Isoreticular Investigation into the Formation of Four New Zinc

Jun 10, 2014 - ABSTRACT: Through the systematic investigation of zinc alkylbisphosphonates, four new structural families have been obtained...
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Isoreticular Investigation into the Formation of Four New Zinc Alkylbisphosphonate Families Kevin J. Gagnon,*,†,‡ Simon J. Teat,‡ Zachary J. Beal,†,§ Alyssa M. Embry,†,§ Megan E. Strayer,†,§ and Abraham Clearfield† †

Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States

‡

S Supporting Information *

ABSTRACT: Through the systematic investigation of zinc alkylbisphosphonates, four new structural families have been obtained. These families are named zinc alkyl-tunnel, -gate, -cation, and -sheet (ZAT, ZAG, ZAC, and ZAS) for convenience and have been synthesized and further extended through isoreticular design utilizing alkylbis(phosphonic acid) ligands of the formula H2O3PCnH2nPO3H2 (n = 3−6) (H4Ln). Both even- and odd-length chains were utilized to help determine the effect of chain conformation on structure formation. The investigation lead to two known compounds (ZAG-4, and ZAS-3) and nine new compounds, two of which contain large 1-D channels. The crystal structures of all compounds were determined by single-crystal X-ray diffraction. Of the nine new compounds, only seven of them fall into the new families. In three of the four families, the structure is controlled by alkyl-chain length and conformation (i.e., odd vs even), and in the fourth, a conformational distortion allows both odd and even lengths to form the given structure. Isoreticular species using n = 3 and 5 were obtained in both the ZAT and the ZAS families; using n = 4 and 6 were obtained in the ZAG family; and n = 4−6, in the ZAC family.

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INTRODUCTION Over the past decade, there has been a growing interest in coordination polymer framework materials.1−6 After the subset of these materials termed “metal−organic frameworks (MOFs)” was defined, a research effort of monumental proportions developed, which, at first, did not focus much on application but rather predominately on design and structural prediction. While much more research has developed on various applications of these materials since (i.e., gas storage/ separations,6 catalysis,7 ion exchange,8 etc.), the ability to design these materials has remained the driving force. Organic linkers utilizing carboxylic acid moieties have been the major focus; however, as early as the mid 1980s, there has been a similar research effort into utilizing phosphonic acids as the linker molecules.8−17 As a result of this effort, new applications for phosphonate materials have surfaced, such as nuclear waste remediation.18 Recent review papers have highlighted the similarities and contrasts between these two classes of linkers.14,19 What is most evident is the difference in the number of compounds falling into each group, with carboxylate frameworks far outweighing the others (i.e., phosphonic acids, sulfonic acids, imidazoles, etc.). Structural knowledge has been a cornerstone to metal− organic framework (MOF) synthesis and the development of the field; therefore, crystalline materials have been preferred. The difference between the carboxylates and the phosphonates has been the ability not only to form crystalline materials but also to design bonding models, both of which are significantly easier for carboxylates, and subsequently both being difficult for phosphonates. For phosphonate materials, authors make claims of rational design and structural control,20,21 and no subclass has been more subject to these generalizations than the © 2014 American Chemical Society

alkylbisphosphonates. It has in the past been suggested that the chain length determines structural type. Herein, a series of compounds are presented, which are all made with zinc acetate dihydrate as a starting material and four ligands of similar length, H2O3PCnH2nPO3H2 (n = 3−6) (H4Ln). It is shown that neither the metal nor the ligand chain length has much effect on the structure type, but rather, the synthetic conditions have the greatest effect. With the exception of one of the presented families (ZAC), there does appear to be a trend of odd versus even chain length adopting a given structural type.

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EXPERIMENTAL SECTION

Methods and Materials. Zinc acetate dihydrate, zinc sulfate heptahydrate, 1,3-dibromopropane, 1,4-dibromobutane, 1,5-dibromopentane, 1,6-dibromohexane, and triisopropylphosphite were all obtained from Sigma-Aldrich and used without further purification. All solvents were obtained through VWR International and used without further purification. The alkylbis(phosphonic acids) were synthesized similarly to preparations previously described in the literature,22 with modifications outlined in the Supporting Information. The deionized distilled water (ddi H2O) used was purified with a Barnstead Nanopure II System to a resistivity of 17.6 MΩ cm. Elemental analyses were performed by Atlantic Microlab Inc. (Norcross, GA). The steel pressure vessels used in these experiments were designed and manufactured in-house. Synthesis of Zinc Alkyl Gate Compounds (ZAG)(Zn(H2L)· H2O). In a typical experiment, Zn(Oac)2·2H2O (0.1 mmol in 2 mL ddi H2O), in a 20 mL glass scintillation vial, was adjusted to a pH of ∼1.8 using glacial acetic acid. In a separate vial, the alkylbis(phosphonic Received: April 23, 2014 Revised: June 4, 2014 Published: June 10, 2014 3612

