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CRYSTAL GROWTH & DESIGN

Role of Hydrogen-Bonded Interactions in the Crystal Packing of Phenylenediammonium Phosphomolybdates

2006 VOL. 6, NO. 9 2066-2071

Shailesh Upreti and A. Ramanan* Department of Chemistry, Indian Institute of Technology, New Delhi 110016, India ReceiVed March 22, 2006; ReVised Manuscript ReceiVed June 19, 2006

ABSTRACT: Self-assembled synthesis of three new organic/inorganic hybrid solids (1-3) in the presence of isomeric phenylenediammonium ions (p-, m-, and o-) demonstrates the role of nonbonding interactions in the construction of phosphomolybdatebased solid-state assemblies. In 1, crystal packing is dominated by two kinds of supramolecular assemblies: an unusual pentadecameric water cluster linked through p-phenylenediammonium (ppda) cations and a hydrogen-bonded assembly of a ppda cation with four phosphomolybdate anions. In 2, a pair of m-phenylenediammonium (mpda) cations linked through π‚‚‚π interactions envelops a phosphomolybdate anion from either side through hydrogen bonding mediated by water molecules forming butterfly-like supramolecular motifs. In 3, a new kind of O-W-O-W (organic-water) linker formed by alternate hydrogen bonding between ophenylenediammonium (opda) and water molecules; such motifs are well decorated around a pentamolybdate cluster anion leading to the formation of the three-dimensional architecture. Crystal structures of 1-3 reveal that supramolecular synthons earlier observed between water and organic ammonium cations in the presence of octamolybdate anions are broken in the presence of diphosphopentamolybdates due to the latter anion’s strong affinity for hydrogen bonding with the cations. Introduction Polyoxomolybdate (POM) clusters are attractive molecular building blocks for the formation of multidimensional organicinorganic hybrid networks in self-organization processes.1 Earlier studies have shown the structure-directing role of organic amines through electrostatic interactions or coordinating ability with molybdenum atoms, although the significant role of weak interactions influencing crystal packing of hybrid solids based on POM is rarely studied.2 In a recent paper, we have demonstrated the structure-directing role of three hydrogenbonded phenylenediammonium dimers (supramolecular synthons) in the construction of octamolybdate solids.3 In acidified aqueous molybdate solution, isomeric phenylenediammonium ions form dimers through strong hydrogen-bonding and/or π-π interactions, which in turn dictate the supramolecular assemblies between themselves or with octamolybdate anions (β-Mo8O26) that eventually result in the growth of hybrid solids. Since the cluster anion β-Mo8O26 (Figure 1a) rarely gets protonated and competes less for hydrogen bonding, crystal packing of the solids was dictated by the dominating role of weak interactions occurring between organic cations and water molecules.4 To explore this problem further, we examined the crystal structures of another stable molybdate cluster anion, diphosphopentamolybdate [HxP2Mo5O23]-(6-x) (P2Mo5)-based solids with the same isomeric phenylenediammonium as countercation. We preferred P2Mo5 (Figure 1a) as it is a stable cluster anion occurring in aqueous solution under self-assembly conditions over a wide range of pH values in the presence of phosphoric acid.5 Unlike β-Mo8O26, it is quite susceptible to protonation and also strongly competes for hydrogen bonding.6 Even though a number of hybrid solids based on P2Mo5 as building blocks are reported in the literature,5-7 this is the first study that has systematically analyzed the influence of nonbonding interactions among organic diammonium and water molecules in the presence of a competing inorganic cluster anion. Ammonium molybdate was preferred instead of sodium molybdate as hydrated sodium ions appear to be competing countercations along with organic * To whom correspondence should be addressed. E-mail: aramanan@ chemistry.iitd.ac.in.

