Journal of Structural Chemistry. Vol. 47, No. 4, pp. 735-739, 2006 Original Russian Text Copyright © 2006 by K. V. Yusenko, A. V. Zadesenets, I. A. Baidina, Yu. V. Shubin, D. B. Vasil’chenko, and S. V. Korenev
IN BLESSED MEMORY OF OUTSTANDING CHEMISTS G .K. BOKII (1909-2000) AND L. A. CHUGAEV (1873-1922)
RE-DETERMINATION OF THE CRYSTAL STRUCTURE AND INVESTIGATION OF THERMAL DECOMPOSITION OF THE CHUGAEV’S SALT, [Pt(NH3)5Cl]Cl3H2O K. V. Yusenko,1 A. V. Zadesenets,1 I. A. Baidina,1 Yu. V. Shubin,1 D. B. Vasil’chenko,2 and S. V. Korenev1
The crystal structure of the salt [Pt(NH3)5Cl]Cl3H2O has been re-determined by single crystal X-ray diffraction and the salt has been studied by the thermal analysis. It is shown that one molecule of crystallization water enters into the salt composition. Intermediate products of thermal decomposition of the salt have been isolated and explored by IR spectroscopy and powder X-ray diffraction. Keywords: Chugaev’s salt, thermal analysis, X-ray diffraction study.
INTRODUCTION Platinum(IV) chloropentaammine chloride was originally prepared and characterized by L. A. Chugaev in the beginning of the XX century . G. B. Bokii conducted a crystal structure investigation of this salt . The results of these authors appeared to be contradictory: Bokii considered the compound as [Pt(NH3)5Cl]Cl3H2O, while Chugaev concluded on the basis of elemental analysis data that the salt contains no water of crystallization. Later in the work  there was performed a thorough comparison of the elemental analysis results for the salt samples prepared by different methods, and the conclusion was made that the compound did not include water of crystallization. DTA curves obtained by A.V. Nikolaev for the Chugaev’s salt also supported the absence of the crystallization water . However, spectroscopy studies  have reported the IR spectrum and elemental analysis data indicating the presence of water of crystallization. Apparently, in the literature there is no agreement on the composition of the Chugaev’s salt. Therefore we have carried out an additional investigation of the structure, the composition, and the properties of the Chugaev’s salt by the methods of single crystal and powder X-ray diffraction (XRD), thermal analysis (TA), and IR spectroscopy.
EXPERIMENTAL The compound was prepared by the phosphate method . To enhance the yield of [Pt(NH3)5Cl]Cl3, the mother liquor, remained after filtration of the major part of the salt, was added with an excess of concentrated hydrochloric acid, and
A. V. Nikolaev Institute of Inorganic Chemistry, Siberian Division, Russian Academy of Sciences, Novosibirsk; [email protected]
2Novosibirsk State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 47, No.4, pp.749-753, July-August, 2006. Original article submitted October 5, 2005. 0022-4766/06/4704-0735 © 2006 Springer Science+Business Media, Inc.
the solution was left for a week. During this time the solution yielded colorless transparent needle-shaped crystals. The precipitate was filtered off with suction, rinsed with ice water, acetone, and dried in air. Yield was 70-75%. The density of the crystals measured by flotation in a bromoform-toluene mixture was 2.57r0.01 g/cm3. Platinum content, determined by heating of weighted portions of the salt in hydrogen or helium at 600qɋ, was equal to 44.3r0.2 %. For H17N5OCl4Pt calculated, %: Pt 44.33; for H15N5Cl4Pt calculated, %: Pt 46.22. Thermal properties were studied with a Q-1000 derivatograph modified to operate with different gases (air, helium). A sample of the material (a100 mg) was placed in an open quartz crucible, which was heated with 10 deg/min rate in a helium stream (150 ml/min). Single crystal X-ray diffraction study (unit cell refinement and intensity data acquisition) was performed on a Bruker AXS P4 automated diffractometer (MoKD radiation, graphite monochromator, room temperature, T/2T-scanning within T range 3.2-27.5q, 1113 measured reflections). The full-matrix crystal structure refinement using 1102 independent reflections converged to R = 0.0129, R = 0.0126 for I > 2V(I ). Crystal data are: a = b = 20.8361(3) Å, c = 6.7704(2) Å, V = 2545.53(1) Å3, space group R3m, Z = 9, dx = 2.572 g/cm3. The structure was solved by the standard heavy atom method and refined anisotropically; all calculations were performed with SHELX-97 program package . Atomic coordinates, thermal parameters, bond lengths, and angles are listed in Tables 1 and 2. Powder diffraction patterns of the Chugaev’s salt and the products of its thermal decomposition were recorded with a DRON-SEIFERT-RM4 diffractometer (CuKD radiation, graphite monochromator at the diffracted beam, scintillation detector with amplitude discrimination). The samples were prepared by deposition of ethanolic suspensions on the polished side of a molten quartz sample holder. A similarly prepared polycrystalline silicon sample (a = 5.4309 Å) was used as the external standard. Stepwise data collection was performed within 2T range from 5q to 60q. IR spectra (400-4000 cm–1, KBr pellets, and 1400-4000 cm–1, thin film of fluorinated oil) were recorded on a Fourier spectrometer Scimitar FTS 2000.
