3.3458(1) Å. There are two types of coordination modes for L-tartrate ligands (Figure S2). In both types, the ligand further coordinates to one more iron cation with its carboxylate and hydroxyl groups. One tartrate ligand uses one oxygen atom from the carboxylate group to connect another iron cation, which causes a significant difference between the extended structures in 1 and 2: the water molecule coordinated to Na+ in 1 is replaced by the oxygen atom (O14) from one carboxylate group of an adjacent cluster; the Fe1O bond length is 2.098(5) Å, indicating relatively strong bonding between the clusters. The mixed-valence heterometallic cluster is linked to three other clusters by six Fe1O14 bonds to form a negatively charged honeycomb layer (Figures 4, S6). The layers are stacked along the c-axis in an ABAB arrangement (A and B are related by 2-fold axis in ab plane); large chiral hexagonal channels are formed along the c-axis, with a radius of 5.544 Å (distance between the center of channel and the nearest O5 atom in the inner wall). K+ cations are located in the channel walls and are bonded to oxygen atoms from L-tartrate ligands and water molecules. Additional protonated [(CH3)2NH2]+ cations, probably formed by decomposition of DMF, are required to balance the charge; the number was determined by elemental analysis and thermogravimetric analysis. Further experiments showed that no products could be obtained in the absence of DMF, even when Fe2+ salts were added to the reactions. Na3K5[(CH3)2NH2]4[Fe6Sb6(μ4-O)6(μ3-O)2( L-tartrate)6]2 3 30 H2O (3). 3 crystallizes in the chiral space group P6122. As compared to 2, the length of c axis of 3 is tripled, due to change from 63 to 61 screw axis (Figure S7). The structure of 3 is similar to that of 2, but the noncentral Fe2+ cation positions in the clusters are ∼2/3 occupied in the asymmetric unit, which suggests that Fe4 and Fe7 clusters are both present in the structure, with a ratio of close to 1:2. To attempt to find out whether the disorder occurs within a single layer or arises from random stacking of
Figure 4. View of the hexagonal channel of 2. C, Fe, K, O, H2O (O+0 in legend), and Sb are shown as black, yellow, pink, red, light blue, and blue spheres, respectively.
layers containing either all Fe4 or all Fe7 clusters, the symmetry was lowered to P1. Refinement in this space group did not resolve the disorder, and we conclude that most likely the disorder is present within a single layer. Bond valence sums calculated for the four ordered Fe positions are 2.873.31 v.u., indicating that the iron atoms are trivalent similar to those in 1. The K ion positions between neighboring clusters of 2 (Figure 4) are replaced by Na ions that are coordinated only to tartrate oxygen atoms in 3 (Figure 5). The replacement of K+ by Na+ and the central Fe2+ by Fe3+ causes a contraction of the Fe3+O coordination polyhedra, accompanied by expansion of the Fe2+O polyhedra. The three disordered Fe sites in 3 are each coordinated by seven oxygen atoms at 2.1302.549 Å similar to the corresponding sites in 2. The BVSs for the three sites in 3 are 1.35, 1.52, and 1.58 v.u., due to the partial occupancy. Unfortunately, pursuit of further structural details is limited by the quality of the current crystals. 4635
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Figure 6. Thermogravimetric analysis of 2 showing the uptake of 2-butanol. The weight and temperature changes are shown as red and black lines.
Figure 5. The FeO cluster in 3. The ordered FeO6 are plotted as polyhedral, and the partially occupied positions are indicated as yellow spheres. The connections of the central cluster ions to iron atom in surrounding clusters are also indicated. C, Sb, Fe, K, Na, and O are shown as black, blue, yellow, pink, green, and red spheres, respectively.
The locations of Na+ cations in 3 are quite different from that in 1; they protrude outward from the cluster. Distances between Na+ cations and the central Fe cations are much longer: 5.558 and 5.603 Å, respectively. The layers are assembled into a framework containing Na+ cations, K+ cations, and guest water molecules in a way similar to that found in 2. Synthesis. The assembly of this series of compounds can be hypothetically divided into the following steps (Figures S3, S5). The antimony tartrate dimers present in the starting material dissociate and then recombine to form a trinuclear Sb3 unit linked by a μ3-O atom. Similar M3O units have been observed in compounds containing the lone pair cation Sn2+.12c Two of these units and the remaining coordination sites of L-tartrate ligands released by the dimer dissociation define the central Fe4 cluster. The Sb3(μ3-O) units terminate the clusters in one direction, and all the electron lone pairs point outward. The Sb3(μ3-O)(L-tartrate)3 units on both sides act as a scaffold for the iron oxo-cluster; six μ4-O atoms from above and below define the central FeO6 octahedron. [Fe4Sb6(μ4-O)6(μ3-O)2(L-tartrate)6]10 cluster contains three additional Na+ cations in polyhedra that share edges with FeO6 octahedra. These Na+ ions can be completely replaced by Fe2+ cations to give [Fe7Sb6(μ4-O)6(μ3-O)2(L-tartrate)6]5 cluster in 2 or partially replaced as found in 3. Note that a lower [Na]/[Fe] ratio is used in the synthesis of 3 as compared to 1, which results in a lower [Na]/[Fe] ratio in the product. In compound 3, only two-thirds of the Na+ ions found in 1 are replaced by Fe2+ ions, indicating that the structure of [Fe4Sb6(μ4-O)6(μ3-O)2(L-tartrate)6]10 is sufficiently flexible to permit incorporation of additional ions of different charges, which in turn suggests that other members of this family of homochiral heterometallic clusters can be synthesized. Thermogravimetric Analyses. Thermogravimetric analyses (TGA) were performed under N2 flow (for 1) or air flow (for 2 and 3) to investigate compound stabilities (see Figures S11S13). The results show that 1 loses guest water molecules in two steps