cations in dictating the solid-state structure of the hybrid solids.7 Here, we report the formation of three hybrid solids {NH3C6H4NH3}7[Mo5O15{PO3(OH)}2]2[HMo5O15{PO3(OH)}2]2‚21H2O (1), {NH3C6H4NH3}2[Mo5O15{PO3(OH)}2]‚4H2O (2), and {NH3C6H4NH3}2{NH2C6H4NH3}[Mo5O15{PO4}{PO3(OH)}]‚3H2O (3) crystallized in the presence of p-phenylenediammonium (ppda), m-phenylenediammonium (mpda), o-phenylenediammonium (opda) ions, respectively. A detailed crystal structure analysis of the three solids revealed that hydrogen-bonding interactions and supramolecular assemblies formed are entirely different from octamolybdate-based solids 4-6 reported in a previous study.3 The examples clearly demonstrate the importance of understanding nonbonding interactions between cluster anions with water as well as templating organic cations for designing a hybrid solid based on POM as building blocks. Experimental Section General Methods and Materials. The reagents were purchased commercially and used without further purification. Elemental analysis (C, H, and N) was performed on a Perkin-Elmer 2400 CHN Elemental analyzer. Fourier transform IR (FTIR) spectra of 1-3 were recorded on Nicolet 5DX spectrophotometer with pressed KBr pellet in the region 4000-400 cm-1. Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D-8 Advance diffractometer with CuKR (λ ) 1.5418 Å). Thermogravimetric analysis (TGA) was carried out using a Perkin-Elmer TGA7 and DTA7 system on well-ground samples in nitrogen atmosphere with a heating rate of 5 °C/min. Synthesis of 1-3. Crystals of 1-3 were grown from an aqueous solution containing ammonium heptamolybdate (0.5 mmol, 0.6175 g) with ppda, mpda, and opda (1.5 mmol, 0.162 g), respectively, in a molar ratio 1:3. Solutions were then subjected to mild heating at 60 °C in a microwave oven for complete dissolution and after cooling until 25 °C these were acidified with 85% H3PO4 to bring pH down to 2.3-2.5. In all cases, colored crystals (1, brown; 2, light green; 3, brown) appeared within 20 h. The procedure was identical to the one we employed in our previous study to prepare β-Mo8O26-based solids.3 All reactions yielded monophasic solids with high yield (1, 79%; 2, 81%; 3, 72%; based on molybdenum). The solids 1-3 obtained in the presence of ppda, mpda, and opda, were later identified as salts containing the diphosphopentamolybdate anion, (HxMo5P2O23). The experimental and simulated powder XRD patterns of 1-3 revealed that their peak positions are in good agreement with each other, indicating

10.1021/cg0601610 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/21/2006

Phenylenediammonium Phosphomolybdates

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Figure 1. (a) A ball-and-stick representation of the cluster anions: (i) β-Mo8O26 and (ii) P2Mo5O23 (Mo: green, O: pink, and P: blue). (b) CPK representation of pentadecameric water cluster linked to four ppda molecules through hydrogen bonding (shown with dotted green line in a ballstick model). P2Mo5 clusters are not shown for clarity. (c) A single ppda unit (N7N7) in 1 bridges four P2Mo5 through strong N-H‚‚O interactions. (d) In 1, ppda at one end (N3) connected to supramolecular 2O-W15-2O linkers through water (O6) mediation on the other end (N4) linked to two P2Mo5 via N-H‚‚O interactions. (e) A complex hydrogen bonding (dotted green line) among water molecules, ppda and P2Mo5 (shown in green and pink polyhedra) leading to a 3D network in 1 viewed along [100].

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Upreti and Ramanan

Table 1. Crystal Data and Structure Refinement for 1-3 1 formula

formula weight temperature crystal system space group a/Å b/Å c/Å R/° β/° γ/° V/Å3 Z Fcalc/g cm-3 µ/mm-1 R1, wR2 [I > 2σ (I)] R1, wR2 all CCDC No.

2

3

[H2.5Mo5P2O23][H2P2Mo5O23]- [HP2Mo5O23][H3NC6H4NH3]1.75‚ [C6N2H10]2‚ [C6H10N2]2[C6H9N2]‚ 5.25H2O 4H2O 3H2O 1188.74 1204.90 1294.17 298 triclinic P1h 12.8483(9) 13.6336(10) 19.7751(14) 106.2300(10) 96.0820(10) 99.3700(10) 3239.3(4) 4 2.433 2.092 0.0367, 0.1007

298 monoclinic C2/c 12.3467(17) 18.251(2) 15.975(2) 90 111.794(2) 90 3342.5(7) 4 2.393 2.026 0.0377, 0.0705