RESULTS AND DISCUSSION Description of the crystal structure. The crystal structure of the prepared phase is identical to that found by Bokii. However, in his study he has concentrated on the structure of the coordination moiety, but did not discuss in detail the general structural motif. TABLE 1. Positional and Thermal Parameters of Atoms in the Crystal Structure of [Pt(NH3)5Cl]Cl3H2O Atom Pt(1) Cl(1) Cl(2) Cl(3)
0.745075(11) 0.872537(5) 0.333228(4) 18.50(6) 0.64407(8) 0.82203(4) 0.1252(2) 29.3(3) 0.92872(7) 0.85744(13) 0.2430(4) 61.0(6) 0.75210(9) 0.99164(7) 0.8198(2) 40.3(3)
N(1) N(2) N(3) N(4) O(1W)
0.8140(3) 0.90700(15) 0.0946(7) 0.8357(3) 0.91783(16) 0.5207(10) 0.6732(3) 0.83661(14) 0.5695(7) 0.7439(2) 0.7734(2) 0.3265(6) 0.9486(5) 0.8972(9) 0.7705(15)
26.0(11) 43.5(15) 27.8(12) 31.2(8) 174(6)
TABLE 2. Bond Lengths d, Å, Selected Valence Angles Z, deg in the Crystal Structure of [Pt(NH3)5Cl]Cl3H2O
Pt(1)–N(1) Pt(1)–N(2) Pt(1)–N(3) Pt(1)–N(4) Pt(1)–Cl(1)
2.039(5) 2.069(6) 2.059(5) 2.054(3) 2.3035(15)
N(1)–Pt(1)–N(4) N(1)–Pt(1)–N(3) N(4)–Pt(1)–N(3) N(4)–Pt(1)–Cl(1) N(2)–Pt(1)–Cl(1)
89.36(12) 178.6(2) 90.61(12) 88.73(13) 179.86(19)
Fig. 1. General view of the crystal structure of the salt [Pt(NH3)5Cl]Cl3H2O along the Z direction. Cations are presented as octahedra and are conventionally hatched; diagonal hatched are water molecules, cross-hatched are Cl(3) atoms; open circles are Cl(2) atoms; dashed lines are the shortest contacts O…O and Cl(2)…Cl(2).
Fig. 2. Fragment of the layer of coordination moieties in the structure of the [Pt(NH3)5Cl]Cl3H2O salt (notation is the same as in Fig. 1). The crystal structure of the Chugaev’s salt is built from complex cations [Pt(NH3)5Cl]3+, two crystallographically independent anions Cl– and the molecules of crystallization water. The complex cation lies on the m plane in such a way that atoms Cl(1), N(1), N(2), and N(3) lie on the plane, while two other atoms, N(4), are linked with it. One of the noncoordinated chloride ions and the water molecule also lie on the mirror plane. Fig. 1 illustrates the overall view of this salt crystal structure projected along the z axis. General structural motifs of chloropentaammine chlorides of Rh(III), Ir(III), Co(III) and Cr(III) are reported in . Their structures can be represented as a hexagonal close packing with outer-sphere chloride ions residing in tetrahedral interstices. The structure of the Chugaev’s salt does not match the close packing model. In Fig. 1 one can see close-packed layers of the complex cations arranged in the ABC motif, but each of the layers has lacunas filled with triples comprising water molecules and chloride ions. It is these triples that make channels running along the axis z. Interlayer Pt…Pt separation is 6.87 Å, intralayer 6.77-6.87 Å. Other outer sphere ions (Cl(3)) are arranged between the layers in tetrahedral interstices (Fig. 2). The structure is hydrogen bonded; the distances O(H2O)…N(NH3) are 3.098 Å, O(H2O)…O(H2O) 3.213 Å, Cl…N(NH3) 3.203 Å, and Cl(3)…Cl(3) 4.456 Å. Thermal decomposition of the Chugaev’s salt. Thermolysis of the Chugaev’s salt was studied in [4, 8]. The authors
Fig. 3. DTA and TG curves of [Pt(NH3)5Cl]Cl3 H2O in helium atmosphere. TABLE 3. Maxima of Absorption Bands in IR Spectra of the Chugaev’s Salt and Intermediate Phases Isolated During Thermolysis Compound
[Pt(NH3)5Cl]Cl3·H2O [our data] [Pt(NH3)5Cl]Cl3·H2O 
1545 1466 1550
970 940 900 915
1380(m) 1356(s) 1385(m) 1349(s) 1348
Q(PtN) 560 525 555 523 512 525 548 524 567 554 524
postulated the following basic reactions occurring in thermal decomposition of the salt: D
180 C 315 C o trans-[Pt(NH3)4Cl2]Cl2 o trans-[Pt(NH3)2Cl2] o Pt. [Pt(NH3)5Cl]Cl3
