Role of Nonbonding Interactions in the Crystal Growth of

Department of Chemistry, Indian Institute of Technology, Delhi, Hauz Khas, India, and Chemistry and. Physics of Materials Unit, Jawaharlal Nehru Centr...
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CRYSTAL GROWTH & DESIGN

Role of Nonbonding Interactions in the Crystal Growth of Phenazinediamine Tetrahydrate: New Insights into the Occurrence of 2D Water Layers in Crystal Hydrates

2007 VOL. 7, NO. 5 966-971

Shailesh Upreti,† Ayan Datta,‡ and Arunachalam Ramanan*,† Department of Chemistry, Indian Institute of Technology, Delhi, Hauz Khas, India, and Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for AdVanced Scientific Research, Bangalore-64, India ReceiVed December 29, 2006; ReVised Manuscript ReceiVed February 15, 2007

ABSTRACT: Long fibers of 2,3-phenazinediamine tetrahydrate, 1, were grown from aqueous ammoniacal solution containing o-phenylenediamine and catalytic amounts of ammonium molybdate under ambient conditions. Single-crystal analysis of 1 revealed the presence of 2D water layers made of hexameric clusters that are exclusively in the chair conformation similar to that found in hexagonal ice (Ih) on the {0001}; organic molecules were stacked as columns in between water layers dictated through strong supramolecular interactions. We hereby propose an intuitive mechanism to show how nonbonding interactions lead to the crystallization of 1 and other solids exhibiting 2D water sheets. Ab initio calculations at the density functional level for 1 further confirm the origin of their stability. Introduction The structure of water is still an enigma to researchers from all branches of natural sciences. The dynamic nature of hydrogen-bonding interactions and their fluctuations pose a major hurdle to evaluate the anomalous properties of bulk water in terms of its structure.1-6 This has led to an intense experimental and theoretical work focusing on unraveling structures of hydrogen-bonded water clusters.7-16 It is generally believed that understanding of the numerous possible structures and stabilities of water oligomers in diverse surroundings can provide insight into the nature of water-water interactions in bulk water and crystalline ice.17-21 The past few years have seen an unprecedented number of papers reporting the occurrence of hydrogen-bonded water clusters as tetramers, pentamers, hexamers, octamers, decamers, etc., in crystalline organic, inorganic, or metal-organic hosts.5,10-16,22 In selected cases, extended water interactions containing 1D infinite chains consisting of cyclic tetramers,22-24 1D water chains with regularly ordered columnar structures,25 2D layers with large 12- or 18-membered water rings,8,26 and 2D water sheets containing hexameric units in different confirmations were also observed.27-31a However, the novelty of these water structures reported in several crystal hydrates still remain a puzzle as questioned by Mascal et al. in a recent article.32 During our systematic analysis on the influence of nonbonding interactions in the growth of phenylenediammonium-based molybdate solids,33,34 we encountered the growth of single crystals of 2,3-phenazinediamine tetrahydrate, 1. A detailed investigation of 1 showed the presence of 2D sheets made of hexameric water clusters exclusively in chair conformation similar to that found in hexagonal ice (Ih) on the {0001}; the medium of water as solvent appears to dictate specific supramolecular interactions. In 1, all potential H-bonding sites of water and organic molecules are involved in nonbonding interactions. To get better insight into the problem, we analyzed nonbonding interactions of several crystal hydrates reported in the recent literature as well as Cambridge Crystallographic Database. We * Corresponding author. E-mail: [email protected]. Fax: 91 11 2658 2277. Tel: 91 11 2659 1507. † Indian Institute of Technology. ‡ Jawaharlal Nehru Centre for Advanced Scientific Research (NCASR).

