Chemical and Structural Diversity in Chiral Magnesium Tartrates and

Chemical and Structural Diversity in Chiral Magnesium Tartrates and their Racemic and Meso Analogues Kinson C. Kam, Karen L. M. Young, and Anthony K. Cheetham* Materials Research Laboratory and International Center for Materials Research, UniVersity of California, Santa Barbara, California 93106-5121

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 8 1522-1532

ReceiVed April 23, 2007

ABSTRACT: Nine crystalline modifications of magnesium tartrate (tart2- ) C4H4O6) have been synthesized by hydrothermal reactions between magnesium acetate tetrahydrate and the D,L-, D-, and meso-forms of tartaric acid in the temperature range 100200 °C. Two racemic crystals, a dimeric Mg(D,L-tart)(H2O)2‚3H2O in space group Pban, (1), and a two-dimensional coordination polymer Mg(D,L-tart)(H2O)‚3H2O, P2/c, (2), were obtained from the D,L-acid reactions at 100 °C and 125-150 °C, respectively. Two chiral structures were obtained from the D-acid reactions at these temperatures: a layered Mg(D-tart)(H2O)‚1.5H2O, P212121, (3), and a three-dimensional (3-D) network, Mg(D-tart)‚1.5H2O, C2221, (4). In addition, an anhydrous 3-D coordination polymer, Mg(D-tart), I222, (5), was obtained by dehydrating 3 under vacuum at 85 °C. Finally, four achiral structures were obtained from reactions with the meso-acid at 100 °C, 125-150 °C, 180 °C, and 200 °C, respectively: three one-dimensional (1-D) chains, Mg(meso-tart)(H2O)2‚H2O (6, P21/n, and 7, Pbca) and Mg(meso-tart)(H2O)2, P21/c, (8), and one 3-D framework, Mg(meso-tart), C2/c, (9). Within each family, the structures become less hydrated and more condensed with increasing temperature, indicating that the reactions proceed under thermodynamic control. The chiral D-tartrates, 3 and 4, adopt entirely different architectures compared to the racemic D,L-tartrates, 1 and 2, with the former exhibiting higher dimensionalities and lower levels of hydration than their D,Lanalogues formed at the same temperatures. At 180 and 200 °C, the D,L-magnesium tartrate undergoes spontaneous resolution to form a conglomerate corresponding to one of the lower temperature D-phases (4). The unexpected complexity of this ostensibly simple system points to the extraordinary richness of the emerging field of chiral inorganic-organic framework structures. Introduction There has been an explosion of interest in hybrid inorganicorganic framework materials during the past decade, especially in the fields of carboxylates and phosphonates.1-4 The high level of interest has been driven by both the diversity of structural and chemical types and the possible applications of hybrid framework materials in areas such as catalysis,5,6 separations,7,8 and hydrogen storage.9,10 One particularly interesting subset of hybrid frameworks concerns those that have chiral structures, such as tartrates and systems based upon naturally occurring amino acids.11 They represent a unique class of extended (i.e., non-molecular) solids that can be readily synthesized as chiral solids having potential applications in enantiomerically selective catalysis and separations, and other areas.12-14 By contrast, purely inorganic materials, such as aluminosilicate zeolites, can only be obtained in chiral forms by physical separation of mixtures of enantiomers (conglomerates). The structural diversity of chiral framework structures and their racemic analogues is only just beginning to be explored. One of the chiral forms of nickel aspartate, for example, contains a single helix, while its racemic analogue contains both hands of the same helix.15 A second form of nickel aspartate contains chiral layers, while its racemic analogue contains both hands of the layer.16 In the present work, we describe a series of magnesium tartrate coordination polymers in which the relationship between the homochiral structures and their racemic and meso analogues is much more complex and subtle. The existence of four crystalline forms of tartaric acidsthe chiral D- and L-forms, the racemic D,L-form, and the achiral meso-formshas been known since the remarkable discoveries of Louis Pasteur over 150 years ago.17,18 They contain closely related molecular structures (Scheme 1) but can have different * To whom correspondence should be addressed. E-mail: cheetham@ mrl.ucsb.edu.

Scheme 1.

Configurations of D-, L-, and meso-Tartaric Acida

a Gray atoms: C, white atoms: H, red atoms: O. The sizes of the atoms are scaled to the corresponding van der Waals radii.

physical properties both in the solid state and in solution. For example, the D- and L-acids are optically active, while the racemic D,L-form and the meso form are optically inactive. Our study of magnesium tartrate framework materials has yield nine crystalline modifications: two racemic crystals obtained from the D,L-acid, three chiral structures obtained from the D-acid, and four achiral structures containing the acid in the meso form. Unlike the crystalline forms of tartaric acid, these new materials, which were formed at a variety of temperatures, exhibit a wide variety of strikingly different structures with varying dimensionalities, connectivities, and chemical compositions. Experimental Procedures All reagents, magnesium acetate tetrahydrate [Mg(CH3COO)2‚4H2O] (99%, Strem Chemicals), and D,L-tartaric acid (99%, Aldrich), D-tartaric acid (99%, Aldrich), and meso-tartaric acid (98+%, TCI America) were purchased and used as received under aerobic conditions.

