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Solid State Sciences 7 (2005) 1542–1548www.elsevier.com/locate/sssc

Hydrothermal synthesis and structures of two zero-dimensional zinphosphate polymorphs

Srinivasan Natarajan∗

Framework Solids Laboratory, Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India

Received 31 July 2005; accepted 27 August 2005

Available online 24 October 2005

Dedicated to Professor C.N.R. Rao, FRS

Abstract

Two polymorphs of zero-dimensional zinc phosphate with the formula,0∞[Zn(2,2′-bipy)(H2PO4)2], have been synthesized employing hdrothermal technique and their structure determined by single crystal X-ray diffraction. Both the structures consists of ZnO3N2 distortedtrigonal-bipyramidal and PO2(OH)2 tetrahedral units linked through their vertices giving rise to a zero-dimensional molecular zinc phoThe structures are stabilized by extensive hydrogen bond interactions between zero-dimensional monomers. The structures display sences in their packing created by hydrogen bond interactions. Crystal data: polymorphI, triclinic, space groupP 1̄ (No. 2),a = 7.5446(15) Å,b = 10.450(2) Å, c = 10.750(2) Å, α = 67.32(3)◦, β = 81.67(3)◦, γ = 69.29(3)◦, V = 731.4(3) Å3, Z = 2; polymorphII, triclinic, space groupP 1̄ (No. 2),a = 8.664(1) Å, b = 8.849(2) Å, c = 10.113(2) Å, α = 97.37(2)◦, β = 100.54(2)◦, γ = 100.98(2)◦, V = 737.5(3) Å3, Z = 2. 2005 Elsevier SAS. All rights reserved.

Keywords: Zinc; Phosphorus; N ligands; Structure elucidation

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1. Introduction

The synthesis and study of the compounds with extenstructures constitute an important area of research in inorgchemistry[1,2]. Though many new compounds with a wistructural variety and diversity have been prepared and cacterized, our understanding of the formation of these phcontinues to be primitive. There have been some attempidentifying intermediates by employing a variety of in sexperimentation under hydrothermal conditions, but the cplete understanding is still elusive[3–9]. It is clear that thelower-dimensional structures are important in the build-upthe structures of higher dimensionality. In a sense, the moular zero-dimensional compound of the composition,M2P2O4(M = metal), could be one of the fundamental building un[8–13]. Such zero-dimensional molecular compounds hbeen isolated and shown to be reactive[13,14]. We have nowisolated two zero-dimensional polymorphic zinc phospha

* Corresponding author.E-mail address: snatarajan@sscu.iisc.ernet.in(S. Natarajan).

1293-2558/$ – see front matter 2005 Elsevier SAS. All rights reserved.doi:10.1016/j.solidstatesciences.2005.08.021

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with the formula, 0∞[Zn(2,2′-bipy)(H2PO4)2]. In this paper, wepresent the synthesis and structure of the compounds.

2. Experimental

The compounds were synthesized employing hydrothemethods. Typically for polymorphI, 0.08 g of ZnO was dispersed in 2 ml of water. To this 0.2 ml of H3PO4 (85 wt%),0.143 g of [Co(en)3]Cl3 and 0.156 g of 2,2′-bipyridine (2,2′-bipy) were added. The mixture was homogenized at room tperature for 30 min. The reaction mixture with the molartio, 1.0ZnO: 3.0H3PO4 : 0.5[Co(en)3]Cl3 : 1.0(2,2′-bipy) andwater, was filled in a PTFE autoclave of 20 ml capacity (fifactor= 20%) and heated at 140◦C for 96 h. For synthesis opolymorphII the amount of ZnO in the reaction mixture wdoubled. The initial pH of the reaction mixture in both the cawas∼ 3.5 and after the completion of the reaction the pHnot show appreciable change. The resulting product contacolorless rod-like single crystals ofI admixed with some yellow polycrystalline powder was filtered, washed with deionizwater and dried at ambient conditions. The yield of the sincrystalline product was about 30% based on ZnO. We have

S. Natarajan / Solid State Sciences 7 (2005) 1542–1548 1543

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been able to identify the yellow powder, as it does not cospond to any known solids including the starting materials.were also able to synthesize the polymorphI as the majorityphase without the addition of [Co(en)3]Cl3 in the reaction mix-ture. We have also been able to prepare the polymorphII asa majority phase by including boric acid (H3BO3) in the re-action mixture. An EDAX analysis on single crystals, in bothe cases, indicated that the Zn: P ratio is 1: 2, which agreevery well with the results of the crystal structure analysis. TEDAX analysis also indicated the absence of cobalt and crine in the single crystals.

