Mendeleev Communications Electronic Version, Issue 5, 1999 (pp. 171–212) Generation of platinum(III) species by mechanical treatment of solid K2PtX6 (X = Cl, Br) salts Sergey A. Mitchenko,*a Eugenii V. Khomutov,b Vitalii V. Kovalenko,b Anatolii F. Popovb and Irina P. Beletskayac a A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 117813 Moscow, Russian Federation.Fax.: +7 095 135 5085; e-mail: mitchen@samit.donetsk.ua b Institute of Physico-Organic and Coal Chemistry, National Academy of Sciences of Ukraine, 340114 Donetsk, Ukraine c Department of Chemistry, M. V. Lomonosov Moscow State University, 119899 Moscow, Russian Federation. Fax: +7 095 939 3618 A mechanical treatment of the solid salts K2PtX6 containing a small admixture of K2PtX4 (X = Cl, Br) in the atmosphere of air or argon leads to the formations of paramagnetic platinum d7-complexes.The most stable platinum complexes are diamagnetic and contain platinum in the +2 and +4 oxidation states. Paramagnetic compounds in the intermediate +3 oxidation state are much rarer1 than species containing platinum ions in the even-electron configurations. Probably, owing to this reason the information on PtIII compounds with simple inorganic ligands is limited.A number of compounds and complexes which were originally thought to involve the trivalent metal are now known to contain two platinum atoms, one in the +2 oxidation state and the other in the tetravalent state (see, for example, ref. 2). Therefore, the existence of paramagnetism with the magnetic moment close to the spin only value and hyperfine structure in the EPR spectra due to 195Pt nuclei (nucleus spin I = 1/2; natural abundance, 33.7%) may be considered as evidence† for PtIII compound formation.One of the first unequivocal evidence for the formation of platinum(III) complexes has been obtained4 in the case of cis-Pt(NH3)2(SCN)2 oxidation by iodine into Pt(NH3)2(SCN)2I. The EPR spectrum of the latter is a broad intense signal with gef = 2.18 and the line width DH = 56 mT at 20 °C.The formation of platinum(III) complexes has also been observed5 under H2PtCl6·6H2O thermolysis. The EPR spectrum of the species obtained corresponds to the spin-Hamiltonian of the axial symmetry with the parameters g|| = 1.98, g^ = 2.18, A|| = 14 mT, and A^ = 6mT, where A|| and A^ are the hyperfine splitting constants on 195Pt nuclei in the parallel and perpendicular orientations, respectively.The interaction of H2PtCl6·6H2O with concentrated sulfuric acid leads6 to the formation of the platinum(III) sulfate complex [PtO(SO4)]–. The platinum(III) dithiolate complexes [Pt(S2C2R2)2]– (R = CN,7,8 Ph9) have also been studied.For these compounds, the hyperfine structures have been resolved, and the EPR spectra may be fitted to the spin-Hamiltonian of the rhombic symmetry. The paramagnetic resonance of Pt3+ ions in Al2O3,10 BaTiO3,11 and YAl garnets12 has been investigated. The EPR spectra correspond to the spin-Hamiltonian of the axial symmetry for the first two cases, and the spin-Hamiltonian is of the rhombic symmetry in the third case.In the latter two cases, the hyperfine structure due to 195Pt nuclei has been observed. To our best knowledge this is the only information on the paramagnetic platinum(III) complexes with simple inorganic ligands. It is well known that mechanical treatment in mills (where destruction is accompanied by the friction of particles with each other) of covalent crystals such as diamond, graphite, silica, etc., generates free radicals predominantly located at the surface13 of particles.The mechanical treatment of magnesium oxide ionic crystals under frictional conditions also leads to electron transfer from an anionic lattice point to a cationic one with the formation of reactive radical-ion pairs13 {Mg2+O2–} ® {Mg+· O–·}.By analogy with the above data, we may expect that cooperative lattice vibrations can induce oscillations of separate atoms that constitute the complex anion [PtCl6]2–. If the intensity of † The platinum(III) complexes can also form dimeric complexes with a Pt–Pt bond, which are diamagnetic due to this bonding.3 the vibrations is sufficiently large, it results in the Pt–Cl bond cleavage with the loss of a chlorine atom and the formation of the trivalent platinum ion [PtCl5]2– at an anionic lattice point: The aim of this study was to prove this hypothesis.Platinum(II) can serve as a trap for chlorine and shift equilibrium (1) to the right. Thereby, the formation of platinum(III) in appreciable amounts should be expected in the presence of platinum(II) salt admixtures to the treated K2PtCl6.The samples were activated in a dry air atmosphere‡ for 1 h at room temperature in a closed glass vibroreactor containing K2PtCl6 K2PtCl5 + Cl· mechanical treatment (1) 260 300 340 H/mT 240 280 320 (a) (b) 1 2 1 2 H/mT Figure 1 EPR spectra, measured at (a) room temperature and (b) 77 K, of Pt3+ complexes generated by mechanical treatment of a K2PtCl6–K2PtCl4 mixture.(1) The experimental spectrum and (2) the simulated spectrum. The arrow indicates the EPR signal with g = 2.0036.Mendeleev Communications Electronic Version, Issue 5, 1999 (pp. 171–212) grinding bodies of glass. The working frequency used in an MMVE 0.005 vibratory micromill was 50 Hz. The amplitude was 5.5 mm. The specific energy intensity was 15 W kg–1.The K2PtX6 and K2PtX4 (X = Cl, Br) salts were prepared according to the standard procedure14 and carefully dried. The EPR measurements were performed using an SE/X-2544 spectrometer with the working frequency 9.4 GHz. An MnII sample in an MgO matrix was used as a standard for quantitative measurements. The spectra were taken in the solid polycrystalline samples at room temperature and 77 K. We have found that mechanical treatment of the K2PtCl6 salt results in the appearance of a relatively weak EPR signal with the resolved hyperfine structure due to 195Pt nuclei.As we expected, more intense signals were observed in the samples obtained by mechanical treatment of a K2PtCl6 powder which contained a small K2PtCl4 additive (5–15 mol%). The mechanical activation of K2PtCl6 in an atmosphere of Cl2 does not lead to ‡ The reaction also takes place in an argon atmosphere.the appearance of the characteristic EPR signal. Analogous effects occurred when K2PtBr6 with an admixture of K2PtBr4 was treated. The observed spectra can be fitted to the spin- Hamiltonian of the axial symmetry: where S = 1/2 and I = 1/2 are electron and nucleus spins, respectively; b is the Bohr magneton; the other parameters are given in Table 1.The EPR spectra parameters for other known platinum(III) compounds, which are characterised by the spin- Hamiltonian of the axial symmetry, are also presented in Table 1. The EPR spectra for the case of the axial symmetry of the spin-Hamiltonian have been simulated. The values of g-factors and hyperfine interaction constants were obtained by using the perturbation theory of the second order (Figures 1 and 2). The good agreement between experimental and simulated spectra confirms the correctness of the spectra interpretation.The occurrence of hyperfine structures in the perpendicular orientation due to 195Pt isotope proves that the spectra correspond to PtIII compounds.We have observed an appreciable temperature dependence of the line width [Figure 1(b)]. This can be attributed to the temperature-dependent spin–lattice relaxation time. No hyperfine splitting can be detected for the parallel orientation of the magnetic field even at 77 K. Hence, the hyperfine parameter A|| must be smaller than the line width. This gives a value of (0±35)×10–4 cm–1 at 77 K or <65×10–4 cm–1 at room temperature. Let us compare the spectra parameters for different PtIII compounds with the axially symmetric spin-Hamiltonian (Table 1).For these complexes, the g|| values are close to 2, and the values of g^ are higher than 2. The axial character of the spectra and the g-values point10 to the localisation of an unpaired electron at the 5dz2 orbital, and hence to a square-pyramidal structure of the complex.Unusually high values found for the hyperfine constants of PtIII compounds formed in the K2PtCl6 matrix probably arise from the appreciable admixture of the 6s orbital to the 5dz2 orbital.10 The initial amount of Pt3+ was estimated§ at 1×1018 spin g–1 from the EPR spectra. The intensity of the EPR signals corresponding to the Pt3+ ions decreased with time.The decrease of the Pt3+ signal intensity was accompanied by the appearance and intensity growth of the singlet with g = 2.000 