298 monoclinic P21/n 14.9375(15) 7.8764(8) 31.028(3) 90 100.423(2) 90 3590.3(6) 4 2.394 1.895 0.0325, 0.0759

0.0395, 0.1041 233413

0.0474, 0.0737 0.0361, 0.0777 233448 233449

the phase purity of the products. Characteristic FTIR bands at 3436 (br), 2902 (w), 1626, 1552, 1513, 1482 (w), 1435 (w), and 1078 cm-1 confirmed the presence of phenylenediammonium cation and water, and bands at 947, 912, 846, 715, 668, and 553 cm-1 are characteristic of phosphomolybdate anion. CHN analysis gave the following results: For 1: C, 10.318; H, 1.218; N, 3.907. Calc: C, 10.514; H, 1.470; N, 4.087. For 2: C, 11.729; H, 1.591; N, 4.586. Calc: C, 11.971; H, 1.674; N, 4.653. For 3: C, 16.643; H, 2.217; N, 6.377. Calc: C, 16.706; H, 2.259; N, 6.494. Single-Crystal X-ray Diffraction. Single-crystal diffraction studies were carried out on a Bruker Smart Apex CCD diffractometer with a MoKR sealed tube. Crystal structures were solved by direct methods and in anisotropic approximation refined using the SHELXTL package.8 Hydrogen atoms were constrained to the organic part and for water molecule it was first located from the difference Fourier map and were later fixed during subsequent refinement cycles. For 1, H atoms attached to pentamolybdate ring of the cluster could not be fixed. Further details of the X-ray structural analysis for 1-3 are provided in Table 1.

Results and Discussion Crystal Structures. Single-crystal XRD analysis of 1 revealed the presence of four P2Mo5 units for 7 diprotonated ppda cations and 21 water molecules (Table 1). The cluster anion P2Mo5 consists of edge and corner sharing MoO6 octahedra forming a Mo5O15 ring capped by two PO4 tetrahedra; the anion is identical to the one found in many solids.5-7 As in the previous study,3 all ppda cations are diprotonated. On the basis of countercations and bond valence sums, it was inferred that all the phosphate groups and a few pentamolybdate rings are monoprotonated giving the overall composition {NH3C6H4NH3}7[Mo5O15{PO3(OH)}2]2[HMo5O15{PO3(OH)}2]2‚21H2O. Protonation of the phosphate groups as well as the pentamolybdate rings have been observed earlier.5 Bond valence sums clearly confirm that molybdenum atoms are in the fully oxidized state (+6). In 1, two distinct building blocks can be recognized: An unusual pentadecameric water cluster (Figure 1b) connected to four ppda molecules through a 2O-W15-2O (two organic-pentadecameric water cluster-two organic) linker and a ppda unit bridged to four P2Mo5 through N-H‚‚‚O (Figure 1c). The pentadecameric unit is made of three sets of pentameric water clusters: two linear units made of O4w, O5w, and O11w and a third planar one from O1w, O2w, and O3w. The octahedrally coordinated center water molecule (O1w) is surrounded by two O2w and O3w each on the plane, and the apical positions are taken by the linear water pentamers centered at

Figure 2. (a) A pair of mpda dimers formed by a displaced π‚‚‚π interaction (shown in dotted green lines) are further connected to O1w and O2w (via N-H‚‚O in dotted cyan lines) on either side leading to the formation of a butterfly motif in 2. (b) In 2 the butterfly motifs are uniformly decorating the solid; six such motifs envelope each P2Mo5. (c) Supramolecular packing of 2 viewed along [001] shows the existence of 1D channels occupying hydrogen-bonded water molecules.

Phenylenediammonium Phosphomolybdates

Crystal Growth & Design, Vol. 6, No. 9, 2006 2069

Figure 3. (a) Alternate hydrogen-bond interactions among two water (O2w; O3w) and two opda (N1N2; N5N6) molecules forming a new O-WO-W linker; O2w mediates N1 and N6 via O-H‚‚‚N and N-H‚‚O, respectively, while O3w bonds only to N5. Such motifs decorate the lattice such that all hydrophilic parts (N ends) of opda molecules are projected toward the P2Mo5 cluster. (b) P2Mo5 are brought significantly closer through O-W-O-W linkers in 3. (c) Water (O1w) mediates a pair of P2Mo5 in 3, while opda linked to water as well as cluster anion via N-H‚‚O interactions.