hereby propose an intuitive mechanism to explain the occurrence of water sheets in 1 as well as a few other organic and inorganic hydrates and metal-organic frameworks in light of the hypothesis recently proposed by Ramanan and Whittingham to explain the formation of hybrid metal-organic frameworks and molybdates.35,36 Our intuitive model for the crystal structure analysis of crystal hydrates is based on the well-known supramolecular chemistry principles proposed by Desiraju, Etter, Fujita, and others.6,43,52 Experimental Section Bright red fibrous crystals of 2,3-phenazinediamine tetrahydrate were grown from an aqueous ammonia solution containing 1.08 g (10 mmol) of o-phenylenediamine and a catalytic amount of ammonium heptamolybdate under ambient conditions.37 X-ray diffraction studies of suitably sized crystals mounted on a capillary were carried out on a BRUKER AXS SMART-APEX diffractometer with a CCD area detector (Mo KR ) 0.71073 Å, graphite monochromator).48 Frames were collected at T ) 298 K as well as at T ) 100 K by ω, φ, and 2θ rotation at 10 s per frame with SMART.48 The measured intensities were reduced to F2 and corrected for absorption with SADABS.49 Structure solution, refinement, and data output were carried out with the SHELXTL program.50 Non-hydrogen atoms were refined anisotropically. C-H hydrogen atoms were placed in geometrically calculated positions by using a riding model. O-H hydrogen atoms were located in difference Fourier map and refined isotropically by fixing the observed position in subsequent refinement cycles. Images were created with the Diamond program.51 Hydrogen bonding interactions in the crystal lattice were calculated with SHELXTL and Diamond.50,51 TG analysis of 1 indicated weight loss due to water in a broad step in the range 323-393 K. The FTIR spectrum of 1 (see the Supporting information) showed sharp features of bands associated with bending and stretching modes of water molecules. We also assessed the thermal stability of 1 and examined its crystal structure at 100 K. There is very little change in the structural parameters except that the volume of the cell is slightly decreased at 100 K.46 Powder X-ray diffraction studies on 1 before and after water elimination showed remarkable changes in the diffraction patterns and the changes could be attributed to the complete collapse of the host lattice.

Results Crystal Structure. Single-crystal X-ray analysis revealed that 1 contains one asymmetric unit of 2,3-phenazinediamine (2,3phda) and four lattice water molecules.44 The structure shows

10.1021/cg060958r CCC: $37.00 © 2007 American Chemical Society Published on Web 03/24/2007

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Figure 1. Fragment of the crystal lattice showing (a) the structure of the 2,3-phda dimer formed by N-H‚‚‚N hydrogen bonding (shown in dotted cyan lines) sandwiched between hexameric 2D water layers by O-H‚‚‚N (shown in dotted blue lines) and N-H‚‚‚O (shown in dotted red lines) hydrogen bonds. (b) Two 2,3-phda molecules are normal to each other and further linked by N-H‚‚‚N, forming a herringbone motif; each motif (shown in red and green) is separated by a hydrophobic interaction (3.2 Å). (c) H-bonding and π‚‚‚π interactions between 2,3-phda result in supramolecular columns of organic stacks shown in red, green, yellow, and cyan color. (d) Each hexameric unit in the 2D water layer is linked to five different 2,3-phda molecules.

the occurrence of pairs of 2,3-phda molecules (Figure 1a) forming dimers linked through intermolecular hydrogen bonds (N3-H2N′‚‚‚N3′). In addition, the dimers show a herringbone motif (Figure 1b) typically seen in many planar aromatic systems.6 Such L-shaped motifs are further linked to each other through hydrogen bonding (N-H‚‚‚N) and π‚‚‚π interactions, resulting in columns of organic stacks (Figure 1c). In the crystal lattice, these supramolecular columns lie parallel to the c-axis holding the 2D water sheets on an ac-plane through hydrogen bonding, as shown in Figure 2. The organic stacks are aligned parallel to each other, with neighboring stacks being separated by a distance of 3.2 Å. The water sheets are stacked one over the other along the b-axis. The separation between adjacent water layers (AB and CD in our case) is 9.6 Å (Figure 2). O-H‚‚‚O hydrogen bonding is responsible for the 2D water sheets. When viewed along the b-axis, the six-membered water rings are arranged in a distorted honeycomb-like manner (Figure 3); however, the hexameric units on adjacent AB and CD layers are slightly displaced from each other on the ac-plane. Each cyclic hexameric water cluster (type A and type C) present in a layer (Figure 4a) is made from two crystallographically independent water molecules (O1 and O2 in layer AB; O3 and O4 in layer CD; structures b and c in Figure 4). All the hexamers have a distorted chair conformation (structures d and e in Figure 4) like ice Ih on the {0001}. The O‚‚‚O distances found in the water sheets range from 2.82 to

