Open-Framework Zinc Phosphates with Unusual Architectures

Materials possessing framework architectures with open channels have attracted immense interest during the last two decades. Intense research in this ...
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Open-Framework Zinc Phosphates with Unusual Architectures S. Neeraj and Srinivasan Natarajan* Framework Solids Laboratory, Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560 064, India

CRYSTAL GROWTH & DESIGN 2001 VOL. 1, NO. 6 491-499

Received April 30, 2001

ABSTRACT: Open-framework zinc phosphates, I, [HMTA(H)3)]2[Zn3(PO4)2(HPO4)2][(H2PO4)2]‚10H2O, and II [HMTA(H)3][Zn2(PO4)(HPO4)(H2PO4)][(H2PO4)], with unusual layered architectures, have been synthesized under ordinary conditions and characterized. The amine phosphate, HMTA-P, also has been synthesized and characterized. The structures of I and II contain strictly alternating ZnO4, PO4, H/H2PO4 tetrahedral units linked through their vertices forming macroanionic layers with a large interlayer separation of ∼20 Å. While I contains the charge compensating amine, the water, and the isolated free H2PO4 units in the interlamellar region, II possesses only the amine and the free H2PO4 units. In II, one H2PO4 units hangs from the layer in a direction perpendicular to the layers. All these structural features have been observed for the first time in open-framework zinc phosphates, in addition to the large interlamellar separation. The structure has extensive O-H...O, N-H...O, C-H...O hydrogen bonds that lend structural stability and support the extra-large separation between the inorganic sheets. The removal of the amine molecules in I and II causes the collapse of the framework forming condensed zinc phosphates. The structural details garnered from this study might be of some use in the design of more complex inorganic systems that involve larger amine molecules, especially for the synthesis of mesoporous solids. Introduction Materials possessing framework architectures with open channels have attracted immense interest during the last two decades. Intense research in this area resulted in the formation of a large number of new materials possessing zeolitic and related structures.1,2 Recent research has clearly established that solids based on phosphate networks provide more variety and novelty.3 The structural diversity, chemical and physical properties, and the whole gamut of promising applications these materials offer are the driving force for continued active research in this area. Among the phosphate structures, the zinc phosphates appear to be unique. Thus, zinc phosphates exhibiting novel and unusual structural features such as Zn2O2 dimers,4 Zn2PO3 trimers,5 OZn4 tetrahedral clusters,6 and the four-membered Zn4O4 tetramers7 have been isolated and characterized. In addition to the above, some of the zinc phosphates have structures that are analogues to the aluminosilicate zeolites.8 The zinc phosphates, in general, are synthesized hydrothermally in the presence of either alkali metal cations or short chain organic amines.4-9 In recent years, a new family of solids with pore sizes in the range 20-100 nm has been prepared through the templating of lyotropic liquid crystal phases and are classified as mesophases.10-14 The mesophases are synthesized employing surfactant molecules with long chains, viz., cetyltrimethylammmoniumbromide (CTAB, C-16),10 sodiumdodecyl sulfate (SDS, C-12),15 decylamine (C-10), dodecylamine (C-12), tetradecylamine (C14), and hexadecylamine (C-16).16 These organic molecules exhibit ordered liquid-crystalline arrays in solution, which interact with the inorganic species forming * Corresponding Author. E-mail: [email protected], Fax: +91-80846-2766.

the mesophases with lamellar, hexagonal, and cubic stuctures.17 One of the most distinct structural features of mesostructured materials is their amorphous wall structure. The paracrystalline nature of the mesophases is clearly evident in 1-D ordered MCM-50 type lamellar phases. Even though the interlayer spacing is well defined, there are no registries between adjacent layers. Similar amorphous nature of the inorganic wall is also observed for hexagonally ordered mesostructures with 2-D or 3-D ordering or cubic phases with 3-D ordering. The amorphous walls of the mesophases have been related to the loose assembly of micelles and liquid crystals, as compared to the smaller and more rigid molecular templates used in the formation of zeolitic and other related materials. As a consequence of this, the interaction between the self-assembly processes occurring at different ends of the micelles or liquid crystals is not adequate to generate a perfectly ordered structure. To enhance the interactions between the polymeric species and to transfer the structural ordering information at one location to other sites, the synthesis strategy could be to employ long chain (di)amines of comparable length for the synthesis of solids with extended framework architectures. One advantage of using long-chain (di)amines is that they can form relatively strong N-H...O hydrogen bonds with the inorganic host. Although the idea appears to be simple, the practical difficulty due to coiling of the long chain amines, under hydrothermal conditions, invariably results in thermodynamically stable condensed phases without the amine molecule. Recently, Feng et al.18 observed that the use of long chain diamines such as 1,9-diaminononane (C-9), 1,10-diaminodecane (C-10), 1,11-diaminoundecane (C-11), and 1,12-diaminododecane (C-12), under hydrothermal conditions (180 °C), results in the formation of two-dimensional aluminophosphates with precise peri-

10.1021/cg010011j CCC: $20.00 © 2001 American Chemical Society Published on Web 10/03/2001

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odicity. Alhough the interlayer spacings were commensurate with the length of the backbone carbon chain, the exact location and conformation of all the carbon atoms were not possible.18 Presently, we investigated the formation of open-framework zinc phosphates in the presence of an amine molecule possessing a central nitrogen atom as part of a C-12 carbon backbone, employing ordinary temperatures (typically e 100 °C). The reasoning behind this approach is that the stronger N-H...O interactions involving the middle nitrogen atom, probably, would facilitate the complete location of the carbon backbone, and the structural details, thus obtained, may help in our understanding of nanocrystalline, mesostructured phases. In this paper, we present the results of our investigations on the formation of open-framework zinc phosphates in the presence of bis-hexamethylenetriamine, (NH2(CH2)6NH(CH2)6NH2, C-12, HMTA). Our studies resulted in the discovery of two new zinc phosphates, [HMTA(H)3)]2[Zn3(PO4)2(HPO4)2][(H2PO4)2]‚10H2O, I, and [HMTA(H)3][Zn2(PO4)(HPO4)(H2PO4)][(H2PO4)], II, possessing layered architectures. The zinc phosphates I and II are novel in more ways than one. The unique feature is the large separation between the inorganic layers (∼20 Å) that show perfect periodic registry despite the large interlayer distance (equal to ∼1/2 of the c axis length in the case of I). The linear amine molecule, situated between the two inorganic layers, gets stabilized by strong N-H...O interactions through an additional free phosphoryl group located at the center of two inorganic sheets. Both these structural features, to our knowledge, have been observed for the first time in open-framework Zn phosphates. Isolated phosphate groups of the type observed in the present structures have also been observed earlier in other open-framework phosphates.19,20 Additionally, the unit-cell parameter of ∼43 Å is close to the typical lattice parameters observed normally in mesoporous solids. Because of the uniqueness of the architectures of I and II, and to understand the solids better, we have also synthesized and characterized the hydrogen bonded adduct between the HMTA and phosphoric acid [amine phosphate, HMTA-P (III)]. Experimental Section Synthesis of Zn Phosphates, I and II. The compounds I and II were synthesized under ambient conditions. A total of 0.72 g of ZnSO4‚7H2O (2.5 mM) was dispersed in a mixture of 0.5 mL of deionized distilled water (27 mM) and 0.65 mL of H3PO4 (85 wt %) (11.3 mM) to produce a clear solution. Five millimolar bis-hexamethylenetriamine, HMTA, was added dropwise to the above solution under continuous stirring. The resulting gel, with a pH of ∼3, was homogenized for 30 min at room temperature, sealed in a 60-mL polypropylene bottle, and left to crystallize at room temperature. Crystals start to appear after 3 months and crystals suitable for single crystal X-ray diffraction were obtained only after ∼6 months. The transparent crystals were filtered and washed with a minimum quantity of water. The crystals were found to be of two distinct types: a rhomb-like (I) and a rectangular platelike (II), of which the first phase was predominant. Thermogravimetric analysis (TGA) of I and II was carried out in nitrogen atmosphere (50 mL/min) from room temperature to 700 °C using a heating rate of 10 °C min-1. The results indicate a broad mass loss over a wide temperature range in the case of I, and a sharp mass loss followed by a broad tail for II. For I, the total mass loss of 35% in the temperature range 100-500 °C, corresponds with the removal of ex-