O11w. A center of symmetry occurring at O1w relates the unique set of seven pairs of water molecules. The two building blocks are stitched to each other in an interesting fashion through N3N4 rings as shown in Figure 1d. The ppda cations linked to O3w are further connected to another set of ppda cations mediated by a water molecule (O6w) to form the overall assembly. In total, the assembly is made of 17 water molecules and 7 ppda cations. These motifs are further linked through hydrogen-bonding interactions taking place at N3 and N7 to form a three-dimensional (3D) network as shown in Figure 1e.

The remaining four water molecules are present as space fillers stabilizing the solid-state structure. In 2, 4 P2Mo5 units, 8 mpda cations, and 16 water molecules are present per unit cell (Table 1). P2Mo5 is similar to the one present in 1 with only a monoprotonated phosphate group except that the pentamolybdate ring is completely unprotonated. Like ppda in 1, mpda in 2 is also diprotonated giving the overall composition {NH3C6H4NH3}2[Mo5O15{PO3(OH)}2]‚4H2O. A structural analysis of 2 revealed the occurrence of a butterflylike 2W-2O-2W (two water-two organic-two water) su-

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Upreti and Ramanan

Figure 4. TGA/DTG diagrams for (a) 1, (b) 2, and (c) 3.

Scheme 1.

Representative Supramolecular Synthons Found in (a) 4, (b) 5, and (c) 6 (ref 3).

pramolecular motif as shown in Figure 2a. A pair of mpda dimers (N1N2; N1′N2′) formed through a displaced π‚‚‚π stacking is connected to P2Mo5 through water mediation at O1w and O2w (via N-H‚‚O) from either side leading to a supramolecular butterfly motif. A careful analysis of crystal packing suggested that such 2W-2O-2W motifs are uniformly found in the solid (Figure 2b); six such motifs are enveloped around each P2Mo5. Interaction between such motifs and P2Mo5 is from either side through hydrogen bonding; one pair of hydrogen bonding occurs directly to the anion, and another is through a mediated water molecule (O1w). This is another example in which the anion is competing strongly for hydrogen bonding with the cations dictating a new supramolecular architecture. Crystal packing viewed along [001] further showed the existence of one-dimensional (1D) channels along the c-axis occupying hydrogen-bonded water molecules (O2w) as shown in Figure 2c. In 2, all water molecules are involved in strong hydrogen bonding unlike in 1. X-ray diffraction analysis of 3 showed the presence of 4 P2Mo5, 12 opda cations, and 12 water molecules per unit cell. P2Mo5 is similar to those present in 1 and 2 with only one phosphate group protonated. In this structure, each P2Mo5 is enveloped by three opda cations: N1N2 is monoprotonated, while N3N4 and N5N6 are diprotonated. It is important to note that mono9 as well as diprotonated10 opda are reported in the literature, but the presence of both within the same crystal system is unusual. In addition to our earlier study,3 only one other solid is known10 wherein monoprotonated opda cations occur as countercations; all other examples showed the presence of only diprotonated opda countercations. Bond valence sums further corroborate our conclusion. In 3, one can recognize another supramolecular motif formed by alternate hydrogenbond interactions among two water (O2w; O3w) and two opda (N1N2; N5N6) molecules (Figure 3a). O2w mediates N1 and N6 via O-H‚‚‚N and N-H‚‚O, respectively, while O3w bonds N5 leading to another new linker, O-W-O-W (organicwater-organic-water), which is present throughout the structure (Figure 3a). In 1, P2Mo5 are stitched through organic groups, while in 2 these are separated through 2W-2O-2W linkers. In 3, we observed that P2Mo5 are brought significantly closer either through a O-W-O-W linker (Figure 3b) or water (O1w) alone (Figure 3c). We notice that water molecules in 3 appear to have an imperative role in binding either organic-organic