10.1021/cg070388a CCC: $37.00 © 2007 American Chemical Society Published on Web 07/06/2007

Chiral Hybrid Frameworks

Crystal Growth & Design, Vol. 7, No. 8, 2007 1523 Table 1. Compositions, Crystal Data, and Structural Refinement Parameters for 1-9a

empirical formula formula wt crystal size, mm crystal system space group T (K) a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z dcalc, g/cm3 µ, mm-1 Flack parameter reflections collected unique reflections observed data (I > 2σ(I)) Rint R1, wR2 (I > 2σ(I))b R (all data) GOF

empirical formula formula wt crystal size, mm crystal system space group T (K) a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z dcalc, g/cm3 µ, mm-1 reflections collected unique reflections observed data (I > 2σ(I)) Rint R1, wR2 (I > 2σ(I))b R (all data) GOF a

1

2

3

4

5

Mg(D,L-C4H4O6)(H2O)2 ‚3H2O 262.46 0.2 × 0.1 × 0.08 orthorhombic Pban (No. 50) 293 13.576(6) 12.124(5) 13.478(6) 90 90 90 2218.3(16) 8 1.572 0.208

Mg(D,L-C4H4O6)(H2O) ‚H2O 208.41 0.2 × 0.2 × 0.15 monoclinic P2/c (No. 13) 293 10.815(4) 7.332(2) 10.033(3) 90 111.529(5) 90 740.0(4) 4 1.871 0.257

7250 2181 1599 0.0525 0.0451, 0.1168 0.0639, 0.1273 1.081

5745 1551 1274 0.0357 0.0442, 0.1179 0.0548, 0.1271 1.056

Mg(D-C4H4O6)(H2O) ‚1.5H2O 217.42 0.2 × 0.2 × 0.2 orthorhombic P212121 (No. 19) 293 7.9134(6) 11.1318(8) 18.3143(14) 90 90 90 1613.3(2) 8 1.79 0.244 0.0(2) 12895 3527 3369 0.0325 0.0338, 0.0894 0.0352, 0.0918 0.856

Mg(D-C4H4O6) ‚H2O 190.4 0.25 × 0.25 × 0.2 orthorhombic C2221 (No. 20) 293 7.5530(7) 8.8704(9) 10.5761(10) 90 90 90 708.58(2) 4 1.785 0.250 0.1(5) 2651 736 700 0.0256 0.0315, 0.0820 0.0330, 0.0834 1.084

Mg(D-C4H4O6) 172.38 0.25 × 0.2 × 0.1 orthorhombic I222 (No. 23) 293 5.1708(14) 9.359(3) 11.765(3) 90 90 90 569.3(3) 4 2.011 0.287 1342 516 484 0.0241 0.0523, 0.1287 0.0563, 0.1311 1.132

6

7

8

9

Mg(meso-C4H4O6) (H2O)2‚H2O 226.43 0.5 × 0.2 × 0.08 monoclinic P21/n (No. 14) 293 5.8204(8) 9.8167(13) 14.926(2) 90 92.252(2) 90 852.2(2) 4 1.765 0.239 6077 1761 1555 0.0238 0.0414, 0.1069 0.0464, 0.1106 1.073

Mg(meso-C4H4O6) (H2O)2‚H2O 226.43 0.25 × 0.1 × 0.07 orthorhombic Pbca (No. 61) 293 10.0616(14) 11.2358(16) 14.920(2) 90 90 90 1686.7(4) 8 1.783 0.242 8062 1812 1286 0.0452 0.0575, 0.1237 0.0849, 0.1359 1.292

Mg(meso-C4H4O6) (H2O)2 208.41 0.2 × 0.2 × 0.15 monoclinic P21/c (No. 14) 293 5.8316(18) 13.034(4) 10.226(3) 90 99.286(5) 90 767.1(4) 4 1.805 0.248 5808 1610 1365 0.022 0.0412, 0.1173 0.0461, 0.1228 1.091

Mg(meso-C4H4O6) 172.38 0.15 × 0.1 × 0.05 monoclinic C2/c (No. 15) 293 6.866(4) 9.204(5) 9.459(5) 90 108.156(9) 90 568.0(5) 4 2.016 0.288 2203 575 361 0.0748 0.0581, 0.1345 0.0984, 0.1447 1.162

The compositions show coordinated water and extra-framework water separately. b R1 ) ∑ ||Fo| - |Fc||/∑ |Fo|. wR2 ) [∑w(Fo2 - Fc2)2/∑w(Fo2)2]1/2.

Synthesis of [Mg(D,L-C4H4O6)(H2O)2]‚3H2O, 1. A mixture of Mg(CH3COO)2‚4H2O (1.00 mmol), D,L-tartaric acid (1.00 mmol), and 10 mL of deionized (DI) water was placed in a 20 mL scintillation vial. The vial was capped tightly and heated at 100 °C for 48 h (initial and final pH ∼ 4). The final product consisted of colorless rectangular blockshaped crystals. The crystals were then washed with water and dried in air at 60 °C. Elemental analysis (C, H, N) was carried out by the Marine Sciences Institute Analytical Laboratory at UCSB. Anal. found (wt %): C, 18.23; H, 4.33. Calculated (wt %): C, 18.29; H, 5.33. A similar reaction in a 23 mL PTFE lined autoclave was attempted under the same conditions, but only a clear solution was obtained. Synthesis of [Mg(D,L-C4H4O6)(H2O)]‚H2O, 2. A mixture of Mg(CH3COO)2‚4H2O (1.00 mmol), D,L-tartaric acid (1.00 mmol), and 10 mL of DI water was placed in a 23 mL polytetrafluoroethylene (PTFE) lined stainless steel autoclave and stirred for 10 min. The autoclave was then sealed and heated at 125-150 °C for 48 h (initial and final pH ∼ 4). The final product, consisting of colorless crystals, was washed with water and dried in air at 60 °C. Anal. found (wt %): C, 24.84; H, 3.03. Calculated (wt %): C, 23.03, H, 3.84. Synthesis of [Mg(D-C4H4O6)(H2O)]‚1.5H2O, 3. A mixture of Mg(CH3COO)2‚4H2O (1.00 mmol), D-tartaric acid (1.00 mmol), and 10