Neeraj and Natarajan traframework water, the amine, and the elimination of the hydroxyl groups from the PO3OH and PO2(OH)2 units (calc. 33.5%). For II, a mass loss of about 31% in the temperature range 250-350 °C corresponds with the loss of the amine (calc. 29.7%) and a mass loss of about 8% in the range 400-550 °C accounts for the loss of the hydroxyl groups from the PO3OH and PO2(OH)2 units (calc. 7%). The final calcined product, in both the cases, was poorly crystalline with weak diffraction lines that appear to correspond to the condensed zinc phosphate Zn2P2O7 (JCPDS: 43-0488); it seems likely that an amorphous phase with another Zn:P ratio is also present. This clearly shows that the removal of the amine molecule from I and II collapses the framework structure forming condensed phases. Synthesis of bis-(Hexamethylenetriamine) Phosphate (HMTA-P), III. In a typical synthesis, 4.8 mM of H3PO4 (85 wt %) was diluted with 50 mM of deionized water and 4 mM of HMTA was added under continuous stirring. The clear solution, thus obtained, was concentrated over a water bath at 80 °C for 24 h. The resulting concentrated gellike mixture was allowed to crystallize at ambient conditions. Thin flaky plate like crystals appeared after 3-4 days. The crystals were filtered and washed with minimum quantity of water and characterized by single-crystal X-ray diffraction. A preliminary reaction involving the amine phosphate, HMTA-P, and Zn2+ ions under ordinary reaction conditions (typically room temperature) was investigated, resulting in a mixture of phases including I and II. The predominant product of the reaction was the condensed zinc phosphate, Zn2P2O7 (JCPDS:34-623). The amine phosphate route, however, was faster in yielding the products (typically ∼1 week). Single-Crystal Structure Determination. A suitable plate-like single crystal of each compound was carefully selected under a polarizing microscope and glued to a thin glass fiber with cyanoacrylate (superglue) adhesive. Crystal structure determination by X-ray diffraction was performed on a Siemens Smart-CCD diffractometer equipped with a normal focus, 2.4 kW sealed tube X-ray source (MoKR radiation, λ ) 0.71073 Å) operating at 50 kV and 40 mA. A hemisphere of intensity data was collected at room temperature in 1321 frames with ω scans (width of 0.30° and exposure time of 10 s per frame). Pertinent experimental details of the structure determination are presented in Table 1. The crystal structure was solved by direct methods using SHELXS-86,21 which readily revealed all the heavy atom positions (Zn, P) in the case of I and II enabling us to locate the other non-hydrogen positions from the difference Fourier map. Most of the hydrogen positions were located in the difference map. Because of the unusual length of the amine, it was not possible to locate all the hydrogen positions. For the final refinement, the hydrogen atoms were placed geometrically and held in the riding mode. An empirical absorption correction based on symmetry equivalent reflections was carried out using SADABS22 program. The last cycles of refinement included atomic positions, anisotropic thermal parameters for all the non-hydrogen atoms and isotropic thermal parameters for all the hydrogen atoms. Full-matrixleast-squares refinement against |F2| was carried out using the SHELXTL-PLUS23 package of the program. The atomic coordinates, selected bond distances, and angles are presented in Tables 2 and 3 for I, in Tables 5 and 6 for II and in Tables 8 and 9 for III.

Results [NH3(CH2)6NH2(CH2)6NH3]2[Zn3(PO4)2(HPO4)2][(H2PO4)2]‚10H2O, I. The asymmetric unit of I contains 73 non-hydrogen atoms, of which there are three Zn and six P atoms which are crystallographically independent. Of the 34 oxygen atoms, 8 belong to two isolated phosphate tetrahedra, 16 belong to the layered framework involving typical Zn-O-P linkages, and the remaining oxygens are water molecules. The Zn atoms

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Table 1. Crystal Data and Structure Refinement Parameters for [HMTA(H)3)]2[Zn3(PO4)2(HPO4)2][(H2PO4)2]‚10H2O, I, and [HMTA(H)3][Zn2(PO4)(HPO4)(H2PO4)][(H2PO4)], II, [NH3(CH2)6NH2(CH2)6NH3][(HPO4)(H2PO4)]‚6H2O, III empirical formula space group T (K) a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z formula mass dcalc (g cm-3) µ (mm-1) R(Fo2) [s > 2s(I)] a

I

II

III

Zn3P6O34C24N6H90 P21/c (no. 14) 293(2) 8.7820(3) 15.3559(6) 43.080(8) 90.00 90.02(5) 90.00 5809.7(3) 4 1388.85 1.565 1.486 R1a ) 0.068; wR2b ) 0.15

Zn2P4O16C12N3H37 P(-1) (no. 2) 293(2) 8.5148(1) 8.5707(1) 19.6941(2) 87.55(4) 86.05(2) 73.18(7) 1372.1(4) 2 734.11 1.777 2.055 R1 ) 0.045; wR2 ) 0.12

P3O24C24N6H80 C2/c (no. 15) 293(2) 39.567(6) 6.7672(1) 18.2516(2) 90.00 100.31(9) 90.00 4808.04(13) 8 924.81 1.253 0.199 R1 ) 0.077; wR2 ) 0.22

