CRYSTAL GROWTH & DESIGN
Novel Metal-Organic Frameworks Derived from Group II Metal Cations and Aryldicarboxylate Anionic Ligands
2008 VOL. 8, NO. 3 911–922
Colleen A. Williams, Alexander J. Blake, Claire Wilson, Peter Hubberstey,* and Martin Schröder* School of Chemistry, UniVersity of Nottingham, UniVersity Park, Nottingham, U.K. NG7 2RD ReceiVed August 3, 2007; ReVised Manuscript ReceiVed October 25, 2007
ABSTRACT: Reaction of magnesium, calcium, strontium, and barium salts with a range of dicarboxylic acids [benzene-1,4dicarboxylic acid (H2BDC), naphthalene-2,6-dicarboxylic acid (H2NDC), 4,5,9,10-tetrahydropyrene-2,7-dicarboxylic acid (H2TPDC), pyrene-2,7-dicarboxylic acid (H2PDC), 5,10-dihydroanthracene-2,7-dicarboxylic acid (H2DADC)] in N,N′-dimethylformamide (DMF) or N,N′-diethylformamide (DEF) in Teflon-lined stainless steel autoclaves produces a range of metal-organic framework materials. Single crystal X-ray analysis has confirmed that the predominant building block in these materials is a chain of metal centers bridged either by carboxylate moieties alone as in [M(DMF)(µ-BDC)]∞ (M ) Mg or Sr), [Ca1.5(DEF)(µ-BDC)1.5]∞, and [Sr(DEF)(OH2)(µ-BDC)]∞ or bridged by both carboxylate ligands and DMF/DEF molecules as in [M(µ-DMF)(µ-NDC)]∞ (M ) Ca, Sr, or Ba), [M(µ-DEF)(µ-TPDC)]∞ (M ) Ca or Sr), [M(µ-DMF)(µ-DADC)]∞ (M ) Ca or Sr), and [Sr(µ-DEF)(µ-PDC)]∞. In contrast, the isomorphous complexes [Mg3(DMF)4(µ-NDC)3]∞ and [Mg3(DEF)4(µ-NDC)3]∞ contain centrosymmetric trinuclear moieties in which each pair of cations is bridged by three carboxylate anions with two pendant solvent molecules coordinated to each of the terminal Mg2+ cations. These trinuclear building blocks act as six-connected nodes and generate a tilted R-Po type structure. Eleven of the 12 structures based upon cationic chains adopt a common extended architecture in which the aryldicarboxylate anions link the chains to generate diamond-shaped channels. However, in the material [Sr(DMF)(µ-BDC)]∞, the chains are linked to generate a hexagonal motif of triangular channels. In all 12 compounds based on cationic chains, the space within the channels is occupied by coordinated solvent molecules, leading to nonporous materials. Introduction Metal-organic frameworks (MOFs) are molecular architectures comprising metal ion nodes bridged by organic ligands,1 which have attracted much interest recently due to their potential as porous hosts.2,3 Much research has centered on metal carboxylates,4 and in particular on cubic frameworks based upon [Zn4O]6+ nodes linked by linear aryldicarboxylates5 and related paddle-wheel complexes incorporating [M2(O2CR)4] units.6 The anionic nature of the carboxylate linker negates the need for counteranions in the framework, and this increases the potential for porosity in the framework products. These compounds can show high thermally stability and have rigid and extended structures that can reversibly adsorb small organic molecules.7 Furthermore, they have been shown to have significant potential as hydrogen sorption materials.8 In this work, a range of Group II metal cations of diverse size (Table 1) have been used to investigate the effect of metal cation size on the structure and stability of extended structures of aryldicarboxylate-bridged metal organic frameworks. Due to its relatively low atomic mass (Mr ) 24.31), particular focus has been placed on the Mg2+ species since this is a good candidate metal ion for inclusion in hydrogen storage materials owing to the need for light materials to realize the U.S. Department of Energy storage target of 6.5 wt %.9 Multidentate carboxylate functionalities were chosen as they are known to chelate and lock metal ions in clusters, which act as rigid entities.11,12 Benzene-1,4-dicarboxylate (BDC2-), naphthalene2,6-dicarboxylate (NDC2-), 4,5,9,10-tetrahydropyrene-2,7-dicarboxylate (TPDC2-), pyrene-2,7-dicarboxylate (PDC2-), and 5,10-dihydroanthracene-2,7-dicarboxylate (DADC2-) (Scheme 1) were studied to assess the significance of (i) length and (ii) * Corresponding authors. E-mail:
[email protected] (P.H.),
[email protected] (M.S.).