3. Single crystal structure determination

A suitable single crystal of each compound was seleunder a polarizing microscope and glued to a thin glass fiwith a two-component adhesive. Crystal structure determtion by X-ray diffraction was performed on a RIGAKU AFCfour-circle diffractometer, equipped with a Mercury-CCD dtector, and a fine focused sealed tube (Mo Kα radiation,λ =0.71070 Å, graphite monochromator). In case of polymorpItwo measurements with long and short exposure times wperformed to obtain accurate intensities of the strong and

Table 1Crystallographic data, measurement conditions and refinement parametetwo polymorphs of 0∞[Zn(2,2′-bipy)(H2PO4)2] (standard deviation in parenthesis)

PolymorphI PolymorphII

Empirical formula C10H12N2O8P2ZnFormula weight 415.53Space group P 1̄ (No. 2) P 1̄ (No. 2)a/Å 7.5446(15) 8.664(1)b/Å 10.450(2) 8.849(2)c/Å 10.750(2) 10.113(2)α/◦ 67.32(3) 97.37(2)β/◦ 81.67(3) 100.54(2)γ /◦ 69.29(3) 100.98(2)Volume/Å3 731.4(3) 737.5(3)Z 2 2ρcalc/g cm−3 1.887 1.871µ/mm−1 1.942 1.926Crystal size/mm3 0.08× 0.08× 0.12 0.04× 0.04× 0.08Diffractometer RIGAKU AFC7 RIGAKU AFC7Detector Mercury CCD Mercury CCDScans �ϕ = 0.6◦,

600 images, 60 s, 15 s�ϕ = 0.6◦,600 images, 80 s

�ω = 0.6◦,100 images, 60 s, 15 s

�ω = 0.6◦,200 images, 80 s

2θ range up to 58◦ 58◦hkl ranges −10� h � 10 −11� h � 11

−14� k � 14 −12� k � 11−14� h � 14 −13� h � 13

N(hkl) measured 11162 8708N(hkl) unique 3611 3291R(int) 0.033 0.025N(hkl) observed 3155 2769Refined parameters 213 213R1 0.039 0.045wR2 0.082 0.095Residual peaks (e/Å3) −0.49 and 0.44 −0.56 and 0.787

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weak reflections. Both the data sets were scaled and combusing the program XPREP[15]. Lattice constants ofI and IIwere obtained from least-squares fits of 5351 and 2552 retions obtained during the data collection, respectively. Pertiexperimental details for the data collection and the structuretermination are presented inTable 1.

The structure was solved by direct methods. A sufficifragment of the structure was revealed to enable the remder of the non-hydrogen atoms to be located from differeFourier maps. All hydrogen positions were initially locatedthe difference maps and for the final refinement the hydroatoms were placed geometrically and held in the riding moThe last cycles of refinement included atomic positions forthe atoms, anisotropic thermal parameters for all non-hydroatoms and isotropic thermal parameters for all hydrogen atoFull-matrix least-squares refinement against|F |2 was carriedout using the SHELXL-97 suite of programs[16]. The finalatomic coordinates and selected bond distances and anglgiven inTables 2 and 4for polymorphI and inTables 3 and 5for polymorphII.

