ISSN 0036�0236, Russian Journal of Inorganic Chemistry, 2011, Vol. 56, No. 6, pp. 881–887. © Pleiades Publishing, Ltd., 2011.Original Russian Text © R.K. Tukhtaev, S.V. Larionov, R.F. Klevtsova, L.I. Myachina, L.A. Glinskaya, T.A. Udalova, N.Z. Lyakhov, 2011, published in Zhurnal NeorganicheskoiKhimii, 2011, Vol. 56, No. 6, pp. 939–945.

881

Hydrazine carboxylic acid anions NH2NHCOO–

are of interest as ligands having two donor functionalitiesthat are arranged favorably for chelating. Importantly, theligand contains an NH2NH hydrazine moiety, which hasreductive properties. Syntheses of an air�stable nickel(II)complex (NH2NHCOO)2Ni(N2H5)2CO3, as well as of(NH2NHCOO)2Ni ⋅ 2H2O and (NH2NHCOO)2NiN2H4 ⋅H2O, have been described [1]. Braibanti et al. [2] usingan X�ray crystallography experiment performed by aphoto method with a high R value (9.6%), found thatthe actual composition of the first compound is for�mulated as (N2H5)[Ni(NH2NHCOO)3] ⋅ H2O (I).Since then, there has been no more accurate structuralstudies of complex I. Later [3–6], compoundsNi(NH2NHCOO)2 and Ni(NH2NHCOO)2(N2H4)2were described, and another synthesis of complex Iwas proposed. In the context of manufacturing finelydisperse metal particles [7, 8], nickel(II) complexescontaining NH2NHCOО– ions may be treated aspromising molecular precursors for manufacturingfinely disperse nickel particles. For example, thermaldecomposition of complex I in an argon–hydrogenatmosphere produces nanodisperse nickel. However,this nickel is pyrophoric; it is easily oxidized by eventrace oxygen and water contained in an inert gas [9, 10].

Earlier [11, 12], we studied thermal decompositionof nickel(II) formate in benzyl alcohol. Thermolysis ofthis compound in a gas atmosphere also yields pyro�phoric nanosized nickel. In benzyl alcohol, thedecomposition scheme changes on account of theinvolvement of alcohol in reduction of Ni2+ ions.Finally, the product dispersivity changes: the newly

formed nickel powder has an average particle size ofabout 500 nm. Under certain conditions, however,powders having average particle sizes of 50 nm or lesscan be manufactured, and these particles will be stablein air at least at ordinary temperatures [11, 12]. Thehigher oxidation resistance of nickel nanopowdersmanufactured in organic media is due to a protectivefilm that is formed on nickel particles as a result ofdehydrogenation of organic compounds. At suffi�ciently high temperatures, dehydrogenation results inevolution of elementary carbon, which reacts withnickel to form carbide. In benzyl alcohol (bp =205°C), the appearance of a nickel carbide phase wasobserved only upon long (>10 h) boiling of the newlyformed nickel powder.

In light of the foregoing, it was of interest tostudy the possibility of manufacturing oxidation�resistant nickel nanopowders via decomposition ofcomplex I. Our goal was to study the compositionand morphology of decomposition products of(N2H5)[Ni(NH2NHCOO)3] ⋅ H2O upon heating inhigh�boiling organic liquids and to refine the structureof this complex by X�ray crystallography usingadvanced equipment.

EXPERIMENTAL

The precursor complex I was prepared by a one�step procedure [1], which provides high product yields(96%); then, the nickel percentage was determined.Braibanti et al. [2] also prepared complex I in one step;

Synthesis of Finely Disperse Nickel Particles by Thermolysisof Complex (N2H5)[Ni(NH2NHCOO)3] · H2O in Organic

Liquids and Refinement of Crystal Structure of This ComplexR. K. Tukhtaeva, S. V. Larionovb, R. F. Klevtsovab, L. I. Myachinab,

L. A. Glinskayab, T. A. Udalovaa, and N. Z. Lyakhova

a Institute of Solid�State Chemistry and Mechanochemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia

b Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, RussiaReceived July 15, 2009

