Articles

DOI: 10.1002/zaac.200500329

Polymorphism of [Zn(2,2�-bipy)(H2PO4)2]2Srinivasan Natarajan1), Yurii Prots, Bastian Ewald, Rainer Niewa, and Rüdiger Kniep*

Dresden, Max-Planck-Institut für Chemische Physik fester Stoffe

Received August 10th, 2005.

Abstract. Two polymorphs of a zero-dimensional (molecular) zincphosphate with the formula [Zn(2,2�-bipy)(H2PO4)2]2 have beensynthesized by a mild hydrothermal route and their crystal structu-res were determined by single crystal X-ray diffraction (triclinic,space group P1 (No. 2), Z � 2, �-form: a � 8.664(1), b � 8.849(2),c � 10.113(2) A, � � 97.37(2)°, β � 100.54(2)°, γ � 100.98(2)°,V � 737.5(3) A3; β-form: a � 7.5446(15), b � 10.450(2), c �

10.750(2) A, � � 67.32(3)°, β � 81.67(3)°, γ � 69.29(3)°, V �

731.4(3) A3). Both structures consist of distorted trigonal-bipyra-

Introduction

The synthesis and study of compounds with extended struc-tures constitute an important area of research in inorganicchemistry [1, 2]. Though many new compounds with a widestructural variety and diversity have been prepared andcharacterized, and there have been some attempts to ident-ify intermediates by a variety of in-situ experiments underhydrothermal conditions, our understanding of the forma-tion of these phases continues to be vague [3�9]. Lower-dimensional units are important in the formation processof structures of higher dimensionality. In a sense, molecularzero-dimensional synthons of the composition M2P2O4

(M � metal) could be some of the fundamental buildingunits [8�13]. Such zero-dimensional molecular compoundshave been isolated and shown to be reactive [13, 14]. Wehave now succeeded in the preparation of a zero-dimen-sional zinc phosphate with the formula [Zn(2,2�-bi-py)(H2PO4)2]2 and characterized the crystal structures oftwo polymorphic modifications.

Experimental Section

Synthesis

The title compound was synthesized employing hydrothermalmethods. For preparation of the β-polymorph, 0.08 g ZnO weredispersed in 2 ml water and successively 0.2 ml H3PO4 (85 wt%),0.143 g [Co(en)3]Cl3 and 0.156 g 2,2�-bipyridine (2,2�-bipy) (molarratio 1.0 ZnO : 3.0 H3PO4 : 0.5 [Co(en)3]Cl3 : 1.0 (2,2�-bipy)) were

* Prof. Dr. R. KniepMax-Planck-Institut für Chemische Physik fester StoffeNöthnitzer Straße 40D-01187 Dresden, Germanye-mail: Kniep@cpfs.mpg.de

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midal ZnO3N2 units condensed with PO2(OH)2 tetrahedra throughcommon vertices giving rise to dimers [Zn(2,2�-bipy)(H2PO4)2]2.The structures are stabilized by extensive inter- and intramolecularhydrogen bond interactions. Both modifications display subtle dif-ferences in their packing originating from the hydrogen bond inte-ractions as well as π...π interactions between the organic ligands.

Keywords: Zinc; Phosphates; Structure elucidation; Polymorphism

added. This mixture was homogenized at room temperature for 30min, filled in a PTFE autoclave (Carl Roth GmbH, Karlsruhe, Ger-many, 20 ml capacity, fill-factor � 20 %) and heated at 140 °C for96 h. For synthesis of �-[Zn(2,2�-bipy)(H2PO4)2]2 the amount ofZnO in the reaction mixture was doubled. The initial pH of thereaction mixture in both the cases was �3.5 and after the com-pletion of the reaction the pH did not show any appreciable change.The resulting products contain colorless rod-like single crystals ofthe title compound admixed with some yellow polycrystalline pow-der presumably containing the cobalt. The products were consecu-tively filtered, washed with deionized water and dried at ambientconditions. The yield of the single crystalline product was about30 % based on ZnO. The X-ray powder diffraction pattern of theyellow powder does not correspond to any starting material. Wewere able to prepare the a-polymorph as the majority phase byincluding boric acid (H3BO3) in the reaction mixture. We have alsobeen able to synthesize the β-form as the majority phase withoutadding [Co(en)3]Cl3 to the reaction mixture.