R1 ) Σ||F0| - |FC||/Σ|F0|. b wR2 ) {Σ [w(F02 - FC2)2]/Σ[w(F02)2]}1/2. W ) 1/σ2(Fo)2 + (aP)2 + bP]. P ) [max(Fo2,0) + 2(Fc)2]/3.

are tetrahedrally coordinated with their oxygen neighbors with Zn-O distances in the range 1.932(6)-1.977(6) Å [(Zn-O)av. ) 1.950 Å] and O-Zn-O angles in the range 102.4(3)-116.8(3)° [(O-Zn-O)av. ) 109.5°]. The four distinct phosphorus atoms of the framework make three P-O-Zn bonds and possess one terminal P-O linkage. The P-O distances are in the range 1.511(6)1.586(6) Å [(P-O)av. ) 1.536 Å], and the O-P-O bond angles are in the range 106.4(4)-112.3(4)° [(O-P-O)av. ) 109.4°]. The remaining P atoms, P(5) and P(6) are tetrahedrally bonded with O atoms with marginally shorter P-O bond distances [(P(5)-O)av. ) 1.460 and (P(6)-O)av. ) 1.514 Å]. It is likely that the shorter P-O distances could be the result of the free hanging nature of these phosphate tetrahedra in the interlamellar region. Assuming the usual valences of Zn, P, and O to be +2, +5, and -2, respectively, the framework stoichiometry of Zn3(PO4)6 creates a net charge of -12. Taking into account the presence of two completely protonated HMTA molecules, 2[NH3(CH2)6NH2(CH2)6NH3], the excess negative charge of -6 can be balanced by the protonation of the phosphate tetrahedra. One hydrogen position for each of the oxygens, O(15) and O(16), has been observed in the difference Fourier maps. Thus P(3)-O(15) and P(4)-O(16) with distances of 1.586(4) and 1.580(6) Å are P-OH units. The other terminal P-O linkages of the framework, in the case of P(1) and P(2) with distances of 1.528(6) Å [P(1)-O(13)] and 1.534(6) [P(2)-O(14)] are P ) O units. The P-O distances associated with P(5) and P(6) (isolated phosphates) are not well defined, and two of the oxygen atoms involved with each phosphorus [O(17), O(20) for P(5) and O(21) and O(23) for P(6)] has been assigned a proton to maintain charge neutrality. Thus, both P(5) and P(6) are PO2(OH)2 units. These assignments are also consistent with bond valence sum calculations.24 Selected bond distances and angles observed in I are presented in Tables 2 and 3. The framework structure of I is built up of linkages involving ZnO4, PO4, and HPO4 tetrahedra sharing their vertices giving rise to a layered architecture. The connectivity between the strictly alternating ZnO4, PO4, and PO3(OH) tetrahedral moieties results in a macroanionic layered topology consisting of apertures surrounded by 4-T, 6-T, and 8-T atoms (T ) tetrahedral center, Zn or P), in which the Zn and the P atoms strictly alternate. The connectivity between ZnO4, PO4, and

Table 2. Selected Bond Distances in I, [HMTA(H)3)]2[Zn3(PO4)2(HPO4)2][(H2PO4)2]‚10H2Oa bond

distance, (Å)

bond

distance, (Å)

Zn(1)-O(3) Zn(1)-O(1) Zn(1)-O(2) Zn(1)-O(4) Zn(2)-O(5) Zn(2)-O(7) Zn(2)-O(6) Zn(2)-O(8) Zn(3)-O(10) Zn(3)-O(11) Zn(3)-O(12) Zn(3)-O(9) P(1)-O(5)#1 P(1)-O(13) P(1)-O(7)#2 P(1)-O(11) P(2)-O(14) P(2)-O(3)#1

1.932(6) 1.948(6) 1.952(6) 1.964(6) 1.949(6) 1.949(6) 1.950(6) 1.977(6) 1.936(6) 1.939(6) 1.944(6) 1.955(6) 1.528(6) 1.528(6) 1.532(6) 1.538(6) 1.534(6) 1.539(6)

P(2)-O(10) P(2)-O(2)#3 P(3)-O(9) P(3)-O(6) P(3)-O(4) P(3)-O(15) P(4)-O(12) P(4)-O(8) P(4)-O(1) P(4)-O(16) P(5)-O(17) P(5)-O(18) P(5)-O(20) P(5)-O(19) P(6)-O(21) P(6)-O(22) P(6)-O(24) P(6)-O(23)

1.541(6) 1.543(6) 1.511(6) 1.518(6) 1.531(6) 1.586(6) 1.520(6) 1.521(6) 1.526(6) 1.580(6) 1.391(11) 1.470(8) 1.484(11) 1.494(11) 1.480(8) 1.510(8) 1.529(9) 1.538(9)

N(1)-C(1) N(2)-C(7) N(2)-C(6) N(3)-C(12) N(4)-C(13) N(5)-C(18) N(5)-C(19) N(6)-C(24) C(1)-C(2) C(2)-C(3) C(3)-C(4) C(4)-C(5) C(5)-C(6) C(7)-C(8)

Organic Moiety 1.477(9) C(8)-C(9) 1.458(11) C(9)-C(10) 1.494(11) C(10)-C(11) 1.486(11) C(11)-C(12) 1.472(12) C(13)-C(14) 1.45(2) C(14)-C(15) 1.47(2) C(15)-C(16) 1.48(2) C(16)-C(17) 1.52 C(17)-C(18) 1.509(11) C(19)-C(20) 1.519(14) C(20)-C(21) 1.538(13) C(21)-C(22) 1.503(13) C(22)-C(23) 1.500(13) C(23)-C(24)

1.560(13) 1.531(13) 1.527(13) 1.505(13) 1.33 1.46(2) 1.58(2) 1.29(2) 1.58(2) 1.54(2) 1.31(2) 1.56(2) 1.50(2) 1.33(2)

a Symmetry transformations used to generate equivalent atoms: #1 x + 1, y, z; #2 -x + 2, -y + 1, -z; #3 -x + 2, -y + 2, -z.