Table 2Atomic coordinates and displacement parameters (in Å2) for I, 0∞[Zn(2,2′-bipy)(H2PO4)2]

Atom Site x y z Ueq/Uisoa,b

Zn(1) 2i 0.6778(1) 0.9272(1) 0.6895(1) 0.021(1)

P(1) 2i 0.3454(1) 0.8374(1) 0.6221(1) 0.022(1)

P(2) 2i 0.9002(1) 0.7042(1) 0.5193(1) 0.024(1)

O(1) 2i 0.4171(3) 0.9261(2) 0.6744(2) 0.026(1)

O(2) 2i 0.8766(3) 0.7898(2) 0.6093(2) 0.027(1)

O(3) 2i 0.6672(3) 0.1080(2) 0.5269(2) 0.034(1)

O(4) 2i 0.7951(3) 0.5945(3) 0.5686(3) 0.042(1)

O(5) 2i 0.8390(4) 0.8105(3) 0.3723(2) 0.049(1)

O(6) 2i 0.4673(3) 0.6716(2) 0.6781(2) 0.032(1)

O(7) 2i 0.1494(3) 0.8392(3) 0.6953(2) 0.033(1)

O(8) 2i 0.1161(3) 0.6252(2) 0.5085(2) 0.031(1)

N(1) 2i 0.7096(3) 0.0373(3) 0.8091(2) 0.027(1)

N(2) 2i 0.7304(3) 0.7599(3) 0.8879(2) 0.027(1)

C(1) 2i 0.7035(5) 0.1775(4) 0.7619(3) 0.038(1)

C(2) 2i 0.7252(6) 0.2445(4) 0.8442(4) 0.046(1)

C(3) 2i 0.7518(6) 0.1656(4) −0.0207(4) 0.045(1)

C(4) 2i 0.7606(5) 0.0211(4) 0.0294(3) 0.036(1)

C(5) 2i 0.7401(4) 0.9585(3) 0.9416(3) 0.026(1)

C(6) 2i 0.7489(4) 0.8029(3) 0.9861(3) 0.026(1)

C(7) 2i 0.7740(5) 0.7076(4) 0.1199(3) 0.040(1)

C(8) 2i 0.7739(6) 0.5664(4) 0.1521(3) 0.049(1)

C(9) 2i 0.7543(6) 0.5228(4) 0.0518(4) 0.048(1)

C(10) 2i 0.7339(5) 0.6218(4) 0.9202(3) 0.040(1)

H(50) 2i 0.7533 0.8845 0.3744 0.073H(60) 2i 0.5677 0.6587 0.6360 0.048H(70) 2i 0.0843 0.8232 0.6518 0.049H(80) 2i 0.1324 0.5571 0.4834 0.047H(1) 2i 0.6839 0.2316 0.6703 0.046H(2) 2i 0.7218 0.3418 0.8085 0.056H(3) 2i 0.7638 0.2095 0.0367 0.054H(4) 2i 0.7800 −0.0341 0.1208 0.044H(7) 2i 0.7906 0.7386 0.1864 0.048H(8) 2i 0.7871 0.5017 0.2412 0.058H(9) 2i 0.7546 0.4279 0.0716 0.058H(10) 2i 0.7223 0.5914 0.8520 0.048

a Ueq is defined as one third of the trace of the orthogonalizedUij tensor.b The hydrogenUiso values was hold at 1.5 times the equivalent isotropicU

of the atoms to which they are attached.

1544 S. Natarajan / Solid State Sciences 7 (2005) 1542–1548

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Table 3Atomic coordinates and displacement parameters (in Å2) for II, 0∞[Zn(2,2′-bipy)(H2PO4)2]

Atom Site x y z Ueq/Uisoa,b

Zn(1) 2i 0.4383(1) 0.2831(1) 0.2833(1) 0.029(1)

P(1) 2i 0.6878(1) 0.4097(1) 0.5743(1) 0.027(1)

P(2) 2i 0.2052(1) 0.0661(1) 0.4198(1) 0.031(1)

O(1) 2i 0.5794(3) 0.2757(3) 0.4712(2) 0.031(1)

O(2) 2i 0.2635(3) 0.1091(3) 0.2961(2) 0.035(1)

O(3) 2i 0.6073(3) 0.5144(3) 0.6513(2) 0.039(1)

O(4) 2i 0.1034(3) −0.0962(3) 0.4009(3) 0.039(1)

O(5) 2i 0.1027(4) 0.1856(3) 0.4618(3) 0.046(1)

O(6) 2i 0.7943(4) 0.3417(3) 0.6833(3) 0.046(1)

O(7) 2i 0.8009(4) 0.4989(3) 0.4927(3) 0.053(1)