mL of DI water was placed in a 23 mL PTFE lined autoclave and stirred for 10 min. The autoclave was then sealed and heated at 100 °C for 48 h (initial and final pH ∼ 4). The final product, consisting of colorless crystals, was washed with water and dried in air at 60 °C. Anal. found (wt %): C, 22.10; H, 4.11. Calculated (wt %): C, 22.08; H, 4.14. Synthesis of [Mg(D-C4H4O6)]‚H2O, 4. A mixture of Mg(CH3COO)2‚ 4H2O (1.00 mmol), D-tartaric acid (1.00 mmol), and 10 mL of DI water was placed in a 23 mL PTFE lined stainless steel autoclave and stirred for 10 min. The autoclave was then sealed and heated at 125-150 °C for 48 h (initial and final pH ∼ 4). The final product, consisting of colorless crystals, was washed with water and dried in air at 60 °C. Anal. found (wt %): C, 25.14; H, 2.97. Calculated (wt %): C, 25.21; H, 3.15. Synthesis of [Mg(D-C4H4O6)], 5. Crystals of structure 5 were formed by heating crystals of 3 under vacuum at 85 °C for 12 h in an alumina boat. Anal. found (wt %): C, 27.75; H, 2.34. Calculated (wt %): C, 27.84; H, 2.32. Synthesis of [Mg(meso-C4H4O6)(H2O)2]‚H2O, 6. A mixture of Mg(CH3COO)2‚4H2O (1.00 mmol), meso-tartaric acid (1.00 mmol), and 10 mL of DI water was placed in a 23 mL PTFE lined stainless steel

1524 Crystal Growth & Design, Vol. 7, No. 8, 2007

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Table 2. Resulting Compositions, Space Groups, Dimensionalities, and Number of Magnesium Atoms Per 1000 Å3 are Presented as a Function of Temperature for the Three Families of Magnesium Tartrates

autoclave and stirred for 10 min. The autoclave was then sealed and heated at 100 °C for 48 h (initial and final pH ∼ 4). The final product, consisting of colorless crystals, was washed with water and dried in air at 60 °C. Anal. found (wt %): C, 21.41; H, 4.16. Calculated (wt %): C, 21.20; H, 4.41. Synthesis of [Mg(meso-C4H4O6)(H2O)2]‚H2O, 7. The same procedure as 6 is followed. The PTFE autoclave was then sealed and heated at 125-150 °C for 48 h (initial and final pH ∼ 4). The final product, consisting of colorless crystals, was washed with water and dried in air at 60 °C. Anal. found (wt %): C, 21.22; H, 4.09. Calculated (wt %): C, 21.20; H, 4.41. Synthesis of [Mg(meso-C4H4O6)(H2O)2], 8. A mixture of Mg(CH3COO)2‚4H2O (1.00 mmol), meso-tartaric acid (1.00 mmol), and 10 mL of DI water was placed in a 23 mL PTFE lined stainless steel autoclave and stirred for 10 min. The autoclave was then sealed and heated at 180 °C for 48 h (initial and final pH ∼ 4). The final product, consisting of colorless crystals, was washed with water and dried in air at 60 °C. Anal. found (wt %): C, 24.08; H, 3.45. Calculated (wt %): C, 23.03; H, 3.84. Synthesis of [Mg(meso-C4H4O6)], 9. A mixture of Mg(CH3COO)2‚ 4H2O (1.00 mmol), meso-tartaric acid (1.00 mmol), and 10 mL of DI water was placed in a 23 mL PTFE lined stainless steel autoclave and stirred for 10 min. The autoclave was then sealed and heated at 200 °C for 48 h (initial and final pH ∼ 4). The final product, consisting of colorless crystals, was washed with water and dried in air at 60 °C. Anal. found (wt %): C, 27.52; H, 2.47. Calculated (wt %): C, 27.84; H, 2.32. 9 can also be prepared at higher temperature at 220 °C to obtain colorless crystallites, as verified by X-ray diffraction analysis. Characterization. Crystal Structure Determination. For ease of reference, the crystallographic data for all nine magnesium tartrates coordination polymers 1-9 are assembled in Table 1. A suitable single crystal of each of the nine compounds was carefully selected under a polarizing microscope and glued to a thin glass fiber with cyanoacrylate adhesive. Crystal structure determination by X-ray diffraction was performed on a Siemens SMART-CCD diffractometer equipped with a normal focus, 2.4 kW sealed tube X-ray source (Mo KR radiation, λ ) 0.71073 Å) operating at 50 kV and 40 mA. A hemisphere of intensity data was collected at room temperature. An empirical correction based on symmetry equivalent reflections was applied using the SADABS program.19 All structures were solved by direct methods using SHELX97 and difference Fourier syntheses.20 The relevant details of structure determinations are presented in Table 1. Full matrix least-squares refinements against |F2| were carried out using the SHELXTL-PLUS package of programs.21 The last cycles of refinement included atomic positions and anisotropic thermal parameters for all atoms. The final atomic coordinates and thermal parameters are presented in Tables S1-

