Article pubs.acs.org/crystal
Dihydrogen Phosphate Clusters: Trapping H2PO4− Tetramers and Hexamers in Urea-Functionalized Molecular Crystals Arbin Rajbanshi, Shun Wan, and Radu Custelcean* Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6119, United States S Supporting Information *
ABSTRACT: Co-crystallization of two urea-functionalized ligands with tetrabutylammonium (TBA) dihydrogen phosphate resulted in the isolation of discrete (H2PO4−)4 and (H2PO4−)6 clusters stabilized in the crystalline state by multiple urea hydrogen bonds. Structural analysis by single-crystal X-ray diffraction, combined with a Cambridge Structural Database survey of (H2PO4−)n aggregates, established that these clusters display unique topologies and hydrogen-bonding connectivities.
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INTRODUCTION Phosphate is one of the most ubiquitous and important anions in nature and, as a result, has been the subject of numerous anion coordination studies.1 In neutral aqueous solutions, phosphate is present preponderantly in its diprotonated H2PO4− form (along with HPO42−). As dihydrogen phosphate has both proton donors and acceptors, it tends to aggregate into oligomeric and polymeric structures in concentrated solutions and in the solid state. Raman spectroscopy studies found evidence for discrete (H2PO4−)n (n = 2−4) clusters in highly concentrated solutions, alongside monomer and higher-order oligomers.2 In crystals, on the other hand, one can find relatively few examples of discrete dihydrogen phosphate clusters compared to polymeric or extended-network structures. Examples of discrete crystalline (H2PO4−)n clusters include dimers,3 a trimer,4 tetramers,5 and a hexamer.4 Many of these structures contain ligands functionalized with hydrogen-bonding groups that can effectively engage in hydrogen bonds with H2PO4−, thereby disrupting the formation of extended networks. One hydrogen-bonding group that can effectively bind tetrahedral oxoanions, including phosphate, both in solution and the crystalline state, is urea.6 Introduction of multiple urea groups in a ligand often leads to enhanced binding and recognition of this class of oxoanions. For example, ortho-phenylene-bisurea has proven very efficient in binding anions like sulfate and phosphate by chelating two of their adjacent O−X−O edges.7 Here, we show that cocrystallization of the urea-functionalized ligands L1,2 with tetrabutylammonium (TBA) dihydrogen phosphate results in the isolation of discrete H2PO4− tetrameric and hexameric clusters with unique hydrogen-bonding motifs, stabilized in the crystalline state by multiple urea hydrogen bonds. To place these results in the appropriate context, we also performed a Cambridge Structural Database (CSD) survey of (H2PO4−)n aggregates, in order to identify the types and incidences of various clusters and networks formed by dihydrogen phosphate in organic crystals. © 2013 American Chemical Society
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RESULTS AND DISCUSSION CSD Survey of (H2PO4−)n Clusters and Networks. A CSD search (2013 release, version 5.34) found 276 unique structures containing the phosphate anion solely in its diprotonated H2PO4− form (i.e., no H3PO4, HPO42−, or PO43−), which were analyzed to determine the propensity of this anion to selfassociate in the crystalline state and to identify the various hydrogen-bonding motifs present. We found that dihydrogen phosphate is self-associated in the majority of cases, with only 5% of the structures containing the H2PO4− anion as a monomer. Discrete dihydrogen phosphate clusters are present in 15% of the structures analyzed, whereas extended aggregates including onedimensional (1D) chains and tapes, two-dimensional (2D) layers, and three-dimensional (3D) frameworks account for 80% of the structures. A more detailed analysis revealed a wide variety of hydrogen-bonding motifs formed by H2PO4−, which are depicted schematically in Figure 1. Figure 2 shows examples of the different discrete clusters identified in the CSD. The dimer is by far the most frequently encountered discrete motif, with the two H2PO4− anions linked by two hydrogen bonds (32 examples) or three hydrogen bonds (one example). The larger cluster motifs are quite rare, with only Received: March 4, 2013 Revised: April 9, 2013 Published: April 10, 2013 2233
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Figure 1. The most representative hydrogen-bonding motifs found in crystalline (H2PO4−)n aggregates (A = H2PO4−). Single, double, and triple bonds correspond to the observed hydrogen-bonding multiplicities between the anions. The percentage values correspond to probabilities of formation calculated by dividing the number of examples found for each motif to the total number of structures containing H2PO4− in the CSD.