O(8) 2i 0.3500(4) 0.0844(4) 0.5412(3) 0.051(1)

C(1) 2i 0.2053(6) 0.3617(5) 0.428(4) 0.053(1)

C(2) 2i 0.1542(7) 0.3875(6) −0.0872(5) 0.065(1)

C(3) 2i 0.2469(7) 0.3640(6) −0.1807(5) 0.065(2)

C(4) 2i 0.3838(7) 0.3097(5) −0.1450(4) 0.056(1)

C(5) 2i 0.4283(5) 0.2818(4) −0.0125(4) 0.038(1)

C(6) 2i 0.5711(5) 0.2205(4) 0.0382(4) 0.038(1)

C(7) 2i 0.6664(6) 0.1666(5) −0.0463(4) 0.054(1)

C(8) 2i 0.7954(6) 0.1087(6) 0.0078(5) 0.062(1)

C(9) 2i 0.8295(6) 0.1064(6) 0.1460(5) 0.063(1)

C(10) 2i 0.7306(5) 0.1614(5) 0.2246(5) 0.051(1)

N(1) 2i 0.3385(4) 0.3091(4) 0.0799(3) 0.036(1)

N(2) 2i 0.6042(4) 0.2165(4) 0.1732(3) 0.035(1)

H(50) 2i 0.0416 0.1471 0.5077 0.070H(60) 2i 0.8238 0.2680 0.6455 0.069H(70) 2i 0.8272 0.5917 0.5267 0.079H(80) 2i 0.4313 0.1344 0.5234 0.076H(1) 2i 0.1450 0.3817 0.1076 0.063H(2) 2i 0.0586 0.4203 −0.1113 0.078H(3) 2i 0.2168 0.3848 −0.2684 0.077H(4) 2i 0.4464 0.2916 −0.2085 0.068H(7) 2i 0.6425 0.1699 −0.1392 0.064H(8) 2i 0.8590 0.0714 −0.0479 0.074H(9) 2i 0.9170 0.0688 0.1855 0.076H(10) 2i 0.7536 0.1598 0.3178 0.061

a Ueq is defined as one third of the trace of the orthogonalizedUij tensor.b The hydrogenUiso values was hold at 1.5 times the equivalent isotropicU

of the atoms to which they are attached.

4. Results and discussion

The asymmetric unit of both polymorphs is similar and ctain 23 non-hydrogen atoms, of which one Zn and two P atare crystallographically independent (Fig. 1). For describingthe distances and angles the values of polymorphI is givenwith those of polymorphII given in the parenthesis. The Zatom is five coordinated with three oxygen and two nitrogatoms with average Zn–O distances of 2.010 Å [2.008 Å]Zn–N distance of 2.134 Å [2.118 Å], respectively. The geoetry around the Zn atom is distorted trigonal bipyramid. TO–Zn–O bond angles are in the ranges 76.41(9)–163.43◦(av. 106.5◦) [88.43(11)–168.48(11)◦; av. 107.0◦]. The Zn atomis linked through the N atoms to 2,2′-bipyridine and throughthe O atoms to two distinct P atoms with average bond anof 120.6◦ and 138.4◦ [120.3 and 137.4◦], respectively, for theZn–N–C and the Zn–O–P bonds. While all the O atoms bonto Zn are connected to P atoms, only two oxygen atoms attato P atoms make P–O–Zn bond and the other two are term

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Table 4Selected bond distances and angles in the polymorphI, 0∞[Zn(2,2′-bipy)-(H2PO4)2]

Bond Distance, Å Bond Distance, Å

Zn(1)–O(1) 2.001(2) P(1)–O(6) 1.562(2)

Zn(1)–O(3) 2.005(2) P(1)–O(7) 1.569(2)

Zn(1)–O(2) 2.027(2) P(2)–O(2) 1.508(2)

Zn(1)–N(1) 2.113(2) P(2)–O(4) 1.509(2)

Zn(1)–N(2) 2.155(3) P(2)–O(8) 1.555(2)

P(1)–O(3) 1.484(2) P(2)–O(5) 1.561(2)

P(1)–O(1) 1.521(2)