S18 (Supporting Information). PLATON was used to determine that no obvious space group change is needed or suggested.22 Powder X-ray Diffraction. X-ray powder diffraction data were collected on a Philips X’PERT powder diffractometer using Cu KR radiation (λ ) 1.5418 Å). The powder diffraction patterns were scanned over an angular range of 5-60° (2θ) with a step size of 0.0167° and a counting time of 5 s step-1. The phase purity of the products was confirmed by comparison of the observed X-ray powder patterns with the simulated powder patterns calculated using the single-crystal X-ray structure and the PowderCell 2.4 program. Thermal Stability Studies. Thermogravimetric analysis (TGA) measurements were carried out in air at a heating rate of 5 °C/min using a Mettler-Toledo TGA/SDTA851 instrument. Variable temperature X-ray powder diffraction, VT-XRD (Cu KR radiation, λ ) 1.5418 Å), was performed under static air using a Bruker D8 Advance diffractometer outfitted with a M. Braun Position Sensitive detector and an Anton Parr HTK 16 high-temperature stage. Patterns were scanned with a temperature ramp of 0.5 °C min-1.

Results Single crystals of eight magnesium tartrate phases, 1-4 and 6-9, were obtained by hydrothermal synthesis, and crystals of an additional phase, 5, were formed by thermal treatment of 3. All crystal structures were determined by single-crystal X-ray diffraction. The compositions, synthesis temperatures, space groups, dimensionalities, and densities (in terms of numbers of magnesium ions per 1000 Å3) are summarized in Table 2. X-ray powder diffraction patterns for the racemic, chiral, and meso structures are shown in Supporting Information, Figures S1S3, respectively. Racemic Structures. Molecular Dimers of Structure 1, [Mg(D,L-C4H4O6)(H2O)2]‚3H2O. The asymmetric unit of 1 is made of two crystallographically independent magnesium atoms, a single tartrate ligand, and several water molecules (Figure 1). Two of the water molecules (O4 and O8) are coordinated to the magnesium atoms, and the four others are hydrogen bonded to the rest of the structure (O9, O10, O11, and O12). Most of the M-O distances are in the range of 2.015-2.062 Å, but the Mg-O bonds to the alkoxy oxygens are somewhat longer at around 2.1 Å. The Mg-O distances to the terminal aqua groups are the shortest ones in this structure.


Figure 1. Asymmetric unit of [Mg(D,L-C4H4O6)(H2O)2]‚3H2O, 1. Thermal ellipsoids are given at 50% probability with 20% probability for hydrogen.

Crystal Growth & Design, Vol. 7, No. 8, 2007 1525

Figure 3. Asymmetric unit of [Mg(D,L-C4H4O6)(H2O)]‚H2O, 2. Thermal ellipsoids are given at 50% probability with 20% probability for hydrogen.

Figure 2. Polyhedral representation of structure 1 as molecular dimers in the unit cell. Lighter shades represent Mg1 pairs, and dark shades of dimers represent Mg2 pairs.

Figure 4. View of the layered structure of 2 along the crystallographic b-axis showing a tetranuclear motif forming the framework channels.

The structure of 1 contains neutral molecular dimers rather than an extended network (Figure 2). As with all the other structures described below, the magnesium ions are octahedrally coordinated and are represented as MgO6 octahedra in the figure. 1 is the low temperature (100 °C) form of the D,L-tartrate, and its centrosymmetric space group (Pban) ensures that both hands of the tartrate ligand are present. The two magnesium atoms lie on a mirror plane. Each dimeric structure consists of two MgO6 octahedra bridged by the tartrate ligands; one dimer contains only Mg1 ions, while the other contains only Mg2. Two terminal water molecules coordinated to each metal octahedron in 1 and two dangling oxygen atoms (O2 and O6) from the carboxylate ends of each tartrate ligand ensure that the structure is molecular rather than extended, contrary to what we observe in 3 and 4 (see below). Each dimer is surrounded by water molecules and is oriented in the same fashion in the ab plane, and all neighboring units alternated along the c-axis by a rotation of 90°. 2-D Framework of Structure 2, [Mg(D,L-C4H4O6)(H2O)]‚ H2O. Reactions with the D,L-acid at higher temperatures (125150 °C) yield structure 2. The asymmetric unit consists of one crystallographically independent magnesium atom and one crystallographically independent tartrate ligand (Figure 3). In addition, there are two water molecules, one coordinated to magnesium (Mg-O7) and the other lying in a small cavity (O8). The MgO6 octahedron involves three oxygens from carboxylate

groups, one water molecule, and two alkoxy oxygens (O3 and O6). As in the previous structure, the Mg-O distances to the alkoxy ligands are longer than the others. All oxygen atoms from the tartrate ligand coordinate to the MgO6 octahedra, except for the carboxylate oxygen, O2, which is dangling and prevents any coordination to neighboring layers. This is reminiscent of the layer morphology observed in 3, where the dangling oxygen atoms from the tartrate ligand preclude the formation of higher dimensionality structures. Again, the structure is centrosymmetric (P2/c) and contains both hands of the tartrate anion. The layered structure of 2 exhibits two types of small channels when viewed perpendicular to the layer along the b-axis (Figure 4). One channel is made of tetrameric rings, which consist of four MgO6 octahedra connected to each other by four tartrate ligands; two of the tartrate ligands chelate to the metal via the carboxylate and alkoxy groups, while in the other two only one terminal carboxylate from each ligand coordinates to two MgO6 octahedra. The terminal water molecules from the MgO6 octahedra protrude into the ring. The other channel, an elongated dimeric ring, consists of two individual MgO6 octahedra connected to each other through two tartrate ligands. The free guest water molecules lie close to the wall of this elongated channel. There is a hydrogen bond between one of the alkoxy groups of the tartrate ligand and the water oxygen, with an O8-O6 distance of 2.62 Å. The


Kam et al.