2.92 Å, with an average value of 2.85 Å. This is comparable to O‚‚O separation found in ice Ih and liquid water, with average distances of 2.75 and 2.85 Å, respectively. The O‚‚‚O‚‚‚O bond angles vary from 90 to 129°, thus deviating only up to 20° from the ideal tetrahedral angle found in ice Ih.5,21 The overall conformation of the 2D sheets of water molecules shown in Figure 4 results from the alternate combination of hexamericring tapes that have distorted chair conformations. The four independent water molecules present in the crystal lattice (two in each AB and CD layers) exhibit very interesting H-bonding interactions. A careful analysis of the structure reveals that every water molecule in a hexameric unit in 2D water layers is linked to five different 2,3-phda molecules (Figure 1d). Out of six water molecules that form a hexamer, four are connected to four different organic molecules, whereas two are linked to the same organic unit. The H-bonding scheme becomes more obvious if we consider that each organic molecule is linked to four water molecules through the four potential H-bonding centers available on it (two each of azide and amine N). Three such units from above and two from below or vice versa are brought together through strong supramolecular interactions. Crystallization of 1 exhibiting 2D water layers can be rationalized in terms of molecular building blocks that may occur because of primary H-bonding in aqueous solution. Formation of 2,3-phda from the oxidation of o-phenylenediamine under oxidizing condition is well-known.45,47 Self-

968 Crystal Growth & Design, Vol. 7, No. 5, 2007

Figure 2. In the crystal lattice, supramolecular columns of 2,3-phda (in green color) lie parallel to the c-axis holding the 2D water sheets (in red color) on the ac-plane through hydrogen bonding. Each water layer is separated by a distance 9.6 Å. All water molecules on acplanes are exclusively in distorted chair confirmations similar to that found in hexagonal ice (Ih) on {0001}.

Figure 3. Honeycomb arrangement of water layers (red color) on the ac-plane: L-shaped dimers of 2,3-phda are lying perpendicular to the water sheets held by strong N-H‚‚‚O and O-H‚‚‚N hydrogen bonds, leading to a sandwiched network.

assembly of molecules is dictated by well-known supramolecular interactions discussed by Lehn, Desiraju, and others.6,52,54 Here, we have extended a similar approach to understand the formation of hexameric water layers and herringbone type motifs in 1. As soon as 2,3-phda is formed in aqueous solution, each organic group (Scheme 1a) is bonded to four water molecules as shown in Scheme 1b. Two such units further interact to form dimers (supramolecular synthons)43a due to strong intermolecular N-H‚ ‚‚N (Scheme 1c). Meanwhile, aggregation of water molecules projected above and below from each supramolecular synthon favors the formation of 2D water layers made of puckered hexagonal rings (the stability of water sheets is further supported by DFT calculations discussed below); also, this geometry is suitable for the occurrence of herringbone-type motifs appearing as supramolecular columns. It is important to note 2,3-phda grown from a nonaqueous solvent(methanol) has resulted only in an anhydrous solid.45 Here, the solvent medium favors a

Upreti et al.