Angle Amplitude,◦ Angle Amplitude,◦

O(1)–Zn(1)–O(3) 97.26(10) O(3)–P(1)–O(7) 113.13(13)O(1)–Zn(1)–O(2) 111.67(8) O(1)–P(1)–O(7) 104.45(12)O(3)–Zn(1)–O(2) 94.44(8) O(6)–P(1)–O(7) 103.03(13)O(1)–Zn(1)–N(1) 117.40(9) O(2)–P(2)–O(4) 113.83(13)O(3)–Zn(1)–N(1) 88.31(9) O(2)–P(2)–O(8) 107.12(12)O(2)–Zn(1)–N(1) 130.07(9) O(4)–P(2)–O(8) 110.36(12)O(1)–Zn(1)–N(2) 95.46(10) O(2)–P(2)–O(5) 110.47(13)O(3)–Zn(1)–N(2) 163.44(9) O(4)–P(2)–O(5) 109.76(16)O(2)–Zn(1)–N(2) 90.65(9) O(8)–P(2)–O(5) 104.89(14)N(1)–Zn(1)–N(2) 76.42(9) P(1)–O(1)–Zn(1) 132.25(12)O(3)–P(1)–O(1) 114.25(12) P(2)–O(2)–Zn(1) 142.39(13)O(3)–P(1)–O(6) 110.59(13) P(1)–O(3)–Zn(1) 140.51(14)O(1)–P(1)–O(6) 110.69(12)

Table 5Selected bond distances and angles in polymorphII, 0∞[Zn(2,2′-bipy)-(H2PO4)2]

Bond Distance, Å Bond Distance, Å

Zn(1)–O(3) 1.966(2) P(1)–O(7) 1.560(3)

Zn(1)–O(2) 1.979(2) P(1)–O(6) 1.561(3)

Zn(1)–O(1) 2.078(2) P(2)–O(2) 1.498(3)

Zn(1)–N(2) 2.096(3) P(2)–O(4) 1.504(3)

Zn(1)–N(1) 2.140(3) P(2)–O(8) 1.554(3)

P(1)–O(1) 1.509(2) P(2)–O(5) 1.571(3)

P(1)–O(3) 1.487(2)

Angle Amplitude,◦ Angle Amplitude,◦

O(3)–Zn(1)–O(2) 110.76(11) O(3)–P(1)–O(6) 106.18(15)O(3)–Zn(1)–O(1) 92.92(10) O(1)–P(1)–O(6) 108.58(14)O(2)–Zn(1)–O(2) 94.04(10) O(7)–P(1)–O(6) 107.71(19)O(3)–Zn(1)–N(2) 133.64(12) O(1)–P(2)–O(6) 115.10(15)O(2)–Zn(1)–N(2) 114.43(11) O(2)–P(2)–O(8) 110.30(15)O(1)–Zn(1)–N(2) 93.72(11) O(4)–P(2)–O(8) 108.23(17)O(3)–Zn(1)–N(1) 88.44(11) O(2)–P(2)–O(5) 107.53(15)O(2)–Zn(1)–N(1) 96.20(11) O(4)–P(2)–O(5) 108.24(15)O(1)–Zn(1)–N(1) 168.45(11) O(8)–P(2)–O(5) 107.15(19)N(2)–Zn(1)–N(1) 77.17(13) P(1)–O(1)–Zn(1) 128.40(14)O(3)–P(1)–O(1) 116.59(16) P(2)–O(2)–Zn(1) 129.04(15)O(3)–P(1)–O(7) 112.49(16) P(1)–O(3)–Zn(1) 154.65(19)O(1)–P(1)–O(7) 104.98(15)

ones. The P–O distances are in the range 1.484(2)–1.569(av. 1.533 Å) [1.487(2)–1.571(3) Å; av. 1.531 Å]. The O–Pbond angles have an average value of 109.4◦ in both the casesThe torsion angle suspended between the 2,2′-bypiridine andthe phosphate groups have the following values

• for I: N(1)–Zn(1)–O(1)–P(1)= −166◦, N(2)–Zn(1)–O(1)–P(1)= − 89.1◦, N(1)–Zn(1)–O(2)–P(2)= −175◦, N(1)–

S. Natarajan / Solid State Sciences 7 (2005) 1542–1548 1545

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Fig. 1. ORTEP diagram of building unit of both polymorph modifications0∞[Zn(2,2′-bipy)(H2PO4)2] (a) I and (b)II. Thermal ellipsoids are given a

50% probability.