below 305 °C (calcd 10.62%, obs 10.82%), and then collapses to yield a residue of a mixture of Na2CO3, Fe2O3, and Sb2O3 (calcd 65.90%, obs 66.90%). Compound 2 displays a two step weight loss from room temperature to 243 °C, corresponding to loss of guest water molecules and (CH3)2NH (calcd 13.70%, obs 14.22%), and then the framework decomposes to produce a mixture of K2CO3, Fe2O3, and Sb2O3 (calcd 61.56%, obs 62.27%). The thermal behavior of 3 is similar to that of 2; the weight loss below 267 °C indicates the loss of free water molecules and (CH3)2NH (calcd 14.04%, found 13.36%), and then the network collapses and a residue of mixture of Na2CO3, K2CO3, Fe2O3, and Sb2O3 is obtained at 598 °C (calcd 62.62%, obs 62.79%). Thermogravimetric Sorption Measurements. The microporosity of 2 was confirmed by the adsorption of 2-butanol in a thermogravimetric sorption experiment (Figure S17). The results are shown in Figure 6. A sample was heated in a helium gas flow to 120 °C to remove guest molecules and then cooled to ambient temperature. A small weight uptake is observed due to readsorption of residual guest molecules in the system. At the point indicated in Figure 6, the gas flow was switched to helium saturated with 2-butanol at room temperature. An immediate weight uptake is observed (8%) corresponding to the adsorption of 2-butanol. The 2-butanol can be completely removed by raising the temperature to 130 °C in helium and then readsorbed by cooling in a helium/butanol gas flow. A racemic mixture of 2-butanol enantiomers was used. Further experiments to investigate the enantiomeric selectivity of the adsorption process are in progress. Magnetic Properties. The magnetic susceptibility data for 1 are show in Figure 7a. The data closely resemble the results for the Fe4 cluster with the same geometry found in the compound Fe4(OCH3)6(dpm)6 (Hdpm is dipalvolylmethane).21 The temperature dependence of χMT is characteristic of an antiferromagnetically coupled system where the geometry prevents complete cancellation of the moments. The value of χMT at 300 K of 16.3 emu mol1 K is lower than the expected value of 17.5 emu mol1 K for 4 uncoupled ions each with S = 5/2. At the lowest temperature, χMT approaches the expected value of 15 emu mol1 K for an S = 5 ground state where the three Fe3+ cations on the triangle vertices have parallel spins and the one in the center is antiparallel. The minimum in χMT occurs at 40 K lower than observed for Fe4(OCH3)6(dpm)6 (155 K), indicating 4636
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Figure 7. The magnetic susceptibility of (a) compound 1 and (b) compound 2.
weaker exchange coupling perhaps due to the longer (0.04 Å on average) FeO distances in 1. The data were fit using the program MAGMUN 4.1. A quantitative analysis of the magnetic data assuming one exchange coupling constant, using the exchange Hamiltonian HEx = J(S1 3 S2 + S1 3 S3 + S1 3 S30 ), gives a good fit to the experimental data (g = 2.05 and J = 5.5 cm1) (Figure 7a), and no significant improvement was obtained by introducing two different J’s to take into account the C2 symmetry of the cluster.
cations in the middle layer of the sandwich can be varied, suggesting the possibility of a series of related heterometallic cluster compounds with different cations in the middle layer and different physical properties. The accessibility of the microporosity of 2 was demonstrated by adsorption of 2-butanol. Further work to investigate the enantiomeric selectivity is ongoing.
’ ASSOCIATED CONTENT
bS
Supporting Information. X-ray crystallographic information files (CIF) for compounds 13, additional structure figures, physical characterization data for 13, details of the thermogravimetric sorption study for 2, and magnetic data for 3. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION The magnetic susceptibility of 2 (Figure 7b) also indicates antiferromagnetic exchange interactions. For compound 2, the value of χMT at 300 K is 22.4 emu mol1 K intermediate between the values expected for 7Fe3+ and 7Fe2+ consistent with the mixed valence characteristic of 2 (alternating Fe2+ and Fe3+ cations in the hexagon and Fe2+ in the center) as indicated by the FeO bond lengths. At lower temperature, χMT decreases indicative of overall antiferromagnetic interactions. The magnetic behavior of 2 is different from that of previously reported Fe7 clusters, which have different average iron oxidation states.22 As expected from the structure, the magnetic behavior of 3 is approximately a combination of the behavior of 1 and 2 (see Figure S18). Above 50 K, the data fit reasonably well to a combination of the data for the Fe7 and Fe4 clusters in the ratio 2.1:1 in good agreement with the structural data. The detailed agreement is not perfect, and the simple linear combination does not fit the data at