PO3(OH) units are such that it forms a bifurcated eightmembered ring along the ab plane separated by the four- and eight-membered rings (Figure 1). What is remarkable about I is the large interlamellar separation of ∼20 Å in the c direction (half of the c axis parameter). The triply protonated amine molecule, HMTA, occupies the interlamellar region and interacts with the inorganic layers through hydrogen bonds. The position of the amine molecule is reminiscent of pillared clays, where inorganic pillars support the silicate layers.25 The isolated PO2(OH)2 tetrahedral units also occupy the interlamellar region and are located near the central nitrogen atom of the amine molecule as shown in Figure

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Table 3. Selected Bond Angles in I, [[HMTA(H)3)]2[Zn3(PO4)2(HPO4)2][(H2PO4)2]‚10H2Oa moiety

angle (°)

moiety

angle (°)

O(3)-Zn(1)-O(1) O(3)-Zn(1)-O(2) O(1)-Zn(1)-O(2) O(3)-Zn(1)-O(4) O(1)-Zn(1)-O(4) O(2)-Zn(1)-O(4) O(5)-Zn(2)-O(7) O(5)-Zn(2)-O(6) O(7)-Zn(2)-O(6) O(5)-Zn(2)-O(8) O(7)-Zn(2)-O(8) O(6)-Zn(2)-O(8) O(10)-Zn(3)-O(11) O(10)-Zn(3)-O(12) O(11)-Zn(3)-O(12) O(10)-Zn(3)-O(9) O(11)-Zn(3)-O(9) O(12)-Zn(3)-O(9) O(13)-P(1)-O(5)#1 O(5)#1-P(1)-O(7)#2 O(13)-P(1)-O(7)#2 O(5)#1-P(1)-O(11) O(13)-P(1)-O(11) O(7)#2-P(1)-O(11) O(14)-P(2)-O(3)#1 O(14)-P(2)-O(10) O(10)-P(2)-O(3)#1 O(14)-P(2)-O(2)#3 O(2)#3-P(2)-O(3)#1 O(10)-P(2)-O(2)#3 O(9)-P(3)-O(6) O(9)-P(3)-O(4) O(6)-P(3)-O(4)

109.9(3) 112.8(3) 110.6(3) 108.2(3) 112.4(3) 102.8(3) 112.7(2) 110.1(3) 111.0(3) 108.2(3) 102.4(3) 112.3(3) 116.8(3) 107.4(3) 105.3(3) 104.9(3) 107.6(3) 115.3(3) 112.0(4) 108.7(4) 109.4(4) 107.7(4) 111.2(3) 107.8(3) 111.7(4) 111.1(4) 108.2(4) 110.0(4) 108.5(4) 107.3(3) 112.4(4) 112.3(4) 111.7(4)

O(9)-P(3)-O(15) O(6)-P(3)-O(15) O(4)-P(3)-O(15) O(12)-P(4)-O(8) O(12)-P(4)-O(1) O(8)-P(4)-O(1) O(12)-P(4)-O(16) O(8)-P(4)-O(16) O(1)-P(4)-O(16) O(17)-P(5)-O(18) O(17)-P(5)-O(20) O(18)-P(5)-O(20) O(17)-P(5)-O(19) O(18)-P(5)-O(19) O(20)-P(5)-O(19) O(21)-P(6)-O(22) O(21)-P(6)-O(24) O(22)-P(6)-O(24) O(21)-P(6)-O(23) O(22)-P(6)-O(23) O(24)-P(6)-O(23) P(4)-O(1)-Zn(1) P(2)#3-O(2)-Zn(1) P(2)#4-O(3)-Zn(1) P(3)-O(4)-Zn(1) P(1)#4-O(5)-Zn(2) P(3)-O(6)-Zn(2) P(1)#2-O(7)-Zn(2) P(4)-O(8)-Zn(2) P(3)-O(9)-Zn(3) P(2)-O(10)-Zn(3) P(1)-O(11)-Zn(3) P(4)-O(12)-Zn(3)

106.3(4) 106.9(4) 106.8(4) 112.3(4) 112.0(4) 111.7(4) 106.5(4) 107.0(4) 106.9(4) 111.8(6) 114.4(12) 108.7(6) 110.7(12) 109.3(6) 101.5(6) 109.5(5) 113.4(5) 107.7(6) 111.9(5) 108.8(6) 105.3(5) 124.4(4) 127.3(4) 137.7(4) 125.0(4) 137.1(4) 124.5(4) 127.9(4) 124.9(3) 122.7(3) 133.1(4) 133.4(4) 122.6(4)

C(7)-N(2)-C(6) C(18)-N(5)-C(19) N(1)-C(1)-C(2) C(3)-C(2)-C(1) C(2)-C(3)-C(4) C(3)-C(4)-C(5) C(6)-C(5)-C(4) N(2)-C(6)-C(5) N(2)-C(7)-C(8) C(7)-C(8)-C(9) C(10)-C(9)-C(8) C(11)-C(10)-C(9) C(12)-C(11)-C(10)

Organic Moiety 115.9(8) N(3)-C(12)-C(11) 111.6(11) C(14)-C(13)-N(4) 113.4(4) C(13)-C(14)-C(15) 111.4(5) C(14)-C(15)-C(16) 114.8(9) C(17)-C(16)-C(15) 110.9(9) C(16)-C(17)-C(18) 111.3(9) N(5)-C(18)-C(17) 110.6(9) N(5)-C(19)-C(20) 110.8(8) C(21)-C(20)-C(19) 112.4(9) C(20)-C(21)-C(22) 110.6(9) C(23)-C(22)-C(21) 113.7(9) C(24)-C(23)-C(22) 110.7(9) C(23)-C(24)-N(6)

112.6(8) 120.7(5) 121.1(8) 110(2) 117(2) 114(2) 104.3(13) 103.6(14) 113(2) 117(2) 110(2) 118(2) 119.7(13)

Figure 1. The polyhedral view of a single layer in I. Note that the layers comprise four-, six- and eight-membered apertures.

a Symmetry transformations used to generate equivalent atoms: #1 x + 1, y, z; #2 -x + 2, -y + 1, -z; #3 -x + 2, -y + 2, -z; #4 x - 1, y, z.

2. To our knowledge, this is the first time isolated phosphate tetrahedra have been observed in an openframework zinc phosphate. In addition to the presence of the amine molecule and isolated PO2(OH)2 tetrahedra, the interlamellar region also possesses 10 molecules of water. It is well established that the structural stability of the low-dimensional solids depends on the hydrogen bond interactions between the host and the guest.26 In I, the layered nature coupled with the large interlayer separation suggests the presence of strong hydrogen bond interactions. The predominant hydrogen interactions are between the amine molecule, HMTA, with the framework and with the free isolated phosphate groups through O-H...O, N-H...O, and C-H...O hydrogen bonds. The water molecules, present between the layers, also participate in hydrogen bond. The important hydrogen bond interactions in I are listed in Table 4.

Figure 2. Structure of I showing the layer architecture with the amine molecule. Note the presence of the water molecule and free H2PO4 within the interlamellar region. Dotted lines represent the possible hydrogen bond interactions.