different supramolecular synthon (Scheme S1 in the Supporting information) through azide N and amine H of two different organic molecules and hence the crystal packing is dominated by N-H‚‚‚N interactions. Incidentally, π‚‚‚π interactions are not significant. The crystal structures of 1 and the anhydrous form clearly suggest that water plays an important role in mediating the supramolecular interactions between organic molecules and hence the occurrence of 2D water sheets. Further, we decided to examine other structures wherein water molecules occur as 2D layers. CSD analysis coupled with literature search revealed that there were only a few examples in which 2D water layers made of hexameric clusters are known: Sr(C7H7N2O4)2‚4H2O,27 1,4-[B(OH)2]2C6H4‚4H2O,28 [Cd(H2O)2Ni(CN)4]‚4H2O,29 C7H9N5‚2H2O,30a C12H12(CuN2O8)n‚ 4n(H2O),30b and (C2H5N)n‚2n(H2O)31a. Interestingly, in all these examples, a polymer, a 1D chain, a metal coordination polymer, or a supramolecular organic stack was found in between water layers. In Sr(C7H7N2O4)2‚4H2O, all are distorted hexamers in both boat and chair conformations (see Scheme S2 in the Supporting information); here one of the H atoms attached to O6 is disordered. 1,4-[B(OH)2]2C6H4‚4H2O also contain both chair and boat conformations but H-bonding on the water sheet is again disordered, as only one of the protons of hydroxyl groups can satisfy the H-bonding requirement. In [Cd(H2O)2Ni(CN)4]‚4H2O, the hexameric water clusters are only in boat conformations. In this structure, water layers are found to be similar to that of Ih, but on the ac-plane, where water hexamers are in boat conformations; the mixed metal polymeric complex is sandwiched between these water sheets. Unlike other examples, in this case, part of the water molecules forming the 2D sheets is coordinated with the metal. In C7H9N5‚2H2O, all hexamers are in distorted chair confirmation, but only one hydrogen atom was refined for each water molecule. In addition, three water molecules present in each hexamer are linked only to the other three water molecules H-bonded to the organic groups. In [C12H12(CuN2O8)n]‚4n(H2O), the 2D water sheets are made of hexamers in distorted chair confirmation. Like in the previous case, three water molecules also occur here that are connected only to other water molecules. The hexamers in 2D water sheets in [(C2H5N)n]‚2n(H2O) are also found in only chair confirmation. Here again, a few of the H atoms are disordered. A careful examination of all the crystal structures exhibiting 2D water layers reveals that the guest in between the 2D water host is a polymer, a metal complex, a coordination polymer, or a supramolecular organic stack as in 1. It appears that the guest must possess potential sites that show H-bonding with water molecules. The possibility of complexation, coordination polymerization, polymerization, or supramolecular interactions among organic or inorganic molecules facilitates water interactions to a restricted dimension. Formation of 2D water sheets is a compromise of these interactions along with crystal packing effects. If the geometry is not exactly matching, then either additional water can come into play to provide a stable structure or disorder can occur among water molecules as the case may be. To the best of our knowledge, 1 is a rare example in which all water molecules H-bonded to each of the four potential sites (two donor and two acceptors) are ordered and occur in a tetrahedral geometry. In a recent work, Jayaram and coworkers53 have analyzed the structural and functional importance of water molecules in the context of biomolecules. They have classified the hydrogen-bonding patterns depending on the donor-acceptor hydrogen bond relationships into four broad, mutually exclusive classes. In view of their classification, 1 belongs to category 3 of Class I.

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Figure 4. Perspective view of water layers along the b-axis; the organic part is omitted for clarity. (a) Adjacent AB and CD layers are slightly displaced from each other on the ac-plane. (b) The first layer AB (purple color) is made of similar hexameric water cluster (type A) in two different orientations containing O1 and O2 asymmetric oxygen atoms, whereas (c) the neighboring layer CD is made of two other asymmetric water molecule O3 and O4 (type C). A comparative view of hexameric water clusters in the adjacent layers (d) type A and (e) type C, stabilized in distorted chair confirmation.

Scheme 1

(a) Organic amine binds to four water molecules through H-bonding and moves closer to achieve the geometry that favors hexameric chair formation for the water cluster and the herringbone for the organic. (b) Formation of a novel supramolecular synthon (dimer) through a strong intermolecular H-bonding N-H‚‚‚N. (c) Orientation of five 2,3-phda molecules to achieve the geometry that favors a hexameric cluster that forms the 2D layer. (d) Partial view of the crystal structure in 1 to match the scheme proposed.