(O2w) or organic-P2Mo5 (O3w) or P2Mo5-P2Mo5 (O1w) groups; this is attributed to the asymmetric shape of opda. Hydrophilic parts (N ends) of all opda are projected toward P2Mo5 (Figure 3a); thus, the outward hydrophobic part does not allow water molecules to ramble around. Since the charge distribution as well as the affinity of P2Mo5 toward hydrogen bonding is appreciably nonuniform as compared to the Mo8O26 cluster, the protonated ends of the organic groups are more randomly enveloped around P2Mo5, thus contravening the strong π‚‚‚π interactions found in 6.3 A comparison of the supramolecular synthons found in 4-6 (Scheme 1)3 with the linkers observed in 1-3 highlight the significance of nonbonding interactions in the crystal packing of hybrid salts based on POM. In 4, ppda molecules exclusively bridge water molecules, whereas in 1 they also interact with P2Mo5. A compromise between these two forces results in an extended assembly of 1D chains and the unusual water oligomers. In 4, the anion is reasonably quiet, and hence the supramolecular synthons dominate the crystal packing, whereas in 1, these are broken. The occurrence of a large number of water molecules in the hydrophilic pockets leads to clustering (for optimizing intermolecular contacts), and the geometry of the water cluster is just incidental. In 5, the supramolecular synthons were formed by strong π‚‚‚π stacking supported by water mediation at both the N ends of mpda, but the affinity of the P2Mo5 anion for hydrogen bonding breaks the interactions between mpda cations and hence results in the formation of a butterfly motif. In 6, the presence of monoprotonated opda favored a direct hydrogen bonding between two adjacent molecules, which was further supported by a strong π‚‚‚π stacking; but in 3, the competing P2Mo5 anion breaks the strong π‚‚‚π interaction between opda molecules. The presence of both monoprotonated and diprotonated parts is again a compromise between charge and the economical packing of cations and anions. A comparison of packing densities in 1-3 in contrast to 4-6 (Table 1) suggests that crystal packing is essentially directed by the charge distribution around P2Mo5. Positioning of the two Ns in ppda, mpda, and opda is also important for certain nonbonding interactions to dominate. Thermal Stability of 1-3. Thermal dehydration of 1-3 is quite different (Figure 4). All samples showed a weight loss in two gradual but distinct steps corresponding to the decomposition of water molecules and organic groups, respectively: (1:

Phenylenediammonium Phosphomolybdates

obs., 7.150%, 13.92% and calc., 7.88%, 16.07%; for 2: obs., 6.14%, 12.62% and calc., 6.07%, 18.30%; for 3: obs., 3.12%, 15.53% and calc., 4.18%, 25.45%). The loss of organic groups in a broad step (150-600 °C) reflects the strength of nonbonding interactions occurring in the solids. The water loss occurring at lower temperatures can be attributed to the loss of space-filling water molecules. For 1, the overlapping of weight loss for water and organic groups (Figure 4a) further reveals that the cleavage of 2O-W15-2O linkers takes place over a wide range of temperature; a strong bonding between the cations and anions is also reflected in the shape of the thermogram (Figure 1c). In 2, loss of water is intriguingly sharp (Figure 4b) and occurs at a higher temperature (∼180 °C) due to a strong 2W-2O-2W linker with cluster ions. Like 1, in 2 also the loss of organic groups is gradual. In 3, the water loss occurs significantly at a higher temperature (∼200 °C, Figure 4c) as it is strongly bonded to the cluster anions and opda via the O-W-O-W linker. In all the cases, weight loss above 600 °C is also caused by the decomposition as well as volatility of MoO3 as confirmed by PXRD and FTIR studies. The loss of crystallinity in all samples above ∼250 °C suggests that nonbonding interactions are responsible for the stability of the solid-state structure. Acknowledgment. Shailesh Upreti thanks DST & IITD for a research fellowship. A.R. acknowledges DST, Government of India, for financial support. Thanks are also due to DST for funding a powder X-ray diffractometer under IRHPA and a single crystal X-ray diffractometer under FIST to the Department of Chemistry, IIT Delhi, India. Supporting Information Available: Hydrogen bonding table and crystallographic information files (CIF) for 1-3. This material is available free of charge via the Internet at http://pubs.acs.org.

Crystal Growth & Design, Vol. 6, No. 9, 2006 2071

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