one trimer, six tetramers (cyclic and linear), and one hexamer encountered.8 Among the extended hydrogen-bonded motifs identified, the most common are 1D chains (44%) and tapes (9%) and 2D layers (22%). We could only find six examples of 3D frameworks (2%), each of them displaying unique hydrogen-bonding connectivities. Figure 3 illustrates examples of the most representative 1D and 2D motifs. In these structures, each dihydrogen phosphate is engaged in a total of four hydrogen bonds with the neighboring anions, by acting as both a donor and acceptor of two hydrogen bonds, respectively. Novel (H2PO4−)n (n = 4, 6) Clusters in Co-Crystals of TBA H2PO4− with L1 and L2. Crystallization of TBA dihydrogen phosphate in the presence of the urea-functionalized ligands L1 and L2 from acetone/diethyl ether yielded single crystals of
[L1(H2PO4)3](TBA)3(H2O)2(acetone)n (1) and [L2(H2PO4)3](TBA)3(H2O)2(acetone)n (2). Single-crystal X-ray diffraction analysis revealed that 1 and 2 are isostructural and consist of centrosymmetric (H2PO4−)6 hexamers sandwiched between two ligand molecules (Figure 4). The hexamers display an unprecedented topology, with four of the anions interlinked by double hydrogen bonds into a linear chain and the other two anions branching out from the main chain via double hydrogen bonds. The observed intermolecular O···O contact distances between the H2PO4− anions ranged between 2.54 and 2.68 Å (average 2.60 Å) and are comparable with the average value of 2.58(5) Å found in the CSD for hydrogen bonds between dihydrogen phosphate anions. The four terminal dihydrogen phosphate anions in the clusters are each hydrogen bonded by two urea groups of L1 or L2, with the ureas 2234
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Figure 4. Crystal structures of 1 and 2 (TBA cations and acetone solvent are not shown). (a) (H2PO4−)6 hexamers sandwiched between two ligand molecules and anchored by 16 hydrogen bonds from 8 urea groups. (b) Expanded view of the (H2PO4−)6 cluster and schematic representation of its hydrogen-bonding connectivity.
Figure 2. Examples of discrete dihydrogen phosphate clusters. (a) Dimers linked by double or triple hydrogen bonds. (b) Cyclic trimer linked by double hydrogen bonds. (c) Cyclic and linear tetramers linked by either double or alternating single and double hydrogen bonds. (d) Hexamer consisting of two different trimers connected by a single hydrogen bond. CSD reference codes are specified under each represented structure.
for 1 and 2, respectively. Four water molecules are additionally hydrogen-bonded to the anionic clusters. Crystallization of TBA dihydrogen phosphate with L1 from acetone/diethyl ether/triethylamine yielded single crystals of [L1(H2PO4)4](TBA)4 (3). The crystal structure of 3 consists of centrosymmetric (H2PO4−)4 tetrameric clusters linked by L1 into extended chains via hydrogen bonding to the urea groups of the ligand. (Figure 5). As in the previous structures, the urea
Figure 3. Examples of extended 1D and 2D dihydrogen phosphate aggregates. (a) Two examples of the chain motif 1D-B, with each H2PO4− forming two double hydrogen bonds with the neighboring anions. The two structures differ in the relative arrangement of the anions, with either adjacent or opposite O−P−O edges from each tetrahedral H2PO4− engaged in hydrogen bonding. (b) Tape motif 1D−E. (c) Layer motifs 2D−A and 2D−C, consisting of fused four-member and six-member rings, respectively. CSD reference codes are specified under each represented structure.
Figure 5. Crystal structure of 3 (TBA cations are not shown). (a) (H2PO4−)4 clusters linked by L1 into hydrogen-bonded chains. (b) Expanded view of the (H2PO4−)4 cluster and schematic representation of its hydrogen bonding connectivity.
chelating adjacent edges of the tetrahedral anions, as found in other similar ortho-phenylene-bisurea ligands.7 There are a total of 16 urea hydrogen bonds anchoring the clusters, with NH···O contact distances in the range of 1.93−2.34 Å and 1.94−2.30 Å
groups chelate adjacent edges of the terminal dihydrogen phosphate anions in the cluster, with observed NH···O contact distances ranging between 1.89 and 2.22 Å. The (H2PO4−)4 clusters display a linear topology similar to those observed in 2235
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the SQUEEZE routine in PLATON,9 which removed its contribution from the final hkl files. Table 1 summarizes the crystallographic data for 1−3.