Figure 5. View of the extended structure of 2 parallel to the layers along the crystallographic a-axis. Figure 7. View of the extended structure of 3 down the c-axis showing the integrity of the channels in the framework.

Figure 6. Asymmetric unit of [Mg(D-C4H4O6)(H2O)]‚1.5H2O, 3. Thermal ellipsoids are given at 50% probability with 20% probability for hydrogen.

corrugated nature of the layer topology of 2 is apparent when the structure is viewed along the a-axis (Figure 5). Chiral Frameworks. 2-D Framework of Structure 3, [Mg(D-C4H4O6)(H2O)]‚1.5H2O. Using chiral ligands, reactions with D-tartaric acid, under conditions identical to those that were used to form 1 and 2, yielded two entirely different compositions and structures. In both cases, the chirality of the ligand was preserved, as confirmed by both the structures and the circular dichroism measurements. Similar results on the preservation of chirality under hydrothermal conditions have been observed previously by Williams and co-workers.11b Compound 3 was obtained at 100 °C and, as expected, is a chiral structure in space group P212121. It is isostructural with a cobalt analogue that was reported in the L-form.23 The asymmetric unit of 3 consists of two crystallographically independent magnesium atoms bound to each other and to two crystallographically independent tartrate ligands via the carboxylate groups and the adjacent OH groups in each D-tartrate ligand (Figure 6). The two D-tartrate ligands play different roles, as described below. There are also three oxygen atoms in the asymmetric unit corresponding to guest water molecules (O15, O16, and O17). Of the six oxygen atoms octahedrally coordinated to each of the two metal centers, one oxygen atom (Mg1-O13 and Mg2O14) is a terminal water molecule and the other five belong to the D-tartrate ligands. While most of the metal-oxygen (MO) distances are in the range of 2.020-2.048 Å, the oxygen atoms coordinated to the alkoxy groups of the D-tartrate ligands are again slightly elongated in the range 2.109-2.143 Å. It is

Figure 8. Projection of the extended structure of 3 down the a-axis.

interesting to note that of the four elongated M-O distances, the two longer ones (Mg1-O9 and Mg2-O10) appear to experience a trans effect due to the opposite terminal water molecule. The structure can be viewed as being made up from the dimers that comprise the asymmetric unit. The two magnesium atoms in each dimer are connected by two bridges, one involving the tartrate group containing C1-C4 and the other involving the tartrate containing C5-C8 (Figure 6). These dimers are then connected to other dimers through the C5-C8 tartrate via O8 and O11; the corresponding oxygens on tartrate C1-C4 (O2 and O6) are dangling. The extended structure can be viewed as being constructed from dimers that are connected to other dimers through one of the tartrate groups. Overall, 3 comprises neutral layers lying in the ab plane and containing a hexameric ring motif that is formed from the dimers described above (Figure 7). It is a twodimensional (2-D) coordination polymer. There are three H2O guest molecules within the channels formed by the rings, and two terminal water molecules from two of the six MgO6 octahedra point toward the center of each ring (Figure 7). A closer look reveals that the layer topology of 3 is corrugated when viewed from the side along the a-axis (Figure 8). 3-D Framework of Structure 4, [Mg(D-C4H4O6)]‚H2O. Reactions with D-tartaric acid at 125-150 °C produce 4 in the chiral space group C2221. The asymmetric unit of 4 is very simple, consisting of one crystallographically independent



Figure 9. Asymmetric unit of [Mg(D-C4H4O6)]‚H2O, 4. Thermal ellipsoids are given at 50% probability with 20% probability for hydrogen.

Figure 11. View of the extended structure of 4 along the crystallographic a-axis.

Figure 10. Extended structure of 4 along the [110] direction showing a trinuclear motif.

magnesium atom, one D-tartrate ligand, and one free water molecule, O4, that is hydrogen bonded to a carboxylate oxygen, O2 (Figure 9). The M-O distances are in the range 1.9912.119 Å, with the coordination to the oxygen of the alkoxy group being the longest (Mg-O3), as in structure 1. This higher temperature phase is a three-dimensional (3-D) coordination polymer with an anhydrous framework that accommodates guest water molecules within a small channel. When viewed along the [110] direction (Figure 10), we can see a trimeric ring architecture made of three individual MgO6 octahedra connected to each other via D-tartrate ligands. All oxygen atoms in the MgO6 octahedra belong to the D-tartrate ligands; no terminal aqua groups are present. Of the six oxygen atoms in each MgO6 octahedron, two equivalent O1 oxygens, which are cis, are coordinated to the terminal carboxylate ends of two D-tartrate ligands. The other four oxygen atoms (2 × O2 and 2 × O3) are coordinated to the carboxylate group and to the adjacent OH group in each of two D-tartrate ligands. Closer examination reveals that the D-tartrate ligand bridges four neighboring MgO6 octahedra and serves as the backbone when viewed along the a-axis (Figure 11). Unlike 3, there are no dangling oxygen atoms in 4 from the carboxylates of the D-tartrate ligand. 3-D Framework of Structure 5, Mg(D-C4H4O6). Heating crystals of 3 under vacuum yielded crystals of an anhydrous phase, 5. The asymmetric unit consists of one crystallographically independent magnesium atom bound to the carboxylate oxygen, O1, and alkoxy oxygen, O3, of the D-tartrate ligand (Figure 12). The other carboxylate oxygen, O2, is coordinated to the next neighboring magnesium atom. Of the six oxygen

Figure 12. Asymmetric unit of [Mg(D-C4H4O6)], 5. Thermal ellipsoids are given at 50% probability with 20% probability for hydrogen.