Evidently, these authors did not observe the stage of crystallization water loss. TG and DTA curves of the Chugaev’s salt heated in helium are illustrated in Fig. 3. The compound decomposes in three endothermic stages, the final product being platinum metal. Dehydration begins almost at room temperature, so water content in the Chugaev’s salt samples can be reduced. Easiness of dehydration can be explained by specific structural features of the salt. The structure has channels filled with water of crystallization and non-coordinated chloride ions. Partial escape of the water of crystallization would not result in drastic changes in the crystal structure. At high heating rate the last stages of decomposition cannot be separated, but the first two are well divided. The first stage on the TG curve corresponds to the loss of water molecule, the second to the escape of coordinated ammonia molecule according to the scheme D
100-180 C 220-250 C [Pt(NH3)5Cl]Cl3H2O o [Pt(NH3)5Cl]Cl3 o trans-[Pt(NH3)4Cl2]Cl2.
To substantiate this conclusion, we have studied intermediate products corresponding to these two stages. For this purpose a sample of the salt was kept at 120qɋ for 24 h (sample I, weight loss 4.1%, the end of the first step) and at 250qɋ during 10-15 min (sample II, weight loss 4.8%, the onset of the second step), and also at 300qɋ during 10-15 min (sample III, 738
weight loss 6.2%, the end of the second step). Sample I is a white powder, samples II and III are pale-yellow. The samples were studied with IR spectroscopy and powder XRD. Positions and relative intensities of the maxima of absorption bands corresponding to stretching vibrations of water, deformational and librational vibrations of coordinated ammonia, and also to stretching vibrations of the Pt–N bond are listed in Table 3. IR spectrum of sample I is practically identical to the spectrum of the starting salt, but lacks the bands of the water of crystallization. IR spectrum of sample II is similar to that of trans-[Pt(NH3)4Cl2]Cl2, but the bands of the dehydrated Chugaev’s salt are also present. The powder diffraction pattern of the Chugaev’s salt was fully indexed with the single crystal data, thus indicating phase purity of the sample and representativity of the single crystal studied. Sample I does not have reflections of the starting salt, the lines are narrow and do not match to any phase listed in the PDF files . The diffraction pattern of sample II contains reflections of the Gros’s salt (trans-[Pt(NH3)4Cl2]Cl2), along with reflections observed for sample I. To confirm the presence of dehydrated Chugaev’s salt in sample II, the sample was treated with water. The specimen was partially dissolved and partially remained unchanged. According to powder XRD, the insoluble part is a pure Gros’s salt, while the crystals obtained after complete evaporation of the solution is the monohydrate. Therefore, at the second stage the dehydrated Chugaev’s salt decomposes to yield the Gros’s salt without any intermediate products. The powder diffraction pattern of sample III manifests only peaks of the Gros’s salt. In the course of studies of [Pt(NH3)5Cl]Cl3H2O, Chugaev noticed deterioration of the salt during storage. We have also observed this phenomenon with a sample of three-year age salt. During this time the salt turned yellowish and became insoluble in water. According to powder XRD, the sample has completely transformed into the Gros’s salt. Rather wide reflections indicated poor crystallinity of the sample. The conducted study closes the question about the composition of the Chugaev’s salt. Bokii and we used in the studies the air-dry sample, which had not been heated. In the work  the salt had been heated in a vacuum chamber to 105110qɋ before the examination and it caused the escape of the water of crystallization, its removal beginning already at room temperature. On further heating the salt decomposes with evolution of ammonia. In conclusion, the authors are pleased to express their gratitude to N. I. Alferova for recording the IR spectra and her help in their interpretation.
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