Table 1. Ionic Radii of the Group II Metals10 cation
Mg2+
Ca2+
Sr2+
Ba2+
radius (pm)
78
106
127
143
Scheme 1. Dicarboxylate Anions Investigated in This Study
bulk of the linker on the architecture of the resultant framework. The C · · · C separation between the carboxylate carbon atoms of BDC2-, NDC2-, TPDC2-, PDC2-, and DADC2-, which have one, two, four, four, and three six-membered rings, is 5.81, 8.02, 10.05, 10.03, and 10.42 Å, respectively. A major impetus for this work is derived from the fact that previously reported examples of Group II aryldicarboxylate framework polymers are rare and restricted to only three examples with Mg2+ 13–15 and sporadic examples of hydrated polymers of Ca2+, Sr2+, or Ba2+.16 Examples of porous frameworks with Al3+ have been described.17 Dincâ and Long reported13 the isolation of the first magnesium-based metalorganic framework, [Mg3(DEF)4(µ-NDC)3]∞ (DEF ) diethylformamide), 1, which shows a neutral 3D framework in which linear [Mg3]6+ cationic units incorporating two terminal N,N′diethylformamide (DEF) molecules at each end are linked by NDC2- anions. Following treatment at 190 °C under dynamic
10.1021/cg700731d CCC: $40.75 2008 American Chemical Society Published on Web 02/05/2008
Williams et al.
12
13
14
vacuum, its desolvated derivative was found to absorb virtually no N2 but limited quantities of O2 and H2 (0.46 wt % at 880 torr).13 Subsequently, Senkovska and Kaskel reported14 the analogous DMF-solvated species [Mg3(DMF)4(µ-NDC)3]∞ (DMF ) N,N′-dimethylformamide), 2. Following treatment at 195 °C under vacuum, the desolvated derivative of 2 was found to absorb limited quantities of N2, O2, CH4, and H2 (0.78 wt % at 760 torr). Although 2 could be converted into 1, the reverse process was not realized.14 We were particularly interested in investigating frameworks with other Group II metal ions and report herein structural studies on a range of arylcarboxylate complexes of Mg2+, Ca2+, Sr2+, and Ba2+.
10 9 8 7 6 5 4 3 properties
Table 2. Crystal Data for Compounds 3–14
All chemicals, except H2TPDC,18 H2PDC,19 and H2DADC,20 which were prepared following published procedures, were purchased from Aldrich Chemical Co Ltd. and used as received. Elemental analysis (CHN) was performed either by the Nottingham University School of Chemistry Microanalytical Service using a Perkin-Elmer 240B instrument or by Stephen Boyer at London Metropolitan University. Infrared spectra were obtained in the solid state using a Nicolet Avatar 3200 FTIR spectrometer. 1H NMR spectra were obtained by dissolution and breakup of the products in D2O and referenced to residual protio solvent using a Bruker DPX 300 spectrometer. Synthesis of [Mg3(DEF)4(µ-NDC)3]∞, 1.13 Hydrated magnesium nitrate (128 mg, 0.50 mmol) and H2NDC (108 mg, 0.50 mmol) were dissolved in DEF (8 cm3) in a vial. The vial was placed in a Teflonlined stainless steel autoclave and heated to 130 °C for 5 days. This produced colorless block crystals of X-ray quality. Monoclinic, space group C2/c, a ) 15.05, b ) 18.04, c ) 20.80 Å, β ) 101.5°, V ) 5578 Å3. Synthesis of [Mg3(DMF)4(µ-NDC)3]∞, 2.14 Anhydrous magnesium chloride (24 mg, 0.25 mmol) and H2NDC (54 mg, 0.25 mmol) were dissolved in DMF (8 cm3) in a vial. The vial was placed in a Teflonlined stainless steel autoclave and heated to 120 °C for 5 days. This produced colorless block crystals of X-ray quality. Monoclinic, space group C2/c, a ) 13.17, b ) 17.86, c ) 21.04 Å, β ) 100.0°, V ) 4874 Å3. Synthesis of [Mg(DMF)(µ-BDC)]∞, 3. Anhydrous magnesium chloride (24 mg, 0.25 mmol) and H2BDC (42 mg, 0.25 mmol) were dissolved in DMF (8 cm3) in a vial. The vial was placed in a Teflonlined stainless steel autoclave and heated to 140 °C for 5 days. This produced colorless block crystals of X-ray quality. IR (νmax/cm-1): 3417b, 1682m, 1545m, 1393s, 1357m, 788s. 1H NMR δ (ppm): 7.80 (s, 1H, HCON), 7.75 (m, 4H), 2.88 (s, 3H, Me), 2.73 (s, 3H, Me). Synthesis of [Ca1.5(DEF)(µ-BDC)1.5]∞, 4. Hydrated calcium nitrate (100 mg, 0.50 mmol) and H2BDC (84 mg, 0.50 mmol) were dissolved in DEF (8 cm3). The solution was stirred until dissolved and, then, heated without stirring to 110 °C for 4 days in a heavy walled glass vessel with a Teflon screw cap. This produced crystals of X-ray quality. IR (νmax/cm-1): 3366b, 1647m, 1557s, 1399s, 757s. 1H NMR δ (ppm): 7.85 (s, 2H, HCON), 7.74 (s, 12H, H), 3.24 (q, 4H, (CH2), 3J ) 7.2), 3.21 (q, 4H, (CH2)2, 3J ) 7.3), 1.06 (t, 6H, Me, 3J ) 7.2), 0.99 (t, 6H, Me, 3J ) 7.3). Synthesis of [Ca(µ-DMF)(µ-NDC)]∞, 5. Hydrated calcium nitrate (50 mg, 0.25 mmol) and H2NDC (55 mg, 0.25 mmol) were dissolved in DMF (8 cm3) in a vial. The vial was placed in a Teflon-lined stainless steel autoclave and heated to 130 °C for 5 days. This produced colorless crystals of X-ray quality. IR (νmax/cm-1): 3317b, 1607s, 1570s, 1495m, 1402s, 792s. 1H NMR, δ (ppm): 8.29 (d, 2H, 3J ) 6.1), 8.13 (s, 2H), 7.93 (d, 2H, 3J ) 8.5), 7.80 (s, 1H, HCON), 2.89 (s, 3H, Me), 2.73 (s, 3H, Me). Synthesis of [Ca(µ-DEF)(µ-TPDC)]∞, 6. Hydrated calcium nitrate (19 mg, 0.08 mmol) and H2TPDC (24 mg, 0.08 mmol) were dissolved in DEF (3 cm3) in a vial. The vial was placed in a Teflon-lined stainless steel autoclave and heated to 125 °C for 5 days. This produced orange needle crystals of X-ray quality. IR (νmax/cm-1): 3354b, 1657s, 1570s, 1432m, 1398s, 1115m, 796m. 1H NMR δ (ppm): 7.87 (s, 1H, HCON), 7.54 (s, 4H), 3.25 (q, 2H, (CH2), 3J ) 7.2), 3.21 (q, 2H, CH2, 3J ) 7.3), 2.83 (m, 8H), 1.07 (t, 3H, Me, 3J ) 7.2), 1.00 (t, 3H, Me, 3J ) 7.3).