and DH = 3mT (Figure 3). The decrease in the amount of paramagnetic platinum( III) ions and the simultaneous accumulation of paramagnetic centres with the g-factors close to the inherent values for free electrons apparently means that the decay of Pt3+ proceeds through electron transfer from platinum(III) to regenerate diamagnetic platinum(IV) and to form structural defects in the K2PtCl6 ion crystal matrix of the F-centre type.Taking into account the absence of ‘free’ chloride ions in the system and the diffusively retarded mobility in solids, the stoichiometric consequence of the reaction should be the formation of a coordinatively unsaturated platinum(IV) anion. As a result, the creation of the single-charged anion as a point defect in an anionic point of the lattice should be expected: § The concentration of Pt3+ was measured immediately after the mechanical treatment (for 1 h) of K2PtCl6 with a K2PtCl4 additive (15 mol%).The yield of platinum(III) formed under the same conditions but in the absence of K2PtCl4 was approximately equal to 2×1017 spin g–1. 1 2 260 340 H/mT Figure 2 The EPR spectra of Pt3+ complexes generated by mechanical treatment of a K2PtBr6–K2PtBr4 mixture. (1) The experimental spectrum and (2) the simulated spectrum. The arrow indicates the EPR signal with g = 2.0036. 1 2 3 4 5 250 300 350 H/mT Figure 3 EPR spectra of the mechanically treated K2PtCl6 solid salt with the additives of K2PtCl4 (5 mol%).The spectra were measured (1) 1, (2) 14, (3) 134, (4) 157 and (5) 189 h after the completion of mechanical treatment at room temperature. The arrow indicates the EPR signal with g = 2.0036. H = g|| bHzSz + g^b(HxSx + HySy) + A||Sz Iz + A^(Sx Ix + Sy Iy), (2) aHyperfine splitting constant/mT. bAt room temperature.cAt 77 K. Table 1 The EPR spectra parameters for platinum(III) complexes with the axially symmetric spin-Hamiltonian. Compound g^ g|| A^/ 10–4 cm–1 A|| / 10–4 cm–1 Reference Pt3+ in PtCl4 2.18 1.98 6a 14a 5 Pt3+ in Al2O3 2.328 2.011 — — 10 Pt3+ in BaTiO3 2.459 1.950 135 0 11 Pt3+ in K2PtCl6 2.371b 1.983b 500b <65b This work 2.385c 2.000c 511c 0±35c This work Pt3+ in K2PtBr6 2.45 1.95 317 <420 This work [PtCl5]2– [PtCl5]·– + e– (F-centre).(3)Mendeleev Communications Electronic Version, Issue 5, 1999 (pp. 171–212) The electron transfer in reaction (3) to the point of localisation of a structural defect that is able to accept it, probably, proceeds as a relay-race charge-transfer process through the neighbouring platinum(IV) complex anions with intermediate platinum(III) formation.The time dependence of the concentration of paramagnetic centres with g = 2.000 exhibits a maximum. In this case, the platinum(III) can probably serve as a sink for defects of the F-centre type: reduction of platinum(III) by free electrons should give platinum(II). In other words, despite the low mobility and a relatively low concentration of Pt3+ ions, the effective reaction of diffusively forbidden platinum(III) disproportionation into PtII and PtIV reaction should be expected in the K2PtCl6 solid matrix.Thus, mechanical treatment of the solid salts K2PtX6 (X = = Cl, Br) leads to the formation of metastable and hence potentially reactive species: platinum(III) complexes and, possibly, coordinatively unsaturated platinum(IV) complexes.The reactivity of these compounds towards organic substrates of different nature is under study in our laboratories. This work was supported by INTAS (grant no. 97-1874). References 1 F. R. Hartley, The Chemistry of Platinum and Palladium, Wiley, New York, 1973, p. 573. 2 T. D. Ryan and R. E. Rundle, J. Am. Chem. Soc., 1961, 83, 2814. 3 G. S. Muravejskaya, G.A. Kuksha, V. S. Orlova, O. N. Evstaf’eva and M. A. Porai-Koshits, Dokl. Akad Nauk SSSR, 1976, 226, 596 [Dokl. Chem. (Engl. Transl.), 1976, 226, 76]. 4 G. S. 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Khim., 1994, 63, 1031 (Russ. Chem. Rev., 1994, 63, 965). 14 Sintez Kompleksnykh Soedinenii Metallov Platinovoi Gruppy (Synthesis of Complex Compounds of Platinum Group Metals), ed. I. N. Chernyaev, Nauka, Moscow, 1964, p. 239 (in Russian). 15 S. A. Mitchenko, Yu. V. Dadali and V. V. Kovalenko, Zh. Org. Khim., 1998, 34, 1293 (Russ. J. Org. Chem., 1998, 34, 1233). Received: 19th February 1999; Com. 99/1445