[NH 3 (CH 2 ) 6 NH 2 (CH 2 ) 6 NH 3 ][Zn 2 (PO 4 )(HPO 4 )(H2PO4)][(H2PO4)], II. The asymmetric unit of II, [NH3(CH2)6NH2(CH2)6NH3][Zn2(PO4)(HPO4)(H2PO4)][(H2PO4)], contains 37 non-hydrogen atoms (Figure 3), of which 17 atoms belong to the framework (two Zn, three P, and 12 O atoms) and 20 atoms belong to the guest (12 C, three N of the amine molecule, one P, and four O atoms of the isolated phosphate). The framework contains two crystallographically distinct Zn and four P atoms. Of the 16 O atoms, 12 form linkages within the

Open-Framework Zinc Phosphates

Crystal Growth & Design, Vol. 1, No. 6, 2001 495 Table 5. Selected Bond Distances in II, [HMTA(H)3][Zn2(PO4)(HPO4)(H2PO4)][(H2PO4)]a

Figure 3. ORTEP plot of II showing the asymmetric unit. Thermal ellipsoids are given at 50% probability. Table 4. Important Hydrogen Bond Interactions in I, [HMTA(H)3)]2[Zn3(PO4)2(HPO4)2][(H2PO4)2]‚10H2O bond

distance (Å)

moiety

angle (°)

O(13)...H(1) O(8)...H(2) O(7)...H(3) O(18 ...H(16) O(21)...H(17) O(4)...H(30) O(14)...H(31) O(2)...H(32) O(14)...H(34) O(5)...H(35) O(21)...H(48) O(18)...H(49) O(13)...H(62) O(3)...H(64) O(102)...H(102) N(5)...H(103) O(107)...H(104) O(103)...H(106)

2.054(1) 2.130(1) 2.059(8) 1.834(8) 1.851(8) 2.140(3) 2.051(5) 2.050(5) 1.831(8) 2.267(4) 1.872(1) 1.845(9) 1.831(5) 2.252(1) 2.042(6) 1.947(1) 1.924(2) 2.037(3)

O(13)...H(1)-N(1) O(8)...H(2)-N(1) O(7)...H(3)-N(1) O(18)...H(16)-N(2) O(21)...H(17)-N(2) O(4)...H(30)-N(3) O(14)...H(31)-N(3) O(2)...H(32)-N(3) O(14)...H(34)-N(6) O(5)...H(35)-N(6) O(21)...H(48)-N(5) O(18)...H(49)-N(5) O(13)...H(62)-N(4) O(3)...H(64)-N(4) O(102)...H(102)-O(15) N(5)...H(103)-O(21) O(107)...H(104)-O(23) O(103)...H(106)-O(16)

160.9(9) 142.3(1) 141.7(3) 179.1(5) 173.9(9) 142.9(3) 163.0(3) 143.1(4) 178.7(2) 153.2(7) 169.4(4) 158.8(3) 178.2(1) 153.6(4) 156.3(7) 172.0(6) 166.9(6) 156.4(3)

framework, and four are present as part of the isolated PO2(OH)2 group. The Zn atoms are tetrahedrally coordinated by their O atom neighbors with Zn-O bond distances in the range 1.920(3)-1.966(3) Å [(Zn-O)av. ) 1.946 Å] and O-Zn-O angles in the range 98.8(1)118.3(1)° [(O-Zn-O)av. ) 109.4°]. Of the three phosphorus atoms, P(1) makes four P-O-Zn bonds, P(2) makes three such bonds and has one free oxygen atom, and P(3) makes only one P-O-Zn connection and possesses three free oxygen atoms. The P-O distances are in the range 1.498(5)-1.561(4) Å [(P-O)av. ) 1.537 Å] and the O-P-O bond angles are in the range 105.3(2)-113.1(2)° [(O-P-O)av. ) 109.4°]. The fourth independent P atom is bonded with four O atom neighbors with an average P-O distance of 1.532 Å and a O-P-O bond angle of 109.4°. The framework stoichiometry of Zn2(PO4)4 creates a net charge of -8. Taking into account the presence of one completely protonated HMTA molecule, [NH3(CH2)6NH2(CH2)6NH3], the excess negative charge of -5 can be balanced by the protonation of the five oxygen atoms of the phosphate tetrahedra. Thus, one hydrogen position for each of the framework oxygens, O(9), O(11), and O(12), has been observed in the difference Fourier maps. Thus, P(2)-O(9), P(3)-O(11) and P(3)-O(12) with distances of 1.561(4), 1.542(4), and 1.573(5) Å are P-OH units. The third terminal linkage, in the case of P(3) with a P-O distance of 1.498(4) is a P ) O unit. Two of the P-O linkages of the isolated phosphate P(4)O4 unit,

bond

distance (Å)

bond

distance (Å)

Zn(1)-O(1) Zn(1)-O(2) Zn(1)-O(4) Zn(1)-O(3) Zn(2)-O(6) Zn(2)-O(5) Zn(2)-O(7) Zn(2)-O(8) P(1)-O(1)#1 P(1)-O(4) P(1)-O(6) P(1)-O(7)#2

1.920(3) 1.940(3) 1.957(3) 1.960(3) 1.941(3) 1.942(3) 1.942(3) 1.966(3) 1.520(3) 1.533(3) 1.540(3) 1.556(3)

P(2)-O(2)#3 P(2)-O(5) P(2)-O(3) P(2)-O(9) P(3)-O(10) P(3)-O(8) P(3)-O(11) P(3)-O(12) P(4)-O(15) P(4)-O(13) P(4)-O(16) P(4)-O(14)

1.520(3) 1.522(3) 1.537(3) 1.561(4) 1.498(4) 1.517(4) 1.542(4) 1.573(5) 1.488(5) 1.515(4) 1.560(4) 1.565(4)

N(1)-C(1) N(2)-C(6) N(2)-C(7) N(3)-C(12) C(1)-C(2) C(2)-C(3) C(3)-C(4)

Organic Moiety 1.482(11) C(4)-C(5) 1.445(12) C(5)-C(6) 1.495(11) C(7)-C(8) 1.463(11) C(8)-C(9) 1.16(2) C(9)-C(10) 1.63(2) C(10)-C(11) 1.43(2) C(11)-C(12)

1.45(2) 1.53(2) 1.21(2) 1.55(2) 1.33(2) 1.53(2) 1.336(14)

a Symmetry transformations used to generate equivalent atoms: #1 -x + 2, -y + 1, -z; #2 -x + 3, -y + 1, -z; #3 -x + 2, -y + 2, -z.