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Table 1. Crystallographic Data for ZAG-4, ZAG-6, ZAS-3, and ZAS-5 sample

ZAG-4

ZAG-6

ZAS-3

ZAS-5

chemical formula formula mass crystal system space group λ (Å) a (Å) b (Å) c (Å) β (deg) Z V (Å3) temperature (K) density (g/cm3) measured reflections unique reflections parameters restraints Rint θ range (deg) R1, wR2 all data S (GOF) all data max/min res. dens. (e/Å3)

ZnP2O8C4H14 317.46 monoclinic C2/c 0.710 73 18.485(3) 8.2738(15) 8.2600(15) 114.461(2) 4 1149.9(4) 150(2) 1.834 5775 1133 69 0 0.0204 2.75−25.99 0.0177, 0.0468 1.120 0.304/−0.284

ZnP2O8C6H18 345.51 monoclinic C2/c 0.710 73 20.388(4) 8.2663(13) 8.1857(13) 102.720(13) 4 1345.7(4) 120(2) 1.705 7120 1395 84 1 0.0691 2.67−26.48 0.0350, 0.0856 1.037 0.322/−0.483

ZnP2O6C3H8 267.40 monoclinic P21/n 0.774 90 8.595(7) 5.056(4) 18.096(15) 93.344(9) 4 785.1(11) 100(2) 2.262 14 326 2032 109 0 0.0541 4.38−31.56 0.0289, 0.0794 1.084 0.501/−0.710

ZnP2O6C5H12 295.46 monoclinic P21/n 0.774 90 8.651(2) 5.0283(12) 22.598(5) 93.046(3) 4 981.6(4) 100(2) 1.999 13 137 2853 127 0 0.0534 3.81−33.07 0.0357, 0.0936 1.049 0.779/−0.974

acid) (0.2 mmol) was dissolved in 2 mL of ddi H2O and was subsequently layered on top of the zinc acetate solution and set undisturbed for 2 days to allow for crystal growth. The resulting large colorless crystals were filtered and washed with ethanol and dried at room temperature. (EA: ZAG-4: C: 15.13%, H: 4.44%, Found: C: 15.88%, H: 4.49%. ZAG-6: C: 20.86%, H: 5.25%, Found: C: 21.12%, H: 5.14%.) Alternatively, the alkylbis(phosphonic acid) (1.23 mmol in 2 mL of ddi H2O) was adjusted to a pH of 2.0 using 0.1 M NaOH. To this was added ZnSO4·7H2O (2.04 mmol in 2 mL ddi H2O). The resulting mixture was shaken and left to stand for 48 h undisturbed. The resulting large colorless crystals were filtered and washed with ethanol and dried at room temperature. Synthesis of Zinc Alkyl Tunnel Compounds (ZAT)(Zn3(H2L)(L)2(H2O)4. In a typical experiment, Zn(Oac)2·2H2O (0.3 mmol in 3 mL of ddi H2O), in a 20 mL glass scintillation vial, was adjusted to a pH of 1.8 using glacial acetic acid. In a separate vial, the alkylbis(phosphonic acid) (0.2 mmol) was dissolved in 3 mL of ddi H2O and was subsequently layered on top of the zinc acetate solution and set undisturbed for 2 days to allow for crystal growth. The resulting colorless needles were filtered and washed with water and dried at room temperature. (EA: ZAT-3: C: 10.75%, H: 3.31%, Found: C: 10.37%, H: 4.03%. ZAT-5: C: 16.53%, H: 4.16%, Found: C: 17.92%, H: 3.58%. Errors in these values are most likely due to the unknown solvent content in the pores.) Synthesis of Zinc Alkyl Sheet Compounds (ZAS)(Zn(H2L)). In a typical experiment, into a 25 mL PTFE-lined steel pressure vessel were added Zn(Oac)2·2H2 O (0.15 mmol) and the alkylbis(phosphonic acid) (0.15 mmol). To this were added 0.5 mL of ddi H2O and 0.5 mL of glacial acetic acid. The pressure vessel was sealed and heated for 4 days at 140 °C. The resulting colorless crystals were filtered and washed with ddi H2O and dried at room temperature. (EA: ZAS-3: C: 13.47%, H: 3.02%, Found: C: 12.65%, H: 2.35%. ZAS5: ZAS-5 was obtained as a mixture of crystals in an unidentified powder.) Synthesis of Zinc Alkyl Cation Compounds (ZAC)(Zn2(H2L)(L)·Zn(H2O)6·H2O·HO2C2H3). In a typical experiment, Zn(Oac)2· 2H2O (1.2 mmol in 3 mL ddi H2O), in a 20 mL glass scintillation vial, was adjusted to a pH of 1.8 using glacial acetic acid. In a separate vial, the alkylbis(phosphonic acid) (0.5 mmol) was dissolved in 3 mL of ddi H2O and was subsequently layered on top of the zinc acetate solution and set undisturbed for 2 weeks to allow for crystal growth.