Abstract—The crystal structure of the complex (N2H5)[Ni(NH2NHCOO)3] ⋅ H2O (I) was refined using X�raydiffraction data (obtained on an Apex X8 diffractometer, MoK

α radiation, 1830 Fhkl, R = 0.0182). The struc�

ture is built of complex anions [Ni(NH2NHCOO)3]–, cations N2 and water molecules and is character�ized by strong hydrogen bonds N–H…O, O–H…O, and N–H…N. The N3O3 coordination polyhedronaround a Ni atom is a slightly distorted octahedron. The thermal decomposition of complex I was studied inhigh�boiling organic liquids. The feasibility of preparing non�pyrophoric nickel powders having particle sizesof up to 20 nm is shown.

DOI: 10.1134/S0036023611060234

H5+

,

COORDINATION COMPOUNDS

882

RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 6 2011

TUKHTAEV et al.

however, they do not describe the synthesis procedure.The synthesis procedure described by Macek et al. [9]includes an additional step of preparing a solution ofhydrazine carboxylic acid.

For an X�ray crystallography experiment, a well�edged prismatic single crystal was selected from thedark�blue as�synthesized crystals. Reflection intensi�ties were measured by a standard procedure at roomtemperature on a Bruker X8 Apex four�circle auto�mated diffractometer equipped with a CCD areadetector (MoK

α radiation, λ = 0.71073 Å, graphite

monochromator). Crystal parameters and somedetails of the experiment and refinement are listed inTable 1. An acentric space group was selected for crys�tals of complex I based on analysis of absences in theintensity array supported by calculations and a men�tion of their piezoelectric properties [2].

The final refinement of the structure model wascarried out by full�matrix least squares for F2 in theanisotropic approximation for non�hydrogen atomsusing the SHELXL�97 program package [13]. Thepositions of H atoms for the complex anion

[Ni(NH2NHCOO)3]– were calculated geometrically;

the cation N2 and the water molecule were locatedfrom difference electron�density syntheses Δρ(xyz).All H atoms were included into refinement in the iso�tropic approximation together with the non�hydrogenatoms. The absolute configuration of the structuremodel was validated by the value of the Flack parame�ter [14], which was close to zero and equal to0.021(0.010). Selected interatomic distances and bondangles in the structure of complex I are listed in Table 2.Complete tables of atomic coordinates, bond lengths,and bond angles are deposited with the CambridgeStructure Database (No. 692923) and are availablefrom the authors.

As media for thermal decomposition of complex I,we used three organic liquids distinguished by theirlevels of reductive properties, namely, hexadecane (bp =286°C), benzyl alcohol, and ethylene glycol (bp =198°C). A weighed portion of the complex (5 g) wasplaced in a conical flask, flooded with a tenfold weightexcess of the liquid, and heated on a sand bath until itdecomposed completely. The temperature of the reac�tion mixture was monitored with a thermometer. Theaverage temperature elevation rate to reach 160°С was4–5 K/min; then, it was gradually decreased to a levelof 2 K/min in the region of 200°С and above. After theexperiment was over, the excess liquid was decantedand the product was washed with ethanol (in the caseof hexadecane, with hexane), dried, and then charac�terized using powder X�ray diffraction, electronmicroscopy, and thermal analysis. The instrumentsused were a DRON�3M diffractometer, JEM�2000 FXII and JEM�4000 EX electron microscopes, and aNetzsch STA 449 C thermal analyzer.

RESULTS AND DISCUSSION

Structural study. Our high�precision X�ray crystal�lography study of complex I verified the compositionreported by Braibanti et al. [2]. The structure of com�plex I is built of complex anions [Ni(NH2NHCOO)3]

–,

cations N2 , and water molecules (Fig. 1). Thecoordination polyhedron around a Ni atom is aslightly distorted N3O3 octahedron, which is formedon account of coordination of three NH2NHCOO–

bidentate chelating ligands and has an approximatetrigonal symmetry. All coordinated N atoms of theligands, namely, N(1)N(3)N(5), lie on one triangularface of the octahedron; all coordinated oxygen atoms,namely, O(1)O(3)O(5), lie on the opposite face. Thesefaces are almost parallel to each other (the angleformed by their normals is 2.0°). The average Ni–Nbond length is 2.096(2) Å; the average Ni–O bondlength is 2.047(2) Å. In the three NiN2CO five�mem�bered chelate rings, the distances between the constit�uent atoms are practically equal (Table 2). The calcu�lation of planes through the atoms of the NiN2OC che�

H5+

H5+

Table 1. Crystallographic parameters, details of the experi�ment and structure refinement for complex I

Empirical formula C3H16N8NiO7

Formula weight 334.95

Crystal system Monoclinic

Spase group Cc

a, Å 12.0777(4)

b, Å 10.8400(3)

c, Å 10.2449(2)

β, deg 120.836(1)

V, Å3 1151.68(5)

Z, ρ calc, g/cm3 4, 1.932

μ, mm–1 1.736

Crystal size, mm 0.12 × 0.10 × 0.09

θ scan range, deg 2.72–26.00

Measured reflections 3975

Unique reflections 1879

R(int) 0.02001

Reflections with I > 2σ(I) 1830

Refined parameters 200

GOOF on F2 0.919

R value, I >2s(I)

R1 0.0182

wR2 0.0452

R value (for all Ihkl)

R1 0.0191

wR2 0.0457

Absolute structural parameter 0.021(10)

RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 6 2011

SYNTHESIS OF FINELY DISPERSE NICKEL PARTICLES 883

late rings with inclusion of the O(2), O(4), and O(6)atoms showed that all of them are almost planar. Theaverage departure of atoms from the mean�root�square NiN2O2C planes is 0.039(2), 0.032(1), and0.059(2) Å respectively. An inspection of intermolecu�lar distances in the structure of complex I showedstrong hydrogen bonds between all fragments of thetypes of N–H···O, O–H···O, and N–H···N. The short�est hydrogen bonds are displayed in Table 3.

Figure 2 shows the packing of ions and water mol�ecules in the crystal structure of complex I as projectedon plane (010).

A comparison of bond lengths in the structure ofcomplex I and in isostructural compounds containingCo2+ and Zn2+ ions (their structures are solved withgood accuracy [15]), showed analogy with account forthe difference between the ionic radii of Ni, Co, andZn [16]. The average M–O distances in the coordina�tion core are 2.047, 2.072, and 2.082 Å for M = Ni,Co, and Zn, respectively; and the average M–N dis�tances are 2.096, 2.152, and 2.166 Å, respectively. Theother distances in all these structures are virtuallyequal within standard deviations.

Thermal decomposition. According to Macek et al.[9, 10], thermal decomposition of complex I in anargon–hydrogen atmosphere occurs in two steps.The first (endothermic) step occurs at a heating rateof 4 K/min in the range 130–205°С and produces anNi(NH2NHCOO)2 ⋅ N2H4 intermediate. The second(exothermic) step begins when the first step is not yetfinished and ends in the region of 270°С, although asmall weight loss is still observed at ~500°С.

The decomposition of complex I in organic liquidshas some common features with thermal decomposi�tion in a gas atmosphere, but also has strong distinc�tions. The character of distinctions depends on the liq�uid, primarily on the level of its reductive properties.

Table 2. Selected interatomic distances (d) and bond an�gles (ω) in the structure of complex I

Bond d, Å Bond d, Å

Ni(1)–O(5) 2.038(2) N(3)–N(4) 1.419(3)

Ni(1)–O(3) 2.049(2) O(3)–C(2) 1.266(3)

Ni(1)–O(1) 2.054(2) C(2)–O(4) 1.251(3)

Ni(1)–N(1) 2.084(2) C(2)–N(4) 1.363(3)

Ni(1)–N(5) 2.098(2) N(5)–N(6) 1.417(3)

Ni(1)–N(3) 2.106(2) O(5)–C(3) 1.272(3)

N(1)–N(2) 1.419(3) C(3)–O(6) 1.249(3)

O(1)–C(1) 1.261(3) C(3)–N(6) 1.377(3)

C(1)–O(2) 1.264(3) N(7)–N(8) 1.441(3)

C(1)–N(2) 1.379(3)

Angle ω, deg Angle ω, deg

O(1)Ni(1)N(1) 80.10(8) O(2)C(1)N(2) 116.9(2)