previously reported dihydrogen phosphate tetramers but a unique connectivity consisting of alternating triple and double hydrogen bonds among the anions. According to our analysis, there is only one example of a triple hydrogen bond between dihydrogen phosphate anions in the CSD. The observed intermolecular O···O contact distances between the H2PO4− anions in 3 are 2.60, 2.63, and 2.67 Å for the terminal triple hydrogen bonds and 2.57 Å for the two hydrogen bonds in the middle. The (H2PO4−)6 and (H2PO4−)4 structures observed in 1−3 represent rare examples of discrete dihydrogen phosphate clusters. As our CSD analysis indicated, H2PO4− prefers to form extended polymeric structures in the crystalline state to maximize the number of hydrogen bonds per anion. Among the previously reported discrete (H2PO4−)n oligomers, there is only a handful of structures containing clusters larger than the dimer (8 examples in the CSD). Furthermore, the hexameric and tetrameric clusters observed in 1−3 display unique hydrogenbonding connectivities. We tentatively attribute these unique structural features to the effective ability of the urea-functionalized ligands L1,2 to stabilize the clusters in the solid state via hydrogen bonding to the peripheral anions in the clusters. A common structural motif in 1−3 is the chelating of adjacent edges of the terminal H2PO4− anions by the ortho-phenylenebisurea groups, which act as effective “capping” groups that prevent the growth of the clusters into extended networks. Other potential factors determining the structures of the anionic clusters formed are the structure of the organic linker connecting the ortho-phenylene-bisurea groups, the nature of the countercation, and the composition of the crystallizing solution. Future crystallization studies of dihydrogen phosphate with analogous urea-functionalized ligands promise the prospect of discovering other unique (H2PO4−)n clusters.
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Table 1. Crystallographic Data for 1−3 1
2
3
formula
C77H154N11O19P3
C81H162N11O19P3
C45H91N6O10P2
M crystal size (mm) crystal system space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z T (K) 2θmax (°) μ (cm−1) reflections collected independent reflections parameters Rint R1,a wR2(b) (I > 2σ(I)) GOF
1631.02 0.30 × 0.16 × 0.10 triclinic P1̅ 14.0158(12) 17.1001(15) 22.014(2) 79.749(2) 89.457(2) 66.057(2) 4733.6(7) 2 173(2) 50.00 0.129 43011
1687.13 0.32 × 0.18 × 0.04 triclinic P1̅ 13.9538(15) 17.1425(18) 22.245(2) 99.483(2) 90.133(2) 113.454(2) 4801.1(9) 2 173(2) 50.00 0.129 47334
938.18 0.28 × 0.22 × 0.19 triclinic P1̅ 12.5433(14) 15.2724(16) 16.7800(18) 83.775(2) 79.502(2) 68.296(2) 2933.6(5) 2 173(2) 50.00 0.125 18486
16661
16923
10297
1042 0.0317 0.0515, 0.1478
1057 0.0674 0.0876, 0.2330
578 0.0376 0.0934, 0.2727
1.013
1.059
1.094
R 1 = Σ(|F 0| − |F c |)/Σ|F 0|. Σ[w(F02)2]}1/2. a
(b)
wR2 = {Σ[w(F 0 2 − F c 2 ) 2]/
EXPERIMENTAL SECTION
CSD Survey of (H2PO4−)n Aggregates. The survey was performed using the 2013 release of the CSD (version 5.34), employing ConQuest (version 1.15) and Mercury (version 3.1). The search was set up so that the P−O bond type was specified as “any”, and the number of bonded atoms for the H-free O atoms was constrained to 1. This resulted in 437 hits, which were scrutinized to remove duplicate structures, structures containing phosphate anions in other protonation states (H3PO4, HPO42−, or PO43−), structures containing other anions hydrogen bonded to H2PO4−, disordered structures, and structures with no coordinates available. The remaining 276 structures were then thoroughly inspected by removing the countercations, ligands, water, and other solvent molecules present in the crystal, and expanding the intermolecular contacts to identify the hydrogen bonding motifs involving the H2PO4− anion. Crystallization of 1−3. To solutions of TBAH2PO4 (10.2 mg, 30 μmol) and L1 (5.2 mg, 10 μmol) or L2 (5.8 mg, 10 μmol) in 250 μL of acetone was added diethyl ether (250 μL). The resulting mixtures were left overnight to yield diffraction-quality single crystals of 1 and 2, respectively. Crystallization of 3 was done similarly to 1, except triethylamine (10 μL) was added to the final solution, which resulted in large single crystals within a couple of hours. X-ray Crystallography. Single-crystal X-ray data were collected on a Bruker SMART APEX CCD diffractometer with fine-focus Mo Kα radiation (λ = 0.71073 Å), operated at 50 kV and 30 mA. The structures were refined on F2 using the SHELXTL 6.12 (Bruker AXS, Inc., Madison, WI, 1997). Absorption corrections were applied using SADABS. Some of the ethyl and butyl C atoms were severely disordered and therefore were refined isotropically. Hydrogen atoms were placed in idealized positions, except for the protons of the H2PO4− and water molecules, which were located from the difference Fourier maps and their positions were fully refined. Crystals 1 and 3 included highly disordered and diffuse solvent molecules that could not be properly modeled. The corresponding residual electron density in these structures was treated with
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ASSOCIATED CONTENT
S Supporting Information *
Crystallographic data in CIF format, reference codes for the CSD structures analyzed, and synthetic procedures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Address: Radu Custelcean Research Scientist Oak Ridge National Laboratory, P.O. Box 2008, MS 6119 Oak Ridge, TN 37831-6119, United States. Tel: (865) 574-5018. Fax: (865) 574-8559. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy.
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