The reaction produced a mixture of products; long colorless needles of the type ZAT (in the H4L5 case) form as well as the ZAC smaller blade-like colorless crystals. Synthesis of Zinc Alkyl Layered Compounds (ZAL)(Zn2(L)) and (Zn3(H2L)(L)). In a typical experiment, into a 25 mL PTFE-lined steel pressure vessel were added Zn(Oac)2·2H2O (0.15 mmol) and the alkylbis(phosphonic acid) (0.15 mmol). To this were added 0.5 mL of ddi H2O and 0.5 mL of glacial acetic acid. The pressure vessel was sealed and heated for 4 days at 140 °C. The resulting colorless crystals were filtered and washed with ddi H2O and dried at room temperature. (EA: ZAL-5: ZAL-5 was obtained as a mixture of crystals in an unidentified powder. ZAL-6: C: 16.78%, H: 2.54%, Found: C: 17.19%, H: 2.58%.) Single-Crystal X-ray Diffraction. Single-crystal diffraction data for ZAT-5, ZAG-4, and ZAG-6 were collected at 110−120(2) K on a Bruker APEX II diffractometer using Mo−Kα radiation. (λ = 0.71073 Å) The cell constants were indexed from reflections obtained from 36 frames with an exposure time of 20 s/frame. A full sphere (2400 frames at 6 cm detector distance) was collected with scan widths of 0.30° in ω. Data for ZAC-4 were collected as above with the following differences: the cell constants were indexed from reflections obtained from the full data collection using the Bruker APEX-II software suite. The program COSMO23 was used to calculate a data collection strategy based on phi and omega scans and multiple detector positions. Data for the remaining compounds were collected on Beamline 11.3.1 at the Advanced Light Source, Lawrence Berkeley National Lab, using monochromatic radiation (λ = 0.7749 Å (0.6888 Å for ZAC-5)) at 100(2) K. The cell constants were indexed from reflections obtained from 90 frames with an exposure time of 1 s/frame. A full sphere (2400 frames at 5 cm detector distance) was collected with scan widths of 0.30° in ω and an exposure time of 1 s/frame. Data reduction and cell refinement for all compounds were performed with SAINT.24 SADABS25 was used to obtain volume and absorptioncorrected data. The space groups were uniquely determined to be Pmmn, C2/c, P21/n, P21/c, C2/c, and P1̅ for ZAT, ZAG, ZAS, ZAC, ZAL-5, and ZAL-6, respectively. The structures were solved by direct methods and refined by full-matrix least-squares on F2 using the Bruker SHELXTL package.26 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed geometrically on the carbon atoms 3613

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Table 2. Crystallographic Data for ZAT-3, ZAT-5, ZAL-5, ZAL-6

a

sample

ZAT-3a

ZAT-5a

ZAL-5

ZAL-6

chemical formula formula mass crystal system space group λ (Å) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) Z V (Å3) temperature (K) density (g/cm3) measured reflections unique reflections parameters restraints Rint θ range (deg) R1, wR2 all data S (GOF) all data max/min res. dens. (e/Å3)

Zn3P4O16C6H22 670.22 orthorhombic Pmmn 0.774 90 19.397(3) 27.620(5) 5.1655(9) 90.00 90.00 90.00 4 2767.4(8) 100(2) 1.609 28 084 3019 142 0 0.0803 3.53−29.10 0.0450, 0.1212 1.062 1.787/−0.915

Zn3P4O16C10H30 726.33 orthorhombic Pmmn 0.710 73 24.408(4) 27.827(5) 5.1509(7) 90.00 90.00 90.00 4 3498.5(10) 110(2) 1.379 13 383 3009 158 0 0.1288 1.67−24.72 0.0613, 0.1608 1.027 0.564/−0.975

Zn2P2O6C5H9 357.80 monoclinic C2/c 0.774 90 21.775(4) 4.8267(8) 9.5273(17) 90.00 95.233(2) 90.00 4 997.2(3) 100(2) 2.383 4590 1291 69 0 0.0437 4.10−31.57 0.0335, 0.0907 1.078 0.785/−1.071

Zn3P4O12C12H26 682.38 triclinic P1̅ 0.774 90 8.6850(13) 9.9739(15) 12.8831(19) 100.409(2) 104.701(2) 90.945(2) 2 1059.4(3) 100(2) 2.139 10 669 6449 282 0 0.0499 2.79−33.61 0.0354, 0.1017 1.092 0.915/−0.970

Residual density in the pores has been removed via the Olex2 masking algorithm. Details can be found in the CIF file.27