Single Crystal Structure Determination

Suitable colorless single crystals of each polymorph were selectedunder a polarizing microscope and glued to a thin glass fiber witha two-component adhesive. Crystal structure determination by X-ray diffraction was performed on a RIGAKU AFC7 four-circlediffractometer, equipped with a Mercury-CCD detector, and a finefocused sealed tube. In case of β-[Zn(2,2�-bipy)(H2PO4)2]2 twomeasurements with long and short exposure times were performedto obtain accurate intensities of the strong and the weak reflections.Both data sets were scaled and combined using the programXPREP [15]. Unit cell parameters were obtained from least-squaresfits of 2552 (�) and 5351 (β) reflections obtained during the datacollection, respectively. Pertinent experimental details for the datacollection and the structure determination are presented in Table 1.

The structures were solved by direct methods. A sufficient fragmentof the structure was revealed to enable the remainder of the non-hydrogen atoms to be located from difference Fourier maps. Allhydrogen positions were initially located in the difference maps and

S. Natarajan, Yu. Prots, B. Ewald, R. Niewa, R. Kniep

for the final refinement the hydrogen atoms were placed geometri-cally and held in the riding mode. The last cycles of refinementsincluded atomic positions and anisotropic displacement parametersfor all non-hydrogen atoms, and isotropic displacement parametersfor all hydrogen atoms constrained to 1.5 times the value for theH-bearing atom. Full-matrix least-squares refinement against �F�2

was carried out using the SHELXL-97 suite of programs [16]. Thefinal atomic coordinates and selected bond distances and anglesare given in Tables 2 and 4 for the �-form and in Tables 3 and 5for the β-modification.

Results and Discussion

Two polymorphs of [Zn(2,2�-bipy)(H2PO4)2]2 were pre-pared under mild hydrothermal conditions. The differentpolytypes form due to slight variations of the Zn concen-tration in the mother liquor. Since we were able to prepareboth polymorphs in the presence/absence of different addi-tives to the reaction mixture it is to suppose that no stabiliz-ation from foreign elements in the colorless crystals occursand that the additives also have no decisive influence on thecrystallization. The smaller volume of the β-form (about1 %) suggests it to be a lower-temperature or higher-press-ure modification possibly metastable formed in a kineticallyfavored crystallization according to the Ostwald rule.

�-[Zn(2,2�-bipy)(H2PO4)2]2 was previously described [17],a Co containing isotype [Co(2,2�-bipy)(H2PO4)2]2 exists [18]and a phosphonate analogue [Zn(2,2�-bipy)(H2PO3)2]2 [19]is known to crystallize with a remarkably similar structuralarrangement. Since our refinement of the crystal structureof the α-polymorph is considerably more reliable (R1 �

Table 1 Crystallographic data, measurement conditions and refinement parameters for two polymorphs of [Zn(2,2�-bipy)(H2PO4)2]2 (stand-ard deviation in parenthesis)

�-[Zn(2,2�-bipy)(H2PO4)2]2 β-[Zn(2,2�-bipy)(H2PO4)2]2

Space group P1 (No. 2) P1 (No. 2)a /A 8.664(1) 7.5446(15)b /A 8.849(2) 10.450(2)c /A 10.113(2) 10.750(2)� /° 97.37(2) 67.32(3)β /° 100.54(2) 81.67(3)γ /° 100.98(2) 69.29(3)Volume /A3 737.5(3) 731.4(3)Z 1 1ρcalc /g cm�3 1.871 1.887µ /mm�1 1.926 1.942Crystal size /mm3 0.04 � 0.04 � 0.08 0.08 � 0.08 � 0.12Diffractometer RIGAKU AFC7 RIGAKU AFC7Detector Mercury CCD Mercury CCDScans ∆ϕ � 0.6°, 600 images, 80 s ∆ϕ � 0.6°, 200 images, 80 s

∆ω � 0.6°, 600 images, 60 s, 15 s ∆ω � 0.6°, 100 images, 60 s, 15 s2θ range up to 58° 58°hkl ranges �11 � h � 11 �10 � h � 10

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

N(hkl) measured 8708 11162N(hkl) unique 3291 3611R(int) 0.025 0.033N(hkl) observed 2769 3155Refined parameters 213 213

R1 0.045 0.039wR2 0.095 0.082Residual peaks (e/A3) �0.56 and 0.78 �0.49 and 0.44

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0.045 versus 0.075) and for the aid of comparison of bothmodifications we present the complete data of both poly-morphs. The earlier report of one polymorph led us to as-sign it the �-modification, the other is consequently speci-fied β-[Zn(2,2�-bipy)(H2PO4)2]2.

