Synthesis, Structure, and Thermal Decomposition of

Journal of Structural Chemistry, Vol. 43, No. 4, pp. 649–655, 2002 Original Russian Text Copyright @ 2002 by K. V. Yusenko, S. A. Gromilov, I. A. Baidina, Yu. V. Shubin, I. V. Korol’kov, T. N. Drebushchak, T. V. Basova, and S. V. Korenev

SYNTHESIS, STRUCTURE, AND THERMAL DECOMPOSITION OF CHLOROPENTAMMINERHODIUM(III) HEXABROMOPLATINATE(IV) K. V. Yusenko, S. A. Gromilov, I. A. Baidina,

UDC 546.92+546.97)541.49+548.736

Yu. V. Shubin, I. V. Korol’kov, T. N. Drebushchak, T. V. Basova, and S. V. Korenev

The binary complex salt [Rh(NH3 )5 Cl][PtBr6 ] was synthesized and studied by X-ray structural analysis. The crystallographic data are as follows: a = 12.013(2) ˚ A, b = 8.401(2) ˚ A, c = 15.999(3) ˚ A, ◦ 3 3 ˚ β = 91.13(3) , V = 1614.3(6) A , space group P 21 /m, Z = 4, dx = 3.70 g/cm , R = 0.086. The thermal decomposition of the salt in a hydrogen atmosphere is shown to produce a Rh0.5 Pt0.5 solid solution with an FCC cell [a = 3.864(2) ˚ A]. The thermal decomposition of the salt in a helium atmosphere proceeds via the formation of metallic Pt and RhBr3 and finally results in a mixture of several solid solutions.

The thermal decomposition of binary complex salts is an alternative method for producing solid solutions based on high-melting metals [1, 2]. Previously, we showed that the thermal decomposition of [Rh(NH3 )5 Cl][PtCl4 ] in inert or reducing atmospheres yields a Rh0.5 Pt0.5 solid solution [3]. Synthesis of binary complex salts with an octahedral anion offers additional opportunities for modeling both the structure of precursor complexes and the thermaldecomposition processes yielding metal powders. Nevertheless, we were unable to obtain the [Rh(NH3 )5 Cl][PtCl6 ] phase in pure form [4]; this phase was formed in a mixture with [Rh(NH3 )5 Cl]2 [PtCl6 ]Cl2 . For this reason, hexabromoplatinate(IV) [PtBr6 ]2− was used as the anion. The purpose of this work was to synthesize the binary complex salt [Rh(NH3 )5 Cl][PtBr6 ] and study it by X-ray structural analysis. It was also of interest to examine its thermaldecomposition kinetics in comparison with [Rh(NH3 )5 Cl][PtCl4 ].

EXPERIMENTAL The starting compounds [Rh(NH3 )5 Cl]Cl2 and K2 [PtBr6 ] were synthesized by the procedures described in [3, 5]. A polycrystalline sample of [Rh(NH3 )5 Cl][PtBr6 ] was obtained as follows. Warm 0.02 M aqueous solutions of [Rh(NH3 )5 Cl]Cl2 and K2 [PtBr6 ] were poured together. After a time, a fine-grained red precipitate formed, which was filtered in vacuum, washed with water and acetone, and dried in air. The yield was ∼92%. For analysis of total metal content, a polycrystalline sample of the salt placed in a boat of fused quartz was reduced in a hydrogen current. The temperature was raised to 500◦ C for 15 minutes, and the thermal-decomposition products were then cooled in a hydrogen current. The analysis data are as follows: Found, % Calculated for RhPtH15 Br6 ClN5 , %

Rh + Pt Rh + Pt

33.4 ± 0.1 33.18

Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk. Novosibirsk State University. Institute of Chemistry of Solids and Mechanochemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk. Translated from Zhurnal Strukturnoi Khimii, Vol. 43, No. 4, pp. 699–705, July–August, 2002. Original article submitted December 14, 2001. c 2002 Plenum Publishing Corporation 0022-4766/02/4304-0649 $27.00