Table 3. Crystallographic Data for ZAC-4, ZAC-5, and ZAC-6 sample

ZAC-4a

ZAC-5a

ZAC-6

chemical formula formula mass crystal system space group λ (Å) a (Å) b (Å) c (Å) β (deg) Z V (Å3) temperature (K) density (g/cm3) measured reflections unique reflections parameters restraints Rint θ range (deg) R1, wR2 all data S (GOF) all data max/min res. dens. (e/Å3)

Zn5P6O26C12H42 1115.12 monoclinic P21/c 0.710 73 10.5536(9) 17.3130(11) 10.3937(7) 99.502(5) 2 1873.0(2) 110(2) 1.977 20 167 3290 224 0 0.1128 1.96−25.02 0.1187, 0.3346 1.279 9.392/−1.520

Zn5P6O28C17H52 1196.21 monoclinic P21/c 0.6888 23.215(5) 17.112(5) 10.343(2) 94.529(10) 4 4095.9(17) 100(2) 1.974 49 817 12 627 547 366 0.0880 2.72−29.70 0.0614, 0.1725 1.007 3.177/−1.455

Zn5P6O28C20H58 1259.33 monoclinic P21/c 0.774 90 13.045(2) 17.192(3) 10.3905(18) 96.914(3) 2 2313.3(7) 100(2) 1.808 23 278 6217 314 31 0.0596 2.89−32.07 0.0476, 0.1261 0.999 1.471/−0.975

a

Data for ZAC-4 and ZAC-5 were of poor quality and showed signs of twinning and stacking faults that were not fully resolved. The structural features present were enough to determine that they are, in fact, part of the same family as ZAC-6; however, the crystallographic data are not of good quality. of the refinement, but were included in the chemical formula for completeness. Crystal parameters and information pertaining to data collection and refinement for all structures are summarized in Tables 1, 2, and 3. Important bonding distances and angles are listed in tables in the Supporting Information. Further crystallographic details, including CIF files, are available in the Supporting Information.

and refined using a riding model. The hydroxyl and water hydrogen atoms were found in the difference Fourier map. Their distance to their O atom was restrained (0.84(2) Å), but their coordinates were allowed to refine. Hydrogen atom displacement parameters were tied to the atom to which they were bonded. In any instances in which the H atoms could not be found in the difference map, they were left out 3614

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RESULTS Synthesis. A systematic investigation was needed to fully understand how alkylbisphosphonates can be used to design porous framework materials. This work aimed to investigate if isoreticular structures can be designed with divalent alkylbisphosphonates and covers the range of alkyl spacers using the ligand H4Ln, n = 3−6. Two techniques were utilized in investigating this system: (i) varying the pH of the reaction, and (ii) varying the metal-to-ligand starting ratios. The first technique is adapted from the procedure used by Clearfield et al.22 The metal salt was dissolved in a minimal amount of water and its pH adjusted to different amounts; then, a solution of the ligand dissolved in a minimal amount of water was layered on top. The procedure reported herein differs from that reported by Clearfield and co-workers in that it layers the two solutions, whereas they mixed the two solutions and placed them on a shaker for 24−72 h. The metal-to-ligand ratio was initially fixed at 2:1. Since these conditions worked well in the previously studied Cu2+ H4L5 derivative, the first reactions were set up with Zn2+ and the H4L5 ligand. The second technique was to fix the pH and to vary the metal-to-ligand starting ratio. The same process was followed with the metal being dissolved in a minimal amount of water and the pH adjusted to a fixed amount (∼1.8), and then a solution of the ligand dissolved in a minimal amount of water was layered on top. In both cases, once a new compound was synthesized, the conditions were adjusted to produce a phase pure product. After the optimal conditions were determined, then the ligand chain length was changed in an attempt to synthesize the same structural architecture with longer (or shorter) linkers. Single-Crystal X-Ray Diffraction. All of the compounds presented contain tetrahedrally coordinated zinc atoms in which all of the coordination sites are phosphonate oxygen atoms. The Zn−O bond lengths range from 1.8778(19) to 2.018(5) Å, which is in good agreement with those of previously reported zinc phosphonate materials.28 The PO3C tetrahedra in these structures all contain P−O bond distances of 1.484(6)−1.590(2) Å. The large range in the P−O bond distances can be attributed to the presence of protons on some of the oxygen atoms, which results in an elongation of the P−O bond compared to a chelated or double-bonded oxygen atom. Several of the presented compounds also have octahedrally coordinated zinc atoms in which some or all of the sites are water molecules. The Zn−O(w) bond lengths range from 2.004(12) to 2.166(5) Å. Zinc Alkyl Gate (ZAG-4,6). In 2003, Fu and co-workers presented a series of divalent phosphonates using H4L4 including the compound ZAG-4.29 Further investigation shows that this same compound had previously been synthesized by Dierdra Arnold at Texas A&M University and was presented in her Masters thesis with X-ray powder diffraction (XRPD) structural analysis.30 Both of these previous reports were in agreement with the structure of ZAG-4. Much like ZAG-4, ZAG-6 consists of 1-D chains that are hydrogen-bonded together into 2-D supramolecular sheets and cross-linked by the alkyl linkers to form a 3-D network. Figure S1 (Supporting Information) shows the asymmetric unit of ZAG-6, which crystallizes with the cell parameters given in Table 1. Bond lengths and angles are given in Table S1 (Supporting Information).