11
Experimental Details
C22H22Mg2N2O10 C34H34Ca3N2O14 C15H13CaNO5 C23H23CaNO5 C19H17CaNO5 C11H11NO5Sr C13H17NO6Sr C15H13NO5Sr C23H23NO5Sr C23H19NO5Sr C19H17NO5Sr C16.13H15.63BaN1.38O5.38 M 523.04 814.87 327.34 433.50 379.42 324.83 370.90 374.88 481.04 477.01 426.96 452.01 crystal system monoclinic monoclinic orthorhombic orthorhombic orthorhombic trigonal orthorhombic orthorhombic orthorhombic orthorhombic orthorhombic orthorhombic space group P21/c I2/a Pnma Pnma Pnma P32 Pnma P212121 P212121 P212121 Pnma P212121 a/Å 13.627(3) 17.850(4) 6.8850(10) 6.8739(11) 6.953(1) 10.599(1) 12.1864 (14) 7.1452 (8) 7.5731(12) 7.4222 (6) 7.296(1) 8.077(2) b/Å 9.941(2), 10.684(2) 21.065(3) 25.022(4) 24.355(2) 10.599(1) 6.8970 (8) 10.1443 (11) 10.5515 (16) 10.6097(8) 23.907(2) 10.442(2) c/Å 17.686(4) 19.960(4) 9.9679(14) 12.785(2) 9.958(1) 9.806(2) 18.516 (2) 20.695 (2) 25.875 (4) 25.816(2) 10.018(1) 21.327(4) β/deg 92.732(4) 95.160(3) U/Å3 2393(2) 3790.9(14) 1445.7(3) 2199.1(6) 1686.3(3) 954.0(2) 1556.2 (3) 1500.1 (8) 2067.6 (6) 2032.9(3) 1747.4(3) 1799.5(6) Z 4 4 4 4 4 3 4 4 4 4 4 4 -1 µ/mm 0.16 0.50 0.46 0.32 0.40 4.25 3.49 3.62 2.64 2.69 3.12 2.233 reflections, measured 11650 14837 8945 10495 9697 5181 12903 8865 10129 12545 14764 15777 reflections, unique, 4204, 0.072 4314, 0.145 1692, 0.040 1979, 0.034 1965, 0.019 2839, 0.017 1918, 0.031 3351, 0.027 4664, 0.035 4643, 0.030 2048, 0.019 3165, 0.026 Rint reflections, observed 3829 3659 1411 1553 1759 2785 1584 3059 3846 4161 1901 2986 [I g 2σ(I)] R [I g 2σ (I)] 0.112 0.083 0.041 0.087 0.032 0.024 0.039 0.030 0.036 0.034 0.026 0.081 wR2 [all data] 0.200 0.203 0.110 0.231 0.088 0.060 0.114 0.077 0.094 0.078 0.061 0.193
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Table 3. Molecular Formulae of, and Reaction Conditions for, the Group II Framework Materials metal salt
carboxylic acid
temp (°C)
time (days)
solvent
product
number
Mg(NO3)2 · 6H2O MgCl2 MgCl2 Ca(NO3)2 · 4H2O Ca(NO3)2 · 4H2O Ca(NO3)2 · 4H2O Ca(NO3)2 · 4H2O Sr(NO3)2 Sr(NO3)2 Sr(NO3)2 Sr(NO3)2 Sr(NO3)2 Sr(NO3)2 BaCl2 · 2H2O
H2NDC H2NDC H2BDC H2BDC H2NDC H2TPDC H2DADC H2BDC H2BDC H2NDC H2TPDC H2PDC H2DADC H2NDC
130 140 130 110 140 125 150 130 130 140 125 130 150 100
5 5 5 4 5 5 5 5 5 5 5 5 5 5
DEF DMF DMF DEF DMF DEF DMF DMF DEF DMF DEF DEF DMF DMF
[Mg3(DEF)4(µ-NDC)3]∞ [Mg3(DMF)4(µ-NDC)3]∞ [Mg(DMF)(µ-BDC)]∞ [Ca1.5(DEF)(µ-BDC)1.5]∞ [Ca(µ-DMF)(µ-NDC)]∞ [Ca(µ-DEF)(µ-TPDC)]∞ [Ca(µ-DMF)(µ-DADC)]∞ [Sr(DMF)(µ-BDC)]∞ [Sr(DEF)(OH2)(µ-BDC)]∞ [Sr(µ-DMF)(µ-NDC)]∞ [Sr(µ-DEF)(µ-TPDC)]∞ [Sr(µ-DEF)(µ-PDC)]∞ [Sr(µ-DMF)(µ-DADC)]∞ [Ba(µ-DMF)(µ-NDC)]∞
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Synthesis of [Ca(DMF)(µ-DADC)]∞, 7. Hydrated calcium nitrate (50 mg, 0.25 mmol) and H2DADC (67 mg, 0.25 mmol) were dissolved in DMF (8 cm3) in a vial. The vial was placed in a Teflon-lined stainless steel autoclave and heated to 150 °C for 5 days. This produced colorless needle crystals of X-ray quality. Analysis found (calculated for C19H17NO5Ca): C 60.2 (60.1); H 4.6 (4.5); N 3.7 (3.7). IR (νmax/cm-1): 3282b, 1642m, 1591m, 1549m, 1409s, 1107m, 761s. 1H NMR δ (ppm): 7.85 (s, 1H, HCON), 7.77 (s, 2H), 7.65 (d, 2H, J ) 9.0), 7.38 (d, 2H, J ) 9.0), 3.99 (s, 2H), 2.92 (s, 3H, Me), 2.76 (s, 3H, Me). Synthesis of [Sr(DMF)(µ-BDC)]∞, 