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TABLE 1 ˚2 ] Atomic Coordinates [×104 ] and Equivalent Thermal Parameters [×103 , A in the [Rh(NH3 )5 Cl][PtBr6 ] Structure Atom

x

y

z

B

Atom

x

y

z

B

Pt(1) Pt(2) Rh(1) Rh(2) Br(1) Br(2) Br(3) Br(4) Br(5) Br(6) Br(7)

8396(2) 6578(2) 3315(4) 1537(4) 9810(4) 8748(6) 6991(4) 8030(6) 8121(5) 5046(6) 6556(5)

2500 2500 2500 2500 386(6) 2500 403(5) 2500 2500 2500 425(6)

8599(1) 3606(2) 8986(3) 3926(3) 8468(3) 10111(4) 8750(3) 7068(4) 4647(4) 2555(5) 3613(3)

19(1) 21(1) 22(1) 22(1) 36(1) 42(2) 35(1) 34(2) 28(2) 48(2) 46(2)

Br(8) Br(9) Cl(1) Cl(2) N(1) N(2) N(3) N(4) N(5) N(6) N(7)

5320(6) 7984(6) 4565(13) 984(15) 2270(40) 4600(40) 3390(30) 2050(40) 330(30) 2720(30) 2050(40)

2500 2500 2500 2500 2500 2500 10(40) 2500 770(40) 730(40) 2500

4774(5) 2469(4) 10119(10) 5298(11) 7870(30) 8180(30) 8990(30) 9810(30) 3670(20) 4250(30) 2690(30)

41(2) 36(2) 30(4) 39(5) 23(13) 32(15) 53(13) 50(20) 29(10) 38(11) 12(11)

IR spectra were recorded on a Bruker IFS-85 spectrophotometer for samples pressed in pellets with KBr. Raman spectra were taken on a SPEX Triplemate spectrophotometer equipped with a CCD photodetector and a microscope for recording backscattering spectra using laser emission at a wavelength of 488 nm. Thermogravimetric properties were studied on a modified Q-1000 derivatograph. A ∼150-mg sample was placed in a platinum crucible and heated at a rate of 10 K/min in a helium flow of at least 150 ml/min. Single crystals for X-ray structural analysis were obtained as follows. To [Rh(NH3 )5 Cl]Cl2 (7 · 10−5 mole) placed in a shallow dish, we added 0.1 M HBr (a few milliliters), and then a 0.01 M solution (10 ml) of K2 [PtBr6 ] in 0.1 M HBr. After 5–7 days, [Rh(NH3 )5 Cl]Cl2 completely dissolved, and transparent acicular red crystals formed on the bottom of the dish. Part of the crystals was transferred onto filter paper and dried in air. Interestingly, the crystals left in the mother solution dissolved again for a month to finally yield K2 [PtBr6 ] and [Rh(NH3 )5 Cl]Br2 sediments. This can be attributed to changes in the ambient conditions during the experiment (temperature, humidity) or to the strong effect of the concentrations of the starting substances in the solution on the stability of the complex. The synthesis was carried out at room temperature. For X-ray structural analysis, several single crystals were selected from the crystals synthesized and examined by the Laue method in an X-ray chamber for unit cell determination. All crystal samples were of rather poor quality. The best single crystal was studied on an STOE STADI-4 automated diffractometer (MoKα radiation, graphite monochromator, θ/2θ scans in the θ range of 1.7 to 20◦ , 3288 measured reflections, room temperature). The crystallographic data are as follows: a = 12.013(2) ˚ A, b = 8.401(2) ˚ A, c = 15.999(3) ˚ A, β = 91.13(3)◦ , 3 3 V = 1614.3(6) ˚ A , space group P 21 /m, Z = 4, dx = 3.70 g/cm . The crystal structure was refined in the fullmatrix approximation to R = 0.1189 for 1644 independent reflections, and to R = 0.0864 for the reflections with I > 2σI . The atomic coordinates are listed in Table 1, and the interatomic distances and angles are given in Table 2. All calculations were performed with the SHELX-93 program package [6]. An X-ray diffraction study was performed on DRON-3M and DRON-SEIFERT-RM4 diffractometers (R = 192 mm, CuKα radiation, Ni filter, a scintillation counter with amplitude discrimination). Two diffraction patterns for a single-crystal sample and a polycrystalline sample of [Rh(NH3 )5 Cl][PtBr6 ] were obtained in the 2θ range from 5 to 60◦ at room temperature. The single-crystal sample was ground in an agate mortar, and the powder obtained was then poured onto the polished side of a cell coated with a thin layer of vaseline. The polycrystals were ground with heptane, and the resultant suspension was applied onto a cell. After the heptane had dried, the sample was a layer ∼100 µm thick. Both diffraction patterns were completely indexed according to the data obtained for the single-crystal sample, which shows that the sample was monophase and representative. Samples of the thermal-decomposition products were ground with heptane, and the resultant suspension was applied onto a cell. Diffraction patterns were taken in the 2θ range from 5 to 135◦ at room temperature. They were indexed by analogy with pure Rh and Pt metals. The parameters of the metallic phases were refined using single reflections measured in the 2θ range from 120 to 135◦ .