The Zn(1) center is bridged to four symmetry-related zinc ions through O-P-O bridges in two directions to form a 1-D chain of 8-membered rings, and through full-ligand bridges in the other two directions to link the 1-D chains together into the 3-D network, as shown in Figure 1. The ligand only loses one

Figure 1. Corner-shared 8-membered Zn-O-P-O rings that make up the 1-D inorganic chain unit of ZAG-6 (top). Localized hydrogenbonding scheme of ZAG-6 showing the interconnection between 1-D chains via the interstitial water molecules (bottom left). ZAG-6 viewed along the b axis showing the cross-linking of the 1-D chains to form the 3-D network (bottom right).

proton per phosphonic acid, and there are two interstitial water molecules (which are related by symmetry) hydrogen-bonded between the inorganic chains. Together with the water molecules, the 1-D chains are hydrogen-bonded together to form a 2-D supramolecular sheet through hydrogen bonds, {P(2)−O(3)−H(3C)···O(1W) 2.521(3) Å, 160(4)°}, {O(1W)−H(1WA)···O(3) 2.802(3) Å, 179(4)°}, and {O(1W)− H(1WB)···O(2) 2.795(3) Å, 169(4)°}. A localized diagram of this hydrogen-bonded network is shown in Figure 1. When viewed down the c axis (Figure 2), one can see the diamondshaped channels, of the wine-rack topology, filled with interstitial water molecules that lie between the inorganic chains. Identical reaction conditions with either H4L4 or H4L6 results in the production of ZAG-4 or ZAG-6, respectively. The differences between the two compounds lie in the unit-cell parameters and the angle of the alkyl chain with respect to the inorganic 2-D sheets. The connectivity is identical between the two structures; the only reason for the differences is to accommodate the change in linker length, which results in a change in unit-cell orientation to provide the most conventional β-angle. Our structure of ZAG-4 also agrees with those independently determined by Fu et al.29 and Arnold.30 Figure 2 shows a side-by-side comparison of ZAG-4 and ZAG-6. Zinc Alkyl Tunnel (ZAT-3,5). ZAT-3 consists of 1-D chains that are cross-linked in one direction by octahedral tetraaqua zinc centers and in the other direction by the alkyl chain to form a 3-D network with a large and a small 1-D channel. The asymmetric unit is shown in Figure S2 (Supporting Information). ZAT-3 crystallizes with the cell parameters given in Table 2. Bond lengths and angles are given in Table S2 (Supporting Information). The Zn(1) center is tetrahedrally coordinated by four phosphonate oxygen atoms and is bridged to four symmetryrelated Zn(1) centers through O-P-O bridges to form a 1-D 3615

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Figure 2. Side-by-side comparison of ZAG-4 (left) and ZAG-6 (right) viewed down the c axis.

connective array is one intrachain hydrogen bond, {O(3)− H(3)···O(4), 2.583(4) Å, 169(2)°}. This connectivity results in a large 3-D framework that contains 1-D tunnels of ∼0.7 nm in diameter (at its smallest direction based on van der Waals radii) and also parallel, much smaller, 1-D tunnels, as shown in Figure 4. OLEX2 void analysis shows that there are 822 Å3 of available space in the unit cell; this accounts for 31.9% of the cell volume.27 The contents of this tunnel have not been determined but are most likely a mixture of water and acetic acid, or just water; the void space analysis only returns a total of 85.8 e/Å3, which is low compared to the available volume for reasonable assignment. Identical reaction conditions with supplementation of H4L5 for H4L3 results in the production of ZAT-5, which has an identical topology to that of ZAT-3. ZAT-5 crystallizes with slightly different cell parameters due to the increase in the chain length. The large tunnel in ZAT-5 is ∼1.3 nm in diameter (at its smallest direction based on van der Waals radii) and also contains unknown solvent molecules as in ZAT-3. The void analysis of ZAT-5 suggests a void space of 1276.8 Å3 of accessible space containing 94.3 e/Å3, which amounts to 36.5% of the unit cell being solvent. Figure 4 shows a side-by-side comparison of ZAT-3 and ZAT-5. Zinc Alkyl Sheet (ZAS-3,5). In 1999, Clearfield and coworkers presented a pair of zinc frameworks made with H4L3, one of which was ZAS-3.31 This compound was synthesized via a different procedure and was solved via XRPD at room temperature. We have solved the structure of ZAS-3 and ZAS-5 via single-crystal X-ray diffraction at low temperature. ZAS-3 consists of 1-D inorganic chains that are cross-linked in two directions by the alkyl linkers to form 2-D slabs that are hydrogen-bonded together to form a supramolecular 3-D network. The asymmetric unit is shown in Figure S3 (Supporting Information). ZAS-3 crystallizes with the cell parameters given in Table 1. Bond lengths and angles are given in Table S3 (Supporting Information). ZAS-3 contains one symmetry independent zinc atom and one H4L3 ligand, in which each phosphonate group only has one deprotonated hydroxyl group. This combination results in a metal:ligand ratio of 1:1. The Zn(1) center is bridged to four symmetry-related Zn(1) centers through O-P-O bridges to

chain of 8-membered Zn-O-P-O rings, as shown in Figure 3. These 1-D chains are bridged together by an octahedral Zn(2)