Both polymorphs of [Zn(2,2�-bipy)(H2PO4)2]2 containmolecular dimeric units as a common zero-dimensionalbuilding block (Fig. 1). In this entity two central Zn atomsare linked via two dihydrogen phosphate ions at the twonon-hydrogen bearing oxide ligands. At each Zn atom onefurther dihydrogen phosphate ion and a µ2-coordinating2,2�-bipyridine ligand complete the coordination sphere.Distances and angles (Tables 4 and 5) are in good agree-ment with those derived in previous studies on similar com-pounds [8�13, 17�19]. The main differences of both poly-morphs originate in conformational variations of the dihy-drogen phosphate ligands, particularly the proton positionsand the resulting hydrogen bonds. In both polymorphs eachdihydrogen phosphate ion donates one hydrogen to an in-tramolecular hydrogen bond. In this way a cage of two Znatoms and four coordinating and interconnecting dihydro-gen phosphate ions is formed, terminated from two sites atboth Zn by 2,2�-bipyridine molecules. The remaining hydro-gen atoms of the dihydrogen phosphate groups are involvedin intermolecular hydrogen bonding, connecting every[Zn(2,2�-bipy)(H2PO4)2]2 dimeric unit to four further mol-ecules (Fig. 2). This leads to the formation of different two-dimensional infinite layers terminated by the 2,2�-bipyridineligands. Donor-acceptor distances of hydrogen bonds varyin the range of 2.528(4) � 2.711(4) A. The 2,2�-bipyri-

Polymorphism of [Zn(2,2�-bipy)(H2PO4)2]2

diene ligands of two adjacent layers are differently inter-locked, both with the possibility of π...π interactions (Fig.3). The role of π...π interactions for the stability of organiccompounds has been discussed with particular emphasis onthe centroid-centroid distances (d) and the angle (θ) sus-pended between the benzene rings [20]. A correlation be-tween both d and θ has been discussed in terms of a phase-diagram with the distances and angles having a unique re-lationship. From this, the following interactions appear tobe important for different values of the centroid-centroiddistance (d) and angles (θ): (i) d �< 3 A and θ � 50 � 90°,(ii) d � 3 � 7 A and θ � 0 � 50°, (iii) d � 4 � 7.5 A, θ �140 � 180° and (iv) d � 6 � 7 A and θ � 0 � 145°. In thepresent cases we find four types of interactions between thebenzene rings of the 2,2�-bipyridine ligand: For the �-poly-morph: d � 4.49 A, θ � 28.36°; d � 4.40 A, θ � 28.36°;d � 4.02 A, θ � 14.39°; d � 4.04 A, θ � 14.43° and for theβ-form: d � 3.84 A, θ � 3.09°; d � 4.59 A, θ � 3.66°; d �4.35 A, θ � 0°; d � 4.18 A, θ � 0°. These π...π interactionsalong with the hydrogen bond interactions appear to rulethe packing of the molecular zinc phosphate units in thesolid state.

Table 2 Atomic coordinates and displacement parameters (in A2)for �-[Zn(2,2�-bipy)(H2PO4)2]2, all atoms in 2i.

Atom x y z Ueq/Uisoa,b)

Zn(1) 0.4383(1) 0.2831(1) 0.2833(1) 0.029(1)P(1) 0.6878(1) 0.4097(1) 0.5743(1) 0.027(1)P(2) 0.2052(1) 0.0661(1) 0.4198(1) 0.031(1)O(1) 0.5794(3) 0.2757(3) 0.4712(2) 0.031(1)O(2) 0.2635(3) 0.1091(3) 0.2961(2) 0.035(1)O(3) 0.6073(3) 0.5144(3) 0.6513(2) 0.039(1)O(4) 0.1034(3) �0.0962(3) 0.4009(3) 0.039(1)O(5) 0.1027(4) 0.1856(3) 0.4618(3) 0.046(1)O(6) 0.7943(4) 0.3417(3) 0.6833(3) 0.046(1)O(7) 0.8009(4) 0.4989(3) 0.4927(3) 0.053(1)O(8) 0.3500(4) 0.0844(4) 0.5412(3) 0.051(1)C(1) 0.2053(6) 0.3617(5) 0.428(4) 0.053(1)C(2) 0.1542(7) 0.3875(6) �0.0872(5) 0.065(1)C(3) 0.2469(7) 0.3640(6) �0.1807(5) 0.065(2)C(4) 0.3838(7) 0.3097(5) �0.1450(4) 0.056(1)C(5) 0.4283(5) 0.2818(4) �0.0125(4) 0.038(1)C(6) 0.5711(5) 0.2205(4) 0.0382(4) 0.038(1)C(7) 0.6664(6) 0.1666(5) �0.0463(4) 0.054(1)C(8) 0.7954(6) 0.1087(6) 0.0078(5) 0.062(1)C(9) 0.8295(6) 0.1064(6) 0.1460(5) 0.063(1)C(10) 0.7306(5) 0.1614(5) 0.2246(5) 0.051(1)N(1) 0.3385(4) 0.3091(4) 0.0799(3) 0.036(1)N(2) 0.6042(4) 0.2165(4) 0.1732(3) 0.035(1)H(50) 0.0416 0.1471 0.5077 0.070H(60) 0.8238 0.2680 0.6455 0.069H(70) 0.8272 0.5917 0.5267 0.079H(80) 0.4313 0.1344 0.5234 0.076H(1) 0.1450 0.3817 0.1076 0.063H(2) 0.0586 0.4203 �0.1113 0.078H(3) 0.2168 0.3848 �0.2684 0.077H(4) 0.4464 0.2916 �0.2085 0.068H(7) 0.6425 0.1699 �0.1392 0.064H(8) 0.8590 0.0714 �0.0479 0.074H(9) 0.9170 0.0688 0.1855 0.076H(10) 0.7536 0.1598 0.3178 0.061