Zn(1)–O(3)–P(1) = 176.6◦, N(2)–Zn(1)–O(3)–P(1)=154.1◦ and N(2)–Zn(1)–O(2)–P(2)= 111.3◦;

• for II: N(1)–Zn(1)–O(1)–P(2)= −151.5◦, N(1)–Zn(1)–O(2)–P(1) = 62.3◦, N(1)–Zn(1)–O(3)–P(1)= 116.1◦,N(2)–Zn(1)–O(1)–P(2)= 129.8◦, N(2)–Zn(1)–O(2)–P(1)= 100.1◦ and N(2)–Zn(1)–O(3)–P(1)= 173.4◦.

Bond valence sum calculations show that the valence statZn and P to be+2 and+5, respectively.

The structure of bothI andII is a simple molecular solidwhich consists of ZnO3N2 distorted trigonal-bipyramidal an

of

(a)

(b)

Fig. 2. Packing diagram of (a)0∞[Zn(2,2′-bipy)(H2PO4)2], I, in thebc planeand (b) 0∞[Zn(2,2′-bipy)(H2PO4)2], II, in the ac plane. Dotted lines represent hydrogen bonds. For clear representation H-atoms in the molecu2,2′-bypiridine are not shown from here.

PO2(OH)2 tetrahedral units. The polyhedral units are linkthrough their vertices forming the structure. Thus, Zn(1)P(1) form the 4-membered ring of the composition Zn2P2O4

and the other phosphorus, P(2) hangs from the Zn centegive rise to the zero-dimensional monomer. Both the moular structures have close resemblance to the previouslyscribed zero-dimensional zinc phosphates[8–13]. Additionally,the packing arrangement in both the compounds shows inte

1546 S. Natarajan / Solid State Sciences 7 (2005) 1542–1548

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Fig. 3. Packing diagram in theab plane for (a) 0∞[Zn(2,2′-bipy)(H2PO4)2], I, and (b) 0∞[Zn(2,2′-bipy)(H2PO4)2], II. Dotted lines represent hydrogen bonds. Nthe formation of channel structure through hydrogen bonds (see text).

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ing features. The packing arrangement forI andII, in thebc andac plane, indicates the possibility ofπ · · ·π interactions (Fig. 2).In theac andbc plane, the position of the 2,2′-bipyridine lig-and appear to be perpendicular to the molecular units inI andin II, the 2,2′-bipyridine molecules appears to be at an anThe packing view of the molecular zinc phosphate in theab

plane, for both the compounds, shows the formation of a cnel structure (Fig. 3).

As expected, bothI and II possess strong hydrogen bointeractions. Both intra- and intermolecular hydrogen bondteractions have been observed. Two each of intra- and imolecular hydrogen bonds have been observed with aveO· · ·O contact distances of 2.612 Å (I) and 2.614 Å (II) andaverage O–H· · ·O bond angles of 165◦ (I) and 166◦ (II). Theinter- and intramolecular hydrogen bond interactions giveto one-dimensional chains, which are further connected foing the observed channel structure (Fig. 3). The importanthydrogen bond interactions are listed inTable 6. The 2,2′-bipyridine molecules, attached to the Zn center, separatechain units and appears to have strongπ · · ·π interactions. Therole of π · · ·π interactions for the stability of organic compounds have been discussed with particular emphasis ocentroid–centroid distances (d) and the angle (θ ) suspendedbetween the benzene rings[17]. A correlation between bot(d and θ ), has been discussed in terms of a phase-diagwith the distances and angles having a unique relationsFrom this, the following interactions appear to be important

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Table 6Important hydrogen bond interactions in polymorphsI and II, 0∞[Zn(2,2′-bipy)(H2PO4)2]

D–H· · ·A D–H (Å)a H· · ·A (Å) D · · ·A (Å) D–H· · ·A (◦)