650

TABLE 2 Bond Lengths d [˚ A] and Angles ω [deg] in the [Rh(NH3 )5 Cl][PtBr6 ] Structure Bond Pt(1) Pt(1) Pt(1) Pt(1) Pt(2) Pt(2) Pt(2) Pt(2) Pt(2) Rh(1) Rh(1) Rh(1) Rh(1) Rh(1) Rh(2) Rh(2) Rh(2) Rh(2)

Br(1) Br(2) Br(3) Br(4) Br(5) Br(6) Br(7) Br(8) Br(9) N(1) N(2) N(3) N(4) Cl(1) N(5) N(6) N(7) Cl(2)

d 2.469(5) 2.448(7) 2.456(5) 2.480(7) 2.469(7) 2.468(7) 2.458(5) 2.427(8) 2.505(7) 2.16(5) 2.02(5) 2.11(3) 2.03(4) 2.331(16) 2.08(3) 2.11(3) 2.09(4) 2.306(18)

Angle Br(2) Br(3) Br(3) Br(2) Br(3) Br(1)1 Br(2) Br(3) Br(1) Br(8) Br(7) Br(8) Br(7) Br(8) Br(7) Br(6) Br(8) Br(7) Br(6) Br(5)

Pt(1) Pt(1) Pt(1) Pt(1) Pt(1) Pt(1) Pt(1) Pt(1) Pt(1) Pt(2) Pt(2) Pt(2) Pt(2) Pt(2) Pt(2) Pt(2) Pt(2) Pt(2) Pt(2) Pt(2)

ω Br(3) Br(3)1 Br(1)1 Br(1) Br(1) Br(1) Br(4) Br(4) Br(4) Br(7) Br(7)1 Br(6) Br(6) Br(5) Br(5) Br(5) Br(9) Br(9) Br(9) Br(9)

90.46(19) 91.7(2) 179.18(19) 88.74(18) 88.15(16) 92.0(2) 179.7(3) 89.34(18) 91.46(18) 89.40(14) 178.6(3) 93.3(3) 89.72(14) 87.2(2) 90.29(14) 179.5(3) 176.1(2) 90.62(14) 90.6(2) 89.0(2)

Angle N(2) N(2) N(4) N(3)1 N(2) N(4) N(3) N(2) N(4) N(3) N(1) N(5) N(5) N(5) N(5) N(7) N(6)1 N(5) N(7) N(6)

Rh(1) Rh(1) Rh(1) Rh(1) Rh(1) Rh(1) Rh(1) Rh(1) Rh(1) Rh(1) Rh(1) Rh(2) Rh(2) Rh(2) Rh(2) Rh(2) Rh(2) Rh(2) Rh(2) Rh(2)

ω N(4) N(3) N(3) N(3) N(1) N(1) N(1) Cl(1) Cl(1) Cl(1) Cl(1) N(5)1 N(7) N(6)1 N(6) N(6) N(6) Cl(2) Cl(2) Cl(2)

179(2) 88.2(14) 91.8(14) 175(2) 85.1(17) 96.2(18) 91.4(12) 90.4(14) 88.3(16) 88.4(12) 175.5(12) 88(2) 91.7(13) 176.8(15) 91.1(14) 91.5(15) 89.4(19) 88.5(10) 179.8(14) 88.4(12)

Note. 1) x, −y + 1/2, z.