Figure 13. Projection of the extended structure of 5 along the a-axis.

atoms in each MgO6 octahedron, the M-O distances are in the range 2.082-2.119 Å, with the coordination to the oxygen of the alkoxy group being the longest (Mg-O3). The M-O bonds that are trans to the alkoxy group have the shortest bond distance (Mg-O2). 5 is an anhydrous 3-D coordination polymer with an architecture (Figure 13) that is quite different from that of 3 or 4. Again, the D-enantiomer of the tartrate ligand is preserved in space group I222. It is isostructural with a cobalt analogue that was reported in the L-form.24 When viewed along the a direction, we can see two types of dimeric ring architectures made of two MgO6 octahedra connected to each other by two D-tartrate


Figure 14. Projection of the extended structure of 5 along the b-axis.

Kam et al.

Figure 16. Asymmetric unit of [Mg(meso-C4H4O6)(H2O)2]‚H2O, 6. Thermal ellipsoids are given at 50% probability with 20% probability for hydrogen.

Figure 17. View of the extended structure of 6 showing 1-D chains running along the b direction in a zigzag manner.

Figure 15. Projection of the extended structure of 5 along the c-axis.

ligands depending on the positions of the alkoxy groups. When the alkoxy groups are located within the channel, they coordinate to the neighboring magnesium atoms and therefore restrict the size and diameter of the channels. On the other hand, absence of the alkoxy groups prevents further coordination to the metal atoms, and larger channels are created. The role of the D-tartrate ligands can be seen along the b direction, where they act as a bridge and connect neighboring MgO6 octahedra to form a 3-D network with dangling hydrogen atoms (H2 and H3) pointing into a small channel (Figure 14). The organic backbone/layers can be seen when viewing the structure along the c-axis (Figure 15). Meso Structures. Reactions were also carried out with the meso-form of tartaric acid. Since meso-tartaric acid contains a center of symmetry, it is optically inactive. Four products, 6-9, were obtained at 100 °C, 125-150 °C, 180 °C, and 200 °C, respectively. 1-D Chain of Structure 6, [Mg(meso-C4H4O6)(H2O)2]‚H2O. Reactions with meso-tartaric acid at 100 °C yield 6 with space group P21/n. The asymmetric unit consists of one crystallographically independent magnesium atom, one meso-tartrate ligand, and several water molecules (Figure 16). There are two water molecules coordinated to the magnesium atom (O7 and O8) and one free water molecule (O9). The MgO6 octahedron involves two oxygens from carboxylate groups (O1 and O5),

two water molecules, and two alkoxy oxygens (O3 and O4). The M-O distances to the carboxylate ligands (2.094 and 2.084 Å) are longer than to the alkoxy ligands and the coordinated water molecules. All oxygen atoms from the meso-tartrate ligand coordinate to the MgO6 octahedra, except for two carboxylate oxygens, O2 and O6, which are dangling and prevent any coordination to neighboring chains. With two additional water molecules coordinated to the metal center, they preclude the formation of higher dimensionality structures. The low-temperature phase 6 comprises 1-D chains with MgO6 octahedra connected to each other with meso-tartrate ligands in a zigzag fashion running along the b direction (Figure 17). Free water molecules occupy the space between the chains. The effect of the 21 screw axis can be seen clearly when viewed down the ac plane (Figure 18). 1-D Chain of Structure 7, [Mg(meso-C4H4O6)(H2O)2]‚H2O. Reactions with meso-tartaric acid at 125-150 °C yielded crystals of 7 in the space group Pbca. It has the same chain motif and composition identical to 6 with a similar coordination environment. 7 is isostructural with a cobalt analogue.25 The asymmetric unit of 7 consists of one crystallographically independent magnesium atom bound to one meso-tartrate ligand. The MgO6 octahedron involves two oxygens from carboxylate groups (O1 and O5), two water molecules, and two alkoxy oxygens (O3 and O4). There are two water molecules coordinated to the magnesium atom (O7 and O8), and one free water molecule (O9). The M-O distances in each MgO6 octahedron are in the range 2.000-2.116 Å, with the coordination to the oxygen of the carboxylate group being the longest (Mg-O5). The carboxylate oxygens in 7 (O2 and O6), like those in 6, are also dangling and prevent any coordination to neighboring chains (Figure 19).



Figure 21. View of the extended structure of 7 down the b-axis. Figure 18. View of the extended structure of 6 along the chains down the b-axis.

Figure 19. Asymmetric unit of [Mg(meso-C4H4O6)(H2O)2]‚H2O, 7. Thermal ellipsoids are given at 50% probability with 20% probability for hydrogen.

Figure 22. Asymmetric unit of [Mg(meso-C4H4O6)(H2O)2], 8. Thermal ellipsoids are given at 50% probability with 20% probability for hydrogen.

Figure 20. View of the extended structure of 7 showing 1-D chains running along the b direction.