O(3)Ni(1)N(3) 79.66(8) C(1)N(2)N(1) 117.8(2)

O(5)Ni(1)N(5) 80.50(8) N(4)N(3)Ni(1) 107.3(1)

O(5)Ni(1)N(3) 90.14(8) C(2)O(3)Ni(1) 115.1(1)

O(3)Ni(1)N(1) 89.13(9) O(4)C(2)O(3) 123.8(2)

O(1)Ni(1)N(5) 89.75(8) O(4)C(2)N(4) 117.6(2)

O(5)Ni(1)O(3) 93.80(7) O(3)C(2)N(4) 118.5(2)

O(5)Ni(1)O(1) 95.54(7) C(2)N(4)N(3) 118.8(2)

O(3)Ni(1)O(1) 90.02(7) N(6)N(5)Ni(1) 106.1(1)

N(1)Ni(1)N(5) 96.49(10) C(3)O(5)Ni(1) 115.0(2)

N(1)Ni(1)N(3) 94.66(9) O(6)C(3)O(5) 125.2(3)

N(5)Ni(1)N(3) 101.02(9) O(6)C(3)N(6) 117.4(2)

C(1)O(1)Ni(1) 114.4(1) O(5)C(3)N(6) 117.3(2)

O(1)C(1)O(2) 124.0(2) C(3)N(6)N(5) 119.3(2)

O(1)C(1)N(2) 119.1(2)

Table 3. Hydrogen�bond geometry in the structure of complex I

D–H···A D…A, Å DHA, deg Symmetry transformation for A

N(1)–H(1a)···N(8) N(1)···N(8) 3.139(3) 174.8 x, 1 – y, z – 0.5

N(1)–H(1b)···O(1) N(1)···O(1) 3.002(4) 173.2 x + 0.5, y + 0.5, z

N(2)–H(2)···O(4) N(2)···O(4) 2.831(3) 131.1 x – 0.5, 1.5 – y, z – 0.5

N(3)–H(3a)···O(2) N(3)···O(2) 3.068(3) 164.3 x + 0.5, y – 0.5, z

N(3)–H(3b)···O(3) N(3)···O(3) 3.053(3) 169.4 x, 1 – y, z – 0.5

N(4)–H(4)···O(6) N(4)···O(6) 2.944(4) 147.2 x + 0.5, y + 0.5, z

N(5)–H(5b)···O(1w) N(5)···O(1w) 2.938(4) 161.2 x + 0.5, y – 0.5, z

N(7)–H(17)···O(4) N(7)···O(4) 2.733(3) 154.6 x – 1, 1 – y, z – 0.5

N(7)–H(27)···O(2) N(7)···O(2) 2.903(4) 171.2 x, y, z

N(7)–H(37)···O(1w) N(7)···O(1w) 2.904(5) 156.6 x, 1 – y, z + 0.5

N(8)–H(18)···O(5) N(8)···O(5) 3.009(4) 156.6 x – 0.5, 0.5 – y, z – 0.5

N(8)–H(18)···O(6) N(8)···O(6) 3.193(4) 145.0 x – 0.5, 0.5 – y, z – 0.5

N(8)–H(28)···O(1) N(8)···O(1) 3.113(4) 136.9 x, y, z

884

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TUKHTAEV et al.

The decomposition of the complex in hexadecane,which has no prominent reductive properties, has thegreatest resemblance to the decomposition in a gasatmosphere. Here, gas evolution also occurs in twosteps. The first step occurs within 138–175°С, with amaximal rate reached at 167–168°С; the second stepoccurs within 210–240°С with a maximum in theregion of 230–235°С. Within 175–210°С, gas evolu�tion is weak; small gas evolution occurs at tempera�tures above 240°С, too. The experiments were finishedat 245°С. The intermediate isolated at 175°С is a lilacpowder and contains 22.8% Ni, which corresponds tothe greatest degree to the known compound(NH2NHCOO)2NiN2H4 ⋅ H2O (II) [1]. The X�ray dif�fraction pattern of the intermediate also correspondsto the X�ray diffraction pattern of complex II whichwe synthesized using the Gogorishvili and Khone�lidze’s procedure [1]. Moreover, the results of thermalanalysis for the intermediate in an argon atmospherecoincide, with respect to weight loss and DTA peaks,with the relevant data for (NH2NHCOO)2NiN2H4 ⋅H2O. The unusualness of the result consists in the fol�lowing: according to Macek and Rahten [10], thiscompound is the least stable nickel(II) hydrazine car�

boxylate with respect to decomposition in a gas atmo�sphere, and its decomposition starts at as low a tem�perature as 105°С. The final decomposition productisolated at 245°С was identified by X�ray diffraction asnickel carbide Ni3C. The X�ray diffraction pattern forthe product isolated at 230°С contains, along withlarge halos (which indicate its incomplete decomposi�tion), two broad reflections with peaks in the regionsof 2θ = 44.4° and 51.8°, which may be associated withthe formation of nanosized metallic nickel. Even at235°С, however, the reflection 2θ = 51.8° disappears,the reflection 2θ = 44.4° shifts toward 44.8°, and peaksappear at 2θ = 39.6° and 41.8°, which corresponds tothe formation of carbide Ni3C. The simultaneousoccurrence of peaks from carbide and incompletedecomposition products signifies that the newlyformed nickel is rapidly bound in carbide.

Temperature elevation dynamics for the decompo�sition of complex I in hexadecane does not allow anyinferences to be made as regards the thermochemistryof the processes involved. In some experiments carriedout isothermally in a thermostat at 235°С, it was dis�covered that here, as for decomposition in a gas atmo�sphere, too, the second step occurs with heat evolution(the reaction mixture is overheated by 5–7 K relativeto the thermostating liquid).

In alcoholic media having far higher reductivepotentials than hexadecane has, the process occursdifferently.

In benzyl alcohol, gas evolution starts as early as at55°С and is slowly enhanced till 203°С. During heat�ing in the region of 185–190°С the hitherto clear liquidtransforms to a whitish slurry and a powder on the bot�tom of the flask turns lilac. Gas evolution continuesgrowing up; therefore, it is here impossible to detectthe moment when the first decomposition step is com�plete. At 195°С, strong gas evolution raises the powerfrom the bottom; the slurry turns lilac, and it darkenswhen temperature reaches 203°С. At this moment, gasevolution reaches a peak, and the temperature of theslurry drops to 196°С in 5–6 min, signifying heatabsorption in the course of the process. Then, temper�ature again rises to reach the benzyl alcohol boilingtemperature in several minutes; the slurry blackens.10�min exposure at the boiling temperature yields apowder that was identified as metallic nickel with anadmixture of nitride Ni3N. Proceeding from the nitro�gen percentage (1.01 wt %), the nitride fraction in thenickel powder can amount to 13.7%. The powder hasa near�spherical morphology and consists mostly ofparticles having sizes of 200–300 nm (Fig. 3a).Increasing exposure time at the boiling temperature to3 h does not change the nitrogen percentage; the par�ticle morphology and particle size do not practicallychange either. Based on the fact that the process inbenzyl alcohol occurs with heat absorption and ends ina very short time and at a temperature where the inter�mediate in argon and in hexadecane is still stable, wemay infer that benzyl alcohol participates in reduction

O(4)

N(4)

N(3)

C(2)

O(3)

H(2w)

O(1w)

H(1w)

N(1)N(2)

O(2)C(1)

O(1)

Ni(1)O(5)

C(3)

O(6)N(6)

N(5)H(28)

N(8)

H(18)

N(7)

H(27)

H(37)

H(17)

Fig. 1. Fragments of the structure of complex I: anion[Ni(NH2NHCOO)3]– with non�hydrogen atom number�

ing, N2 cation, and water molecules (50% probability

thermal ellipsoids).

H5+

RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 6 2011

SYNTHESIS OF FINELY DISPERSE NICKEL PARTICLES 885

of Ni2+ ions. Ni3N formation can be caused by thereaction of the newly formed nanosized nickel withammonia evolving upon the decomposition of theintermediate. It cannot be ruled out that Ni3N forma�tion also occurs upon decomposition of complex I in agas atmosphere. At any rate, the small weight lossobserved by Macek and Rahten [10] in the region of500°С may arise just from Ni3N decomposition.