Figure 3. Edge-shared fused 8-membered Zn-O-P-O rings that make up the 1-D inorganic chain of ZAT-3 (top). Fused 8-membered and 24-membered rings that make up the 2-D sheets of ZAT-3. Notice the tetrahedral zinc atoms form the 1-D chains, which are cross-linked by the octahedral zinc atoms (bottom).

via a Zn-O-P-O-Zn-O-P-O linkage to form 24-membered rings, as shown in Figure 3. The Zn(1) center is further bridged to 12 symmetry-related completely deprotonated full ligand bridges. The Zn(1) is even further bridged to two symmetry-related Zn(2) centers through both an O-P-O bridge and a partially deprotonated ligand bridging across its length. The last bridge for the Zn(1) center comes through the partially deprotonated, full ligand that bridges to one last Zn(1) center, which totals 19 zinc atoms to which a single Zn(1) is bridged to. This effectively amounts to 2-D sheets of 8- and 24-membered rings being cross-linked by the alkyl linkers. In total, there are two symmetry independent zinc atoms and two independent ligands, one that is fully deprotonated and one in which each phosphonate group is monodeprotonated. This total arrangement results in a metal:ligand ratio of 3:2. Contained in this 3616

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Figure 4. ZAT-3 (left) and ZAT-5 (right) as viewed down the c axis showing the various 1-D tunnels. Hydrogen atoms have been removed for clarity. Solvent molecules have been masked.

form 1-D chains of 12-membered Zn-O-P-O linkages, as shown in Figure 5. The Zn(1) centers are further bridged to six

Figure 6. Side-by-side comparison of ZAS-3 (top) and ZAS-5 (bottom) slabs. Figure 5. Fused 12-membered rings that form the tube-like 1-D chain of ZAS-3 (top). View down the 1-D tube (bottom left). Localized Hbonding in ZAS-3 (bottom right). This shows how neighboring slabs are connected to form the supramolecular 3-D network.

obtain two related compounds (C5H14N2)Zn4(O3P(CH2)nPO3)2(HO3P(CH2)n)PO3H), where n = 2 and 4.32 These compounds are structured with a double layer of tetrahedrally coordinated zinc centers bridged by phosphonate linkages connected to form 8- and 16-membered rings. The edges of these rings are shared, forming larger 24-membered rings. The double layers are cross-linked together by the alkylbis(phosphonic acid), and the large 24-membered rings contain a fully protonated 2-methylpiperazine for charge balance. The ZAC-6 asymmetric unit is shown in Figure S4 (Supporting Information). ZAC-6 crystallizes with the cell parameters given in Table 3. Bond lengths are given in Table S4 (Supporting Information). ZAC-6 consists of a similar double layer of tetrahedrally coordinated zinc centers as in the compound presented by Fu and co-workers. The layer is composed of a 1-D tube of fused 8-membered rings, as shown in Figure 7. The linkages in the chain are alternating “chair” and “boat” type. These chains are linked together by pairs of O-P-O bridges that combine to form 16-membered rings to form corrugated layers, as shown in Figure 7. The top-down view of these layers shows that the chains and bridges are edge-shared to form larger 24-membered

symmetry-related Zn(1) centers through full ligand bridges, resulting in a total of 10 Zn(1)-Zn(1) bridges. This connectivity forms the 2-D slabs, as shown in Figure 6. Neighboring slabs are further hydrogen-bonded together through a pair of P−O−H···O interactions, {P(1)−O(1)− H(6)···O(4) 2.632(3) Å, 157(2)°} and {P(2)−O(5)−H(5A)··· O(1) 2.699(3) Å, 150(3)°}, which are depicted in Figure 5. The chains run parallel to the b axis and hydrogen bond into 2D supramolecular sheets in the ab plane. Identical reaction conditions with supplementation of H4L5 for H4L3 results in the production of ZAS-5, which has an identical topology to that of ZAS-3. ZAS-5 crystallizes with slightly different cell parameters due to the increase in the chain length. Figure 6 shows side-by-side comparisons of ZAS-3 and ZAS-5. Zinc Alkyl Cation (ZAC-4,5,6). In similar work, using 2methylpiperazine as a template, Fu and co-workers were able to 3617

