Platinum complexes in solution

In the majority of catalytic reactions discussed in this chapter it has been possible to rationalize the reaction mechanism on the basis of the spectroscopic or structural identification of reaction intermediates, kinetic studies, and model reactions. Most of the reactions involve steps already discussed in Chapter 21, such as oxidative addition, reductive elimination, and insertion reactions. One may note, however, that it is sometimes difficult to be sure that a reaction is indeed homogeneous and not catalyzed heterogeneously by a decomposition product, such as a metal colloid, or by the surface of the reaction vessel. Some tests have been devised, for example the addition of mercury would poison any catalysis by metallic platinum particles but would not affect platinum complexes in solution, and unsaturated polymers are hydrogenated only by homogeneous catalysts. 

An examination of the chemishy of platinum complexes in solution indicates that, in the conditions used by Bamwenda et al. [1-2] and Kiwi and Itiliano [3] for preparations of Pt catalysts by DP, ion adsorption was indeed taking place more precisely, with the precursors and at the pH conditions used, no platinum... 

Since Fenton s work in the late nineteenth century, the role of transition metals in oxygen chemistry is known, but the formation of oxygen adducts with coordination metal complexes and their importance for O2 activation have been studied much later [1, 97]. The lively interest in ORR catalysis comes from its utmost importance to the development of fuel cells and this justifies that only a few studies have been done with metal complexes in solution most have been devoted to carbon electrodes modified by immobilization of a catalyst. The research for good catalysts that could be efficient substitutes for the expensive platinum naturally moved toward porphyrins. 

Alkanes and arenes can also be activated to other reactions by platinum complexes in aqueous solution (57,58). For arenes in the presence of H2PtCl5, reduction from Pt(IV) to Pt(II) occurs and the arene undergoes chlorination. The reaction is catalyzed by platinum(II) (59). Similarly, if a platinum(IV) catalyst such as HjPtClg is used, chloroalkanes are formed from alkanes. As an example, chloromethane is formed from methane (Eq. 23) (60-62). Linear alkanes preferentially substitute at the methyl... 

Experiment 2 Molar Conductivity Measurements Considering Arrhenius s electrolytic theory of dissociation, Werner noted that evidence for his coordination theory may be obtained by determining the electrolytic conductivity of the metal complexes in solution. Werner and Jprgensen assumed that acid (ionic) residues bound directly to the metal would not dissociate and would thus behave as nonconductors, while those loosely held would be conductors. Molar conductivities of 0.1 molar percent aqueous solutions of some tetravalent platinum and trivalent cobalt ammines are given in Table 2.3. 

To initiate each reaction, completely dissolve 7.5-9 mg of platinum complex in 3 ml DMSO. Pipet the solution into a cuvette and begin the reaction run. 

This equation is in good agreement with experimental results. An analysis of this dependence results in the following ratio of constants k -.k2. k-i = 100 14 0.6 (for cyclohexane at 100 °C), i.e in effect, the uncharged complex S2PtCl2 is the most active. This may be an indication of the importance of the electrophilic properties of the platinum complex in the reaction with hydrocarbons, although an appreciable value of the rate constant of such a moiety as PtCl, in its reaction with alkanes in aqueous solution, does not fit the usual concepts of electrophiles very well. It has been shown that the particular Pt(II) complexes obtained, which should form positive ions upon solution in water, PtCI(H20)j and [Pt(H20)4] react more slowly with alkanes than PtCl2. This demonstrates a more complicated nature of interaction of the bivalent platinum and the hydrocarbon molecule than that of the interaction of an electrophile with a nucleophile. 

R 388 M. A. Fedotov, Nuclear Magnetic Resonance of Donor Atoms as a Tool for Determination of the Structure of Platinum Metal Complexes in Solutions , Russ. Chem. Bull. (Translation of Izviestiya Akademii Nauk, Seriya Khimicheskaya), 2003,52, 781... 

In this context the use of DMSO as carrier has been claimed to be applicable [23]. The clinically used pyrimidine analogue, iodouridine, is only sparingly water-soluble and is used in a DMSO solution for topical application. The reactivity of platinum complexes in DMSO, however, could be a potential problem and the approaches outlined in Section 3.3 may well be applicable for a pharmaceutically suitable formulation of antiviral activity. The toxic side effects of the platinum complexes, if also manifested in a localized application, would be a serious drawback in this present context since the desired selectivity is not present. Again, it is probable that the antiviral effect is manifested by killing of the host cell. 

To a solution of 1,2-epoxy-7-octene (0.50 mg, 4.0 mmol) and methylbis(trimethyl-silyloxy)silane (0.90 g, 4.0 mmol) in xylene (5.5 mL) is added a solution of the carbene-platinum complex in xylene (90 mL, 0.20 pmol, prepared by dissolving 28 mg of the platinum catalyst in 20 mL of xylene) at 72 °C. The reaction mixture is heated at 72 °C for 210 min (the reaction is monitored by GC) and then cooled. The resulting colorless solution is filtered through a short pad of charcoal, and the solvent is removed under vacuum. The crude product was purified by chromatography on silica gel (petroleum ether-5% Et3N) to give the title compound in 90% yield. ... 

The measurements have been carried out by cyclic voltammetry in 0.1 M sodium perchlorate solutions in the absence and in the presence of chloride ions . The latter play a significant role in the stability of the platinum complexes in aqueous solutions, as shown below for cis-platin, JM 8 being much more stable ... 

All the six platinum-group metals are highly resistant to corrosion by most acids, alkalis, and other chemicals. Their high nobility is the main factor determining their chemical resistance, and the formation of. complex ions in solution is principally responsible for their dissolution under certain conditions. 

The solution should be free from the following, which either interfere or lead to an unsatisfactory deposit silver, mercury, bismuth, selenium, tellurium, arsenic, antimony, tin, molybdenum, gold and the platinum metals, thiocyanate, chloride, oxidising agents such as oxides of nitrogen, or excessive amounts of iron(III), nitrate or nitric acid. Chloride ion is avoided because Cu( I) is stabilised as a chloro-complex and remains in solution to be re-oxidised at the anode unless hydrazinium chloride is added as depolariser. 

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