8. Strontium nitrate (53 mg, 0.25 mmol) and H2BDC (42 mg, 0.25 mmol) were dissolved in DMF (8 cm3) in a vial. The vial was placed in a Teflon-lined stainless steel autoclave and heated to 140 °C for 5 days. This produced colorless needle crystals of X-ray quality. IR (νmax/cm-1): 3366b, 1658m, 1560s, 1393s, 1104m, 757s. 1H NMR δ (ppm): 7.80 (s, 1H, HCON), 7.73 (s, 2H, H2/3/5/6), 2.88 (s, 3H, Me), 2.72 (s, 3H, Me). Synthesis of [Sr(DEF)(OH2)(µ-BDC)]∞, 9. Strontium nitrate (53 mg, 0.25 mmol) and H2BDC (42 mg, 0.25 mmol) were dissolved in DEF (8 cm3). The solution was stirred until dissolved and, then, heated without stirring to 120 °C for 5 days in a Teflon-lined stainless steel autoclave. This produced needle crystals of X-ray quality. Synthesis of [Sr(µ-DMF)(µ-NDC)]∞, 10. Strontium nitrate (53 mg, 0.25 mmol) and H2NDC (55 mg, 0.25 mmol) were dissolved in DMF (8 cm3) in a vial. The vial was placed in a Teflon-lined stainless steel autoclave and heated to 140 °C for 5 days. This produced colorless needle crystals of X-ray quality. Analysis found (calculated for C15H13NO5Sr): C 47.3 (48.1); H 3.4 (3.5); N 3.4 (3.7). IR (νmax/cm-1): 3349b, 1600m, 1550s, 1416s, 1353m, 1193m, 762s. 1H NMR δ (ppm): 8.32 (s, 2H), 7.95 (d, 2H, J ) 8.7), 7.85 (d, 2H, J ) 8.6), 7.80 (s, 1H, HCON), 2.88 (s, 3H, Me), 2.73 (s, 3H, Me). Synthesis of [Sr(µ-DEF)(µ-TPDC)]∞, 11. Strontium nitrate (17 mg, 0.08 mmol) and H2TPDC (24 mg, 0.08 mmol) were dissolved in DEF (3 cm3) in a vial. The vial was placed in a Teflon-lined stainless steel autoclave and heated to 125 °C for 5 days. This produced colorless needle crystals of X-ray quality. IR (νmax/cm-1): 3360b, 1663s, 1388s, 1250m, 1095m, 1194s, 760s. 1H NMR δ (ppm): 7.87 (s, 1H, HCON), 7.54 (s, 4H), 3.25 (q, 2H, (CH2), 3J ) 7.2), 3.22 (q, 2H, (CH2), 3J ) 7.3), 2.84 (m, 8H), 1.07 (t, 3H, Me, 3J ) 7.2), 1.00 (t, 3H, Me, 3J ) 7.3). Synthesis of [Sr(µ-DEF)(µ-PDC)]∞, 12. Strontium nitrate (30 mg, 0.14 mmol) and H2PDC (40 mg, 0.14 mmol) were dissolved in DEF (3 cm3) in a vial. The vial was placed in a Teflon-lined stainless steel autoclave and heated to 130 °C for 5 days. This produced colorless needle crystals of X-ray quality. IR (νmax/cm-1): 3358b, 1661s, 1436m, 1383s, 1260m, 1112m, 825m. 1H NMR, δ (ppm): 8.37 (s, 4H), 7.90 (s, 1H, HCON), 3.30 (q, 2H, (CH2), 3J ) 7.2), 3.25 (q, 2H, CH2, 3J ) 7.2), 1.16 (t, 3H, Me, 3J ) 7.2), 1.04 (t, 3H, Me, 3J ) 7.2). Synthesis of [Sr(µ-DMF)(µ-DADC)]∞, 13. Strontium nitrate (53 mg, 0.25 mmol) and H2DADC (67 mg, 0.25 mmol) were dissolved in DMF (8 cm3) in a vial. The vial was placed in a Teflon-lined stainless steel autoclave and heated to 150 °C for 5 days. This produced colorless needle crystals of X-ray quality. IR (νmax/cm-1): 3343b, 1659s, 1548m, 1388s, 1099m, 770m. 1H NMR δ (ppm): 7.84 (s, 1H, HCON), 7.76 (s, 2H), 7.65 (d, 2H, J ) 8.0), 7.38 (d, 2H, J ) 8.0), 3.98 (s, 4H), 2.93 (s, 3H, Me), 2.77 (s, 3H, Me). Synthesis of [Ba(µ-DMF)(µ-NDC)]∞, 14. Barium chloride dihydrate (61 mg, 0.25 mmol) and H2NDC (67 mg, 0.25 mmol) were dissolved