CRYSTAL STRUCTURE OF [Rh(NH3 )5 Cl][PtBr6 ] A general view of the crystal structure of [Rh(NH3 )5 Cl][PtBr6 ] is shown in Fig. 1. Each cation is surrounded by six anions which form a distorted octahedron, and vice versa; in view of this, the structure of the compound studied can be classed as a distorted NaCl-type structure. The Rh. . .Pt, Rh. . .Rh, and Pt. . .Pt distances are 5.90– 6.16, 6.63–6.64, and 7.21–7.25 ˚ A, respectively. The structure is built from two crystallographically independent types of [Rh(NH3 )5 Cl]2+ complex cations and two crystallographically independent types of [PtBr6 ]2− complex anions, whose central atoms lie in the symmetry plane. The Rh atoms have a distorted octahedral environment made up of five nitrogen atoms and one chlorine atom. The locations of the cations in the structure are significantly different. In the environment of Rh(1), three nitrogen atoms and one chlorine atom lie in the symmetry plane, whereas in the environment of Rh(2), only one nitrogen atom and one chlorine atom do so. The mean Rh(1) N and Rh(2) N distances are 2.086 ˚ A and 2.094 ˚ A, respectively, whereas the Rh Cl distance in the first octahedron is much greater than the corresponding distance in the second octahedron. The N Rh Cl angles are also different. A comparison of this structure with the structure of the Claus salt [Rh(NH3 )5 Cl]Cl2 [7] shows that the metal–ligand bond lengths change. For instance, whereas the Rh N bond lengths in the initial Claus salt are roughly identical (mean value 2.056 ˚ A), the Rh Cl distance is much larger, 2.356 ˚ A. The angles approach the angles in the Rh(2) octahedron. The severe distortion of the coordination polyhedrons can be attributed to the strong Cl. . .Br interaction between the cations and the anions. The Cl. . .Br distances are 3.57–3.58 ˚ A, which is much shorter than the sum of the van der Waals radii of the atoms, 3.71 ˚ A. The [PtBr6 ]2− complex anion has symmetry 2/m, and the Pt Br distances range between 2.448 and 2.505 ˚ A, ◦ ˚ their mean value being 2.469 A. The Br Pt Br angles differ from the right angle by 2–3 . Like the cations, the independent anions have different ligand locations relative to the symmetry plane. The corresponding values do not differ too much from the Pt Br distances in K2 [PtBr6 ], where the coordination octahedron is ideal and the Pt Br distance is 2.46 ˚ A [8]. However, in the structure studied, one of the distances in each octahedron, namely, the Pt(1) Br(4) or Pt(2) Br(9) distance, is notably greater than the others. This can be associated with the presence of weak hydrogen bonds Br. . .H N, in which the bromine–nitrogen distance is 3.30 to 3.60 ˚ A. Two bromine atoms 651

Fig. 1. Crystal structure of [Rh(NH3 )5 Cl][PtBr6 ] (general appearance); interionic contacts are shown.

Fig. 2

Fig. 3

Fig. 2. Crystal structure of [Rh(NH3 )5 Cl][PtBr6 ] (fragment), the Br. . .Br interionic contacts are shown by dashes. Fig. 3. Raman spectra of [Rh(NH3 )5 Cl][PtBr6 ] (1), K2 PtBr6 (2), and [Rh(NH3 )5 Cl]Cl2 (3).

in the environment of Pt(1) and four ligands in the environment of Pt(2) lie in the m plane. In Fig. 1, the chains formed by Pt(2) anions are clearly seen; these chains have shortened Br. . .Br interionic contacts between the [PtBr6 ]2− octahedra. Such contacts can be traced in Fig. 2. Whereas the doubled Van der Waals radius for bromine equals 3.80 ˚ A, the Br. . .Br separations, about 3.50 ˚ A, are comparable in length to the Br. . .Br separations inside the [PtBr6 ]2− octahedra. 652