In a recent paper, Ramanan and Whittingham35a postulated an intuitive pathway to understanding the role of nonbonding interactions in triggering a preformed supramolecular assembly between point zero charge (pzc) soluble species that is ultimately responsible for the growth of neutral metal-organic frameworks. They later extended the concept to rationalize the architectures of multidimensional as well as metal-organic frameworks of organically templated vanadium oxides.35b These examples demonstrate that the methodology can be readily applied to ionic frameworks in which the preformed assembly occurs between ion-pairs as well as pzcs. In this paper, we have adopted a similar approach to underline the influence of supramolecular interactions leading to the self-assembly of selected crystal hydrates reported in the literature; for expediency, we have categorized

them as organic, inorganic, and metal-organic hydrate (Schemes S2-S17). Like 1, we proposed plausible mechanisms to rationalize the occurrence of 2D water layers in terms of its crystal structure formation in the Schemes S2-S4 and S6-S8 (see the Supporting Information). In addition, we have explained the formation of water layers made of pentamers and octamers in 6,7-dimethyl-1,4-dihydroquinoxaline-2,3-dione31b (Scheme S5 in the Supporting Information). All the examples including 1 clearly demonstrate that water mediation is responsible for the crystal growth of 2D water layer-based structures. All these mechanisms are based on well-established chemical principles by choosing chemically conceivable molecules as building blocks. The intuitive mechanisms postulated here take into account the well-known H-bonding interactions suggested by

970 Crystal Growth & Design, Vol. 7, No. 5, 2007

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Table 1. Hydrogen-Bond Parameters for 1 D-H‚‚‚A

D-H (Å)

H‚‚‚A (Å)

D‚‚‚A (Å)

D-H‚‚‚A (deg)

N3-H1N‚‚‚O4 N3′-H2N′‚‚‚N3 N4-H3N‚‚‚O2 N4-H4N‚‚‚N3 O1-H1W‚‚‚O1′ O3-H5W‚‚‚N1 O1-H2W‚‚‚N2 O3-H6W‚‚‚O3′ O2-H3W‚‚‚O2′ O4-H8W‚‚‚O3 O2-H4W‚‚‚O1 O4-H7W‚‚‚O4′

0.988(5) 0.854(5) 0.979(5) 0.806(5) 0.954(5) 0.906(6) 0.953(4) 0.846(6) 0.938(5) 0.924(7) 0.783(5) 0.989(7)

2.083(7) 2.546(5) 2.138(5) 2.348(5) 1.939(5) 1.910(4) 1.866(4) 2.134(6) 2.191(6) 2.017(5) 2.063(5) 1.943(8)

3.062(9) 3.115(7) 3.105(8) 2.770(8) 2.873(7) 2.790(7) 2.798(5) 2.845(9) 2.819(8) 2.921(8) 2.838(7) 2.85(1)

170.74(37) 124.99(37) 169.23(35) 113.50(41) 165.62(34) 163.54(36) 165.24(25) 141.46(45) 123.43(38) 165.82 (44) 170.66 (39) 151.36(47)

Lehn, Desiraju, Etter, Fujita, and many other pioneers in this field.6,43,52 The intuitive mechanisms proposed in various schemes (see Schemes S2-S17 in the Supporting Information) clearly bring out the various forces (electrostatic as well as weak interactions) operating between the soluble chemical species (rather than structural fragments); such an assembly also provides better insight into the formation of covalent linkages (as a result of water/solvent elimination), metal coordination, polymerization, and water aggregation. No doubt the solvent medium plays a significant role in dictating a particular supramolecular assembly, though further systematic investigation is required to establish the same. We strongly believe that the intuitive mechanism reported here for 1 and other related examples would serve as an ideal platform to practice supramolecular retrosynthesis envisioned by Desiraju.43a Theoretical Modeling. Ab initio calculations at the density functional level were carried out to look into the stability of the water sheets in the crystal hydrates. We performed calculations on the isolated water clusters (hexamers) at the wellaccepted density functional level of B3LYP with a basis of 6-31G+ (d, pin order to bonding and electronic structure of the water clusters).38 All the calculations have been performed through the Gaussian 03 program suite.39 We selected two different water hexamers: type A and C (Figure 4), as obtained from the crystal structure. We optimized the positions of the H-atoms, keeping the oxygen atoms fixed to that in their crystal structure and found them in good agreement with that of calculated positions from difference Fourier map. The calculations for the stabilization energies in the hexameric rings are calculated as ∆E ) E(hexamer) - 6E(H2O). The energies are corrected for zero-point vibrational energy (ZPVE) corrections and the basis set superposition errors (BSSE). BSSE values were calculated using the counterpoise correction (CP) scheme.40 The stabilization energies (∆E) were calculated to be -3.56 and -4.2 kcal/mol, respectively, for type A and type C. Thus, the formation of cyclic water clusters leads to stabilization of the system through H-bonding interactions. Apart from specific D-A interactions in these cyclic H-bonded systems, one of us has recently shown through theoretical calculations as well as structural database analysis that the formation of cyclic Hbonded clusters leads to weak aromaticity.41 This is due to the overall delocalization of the π-electrons on the oxygen atoms through charge-transfer interactions to the electropositive Hatoms. The signatures of aromaticity in these H-bonded systems are unambiguously determined through the existence of diamagnetic ring currents in NMR calculations. In Figure 5, we show the plots of the electron densities and electrostatic potentials for the two water clusters. As can be clearly seen, there exists strong electronic delocalization through the hexameric rings that leads to aromaticity in these water clusters. The ring currents in these water clusters are computed using the well-accepted GIAO NICS method42 at the center of masses