Decomposition of complex I in ethylene glycol hassome specifics. Here, as in hexadecane, too, decom�position steps are slightly resolved with respect to tem�perature, making it possible to isolate the intermediatewhich is identical to the intermediate obtained inhexadecane as characterized by powder X�ray diffrac�tion and thermal analysis. But these steps occur atlower temperatures. Gas formation during the heating

0

a

c

Fig. 2. Packing of ions and water molecules in the structure of complex I as projected on plane (010). Dashed lines show thehydrogen�bond network.

1 µm(а) 1 µm(b)

Fig. 3. Micrographs of nickel powders prepared by reduction of complex I in (a) benzyl alcohol and (b) ethylene glycol.

886

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TUKHTAEV et al.

of the complex in ethylene glycol starts later than inbenzyl alcohol (at 80°С), but the first step mostly endsas early as in the region of 155–160°С. Within therange 160–168°С, the gas evolution rate abruptlydecreases and then again increases to reach a peak at170°С. Both decomposition steps are endothermic, asevidenced by 2°С drops at the points of the strongestgas evolution (at 160 and 170°С). As temperature risesfurther, the gas evolution rate weakens to almost ceaseat 190°С.

One more distinction consists of that the above�described intermediate of decomposition in ethyleneglycol is not formed as a result of solid�phase decom�position of crystals of the precursor complex I or a het�erogeneous reaction with ethylene glycol; rather, it isformed via their dissolution, although there was novisually noticeable dissolution of the complex in ethyl�ene glycol. Figures 4a and 4b display micrographs ofcrystals of precursor complex I and the intermediateobtained at 160°С in ethylene glycol. Figures 4c and4d display micrographs of complex I after 2�min mill�ing in an AGO�2 planetary mill and the intermediateobtained from it. Clearly, the morphology of the inter�mediate is not affected by the initial state of the com�plex, although the crystals themselves slightly differ insizes.

The final product of decomposition of complex I inethylene glycol is a nickel powder having a two�levelstructure: spherical agglomerates that consist of grains

(а) 10 µm 10 µm(b)

(d)(c) 1 µm 1 µm

Fig. 4. Micrographs of crystals of (a) precursor complex I, (b) an intermediate obtained from the precursor complex, (c) groundcomplex I, and (d) an intermediate obtained from the ground complex.

100 nm

Fig. 5. Micrograph of a nickel powder prepared fromground (NH2NHCOO)2NiN2H4 ⋅ H2O.

RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 6 2011

SYNTHESIS OF FINELY DISPERSE NICKEL PARTICLES 887

about 100 nm in size (Fig. 3b). Nickel carbide ornitride admixtures are virtually undetectable by X�raydiffraction, although chemical analysis shows as littleas 91.0% nickel in the power.

Preliminary grinding of the initial complex I, asexpected from Fig. 4, only weakly affects the sizeparameters of the final nickel powder, but agglomer�ates become looser. However, grinding of the interme�diate from the first step under the same conditions verystrongly affects the dispersivity of the final product.The nickel powder manufactured in this way consistsof aggregated particles, where grain sizes do notexceed 20 nm (Fig. 5). A rise in the specific surface ofgrains leads to an increase in the organic component ofthe powder, so that the metal percentage drops to73.4%. Despite the high dispersivity, the nickel powderis rather resistant to oxidation. By thermal analysis,heat evolution during heating in air starts at 110°С andis accompanied by weight loss, that is, is associatedwith oxidation of the protective film. Weight gain startsabove 300°С and ends at 550°С. Thus, the oxidationstability boundary of the nickel nanopowder liesbetween 110 and 300°С. It is impossible to determineits position more accurately by thermal analysisbecause of the concurrence of two oppositely directedprocesses: oxidation of the protective film decreasesthe weight of the sample, whereas oxidation of nickelleads to weight gain. The final weight change corre�sponds to the metallic nickel percentage in the sample.

Our study has shown the feasibility of manufactur�ing non�pyrophoric nickel powders via thermaldecomposition of a nickel(II) complex containingNH2NHCOO– ligands in high�boiling organic liquids.

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