a) U(eq) is defined as one third of the trace of the orthogonalized Uij tensorb) The hydrogen Uiso values was hold at 1.5 times the equivalent isotropic Uof the atoms to which they are attached.

Z. Anorg. Allg. Chem. 2006, 37�41 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim zaac.wiley-vch.de 39

The central unit of the dimers are rings of four vertex-sharing tetrahedra [Zn2O4/2(PO(OH)2)2]. Such four-ringsynthons were considered the molecular zero-dimensionalfundamental building units for formation of higher dimen-sional frameworks [8]. The occurrence of zinc phosphatescoordinated by organic ligands with 1-, 2-, and 3-dimen-sional structures support this view [8�13]. The variabletopology of the [Zn(2,2�-bipy)(H2PO4)2]2 molecules leadingto the two polymorphs of the title compound indicates thepotential of the [Zn2O4/2(PO(OH)2)2] unit for the formationof various higher-dimensional frameworks via conden-sation. Remarkably, the molecular building units present astriking similarity to the molecular unit of the recently de-scribed zinc phosphonate, [Zn(2,2�-bipyridine)(H2PO3)2]2[19]. Despite the smaller number of oxygen bonded hydro-gen atoms available for hydrogen bonds a layered structurequite similar to those of the title polymorphs was obtainedfor the phosphonate.

In conclusion we have characterized two modifications ofthe zero-dimensional molecular zinc dihydrogen phosphate[Zn(2,2�-bipy)(H2PO4)2]2 and analyzed the differences intopology of the molecular building units and of the two-dimensional hydrogen bonding schemes, leading to the dif-

Table 3 Atomic coordinates and displacement parameters (in A2)for β-[Zn(2,2�-bipy)(H2PO4)2]2, all atoms in 2i.

Atom x y z Ueq/Uisoa,b)

Zn(1) 0.6778(1) 0.9272(1) 0.6895(1) 0.021(1)P(1) 0.3454(1) 0.8374(1) 0.6221(1) 0.022(1)P(2) 0.9002(1) 0.7042(1) 0.5193(1) 0.024(1)O(1) 0.4171(3) 0.9261(2) 0.6744(2) 0.026(1)O(2) 0.8766(3) 0.7898(2) 0.6093(2) 0.027(1)O(3) 0.6672(3) 0.1080(2) 0.5269(2) 0.034(1)O(4) 0.7951(3) 0.5945(3) 0.5686(3) 0.042(1)O(5) 0.8390(4) 0.8105(3) 0.3723(2) 0.049(1)O(6) 0.4673(3) 0.6716(2) 0.6781(2) 0.032(1)O(7) 0.1494(3) 0.8392(3) 0.6953(2) 0.033(1)O(8) 0.1161(3) 0.6252(2) 0.5085(2) 0.031(1)N(1) 0.7096(3) 0.0373(3) 0.8091(2) 0.027(1)N(2) 0.7304(3) 0.7599(3) 0.8879(2) 0.027(1)C(1) 0.7035(5) 0.1775(4) 0.7619(3) 0.038(1)C(2) 0.7252(6) 0.2445(4) 0.8442(4) 0.046(1)C(3) 0.7518(6) 0.1656(4) �0.0207(4) 0.045(1)C(4) 0.7606(5) 0.0211(4) 0.0294(3) 0.036(1)C(5) 0.7401(4) 0.9585(3) 0.9416(3) 0.026(1)C(6) 0.7489(4) 0.8029(3) 0.9861(3) 0.026(1)C(7) 0.7740(5) 0.7076(4) 0.1199(3) 0.040(1)C(8) 0.7739(6) 0.5664(4) 0.1521(3) 0.049(1)C(9) 0.7543(6) 0.5228(4) 0.0518(4) 0.048(1)C(10) 0.7339(5) 0.6218(4) 0.9202(3) 0.040(1)H(50) 0.7533 0.8845 0.3744 0.073H(60) 0.5677 0.6587 0.6360 0.048H(70) 0.0843 0.8232 0.6518 0.049H(80) 0.1324 0.5571 0.4834 0.047H(1) 0.6839 0.2316 0.6703 0.046H(2) 0.7218 0.3418 0.8085 0.056H(3) 0.7638 0.2095 0.0367 0.054H(4) 0.7800 �0.0341 0.1208 0.044H(7) 0.7906 0.7386 0.1864 0.048H(8) 0.7871 0.5017 0.2412 0.058H(9) 0.7546 0.4279 0.0716 0.058H(10) 0.7223 0.5914 0.8520 0.048