PolymorphI

O(5)–H(50)· · ·O(1) 0.82 1.86 2.649(4) 163O(6)–H(60)· · ·O(4)b 0.82 1.77 2.575(4) 169O(7)–H(70)· · ·O(2)b 0.82 1.87 2.651(4) 158O(8)–H(80)· · ·O(4) 0.82 1.76 2.575(3) 171

PolymorphII

O(5)–H(50)· · ·O(4)b 0.82 1.72 2.528(4) 169O(6)–H(60)· · ·O(4) 0.82 1.79 2.601(4) 168O(7)–H(70)· · ·O(5) 0.82 1.93 2.711(4) 159O(8)–H(80)· · ·O(1)b 0.82 1.82 2.618(4) 166

a D–H distances are constrained;b Intra.

different values of the centroid-centroid distance (d) and an-gles (θ ):

(i) d = 0–3 Å andθ = 50–90◦,(ii) d = 3–7 Å andθ = 0–50◦,(iii) d = 4–7.5 Å, θ = 140–180◦ and(iv) d = 6–7 Å andθ = 0–145◦.

In the present cases we find four types of interactionstween the benzene rings of the 2,2′-bipyridine ligand. Thus

S. Natarajan / Solid State Sciences 7 (2005) 1542–1548 1547

(a) (b)

(c) (d)

Fig. 4. Packing diagram of some of the known zero-dimensional structures: (a)0∞[Zn(2,2′-bipy)(H2PO4)2], I; (b) [(C5NH5)(C4N2H10)][Zn(HPO4)(H2PO4)2](Ref. [9]); (c) [C6N2H18][Zn(HPO4)(H2PO4)2] (Refs.[8,13]) and (d) [C6N4H21][Zn(HPO4)2(H2PO4)] (Refs.[8,13]).

ly er-the

wingurethe

es-ts

the following interactions have been observed. For po

morphs

I: d = 3.84 Å,θ = 3.09◦; d = 4.59 Å,θ = 3.66◦; d = 4.35 Å,

θ = 0◦; d = 4.18 Å, θ = 0◦ and

II: d = 4.49 Å, θ = 28.36◦; d = 4.40 Å, θ = 28.36◦; d =4.02 Å, θ = 14.39◦; d = 4.04 Å, θ = 14.43◦.

-The π · · ·π interactions along with the hydrogen bond intactions appears to be are important for the stability ofmolecular zinc phosphates.

The present compounds constitute members of the grofamily of zero-dimensional structures reported in the literat[8-14]. The zero-dimensional solids have been consideredfundamental building unit in open-framework phosphates,pecially in Zn phosphates. InFig. 4 the packing arrangemen

1548 S. Natarajan / Solid State Sciences 7 (2005) 1542–1548

crucogee dosaninc

ea,2

liduc-ionur-ee

aoisciaout o

99)

. 43

re,

e,

00)

ao,.em.

03)

.) 417.629

ter.

.5.1,

e-

/i. 5

of some of the molecular zinc phosphates is presented. Asbe noted, the 4-membered ring forms distinct channel sttures depending on the strength and nature of the hydrbond interactions between them. The present structures arferent from the recently described zero-dimensional Zn phphate structure formed using 1,10-phenonthroline as the lig[12,14]. In the 1,10-phenonthroline ligated structure the zphosphate forms with ZnO3N2 and PO4 building units givingrise to a secondary building unit, 4= 1 commonly observed inmany fibrous zeolites[18]. In polymorphsI andII, the build-ing units form a simple 4-membered ring structure and appto be similar to the recently described zinc phosphite, [Zn(2′-bipyridine)2(H2PO3)4] [19].

The isolation and characterization of simple molecular sois important for our understanding of the formation of strtures of higher dimensionality and complexity. The isolatand stabilization of building units, probably, would help us fther in our goal to understand the formation of complex thrdimensionally extended open-framework structures.

Acknowledgements

The author gratefully acknowledges Professor C.N.R. RFRS, for his support, encouragement and mentorship. Itpleasure and a privilege to contribute this work to this speissue honoring Professor Rao. The author also thanks the Ccil of Scientific and Industrial Research (CSIR), GovernmenIndia, for the award of a research grant.

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