Figure 5. Plots of the electron densities for (a) type A and (b) type C and electrostatic potentials for (c) type A and (d) type C showing substantial delocalization of electrons. Isosurface values of 0.02 Å2 are used.

of these two systems to critically examine the existence of the diamagnetic ring-currents geometries. The ring currents in water hexamers type A and type C are -0.08 and -0.13 ppm, respectively, suggesting weak aromaticity. Thus, weak aromaticity induced in these water clusters that extend in the form of layers of hexagons leads aromaticity in the water sheets. This is in principle similar to the origin of stability of the graphene sheets where the extended π-conjugation through the pz orbital stabilizes the systems, which in turn derives stability through the existence of aromaticity within each benzene ring. Thus, our theoretical modeling suggests similar origin of stability for the water layers, though this nature of conjugation is through the σ-electrons in this case. Conclusions A successful growth of 2,3-phenazinediamine tetrahydrate from aqueous solution highlights the role of water in mediating supramolecular interactions. Theoretical calculations at the density function level for 1 support the possible mode of association for the occurrence of 2D water layers built of hexamers. The intuitive mechanisms proposed here for 1 and other examples suggest that structures of 2D water sheets should be interpreted in terms of appropriate molecular building blocks that are chemically recognizable in a solvent medium and how these are self-assembled in terms of complex formation, coordination polymerization, and other supramolecular interactions that favor crystal packing. We believe that proposal of molecular mechanisms for self-assembly combined with suitable theoretical modeling is imperative for correctly understanding the role of water molecules in the formation of crystal hydrates. Acknowledgment. S.U. thanks DST and IITD for a research fellowship and A.R. acknowledges DST, Government of India, for financial support. Thanks are also due to DST for funding an X-ray powder diffractometer under IRHPA and a singlecrystal diffractometer under FIST to the Department of Chemistry, IIT Delhi, India. AR thanks Professor M. S. Whittingham, SUNY-Binghamton, for his hospitality and encouragement during his sabbatical leave last year. We thank Professors B.

Nonbonding Interactions in Growth of Phenazinediamine Tetrahydrate

Jayaram, Nalin Pant, Anil J. Elias, and N. G. Ramesh for helpful discussion. We acknowledge Dr. S. Pati, JNCASR, Bangalore, for help with DFT calculations. Supporting Information Available: CCDC-268459 contains the supplementary crystallographic data for 1. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax: (+44)1223-336-033; or [email protected]). Crystallographic information files (CIF); TGA and FTIR spectrum for 1; optical photographs for growing large crystals of 1; supramolecular interactions in anhydrous 2,3-phda and intuitive mechanism proposed for Sr(C7H7N2O4)2‚4H2O, 1,4[B(OH)2]2C6H4‚4H2O, [Cd(H2O)2Ni(CN)4]‚4H2O, C7H9N5‚2H2O, C12H12(CuN2O8)n‚4n(H2O) (C2H5N)n‚2n(H2O), and C10H10N2O2‚6H2O. This material is available free of charge via the Internet at http://pubs.acs.org.

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