a) U(eq) is defined as one third of the trace of the orthogonalized Uij tensorb) The hydrogen Uiso values was hold at 1.5 times the equivalent isotropicU of the atoms to which they are attached.

S. Natarajan, Yu. Prots, B. Ewald, R. Niewa, R. Kniep

ferent crystal structures. The preparation and characteriz-ation of simple molecular solids is important for under-standing the formation of structures of higher dimensional-ity and complexity. The isolation and stabilization of build-ing units should help further to understand the formationof complex three-dimensionally extended open-frameworkstructures in more detail.

Fig. 1 Molecular dimeric units [Zn(2,2�-bipy)(H2PO4)2]2: (a) �-form and (b) β-form. Intramolecular hydrogen bonds are indicatedby dashed lines.

Table 4 Selected bond distances (in A) and angles (in deg) in �-[Zn(2,2�-bipy)(H2PO4)2]2

Zn(1)�O(1) 2.078(2) P(1)�O(1) 1.509(2)Zn(1)�O(2) 1.979(2) P(1)�O(3) 1.487(2)Zn(1)�O(3) 1.966(2) P(1)�O(6) 1.561(3)Zn(1)�N(2) 2.096(3) P(1)�O(7) 1.560(3)Zn(1)�N(1) 2.140(3) P(2)�O(2) 1.498(3)

P(2)�O(4) 1.504(3)P(2)�O(5) 1.571(3)P(2)�O(8) 1.554(3)

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

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Acknowledgments. SN gratefully acknowledges financial supportfrom the Max-Planck-Gesellschaft.

Fig. 2 Inter- and intramolecular hydrogen bonds (dashed lines) in[Zn(2,2�-bipy)(H2PO4)2]2 leading to two-dimensional networks: (a)�-form and (b) β-form. Zn: large spheres, H: small spheres, tetra-hedra: PO4-groups, 2,2�-bypiridine ligands at Zn are not shown foraid of clarity.

Table 5 Selected bond distances (in A) and angles (in deg) in theβ-[Zn(2,2�-bipy)(H2PO4)2]2

Zn(1)�O(1) 2.001(2) P(1)�O(1) 1.521(2)Zn(1)�O(2) 2.027(2) P(1)�O(3) 1.484(2)Zn(1)�O(3) 2.005(2) P(1)�O(6) 1.562(2)Zn(1)�N(1) 2.113(2) P(1)�O(7) 1.569(2)Zn(1)�N(2) 2.155(3) P(2)�O(2) 1.508(2)

P(2)�O(4) 1.509(2)P(2)�O(5) 1.561(2)P(2)�O(8) 1.555(2)

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

Polymorphism of [Zn(2,2�-bipy)(H2PO4)2]2

Fig. 3 Arrangement of adjacent hydrogen bonded layers in [Zn(2,2�-bipy)(H2PO4)2]2: (a) �-form and (b) β-form. The arrangement of the2,2�-bipypyridene ligands provides the possibility for π...π interactions. Hydrogen bonds: dashed lines, Zn: large spheres, H: small spheres,tetrahedra: PO4-groups, 2,2�-bypiridine ligands are shown without H atoms for clarity.

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