TABLE 3 Frequencies [cm−1 ] of the Most Intense Bands in the IR Spectra Compound

ν(NH3 )

δd (NH3 )

δs (NH3 )

ρr (NH3 )

ν(RhCl)

ν(RhN)

δ(RhN)

ν(PtBr)

[Rh(NH3 )5 Cl]Cl2

3288(i) 3173(w)

1559

1308(i) 1273(w)

850

320

275 266

—

[Rh(NH3 )5 Cl][PtBr6 ]

3280(i) 3230(m) 3198(m)

1624

1355(w) 1316(i)

818

319

507 492 481 499 464 458

273 255

236

Note. Intense (i), weak (w), and moderate-intensity (m) bands.

RESULTS AND DISCUSSION The IR spectrum of [Rh(NH3 )5 Cl][PtBr6 ] is very similar to that of [Rh(NH3 )5 Cl]Cl2 . The data are listed in Table 3. The bands were identified according to previous results [9, 10]. One can clearly see that the main IR bands are shifted and their appearance changed in the region of stretching vibrations of the coordinated ammonium compared with [Rh(NH3 )5 Cl]Cl2 . Raman spectra of [Rh(NH3 )5 Cl][PtBr6 ] (complex 1), K2 PtBr6 (complex 2), and [Rh(NH3 )5 Cl]Cl2 (complex 3) are shown in Fig. 3 (spectra 1, 2, and 3, respectively). Assignment of the vibrations of complexes 1 and 2 is given in [9] and [10], respectively. In the spectrum of complex 3, four bands in the region 470–510 cm−1 (2A1 , B1 , E) [11] are assigned to the stretching vibrations of the Rh N bond; they are shifted by 12–15 cm−1 toward low frequencies relative to the corresponding vibrations of the Claus salt. The weak band at 318 cm−1 is due to the full-symmetric stretching vibrations of the Rh Cl bond. In the spectrum of complex 3, the 187 and 206 cm−1 bands due to the Pt Br stretching vibrations (Ag and Eg ) are also shifted by 5–7 cm−1 to the low-frequency region as compared to the corresponding vibrations in the Raman spectrum of K2 PtBr6 . This shift correlates fairly well with the changes in the Rh N and Pt Br bond lengths as revealed by X-ray structural analysis. Thermal-analysis curves of the complex are shown in Fig. 4. As can be seen, the substance decomposes in four stages in the temperature interval from 345 to 780–800◦ C. We note that as the temperature increases to 300◦ C, the color of the sample becomes increasingly more saturated, although no weight loss is observed. To prove the stability of the substance, we recorded an IR spectrum of a sample quenched at ∼300◦ C. After quenching, the vibration frequencies in the spectrum remained unchanged and the bands broadened somewhat. It can therefore be concluded that the structure of this substance hardly changed. The first three stages are accompanied by sublimation of ammonium bromide. In the last stage, the substance loses three bromine atoms. To examine the final stages of thermal decomposition in an inert atmosphere, we studied two intermediate products corresponding to the ∼40 and 44% weight losses for samples I and II, respectively. A ∼50-mg sample of the salt was placed, as in the elemental analysis, in a reactor and heated in a helium flow; the degree of conversion was determined by weighing the final products. Sample I was obtained at ∼500◦ C, and the weight loss was found to agree with the stoichiometry of the PtRhBr3 residue. The sample displays a metallic luster; on grinding, the sample turns brown. According to X-ray phase analysis data, the sample contains a metallic-platinum phase with a = 3.923(1) ˚ A, which is responsible for asymmetric reflections with considerable broadening at large angles, suggesting the formation of a number of platinum-based solid solutions. The second phase in sample I displays rather broad reflections. According to the data of [11], this phase can be identified as RhBr3 . Raman spectra of this sample are shown in Fig. 5. Apart from two intense vibrations of the RhBr3 phase (177 and 188 cm−1 ), the spectrum contains weaker bands that were not reliably identified. Sample II was synthesized at ∼560◦ C; the weight loss in this case corresponds to the stoichiometry of the PtRhBr2.6 residue. The diffraction pattern of this sample exhibits reflections due to metallic platinum and an unknown phase. The Raman spectrum of sample II is similar to the spectrum described above, indicating the presence of a compound with a rhodium–bromine bond. This phase can be identified as a decomposition product of rhodium tribromide. The decomposition of [Rh(NH3 )5 Cl][PtBr6 ] in an inert atmosphere yields a two-phase metallic powder. These phases have cubic cells with parameters a1 = 3.878(3) ˚ A and a2 = 3.836(3) ˚ A and belong to rhodium and 653