in DMF (6 cm3). The solution was stirred until dissolved and, then, heated without stirring to 100 °C for 5 days in a heavy walled glass vessel with a Teflon screw cap. Although this route produced colorless needle crystals, they were not of X-ray quality. Such crystals were only produced in the presence of bases such as 2,2′-bipyridine. IR (νmax/ cm-1): 3337b, 1558s, 1493m, 1410s, 1353s, 1190m, 785m. 1H NMR δ (ppm): 8.35 (s, 2H), 7.99 (d, 2H, 3J ) 8.4), 7.89 (d, 2H, 3J ) 8.4), 7.78 (s, 1H, HCON), 1.50 (t, 3H, Me, 3J ) 7.2), 1.44 (t, 3H, Me, 3J ) 7.2). Crystallography. Single crystal X-ray diffraction data were collected at 150 K using either Bruker SMART APEX (3, 8, 12, and 14) or Bruker SMART1000 (4–7, 9–11, and 13) CCD area detector diffractometers equipped with Oxford Cryosystems open flow cryostats.21 The radiation used was graphite monochromated Mo KR radiation (λ ) 0.71073 Å). Pertinent details of crystal data, data collection, and processing are given in Table 2. The structures were solved by direct methods using SHELXS9722 and refined by full-matrix least-squares on F2 using SHELXL97.23 Following location in Fourier difference syntheses, aromatic and aliphatic hydrogen atoms were placed in geometrically calculated positions and refined thereafter using a riding model. In 8, the water H atom was found from the difference Fourier map and refined with restraints. All fully occupied nonhydrogen atoms were refined with anisotropic displacement parameters. In the case of disorder, all atoms were refined with isotropic displacement parameters, and the crystal structures were treated as described below: In 3, the DMF molecule containing N2 was modeled for disorder over two positions, with occupancy 58%:42%. In 5, the DEF molecule was modeled for disorder over two positions, occupancy 72%:28%. In this case, the hydrogen atoms on the solvent molecules were omitted from the model. In 6, the saturated ring of the pyrene was modeled for disorder at C4 and C8 over two positions, occupancy 50%:50%. Disorder was also modeled for the DEF molecule over two positions, occupancy 50%:50%. In 7, the DMF was modeled for disorder over two positions over a symmetry element, occupancy 50%:50%. In 8, the DEF was modeled for disorder over two positions, occupancy 50%:50%. In 11, atoms C22 and C23 of the DEF were modeled for disorder over two positions, occupancy 50%:50%. In 12, the carbon atoms of the DEF were modeled for disorder over two positions, occupancy 71%:29%. In 13, the DMF was modeled for disorder over a mirror plane, with two half-occupied orientations with the nitrogen atom lying in the mirror plane and common to both orientations. In 14, only the barium atom could be refined with anisotropic displacement parameters. O3 was modeled for disorder over two positions, occupancy 50%:50%. PLATON SQUEEZE24 was used to model areas of diffuse electron density, 1.5 molecules of DMF per cell, included in cell contents and all values calculated from them. Structural diagrams were produced using either XP (cationic centers)25 or OLEX (chains and extended views).26
Results and Discussion Slow cooling of reaction mixtures comprising metal chloride or nitrate and aryldicarboxylic acid dissolved in dimethylformamide (DMF) or diethylformamide (DEF), previously heated to 100–150 °C for 5 days in autoclaves (1–3 and 5–13) or pressure tubes (4 and 14), yielded 12 hitherto unreported
914 Crystal Growth & Design, Vol. 8, No. 3, 2008
Figure 1.
Williams et al.