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Figure 7. 1-D tube-like chain of ZAC-6. The top-left image is viewed down the chain showing how it forms a tube; the top-right image is viewed perpendicular to the chain showing the 8-membered linkages. The linkages alternate between “boat” and “chair” conformations. ZAC-6 viewed down parallel 1-D tubes showing how they are bridged via O-P-O pairs to form new 16-membered rings that bridge to form the 2-D sheet (bottom).

axis to accommodate the extra ligand distortion; this results in two distinct inorganic layers in which the major difference is the orientation of the Zn(H2O)62+ ions. Figure 9 shows a side-byside comparison of the different ZAC compounds. Zinc Alkyl Layer (ZAL-5,6). ZAL-5 and ZAL-6 are not isoreticular compounds; however, both are stuffed pillared compounds of zinc alkylbisphosphonates, so they have been combined together for this section. They are synthesized via identical methods with only the ligand differing. ZAL-5 consists of 2-D inorganic sheets that are pillared by the H4L5 ligand to form a 3-D stuffed pillared compound. Figure S5 (Supporting Information) shows the asymmetric unit, which contains one zinc atom and half of a fully deprotonated ligand. ZAL-5 crystallizes with the cell parameters given in Table 2. Bond lengths and angles are given in Table S5 (Supporting Information). The Zn(1) center is connected to eight symmetry-related Zn(1) ions through O-P-O bridges and one additional Zn(1) through a pair of oxo-bridges. This connectivity results in a series of edge-shared 4-, 8-, 10-, and 12-membered rings that come together to form a complex 2-D layer in the bc plane, as shown in Figure 10. The layers are cross-linked by the L5 linker, completing the 3-D framework, as shown in Figure 11 viewed down the c axis. In this direction, it is clear that there are small elliptical tunnels that run along the axis direction. This is similar to the tunnels seen in many other alkylbisphosphonate coordination networks. ZAL-6 consists of 2-D inorganic sheets that are pillared as well to form a 3-D stuffed layered compound. ZAL-6 crystallizes with the cell parameters given in Table 2. The asymmetric unit consists of three different tetrahedral zinc centers and one L6 and two half H2L6 ligands. The whole ligand is fully deprotonated, whereas the two half ligands each only have one proton per phosphonate group. One of the half ligands is in a standard alkyl chain conformation, as is the fully deprotonated one, and the other half ligand is kinked in a corkscrew manner, as shown in Figure 11. The layer is constructed by 1-D chains of fused 8- and 10-membered rings, as shown in Figure 12, which are further bridged to form the 2D sheet by pairs of O-P-O bridges, as shown in Figures 10 and 12.

rings, as shown in Figure 8. The difference from Fu and coworkers’ compound is that the charge balance is obtained by

Figure 8. ZAC-6 viewed down the a axis showing the 24-membered rings. Some of the encapsulated hexaaqua zinc ions have been removed to better show the tunnels.

the inclusion of a Zn(H2O)62+ ion, in the center of the 24membered rings instead of the 2-methylpiperazine. The layer viewed down the a axis shows these rings as well as the encapsulated ion, as shown in Figure 8. The double layers are connected together by a pair of symmetry independent bis(phosphonic acid) linkers. One linker is fully deprotonated, and the other has one proton per phosphonic acid group. Figure 9 shows that, due to the corrugation of the layer, the fully deprotonated ligand is in a normal alkyl chain conformation and the partially deprotonated ligand is disordered over two positions due to a distortion in the torsional angles of the central carbon atoms. The 1-D channels that run along the a axis that are formed by the 24membered rings contain the Zn(H2O)62+ ions as well as one water molecule and one acetic acid molecule. Identical reaction conditions with varying of the ligand from H4L6 carbons to H4L5 to H4L4 carbons results in the production of ZAC-5 and ZAC-4 respectively, which have identical topology to that of ZAC-6. ZAC-5 has an elongated a 3618

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Figure 9. ZAC-4 (top), ZAC-5 (bottom left), and ZAC-6 (bottom right) viewed down the c axis. Disordered chains only have one position shown, and the acetic acid molecules have been removed for clarity.

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DISCUSSION

Zn H4L5 structures in the literature to date, which suggests that the use of these alkyl(phosphonic acids) has stalled in current research. The diversity of the structures we have presented here should be cause for reconsideration as to the usefulness of these flexible linkers. One of the common features between these families is the inclusion of monodeprotonated phosphonic acid groups. This results in all of the compounds, except for ZAL-5, containing 1D inorganic chain building units. In a few cases (ZAT, ZAC, and ZAL), these 1-D chains are further cross-linked to form layers; however, it is the 1-D chains that show some of the most unique structural diversity. The simplest of the chains are found in the ZAG family, which contains corner-fused 8-membered rings. Stepping up in complexity are the ZAT chains, which have edge-fused 8-membered rings forming a ladder-type arrangement. The ZAC family also contains edge-fused 8membered ring 1-D chains, except, in this case, the rings alternate between “chair” and “boat” conformations. The ZAS family has chains made of fused 12-membered rings. Lastly, ZAL-6 has a mixture of fused 8- and 10-membered rings. Figure 13 shows a side-by-side comparison of all of these 1-D chains.