Table 6. Selected Bond Angles in II, [HMTA(H)3][Zn2(PO4)(HPO4)(H2PO4)][(H2PO4)]a moiety

angle (°)

moiety

angle (°)

O(1)-Zn(1)-O(2) O(1)-Zn(1)-O(4) O(2)-Zn(1)-O(4) O(1)-Zn(1)-O(3) O(2)-Zn(1)-O(3) O(4)-Zn(1)-O(3) O(6)-Zn(2)-O(5) O(6)-Zn(2)-O(7) O(5)-Zn(2)-O(7) O(6)-Zn(2)-O(8) O(5)-Zn(2)-O(8) O(7)-Zn(2)-O(8) O(1)#1-P(1)-O(4) O(1)#1-P(1)-O(6) O(4)-P(1)-O(6) O(1)#1-P(1)-O(7)#2 O(4)-P(1)-O(7)#2 O(6)-P(1)-O(7)#2 O(2)#3-P(2)-O(5) O(2)#3-P(2)-O(3) O(5)-P(2)-O(3) O(2)#3-P(2)-O(9)

109.76(14) 104.08(14) 104.86(13) 115.16(13) 111.81(13) 110.41(14) 118.31(13) 115.81(13) 103.11(13) 98.79(14) 110.2(2) 110.71(14) 113.1(2) 107.6(2) 110.2(2) 109.0(2) 108.0(2) 109.0(2) 112.8(2) 108.5(2) 110.8(2) 105.3(2)

O(5)-P(2)-O(9) O(3)-P(2)-O(9) O(10)-P(3)-O(8) O(10)-P(3)-O(11) O(8)-P(3)-O(11) O(10)-P(3)-O(12) O(8)-P(3)-O(12) O(11)-P(3)-O(12) O(15)-P(4)-O(13) O(15)-P(4)-O(16) O(13)-P(4)-O(16) O(15)-P(4)-O(14) O(13)-P(4)-O(14) O(16)-P(4)-O(14) P(1)#1-O(1)-Zn(1) P(2)#3-O(2)-Zn(1) P(2)-O(3)-Zn(1) P(1)-O(4)-Zn(1) P(2)-O(5)-Zn(2) P(1)-O(6)-Zn(2) P(1)#2-O(7)-Zn(2) P(3)-O(8)-Zn(2)

111.6(2) 107.7(2) 115.1(2) 111.3(2) 107.4(2) 108.8(3) 107.3(2) 106.5(3) 112.3(3) 111.2(3) 110.3(3) 111.3(3) 111.0(2) 100.1(3) 132.1(2) 138.2(2) 117.7(2) 126.1(2) 131.2(2) 130.4(2) 121.6(2) 122.3(2)

C(6)-N(2)-C(7) C(2)-C(1)-N(1) C(1)-C(2)-C(3) C(4)-C(3)-C(2) C(3)-C(4)-C(5) C(4)-C(5)-C(6) N(2)-C(6)-C(5)

Organic Moiety 116.7(9) C(8)-C(7)-N(2) 127.2(14) C(7)-C(8)-C(9) 116(2) C(10)-C(9)-C(8) 96(2) C(9)-C(10)-C(11) 110.7(11) C(12)-C(11)-C(10) 114.2(11) C(11)-C(12)-N(3) 108.9(8)

122.4(12) 123.0(14) 114.1(14) 123(2) 112.0(12) 117.0(12)

a Symmetry transformations used to generate equivalent atoms: #1 -x + 2, -y + 1, -z #2 -x + 3, -y + 1, - z; #3 -x + 2, -y + 2, -z.

with distances of 1.560(4) and 1.565(4), correspond to P-OH linkages. These assignments are also consistent with bond valence sum calculations.24 Selected bond distances and angles observed in II are presented in Tables 5 and 6. The framework structure of II is built up of linkages involving the ZnO4, PO4, PO3(OH), and PO2(OH)2 tetrahedra sharing the vertices. The connectivity between these units form layers, which are anionic. The

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Neeraj and Natarajan

Figure 4. The polyhedral view of a single layer in II. Note that the layers are formed by four- and eight-membered apertures only. The hanging phosphate groups from the layers are not shown for clarity.

triply protonated amine molecule occupies the interlamellar space. The connectivity between the strictly alternating ZnO4, P(1)O4, and P(2)O3(OH) tetrahedral units form four-membered rings. The four-membered rings are connected to each other resulting in a onedimensional zigzag ladder-like structure, which are linked together via a four-membered ring forming apertures surrounded by 8-T atoms (T ) Zn, P) within the layer (Figure 4). The third phosphate group, P(3)O2(OH)2, hangs from the Zn center into the interlamellar space. This is the first time, to our knowledge, such a hanging phosphate has been observed in a layered architecture, although hanging phosphates are commonly observed in one-dimensional ladder phosphates.9a As in I, the structure of II also has a large interlayer separation of ∼20 Å. The triply protonated HMTA molecule is present in the interlamellar region along with the isolated H2P(4)O4 molecule. The layered arrangement along with the amine and isolated phosphate molecule is shown in Figure 5. There are, however, no water molecules present in the interlamellar region in the structure of II. The large interlamellar separation in II is supported through strong N-H...O, C-H...O, and O-H...O interactions. It is to be noted that the hanging phosphate group from the layers interacts strongly with the free phosphate, present in the interlamellar region, to form an inorganic pillar-like arrangement in II. This is to be contrasted with that observed in I, wherein the organic amine along with the water molecules is responsible for holding the layers together. The complete list of hydrogen bond interactions in II is listed in Table 7. [NH3(CH2)6NH2(CH2)6NH3][(HPO4)(H2PO4)]‚6H2O, III. It has been established recently that the reaction of the amine phosphate (hydrogen bonded complex between the amine and phosphoric acid) with metal ions facilitates a convenient route for the synthesis of solids with open-framework structures.9c Because of the uniqueness of the structures, we thought it desirable to synthesize the hydrogen bonded adduct between the amine, HMTA, and phosphoric acid and to investigate the structural similarity between the amine phosphate with that of I and II. With that in mind, we have synthesized the amine phosphate and elucidated its structure.

Figure 5. Figure showing the layer assembly in II. Note the hanging and free H2PO4 units. Dotted lines represent possible hydrogen bond interactions. Table 7. Important Hydrogen Bond Interactions in II, [HMTA(H)3][Zn2(PO4)(HPO4)(H2PO4)][(H2PO4)] bond

distance Å

moiety

angle (°)

O(10)...H(9C)a O(7)...H(10D) O(2)...H(10E) O(10)...H(14A) O(13)...H(16A) O(15)...H(20A) O(15)...H(20B) O(8)...H(30B) O(1)...H(30C) O(16)...H(7B) O(12)...H(12A)

1.890(6) 2.126(8) 2.204(7) 1.943(5) 1.863(3) 1.868(5) 1.893(1) 2.109(3) 2.533(5) 2.556(1) 2.373(6)

O(10)...H(9C)-O(9)a O(7)...H(10D)-N(1) O(2)...H(10E)-N(1) O(10)...H(14A)-O(14) O(13)...H(16A)-O(16) O(15)...H(20A)-N(2) O(15)...H(20B)-N(2) O(8) ...H(30B)-N(3) O(1)...H(30C)-N(3) O(16)...H(7B)-C(7) O(12)...H(12A)-C(12)

145.0(1) 144.7(4) 137.7(3) 142.9(9) 156.5(8) 159.9(9) 167.5(1) 162.5(1) 153.0(5) 173.7(1) 174.1(5)

a

Intralayer.