Metal Cation and Anionic Ligand MOFs
Crystal Growth & Design, Vol. 8, No. 3, 2008 915
Figure 1. Coordination geometries of the metal centers in (a and b) 2; (c and d) 3; (e and f) 4; (g) 5; (h) 6; (i) 7; (j) 8; (k) 9; (l) 10; (m) 11; (n) 12; (o) 13; (p) 14. Thermal ellipsoids are drawn at 30% probability.
compounds, together with materials 113 and 214 which have been reported previously. The molecular formulae of, and the synthetic routes leading to, materials 1–14 are collected in Table 3. All of frameworks 1–14 have been identified unambiguously by single crystal X-ray diffraction methods. It is important to emphasize that these crystalline products were often formed as mixtures together with starting materials and/or other products, both amorphous and crystalline, such that single phase bulk materials could often not be isolated. Cation Coordination Geometries, Carboxylate Binding and Building Block Architectures. The coordination geometries of the metal centers in 3–14 are shown in Figure 1 together with those for 2, which are included for comparative purposes. Pertinent interatomic distances in 3–14 are collected in Table 4. All of the Mg2+ cations are 6-coordinate [Figure 1a-d], all of the Sr2+ and Ba2+ cations are 8-coordinate [Figure 1j-p], and the intermediate sized Ca2+ cations are 6-, 7-, or 8-coordinate [Figure1e-i].Aswouldbeexpected,thelengthofthemetal · · · oxygen (M · · · O) bond increases as the radius of the metal cation increases, with average M · · · O bond lengths of 2.08, 2.42, 2.60, and 2.79 Å for Mg2+, Ca2+, Sr2+, and Ba2+ compounds, respectively (Table 4). The M · · · O bond also lengthens within the Ca frameworks as the coordination number increases from 6 to 8 with an average Ca · · · O bond length of 2.35, 2.39, and 2.45 Å for 6-, 7-, and 8-coordinate Ca2+, respectively (Table 4). The cationic building blocks in 3–14 are shown in Figure 2 together with those for 2, which are included for comparison purposes. The metal cations are bridged by carboxylate ligands
and, in some cases, by solvent molecules to form building blocks with either trinuclear as in 2 or polymeric metal-carboxylate chain motifs as in 3–14. In the latter complexes, the carboxylate moieties adopt a range of coordination modes in which they can bridge, chelate, or perform a combination of both, as illustrated in Scheme 2. The differing coordination of the metal cations by the carboxylate moieties can clearly be seen in Figure 2, which depicts the trinuclear cationic building block of 2 and typical examples, taken from 3, 4, 8, 9, and 11, of the polymeric cationic building blocks. In 1 and 2, the building block is a centrosymmetric trinuclear unit comprising three 6-coordinate Mg2+ cations bridged by NDC2- carboxylate moieties with two terminal dialkylformamide molecules at each end [Figure 2a].13,14 The central Mg2+ ion (Mg2) has an almost perfect octahedral geometry provided by four type a carboxylate and two c1 carboxylate oxygen donors [Figure 1b]. The flanking magnesium cations (Mg1) are slightly distorted from octahedral, with each cation having two type a carboxylate, one c2, and one c3 carboxylate oxygen donors, with the coordination sphere completed by the oxygens of two terminal dialkylformamide molecules [Figure 1a]. In 3 [Figure 2b], the building block is a chain of alternating centrosymmetric pairs of cations bridged by BDC2- carboxylate moieties with pendant solvent molecules bound to the metal cations. There are two crystallographically distinct 6-coordinate Mg2+ cations, which are slightly distorted from an octahedral geometry [Figure 1c and d]. Mg1 has two type a, two d2, and