A recent review paper outlined the various work conducted in alkyl(phosphonic acid)-based frameworks.14 Two main examples stand out that have attempted systematic investigation of these materials with modification of chain length, one by Zubieta and co-workers using vanadium and one by Attfield and co-workers using gallium.20,33 Combining our work with the aforementioned, we can come to the conclusion that these alkylbisphosphonates can vary greatly in structure; however, with careful control over reaction conditions, isoreticular extension is possible. For some frameworks, this conclusion is true regardless of the ligand length; for others, it is limited by the conformation of the ligands due to the odd vs even length. What our work has shown is that, unlike the previous systematic investigations, our results do not suggest that structural type is limited to the length of the alkyl linker. (i.e., Zubieta and co-workers’ study, where the structural types are grouped by chain length). We show here, for example, that the H4L5 linker is involved in four different structural archetypes, ZAT, ZAS, ZAC, and ZAL. There are no reported instances of 3619

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Figure 12. 1-D chain in ZAL-6 looking down the tube (top left) and along it (top right). ZAL-6 viewed down the 1-D tubes showing the pairs of O-P-O bridges between neighboring chains (bottom).

potential applications. Several of these families contain ordered channels of hydrogen bonds that may be useful for proton conduction pathways. Shimizu and co-workers have suggested that the MOFs may serve as useful proton conduction materials,34 and phosphonate materials, in particular, have shown this property.35 ZAT, in particular, may be useful for this purpose because the substitution of the octahedral metal may allow for the ability to tune proton conduction. Preliminary work has shown that modification of this octahedral site has a drastic effect on the overall structure of the material. ZAT5(Mg) was synthesized under the same conditions as those of ZAT-5, with a small fraction of the Zn being substituted for Mg. As shown in Figure S6 (Supporting Information), the connectivity of the framework is retained; however, the symmetry breaks from Pmmn to P212121 as the pillars lean over and collapse. This ZAT-5(Mg) appears to be the extreme of structural distortion; however, a future study will show the effect of varying added cation concentration vs structural distortion. The ZAG family also appears to be suited for proton conduction. Recently, the ZAG-4 material had its structural resilience showcased in a high-pressure diffraction experiment that showed that the material survives pressures up to 10 GPa, reversibly.36 A similar study on ZAG-6 is currently underway as well as a detailed study of the hydrogen-bonding array in this material. It is interesting to note that, in all of the presented structures, there exist tetrahedrally coordinated zinc centers. This fact leads to the presumption that there may be even more structural variations with zinc centers that are octahedral and even other coordination environments.

Figure 10. ZAL-5 viewed down the a axis showing the 2-D layer (top). ZAL-6 viewed down the c axis showing the 2-D layer (bottom).

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CONCLUSION Rigid carboxylic and phosphonic acids have played an important role in the development of porous MOFs for gas storage and separation; however, the huge diversity of structures that can be obtained with alkyl(phosphonic acids) suggests that the narrow framing of rigid ligands may be missing potentially useful materials. While permanent porosity may be a challenge with these materials, there exist a number of other potential applications in piezo- and thermofunctionality as well as ion exchange and conductivity. Future work is still necessary to tap into the myriad of applications that these materials may be suited for. It is clear that total control over structural topology is a long way off for phosphonate-based materials; however, isoreticular extension seems to be very capable, given the correct control over reaction conditions.

Figure 11. ZAL-5 viewed down the c axis showing the ligand pillars linking together the 2-D sheets to form the 3-D network. ZAL-6 viewed down the b axis. Notice the two different conformations of the alkyl chain, normal (magenta) and kinked (blue).

The importance of the structural diversity of zinc alkylphosphonates presents itself in the varying materials’ 3620

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Figure 13. Comparison of the 1-D chain building units in ZAG, ZAT, ZAC, ZAS, and ZAL-6 (from top to bottom, respectively).

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ASSOCIATED CONTENT

* Supporting Information

Crystallographic information files (CCDC reference numbers 999118−999128), tables containing pertinent bond distances and angles as well as a figure of the ZAT-5(Mg) compound can be found in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.J.G.). Notes

The authors declare no competing financial interest. § Work conducted as an undergraduate scholar.

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REFERENCES

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ACKNOWLEDGMENTS

The authors would like to thankfully acknowledge the National Science Foundation for providing funding through grants DMR-0652166, HRD-0832993, and DGE-0750732. We would also like to acknowledge the Robert A. Welch Foundation for supplemental funding through grant A0673. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. De-AC02-05CH11231. 3621

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