The asymmetric unit of [NH3(CH2)6NH2(CH2)6NH3][(HPO4)(H2PO4)]‚6H2O, HMTA-P, III, contains 30 nonhydrogen atoms of which nine (two P and seven O) belong to the phosphate tetrahedra, 15 (12 C and three N) to the amine molecule and six oxygen to water molecules. One P and two O atoms are partially occupied with a site occupancy factor (SOF) of 0.5. The P atoms are tetrahedrally coordinated with respect to the oxygen atoms with average P-O bond distances of 1.535 Å and O-P-O bond angles of 109.2°. The two phosphate tetrahedra create a net negative charge of -6. The asymmetric unit contains one amine molecule, and assuming all the nitrogen atoms are protonated, the net negative charge of -3 can be balanced by the protonation of three of the oxygen atoms. Thus, one hydrogen

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Crystal Growth & Design, Vol. 1, No. 6, 2001 497

Table 8. Selected Bond Distances in III, [NH3(CH2)6NH2(CH2)6NH3][(HPO4)(H2PO4)]‚6H2Oa bond

distance, (Å)

bond

distance, (Å)

P(1)-O(1) P(1)-O(2) P(1)-O(3) P(1)-O(4)

1.511(3) 1.522(3) 1.525(3) 1.616(3)

P(2)-O(5) P(2)-O(5)#1 P(2)-O(6) P(2)-O(7)

1.438(5) 1.578(5) 1.472(5) 1.617(8)

N(1)-C(1) C(1)-C(2) C(2)-C(3) C(3)-C(4) C(4)-C(5) C(5)-C(6) C(6)-N(2)

Organic Moiety 1.494(5) N(2)-C(7) 1.516(6) C(7)-C(8) 1.518(6) C(8)-C(9) 1.525(6) C(9)-C(10) 1.523(6) C(10)-C(11) 1.525(6) C(11)-C(12) 1.491(5) C(12)-N(3)

1.504(5) 1.504(6) 1.536(6) 1.517(6) 1.526(5) 1.515(5) 1.498(5)

a Symmetry transformations used to generate equivalent atoms: #1 -x + 1, y, -z + 1/2.

Table 9. Selected Bond Angles in III, [NH3(CH2)6NH2(CH2)6NH3] [(HPO4)(H2PO4)]‚6H2Oa moiety

angle, (°)

moiety

angle, (°)

O(1)-P(1)-O(2) O(1)-P(1)-O(3) O(2)-P(1)-O(3) O(1)-P(1)-O(4) O(2)-P(1)-O(4) O(3)-P(1)-O(4)

112.7(2) 112.2(2) 112.2(2) 106.1(2) 106.4(2) 106.7(2)

O(5)-P(2)-O(6) O(5)-P(2)-O(5)#1 O(6)-P(2)-O(5)#1 O(5)-P(2)-O(7) O(5)#1-P(2)-O(7) O(6)-P(2)-O(7)

119.5(2) 113.3(3) 111.0(2) 106.6(4) 102.2(4) 101.9(3)

N(1)-C(1)-C(2) C(1)-C(2)-C(3) C(2)-C(3)-C(4) C(3)-C(4)-C(5) C(4)-C(5)-C(6) C(5)-C(6)-N(2) C(6)-N(2)-C(7)

Organic Moiety 113.0(3) N(2)-C(7)-C(8) 110.7(4) C(7)-C(8)-C(9) 113.7(4) C(8)-C(9)-C(10) 112.8(4) C(9)-C(10)-C(11) 111.7(4) C(10)-C(11)-C(12) 111.0(2) C(11)-C(12)-N(3) 112.7(3)

112.2(3) 110.4(3) 113.7(4) 111.9(3) 112.2(2) 111.0(3)

a Symmetry transformations used to generate equivalent atoms: #1 -x + 1, y, -z + 1/2.

Figure 6. Figure showing the structure of III, HMTA-P. Note the free H2PO4 unit and the similarity with I and II.

position for each of the oxygens, O(4), O(5), and O(7), has been observed in the difference Fourier maps. Thus, P(1)-O(4) and P(2)-O(7) with distances of 1.616(3) and 1.617(8) Å are P-OH units. Thus, of the two P atoms, one is HP(1)O4 and the other is a H2P(2)O4 unit. The bond distances and angles associated with the amine molecule are in the expected range. The selected bond distances and angles are given in Tables 8 and 9. The structure of the amine phosphate possesses the triply protonated amine molecules aligned parallel to the crystallographic a axis. The HP(1)O4 groups are present at either end of the amine molecule and interact through N-H...O hydrogen bonds. The other phosphate H2P(2)O4 molecule is disordered and is situated close to the central nitrogen atom of the amine. The structure, shown in Figure 6, can be visualized as built-up of HPO4 phosphate sheets separated by the amine molecule. The position of the amine molecule again resembles a pillar. The H2PO4 groups, present near the central nitrogen atom, also form extended networks through N-H...O, O-H...O hydrogen bond interactions. The water molecules are positioned in the pseudo interlamellar region between the inorganic HP(1)O4 layers, formed by hydrogen bond interactions. Thus, HMTA-P has a close structural relationship with I and II. The complete list of hydrogen bond interactions is listed in Table 10.

Table 10. Important Hydrogen Bond Interactions in III, [NH3(CH2)6NH2(CH2)6NH3][(HPO4)(H2PO4)]‚6H2O

Discussion Two new zinc phosphates, [NH3(CH2)6NH2(CH2)6NH3]2[Zn3(PO4)2(HPO4)2][(H2PO4)2]‚10H2O, I, and [NH3-

bond

distance (Å)

moiety

angle (°)

O(3)...H(1) O(3)...H(2) O(2)...H(3) O(5)...H(16) O(6)...H(17) O(1)...H(30) O(3)...H(31) O(2)...H(32) O(100)...H(40) O(4)...H(301) O(7)...H(12) O(500)...H(19)

1.853(3) 2.068(7) 2.071(1) 1.886(8) 1.870(7) 1879(4) 1.954(3) 1.963(8) 1.871(2) 2.191(3) 2.517(6) 2.508(2)