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Table 4. M · · · O Interatomic Distances in the Metal Coordination Spheres of 3–14a (t ) terminal, br ) bridging, p ) pendant) 3
4
Mg1-O1(COO) a Mg1-O6(COO) a Mg1-O3(COO) d1 Mg1-O4(COO) d2 Mg1-O3(COO) d2 Mg1-O5(DMF) p
2.002(5) 2.014(4) 2.028(4) 2.143(4) 2.293(4) 2.070(4)
average Mg2-O9(COO) a Mg2-O8(COO) a Mg2-O7(COO) a Mg2-O2(COO) a Mg2-O4(COO) d1 Mg2-O10(DMF) p
2.09 1.991(4) 2.018(4) 2.044(5) 2.103(4) 2.245(4) 2.103(5)
average
2.08 6
Ca1-O2(COO) c1 Ca1-O2(COO) c1 Ca1-O2(COO) c2 Ca1-O2(COO) c2 Ca1-O1(COO) c3 Ca1-O1(COO) c3 Ca1-O3(DEF) br Ca1-O3(DEF) br average
a
2.315(4) 2.315(4) 2.347(3) 2.347(3) 2.379(3) 2.379(3)
Ca2-O1(COO) a Ca2-O5(COO) c2 Ca2-O6(COO) c3 Ca2-O4(COO) d1 Ca2-O4(COO) d2 Ca2-O3(COO) d2 Ca2-O7(DEF) p
2.35 2.276(4) 2.515(3) 2.370(4) 2.329(3) 2.447(3) 2.507(3) 2.299(5) 2.39
7 Ca1-O1(COO) c1 Ca1-O1(COO) c1 Ca1-O1(COO) c2 Ca1-O1(COO) c2 Ca1-O2(COO) c3 Ca1-O2(COO) c3 Ca1-O1S(DMF) br Ca1-O1S(DMF) br
2.638(2) 2.638(3) 2.482(2) 2.482(2) 2.677(2) 2.677(2) 2.516(4) 2.573(4) 2.59
Sr1-O4(COO) c1 Sr1-O1(COO) c1 Sr1-O1(COO) c2 Sr1-O4(COO) c2 Sr1-O3(COO) c3 Sr1-O2(COO) c3 Sr1-O5(DMF) br Sr1-O5(DMF) br
Sr1-O1(COO) c1 Sr1-O1(COO) c1 Sr1-O1(COO) c2 Sr1-O1(COO) c2 Sr1-O2(COO) c3 Sr1-O2(COO) c3 Sr1-O1S(DMF) br Sr1-O1S(DMF) br
2.321(2) 2.321(2) 2.506(2) 2.506(2) 2.457(2) 2.457(2) 2.495(2) 2.527(2) 2.45
Sr1-O4(COO) c1 Sr1-O4(COO) c2 Sr1-O3(COO) c3 Sr1-O2(COO) d1 Sr1-O1(COO) d1 Sr1-O2(COO) d2 Sr1-O1(COO) d2 Sr1-O5(DMF) p
2.497(3) 2.744(3) 2.554(4) 2.518(3) 2.537(2) 2.684(3) 2.703(3) 2.485(2) 2.59
11 2.475(2) 2.485(2) 2.650(2) 2.683(2) 2.565(2) 2.596(2) 2.580(2) 2.813(2) 2.61
13 2.476(2) 2.486(2) 2.627(2) 2.778(2) 2.532(2) 2.619(2) 2.637(2) 2.723(2) 2.61
Ca1-O1(COO) c1 Ca1-O1(COO) c1 Ca1-O1(COO) c2 Ca1-O1(COO) c2 Ca1-O2(COO) c3 Ca1-O2(COO) c3 Ca1-O3(DMF) br Ca1-O3(DMF) br
8 2.329(1) 2.329(1) 2.527(1) 2.527(1) 2.449(1) 2.449(1) 2.475(1) 2.537(1) 2.45
10
12 Sr1-O3(COO) c1 Sr1-O2(COO) c1 Sr1-O2(COO) c2 Sr1-O3(COO) c2 Sr1-O4(COO) c3 Sr1-O1(COO) c3 Sr1-O5(DEF) br Sr1-O5(DEF) br average
a a c1 c1 d1 d1
2.319(3) 2.319(3) 2.525(4) 2.525(4) 2.429(4) 2.429(4) 2.502(4) 2.540(4) 2.45
9 Sr1-O2(COO) b Sr1-O2(COO) b Sr1-O1(COO) d1 Sr1-O1(COO) d1 Sr1-O1(COO) d2 Sr1-O1(COO) d2 Sr1-O3(DEF) p Sr1-O4(H2O) p average
Ca1-O2(COO) Ca1-O2(COO) Ca1-O5(COO) Ca1-O5(COO) Ca1-O3(COO) Ca1-O3(COO)
5
Sr1-O1(COO) c1 Sr1-O4(COO) c1 Sr1-O4(COO) c2 Sr1-O1(COO) c2 Sr1-O2(COO) c3 Sr1-O3(COO) c3 Sr1-O5(DEF) br Sr1-O5(DEF) br
2.487(3) 2.491(3) 2.650(3) 2.792(3) 2.540(3) 2.604(3) 2.664(3) 2.705(3) 2.62
14 2.473(2) 2.473(2) 2.621(2) 2.621(2) 2.604(2) 2.604(2) 2.626(2) 2.628(2) 2.58
Ba1-O1(COO) c1 Ba1-O1(COO) c2 Ba1-O2(COO) c3 Ba1-O3(COO) c3 Ba1-O4(COO) c1 Ba1-O4(COO) c2 Ba1-O1S(DMF) br Ba1-O1S(DMF) br
2.759(12) 2.785(12) 2.747(16) 2.73(3) 2.662(12) 2.908(14) 2.853(11) 2.856(11) 2.79
For following letters a-d, see Scheme 2.