Fig. 4

Fig. 5

Fig. 4. Thermal-analysis curves of [Rh(NH3 )5 Cl][PtBr6 ] in helium atmosphere. Fig. 5. Raman spectrum of the intermediate product of thermal decomposition of [Rh(NH3 )5 Cl][PtBr6 ] complexes in a helium atmosphere (sample I).

platinum-based solid solutions. The crystallite sizes, determined from the diffraction-peak broadening, are 75–85 and 90–100 ˚ A, respectively. Heating of sample I in an inert atmosphere to a temperature of 800◦ C yields analogous products. We can state that the composition of the final products does not depend on whether the process is interrupted at the intermediate stage or not. In view of this, the thermal conversion in an inert atmosphere is described by the scheme 500◦ C

[Rh(NH3 )5 Cl][PtBr6 ] −→ Pt(met.) + RhBr3

>780◦ C

−→

mixed solid solutions of Rh and Pt.

The above scheme suggests that the thermal decomposition of the complex is preceded by ligand exchange between the cation and anion, with simultaneous reduction of platinum. Since rhodium bromide is more stable than platinum bromides [11, 12], it decomposes the latest. The final product of the thermal decomposition in hydrogen is a Rh0.5 Pt0.5 solid solution, which has an FCC cell with the parameter a = 3.864(2) ˚ A; the peak width shows the phase to be well crystallized. The difference in phase compositions between the final products can be explained based on the above scheme. During the first stages of thermal decomposition in an inert atmosphere, metallic platinum forms and rhodium is present in the form of bromide. Upon further heating, the bromide decomposes to yield metallic rhodium, which partly passes into a platinum-based solid solution, and platinum passes into a rhodium-based solid solution. The thermal decomposition in hydrogen proceeds much faster, and the metals are reduced simultaneously, forming a single-phase solid solution. It is worth noting that the crystal lattice parameter of the rhodium–platinum solid solution formed during thermal decomposition of the previously obtained salt [Rh(NH3 )5 Cl][PtCl4 ] in hydrogen [2] coincides with the crystal lattice parameter of the solid solution described above. It is important that the thermal decomposition of the [Rh(NH3 )5 Cl][PtCl4 ] complex in helium proceeds in two stages, and at 700◦ C it yields the same products as the thermal decomposition in hydrogen. The [Rh(NH3 )5 Cl]Cl2 salt decomposes in one stage in the temperature range 340–410◦ C to yield metallic rhodium [13]. The salt K2 [PtBr6 ] decomposes in one stage in the temperature range 545–640◦ C, forming metallic platinum and potassium bromide [14]. As a result of the present study, the binary complex salt [Rh(NH3 )5 Cl][PtBr6 ] was synthesized and its structure determined. The thermal decomposition of this compound in a hydrogen atmosphere is shown to differ from that in a helium atmosphere. The final reduction product in hydrogen is a Rh0.5 Pt0.5 solid solution. RhBr3 is an intermediate product of thermal decomposition in helium, and the final product is a mixture of two metallic solid solutions. The authors thank Prof. A. B. Venediktov for assistance in the synthesis of the starting compounds. This work was supported by the Civil Research and Development Foundation (CRDF) and the Ministry of Education of the Russian Federation (Grant No. REC-008). 654

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