While the chemical composition is the same as 6, the 1-D chains in 7 have a slightly different conformation. The MgO6 octahedra are connected to each other with meso-tartrate ligands along the b-direction (Figure 20). Because of the mirror planes from 7, the packing orientations of the chains are different than those of 6 when viewed down the b-axis (Figure 21). 1-D Chain of Structure 8, [Mg(meso-C4H4O6)(H2O)2]. At 180 °C, reactions with meso-tartaric acid yield 8 with space group of P21/c. The structure of 8 is very similar to that of 6 and 7 but has no water molecules between the chains. The

asymmetric unit consists of one crystallographically independent magnesium atom, one meso-tartrate ligand, and two terminal water molecules (O7 and O8) (Figure 22). The six oxygens octahedrally coordinated to MgO6 include two carboxylate oxygens (O1 and O6) and two alkoxy oxygens (O3 and O4) of the meso-tartrate ligand. The M-O distances are in the range 1.996-2.135 Å, with the oxygens of the alkoxy group (O3 and O4) being the longest. The remaining ligand oxygens (O2 and O5) are uncoordinated and remain dangled in the structure, which prevents any linkage to neighboring chains. The structure of 8 can be viewed as zigzag 1-D chains with MgO6 octahedra connected to each other through meso-tartrate ligands along the c direction (Figure 23). Since there are no free water molecules in 8, the chains are more densely packed than in 6 and 7. The effect of the 21 screw axis can be seen when the chains are viewed down the c-axis (Figure 24). 3-D Framework of Structure 9, [Mg(meso-C4H4O6)]. Reactions with meso-tartaric acid at the highest temperature (200 °C) yielded 9, which adopts an entirely different structure compared to 6-8. It is anhydrous, like 5, and is an anhydrous 3-D coordination polymer in the space group C2/c. The asymmetric unit of 9 consists of one crystallographically independent magnesium atom coordinated to the meso-tartrate ligand (Figure 25). Each magnesium atom is octahedrally coordinated by six oxygen atoms from the carboxylate oxygens (O1 and O2) and alkoxy oxygen O3 of the meso-tartrate ligand. The M-O distances are in the range 1.996-2.090 Å. No dangling oxygens are present in the asymmetric unit.


Kam et al.

Figure 23. View of the extended structure of 8 along the a-axis showing conformation of 1-D chains running in a zigzag manner along the c direction.

Figure 26. View of the extended structure of 9 down the crystallographic [110] axis.

Figure 24. View of the extended structure of 8 down the c-axis showing the effects of 21 screw axis.

Figure 27. View of the extended structure of 9 down the crystallographic c-axis.

Figure 25. Asymmetric unit of [Mg(meso-C4H4O6)], 9. Thermal ellipsoids are given at 50% probability with 20% probability for hydrogen.

In 9, an elongated tetrameric ring architecture made of four individual MgO6 octahedra connected to each other via mesotartrate ligands can be observed when viewed along the [110] direction, where the organic ligands serve as a backbone of the framework (Figure 26). When viewed along the c direction, slightly larger channels are observed in the 3-D network (Figure 27). As in 5, dangling hydrogen atoms (H2) protrude into the channels. Thermal Analysis. The TGA and VT-XRD results for 1-9 reflect the variations in their water contents. For example, the molecular dimeric complex 1 prepared from D,L-tartaric acid is

Figure 28. Thermogravimetric analysis for the racemic magnesium tartrates, 1 and 2.

the least robust of the structures and becomes poorly crystalline upon modest heating. A gradual weight loss from 80 to 150 °C is due to the liberation of uncoordinated water molecules (Figure 28), while further weight loss, observed up to 285 °C, is attributed to the dehydration of the coordinated water molecules. Additional weight loss between 285 and 330 °C, followed by a



Phases 1, 2, and 4 show reversible dehydration and rehydration of the guest water molecules with little evidence of structural alteration. After the removal of isolated water molecules, rehydration takes place in the presence of flowing water vapor over 3 h in air. X-ray diffraction further confirms that the structures remain intact. Compound 3 does not exhibit a reversible dehydration-rehydration process because it transforms to the anhydrous phase 5 on heating, as described above. Discussion

Figure 29. Thermogravimetric analysis for chiral tartrates, 3-5.

D-magnesium

Figure 30. Thermogravimetric analysis for meso-magnesium tartrates, 6-9.

final step between 425 and 475 °C, is caused by the decomposition of the tartrate ligand and the formation of amorphous MgO. The total weight loss for the decomposition of 1 was 82.6 wt %, compared with an expected value of 84.6 wt %. In the case of structure 2, a more robust behavior is observed compared to 1, with two distinct stages of weight loss (Figure 28). The first change around 200 °C corresponds to loss of the free water molecule, while the coordinated water molecule appears to remain intact until the structure decomposes at around 400 °C. The total observed weight loss (77.0%) agrees reasonably well with the calculated value of 80.6%. Broadly similar results are found for the chiral and mesofamilies of compounds (Figures 29 and 30, respectively), with the lower temperature phases showing weight loss due to dehydration processes and all the frameworks collapsing to amorphous MgO at around 400 °C. The total weight losses were in good agreement with our crystallographic findings: compound 3 obs. 80.7, calc. 81.4%; compound 4 obs. 78.3%, calc. 78.8%; compound 5 obs. 80.3%, calc. 76.6%; compound 6 obs. 81.6%, calc. 82.2%; compound 7 obs. 80.7%, calc. 82.2%; compound 8 obs. 77.6%, calc. 80.6%; compound 9 obs. 76.1%, calc. 76.6%. One interesting point to note from the VT-XRD results is that compound 3 transforms to compound 5 during the first dehydration step (see Supporting Information, Figure S4).