O(3)...H(1)-N(1) O(3)...H(2)-N(1) O(2)...H(3)-N(1) O(5)...H(16)-N(2) O(6)...H(17)-N(2) O(1)...H(30)-N(3) O(3)...H(31)-N(3) O(2)...H(32)-N(3) O(100)-H(40)-O(4) O(4)...H(301)-O(300) O(7)...H(12)-C(5) O(500)...H(19)-C(7)

169.5(4) 157.5(8) 155.3(1) 162.3(7) 174.1(8) 174.4(1) 169.1(6) 171.9(9) 170.0(1) 175.9(3) 152.2(1) 155.8(9)

(CH2)6NH2(CH2)6NH3][Zn2(PO4)(HPO4)(H2PO4)][(H2PO4)], II, have been obtained in good-quality single crystals by room-temperature crystal growth, in addition to the amine phosphate, HMTA-P, [NH3(CH2)6NH2(CH2)6NH3][(HPO4)(H2PO4)]‚6H2O, III. Although it was established that the formation of solids with framework architectures is predominantly kinetically controlled with little predictability, the structures of I and II appear to be closely related. As typical of kinetically controlled reactions, there is no correlation between the starting composition and the majority solidphase product.26 As described earlier, the structures of I and II consist of strictly alternating ZnO4 and (H/H2PO4) tetrahedra connected through their vertexes forming layered architectures. It may be noted that the layer architecture of I, comprising four-, six-, and eight-

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membered apertures, has been observed earlier in a zinc phosphate prepared in the presence of 1,6-diaminohexane.9a The layer architecture of II, however, is unique in that it comprises only four- and eightmembered apertures, and such structures have been encountered recently in the oxalate-phosphates of iron.27 It is well established that layered zinc phosphates are synthesized, generally, in the presence of short chain amine molecules,5a,7,9a but the framework structures encountered in the present study are rather unusual due to the presence of the extra-large interlayer separations of ∼20 Å. It was shown recently that ordered crystalline lamellar aluminophosphates (AlPO) with interlamellar distances in the range of ∼17-21 Å can be prepared employing diamines with long alkyl chain backbone.18 Similar lamellar AlPOs with large interlayer separations have also been synthesized using long alkyl chain primary amine molecules.28 These are typically lamellar mesophase structures with no periodicity. In the AlPO structures reported by Feng et al.,18 although there is consistent periodic registry between the layers, it was not conclusive with respect to the positions of the carbon atoms of the diamine molecule. In the present structures, the presence of the free phosphate group in the interlamellar region and its interactions with the middle nitrogen of the amine molecule through the N-H...O interactions was important in the identification of the complete amine molecule. It may be noted that in many of the layered metal phosphates, the amine molecules generally occupy oblique positions with respect to the layers.29 In I and II, however, the amine molecules are nearly perpendicular to the layer. Similar positioning of the amine molecules, although rare, have been observed earlier.19 The structural details obtained here, especially with respect to the position of the amine, may help in elucidating the structures of noncrystalline, mesostructured materials. It is also important to note that the amine phosphate, HMTA-P, has common structural features with I and II (see Figures 2, 5, and 6). Similar observations of a close structural relationship between the amine phosphate and framework phosphates, obtained by reacting metal ions with the amine phosphates, have also been made in other open-framework solids.9c,30 What is specially noteworthy is the presence of the free H2PO4 group near the middle nitrogen of the amine molecule in I, II, and HMTA-P. It appears that the structures of I and II are formed over the amine phosphate structure by selective interaction between the phosphate and Zn2+ ions and may be considered as a supramolecular selfassembly. Similar self-assembled epitaxial growth has been known in the literature, for example, the growth of hydroxyapatite crystals on self-assembled octadecyl phosphate molecules.31 In addition to the above, the isolated phosphate group near the middle nitrogen described herein may be considered as an unusual amine phosphate itself, possessing a perforated sheetlike structure, as shown schematically in Figure 7a. In light of this, it is worth noting that in other polyamine phosphates, such as diethylenetriamine phosphate (DETA-P),32 it is only the terminal nitrogen of the amine molecule that participates in hydrogen bonding with phosphate tetrahedra

Neeraj and Natarajan

Figure 7. (a) Figure showing the schematic of the structure of II. Note the amine phosphate sheet (out-lined in dotted lines). (b) Figure showing the schematic of a possible 16membered channel structure from II by partial replacement of free H2PO4 tetrahedra with ZnO4 ones (see text).

and the middle nitrogen atom is hydrogen bonded with a water molecule. The absence of a phosphate group near the middle nitrogen in DETA-P may be attributed to the stereochemical consequences of the phosphate anion and the short chain length of the amine molecule. The structural details of I and II are worth discussing within the realm of the amine phosphates and their importance in the formation of open-framework structures, mainly because both the structures possess an amine phosphate-like unit. In trying to understand the formation of framework structures in metal phosphates, Rao and co-workers9c have employed the reaction of metal ions with amine phosphates forming a large variety of open-framework materials. Since I and II possess inorganic layers separated by an amine phosphate assembly, it would be interesting to consider the reactivity of such amine phosphates. One possibility would be to replace the phosphate group, present near the middle nitrogen, selectively by ZnO4 tetrahedra (see Figures 2 and 5), and create new bonds as shown in Figure 7b. Such replacement and reaction would yield a new framework architecture with 16-membered apertures. Framework solids with 16-membered channels, although are rare, have been obtained recently in a zinc phosphate using triethylenetetramine (TETA, C-10).33 If additional ZnO4 units are added without displacing any of the phosphate tetrahedra in I and II, one would obtain a framework solid with a 24-membered channel. Open-framework zinc phosphates with 24-membered channel structure have also been prepared in recent years.34 Conclusions The synthesis and structure of two unusual layered zinc phosphates I and II containing alternating inorganic and organic layers have been accomplished, in addition to the synthesis and structural elucidation of HMTA-P. The hanging phosphate units from the layer in II along with the presence of large interlayer separation and isolated H2PO4 groups in the interlamellar region are noteworthy. All these structural features

Open-Framework Zinc Phosphates

have been observed for the first time in the family of open-framework zinc phosphate structures. The formation of these phases is probably driven by several factors including nanoscale phase separation and intricate hydrogen bonding. It is highly likely that the synthesis strategy employed herein may be extended to include other complex organic amine molecules and to more complex inorganic systems, so that mesoporous materials with atomically ordered frameworks can be more readily prepared. Acknowledgment. The authors thank Professor C. N. R. Rao, FRS for his kind help, support, and encouragement. Supporting Information Available: Crystal data and structure refinement (Table 1), atomic coordinates and equivalent isotropic displacement parameters (Table 2), bond lengths and angles (Table 3), anisotropic displacement parameters (Table 4), and hydrogen coordinates and isotropic displacement parameters (Table 5) for layered zinc phosphate with bis(HMTA)I. This material is available free of charge via the Internet at http://pubs.acs.org.

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