Scheme 2. Schematic Diagram of Coordination Modes of the RCOO- Moiety (a) Bridging, (b) Chelating, (c) Chelating and Bridging with One Oxygen, and (d) Chelating and Bridging with Two Oxygens
one d1 carboxylate oxygen donors, with the sixth site occupied by the oxygen of a pendant DMF molecule, while Mg2 has four type a and one d1 carboxylate oxygen donors, with the sixth site occupied by the oxygen of a pendant DMF molecule. In 4, the asymmetric unit comprises two crystallographically different Ca2+ cations, one on a center of inversion and one in a general position leading to a chain with a Ca(1)Ca(2)Ca(2) repetitive sequence [Figure 2c]. The cation on the center of
inversion [Ca(1); Figure 1e] shows a distorted octahedral geometry provided by two type a, two c1, and two d1 carboxylate oxygen donors. The cation in a general position [Ca(2); Figure 1f] has distorted pentagonal biprismatic geometry provided by one type a, one c2, one c3, one d1, and two d2 carboxylate oxygen donors, with the seventh site occupied by the oxygen of a pendant DEF molecule. In 5–7 and 10–14, all of which have the same 1:1:1 metal cation:solvent:dicarboxylate anion stoichiometry and which adopt similar space groups (Pnma for 5–7 and 13; P212121 for 10–12 and 14), the building blocks consist of a chain of metal cations bridged by carboxylate moieties (NDC2- for 5, 10, and 14, TPDC2- for 6 and 11; PDC2- for 12, DADC2- for 7 and 13) and either DMF (5, 7, 10, 13, and 14) or DEF (6, 11, and 12) molecules [Figure 2f]. All metal cations are equivalent and 8-coordinated with a square antiprismatic geometry provided by two c1, two c2, and two c3 carboxylate oxygen donors, with
Metal Cation and Anionic Ligand MOFs
Crystal Growth & Design, Vol. 8, No. 3, 2008 917
Figure 2. Views of the trinuclear cationic building block of (a) 2 and of the polymeric cationic building blocks of (b) 3, (c) 4, (d) 8, (e) 9, and (f) 11. Table 5. Carboxylate and DMF/DEF Coordination Modes in Compounds 1–14 DMF/DEF 1 Mg1 1 Mg2 2 Mg1 2 Mg2 3 Mg1 3 Mg2 4 Ca1 4 Ca2 5 Ca 6 Ca 7 Ca 8 Sr 9 Sr 10 Sr 11 Sr 12 Sr 13 Sr 14 Ba
coord no.
a
6 6 6 6 6 6 6 7 8 8 8 8 8 8 8 8 8 8
2 4 2 4 2 4 2 1
b
c1
c2
c3
d1
d2
term.
1
1
2
1
1
2
bridg
H2O term.
2 2 2 2 2 2 1
1 2 2 2 1
1 2 2 2 1
2 2 2 2 2
2 2 2 2 2
2 2 2 2 2
2
1 1 2 1
2
1 1
2
1 2 2 2
2 2
2 2
1 1
1 2 2 2 2 2
the final two sites filled with the oxygens of bridging DMF/ DEF molecules [Figures 1g-i and p)]. In 8 and 9, the building block is a zigzag chain of Sr2+ cations bridged by BDC2- carboxylate moieties with pendant DMF (8) [Figure 2d] or DEF and H2O (9) [Figure 2e] molecules. In both 8 and 9, the Sr2+ cations are 8-coordinate with a distorted square antiprismatic geometry. In 8, each Sr2+ cation has one c1, one c2, one c3, two d1, and two d2 carboxylate oxygen donors, with the eighth site occupied by the oxygen of a pendant DMF molecule [Figure 1j]. In 9, each Sr2+ cation has two b, two d1, and two d2 carboxylate oxygen donors, while pendant DEF and water molecules provide the oxygens for the seventh and eighth coordination sites [Figure 1k]. The different modes of coordination of the carboxylates observed in these structures are summarized in Table 5 with pertinent interatomic distances and angles in Table 6. Mode a is common for low coordination number compounds but is not observed for 8-coordinate geometries. Mode b is only observed in the 8-coordinate strontium center in 9. Although only a very limited number of examples of modes c1, c2, and c3 are
918 Crystal Growth & Design, Vol. 8, No. 3, 2008
Williams et al.
Table 6. Geometrical Parameters for Carboxylate Coordination to Mg2+, Ca2+, Sr2+, and Ba2+ in Compounds 1–14
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Crystal Growth & Design, Vol. 8, No. 3, 2008 919
Scheme 3. Schematic Diagram of an sp2-Hybridized Carboxylate Anion Showing the Lone Pairs (Red)
Table 7. Dimensions of the Diamond-Shaped Channels of the Frameworks Adopted by the BDC2- Bridged Compounds 3, 4, and 9 compound
3
4
9
minor diagonal, Å (j) major diagonal, Å (k) ratio (k/j) cross-sect. area, Å2 volume per metal node Å3 depth (chain length) per metal node, Å direction of chains metal node coordinating solvent
9.929 17.686 1.781 87.80 598.2 3.407 a Mg2+ DMF
10.640 17.850 1.678 94.96 631.8 3.327 c Ca2+ DEF
12.186 18.516 1.519 112.82 778.1 3.449 b Sr2+ DEF/H2O
observed for 6-coordinate metal centers, they are quite common for 7- and 8-coordinate ones. Modes d1 and d2, although uncommon, are found for 6-, 7-, and 8-coordinate geometries and are comparable to modes c1 and c2. Whereas terminal solvent (DMF/DEF) molecules have been found in 6-, 7-, and 8-coordinate geometries, bridging solvent molecules occur exclusively in 8-coordinate metal centers. Not unexpectedly, as the coordination number of the metal center increases, the tendency for the ligating oxygen atom, from both carboxylate and solvent, to bridge two metal centers increases. Inspection of the detailed geometrical parameters for the above metal–ligand interactions reveals some interesting trends. First, the M · · · O distances generally increase in length from mode a through mode c1/d1 and c3 to c2/d2 (Table 6). Second, for all four cations, the cation and chelating carboxylate moiety ligating through modes c2/d2 and c3 exhibit near coplanarity, as quantified by the deviation of the cation from the carboxylate plane (Table 6), but the cation often lies some distance from the carboxylate plane when ligated through modes a and c1/ d1. Third, a similar differentiation applies to the