The results obtained for the different magnesium tartrate systems as a function of temperature are summarized in Table 2. The table also includes the results of reactions with D,L- and D- tartaric acid at 180 and 200 °C. Although these did not yield any new phases, they did provide another interesting finding. Structure 4, which forms at 125-150 °C with D-tartaric acid, is also found at the higher temperatures. No racemization took place at 180 and 200 °C, as verified by circular dichroism (see Supporting Information). Surprisingly, however, the higher temperature reactions with the D,L-acid also yielded phase 4. In this case, of course, both D- and L-forms are present, and spontaneous resolution has taken place to form the conglomerate. It is known26 that only 5-10% of molecular systems undergo spontaneous resolution, but the statistics for framework materials are unknown at the present time. This appears to be the first example of such behavior in hybrids. The cobalt analogues of three of our structures (3, 5, and 7) have been observed previously, but they were all reported by different groups who were using different synthetic methods and starting materials. By contrast, the nine magnesium tartrates discovered during the course of the present work offer some remarkable insights into the phase behavior of chiral inorganicorganic framework materials and their achiral analogues. The evolution of the three systems (D,L-, D-, and meso-) with temperature (Table 2) exhibits the general features that have recently been observed in other hybrid systems where thermodynamic control appears to prevail:27-29 the structures become less hydrated with increasing temperature and form more condensed structures with higher degrees of connectivity. The higher temperature phases have lower heats of formation but are stabilized by the entropy gain arising from the loss of water into solution.29 Thus, the lower temperature (100 °C) phase (1) obtained from reactions with D,L-tartaric acid contains a simple molecular (0-D) structure, while reactions at higher temperatures lead to (125-150 °C) the formation of a less hydrated 2-D coordination polymer (2), and finally (180-200 °C) to an even less hydrated 3-D coordination polymer (4). Turning to the D-tartrates, the low temperature (100 °C) phase, 3, is a 2-D layered structure, while the higher temperature product, 4, is a 3-D coordination polymer and contains fewer water molecules. Similarly, the meso-structures, 6-9, become less hydrated with increasing temperature, and the chainlike connectivity of 6-8 gives way to a 3-D architecture for 9 at the highest temperature. The TGA plots in Figures 28-30 reflect these changes in hydration and stability. We can also draw some more subtle conclusions about the relative energies of the D,L- and D-products. The phases obtained with D-tartrate at both 100 °C and 125-150 °C, 3 and 4, preserve the single enantiomer and adopt structures in the chiral space groups P212121 and C2221, respectively. Most interestingly, these have strikingly different structures and even different compositions compared with the two racemic phases, 1 and 2. At 100 °C, the racemic 0-D structure, 1, is not an option for 3, since the former requires both enantiomers of the ligand. Instead,


3 adopts a more condensed 2-D coordination polymer that is based upon a dimer that shows some similarity to that seen in 1. We must conclude that 3 is less stable than 1, since otherwise reactions with the D,L-acid at 100 °C would yield a conglomerate of 3. For the higher temperatures reactions with the D-enantiomer (125-150 °C), the formation of the racemic 2-D structure, 2, is again not a possibility in the absence of racemization. In its place, we find a chiral 3-D coordination polymer, 4, rather than the racemic 2-D coordination polymer, 2. We again must conclude that 4 is less stable than 2. An unexpected consequence of these trends is that the use of the chiral D-ligand results in more condensed and less hydrated structures than the D,Ltartrates, almost as if we were increasing the temperature. This is underlined by the observation that a conglomerate of 4 forms at higher temperatures with the D,L-acid. It is less hydrated than 2 and has a higher dimensionality, as expected. Conclusions The results of the present work reveal that hybrid inorganicorganic frameworks formed from chiral ligands and their achiral analogues can exhibit unexpected complexity. With a single metal in combination with the three different isomers of the tartrate ligand, we have obtained one molecular structure, three chains, two layered structures, and three 3-D frameworks. At the lower temperatures, products obtained with a single enantiomer adopt topologies that are entirely different from those obtained under the same conditions with racemic mixtures of ligands. Such diversity of structures is not normally apparent when comparing molecular organic structures of racemic crystals with their chiral analogues, and nor can it be found in purely inorganic systems. While this complexity appears to be unique to inorganic-organic framework materials, not all chiral hybrids display such behavior, as was discussed earlier for the nickel aspartates. Nevertheless, the opportunity to find such diversity and complexity suggests that this area may be much richer than had previously been thought. Given the potential applications of these materials in areas such as chiral separations, enantiomerically selective catalysis, nonlinear optics, and polarized photoluminescence, the field merits a much greater level of effort than it has received hitherto. Acknowledgment. This work was supported by the MRSEC Program of the National Science Foundation (NSF) under Award No. DMR 05-20415, and the International Center for Materials Research of the NSF under Award No. DMR 04-09848. K.C.K. thanks G. Wu for assistance with preparing single crystal for analysis under inert atmosphere condition. Supporting Information Available: Crystallographic data for compounds 1-9 in CIF format as well as additional figures and tables of bond lengths and angles. This material is available free of charge via the Internet at http://pubs.acs.org.

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CG070388A

Chemical and Structural Diversity in Chiral Magnesium Tartrates and

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