Subject reaction with platinum complexes

In this chapter, we will consider the reactions of C-H compounds, such as alkanes, arenes as well as some others, with platinum complexes containing mainly chloride ligands. The reactions of alkanes with platinum(II) complexes have been the first examples of true homogeneous activation of saturated hydrocarbons in solution. Complexes of Pt(II) exhibit both nucleophilic and electrophilic properties, they do not react with alkanes via a typical oxidative addition mechanism nor can they be regarded as typical oxidants. Due to this, it is reasonable to discuss their reactions in a special chapter which is a bridge between previous chapters (devoted to the low-valent complexes) and further sections of the book that consider mainly complexes in a high oxidation state. Chloride cortplexes of platinum(IV) are oxidants and electrophiles and they will constitute the first subjects in our discussion of processes of electrophilic substitution in arenes and alkanes as well as their oxidation. 

Reductive elimination and oxidative addition are ubiquitous reaction steps in many TM-catalyzed processes. A recent study by Beste and Frenking (82) may serve as example for the general finding that relative energies of TM complexes with different coordination numbers may be subject to systematic errors at the DFT level of theory. Table 16 shows calculated energies at the CCSD(T)/n level and at B3LYP using three different basis sets, II-IV, for platinum complexes... 

From a practical point of view, isocyanates, together with carbamates and ureas (Chapter 3), are the most important organic products discussed in this book. Their synthesis from nitroarenes has indeed been the subject of many patents. There are also limited examples of aliphatic isocyanates obtained by this route. Organic mono- and diisocyanates may be prepared in a continues liquid phase method by treating the appropriate amine with phosgene. However, the reaction is rather complex [6] and, besides the use of the dangerous phosgene, the formation of the corrosive hydrochloric acid creates several problems. Aliphatic isocyanates can also be obtained from olefins with isocyanate ion in the presence of a salt of a coordination compound of palladium or platinum [7], from olefins with isocyanic acid in the vapour phase over Pt/ALOs [8], and from formamides, by oxidation over a silver catalyst [9]. Apparently only the last reaction seems to have some potential practical applications [10]. 

A number of relevant review articles have appeared. Their subjects include the chemistry of antitumor platinum complexes, complexes of platinum metals with weak donor ligands, and transition metal complexes of sulfide, selenide, and telluride ligands (which includes much material on square-planar compounds). A review by Chanon and Tobe " on electron transfer catalysis relates to many reaction types, including ligand replacements at square planes. 

The predominant photoreaction of the [Pt(C2H4)Cl3] anion is loss of ethylene, which is accompanied by a little loss of m-chloride. This behaviour contrasts with the corresponding thermal reaction, where the predominant reaction is loss of the chloride trans to the ethylene. This contrast between thermal and photochemical behaviour is novel most photochemical reactions of platinum(ii) complexes follow the same course as the corresponding thermal reactions, but occur more quickly. The photoaquation of the ethylene ligand is subject to sensitization by acetone or by acetophenone, and therefore probably occurs via the first excited singlet state. A general model for photosubstitution reactions has been described, which is relevant to octahedral as well as to square-planar complexes. ... 

The reaction is a sensitive one, but is subject to a number of interferences. The solution must be free from large amounts of lead, thallium (I), copper, tin, arsenic, antimony, gold, silver, platinum, and palladium, and from elements in sufficient quantity to colour the solution, e.g. nickel. Metals giving insoluble iodides must be absent, or present in amounts not yielding a precipitate. Substances which liberate iodine from potassium iodide interfere, for example iron(III) the latter should be reduced with sulphurous acid and the excess of gas boiled off, or by a 30 per cent solution of hypophosphorous acid. Chloride ion reduces the intensity of the bismuth colour. Separation of bismuth from copper can be effected by extraction of the bismuth as dithizonate by treatment in ammoniacal potassium cyanide solution with a 0.1 per cent solution of dithizone in chloroform if lead is present, shaking of the chloroform solution of lead and bismuth dithizonates with a buffer solution of pH 3.4 results in the lead alone passing into the aqueous phase. The bismuth complex is soluble in a pentan-l-ol-ethyl acetate mixture, and this fact can be utilised for the determination in the presence of coloured ions, such as nickel, cobalt, chromium, and uranium. 

Rate constants for reaction of cis-[Pt(NH3)2(H20)Cl]+ with phosphate and with S - and 5/ -nucleotide bases are 4.6xl0-3, 0.48, and 0.16 M-1s-1, respectively, with ring closure rate constants of 0.17 x 10 5 and 2.55x10-5s-1 for subsequent reaction in the latter two cases 220). Kinetic aspects of interactions between DNA and platinum(II) complexes such as [Pt(NH3)3(H20)]2+, ds-[Pt(NH3)2(H20)2]2+, and cis-[Pt(NH3)2(H20)Cl]+, of loss of chloride from Pt-DNA-Cl adducts, and of chelate ring formation of cis-[Pt(NH3)2(H20)(oligonucleotide)]"+ intermediates implicate cis-[Pt(NH3)2(H20)2]2+ rather than cis-[Pt(NH3)2 (H20)C1]+, as usually proposed, as the most important Pt-binder 222). The role of aquation in the overall scheme of platinum(II)/DNA interactions has been reviewed 223), and platinum(II)-nucleotide-DNA interactions have been the subject of molecular modeling investigations 178). 

In 1942 the resolution of the microscope in the hands of James Hillier of the RCA Laboratories was 20 A. Now in the hands of Joseph H. Wall of the Brookhaven National Laboratory it is 2.5 A permitting visualization of the individual platinum atoms. A survey of catalysts made with the electron microscope in 1942(95) showed a diversity of size, shape and texture of catalytic substances. Many of the precious metals were large and consequently not very efficient—only a very small fraction of the atoms were available for surface reactions. However many of them were of colloidal size,(96) i.e. of one dimension at most of 2000A. The usual method of making the catalyst was to soak the support with a solution of the salt of the precious metal and then subject it to thermal treatment. The complex topoche-mical reactions that take place are difficult to control to obtain monodisperse particles of optimum size. Two questions arose in the 40 s and 50 s. What is the dependence of catalytic activity on particle size Is there a particle size below which there is no catalytic activity It was proposed to synthesize the metal particles in solution in colloidal form check their properties, both physical and chemical in solution then mount them on a suitable support to study their activity in heterogeneous catalytic reactions. However, the colloidal chemistry of platinum and palladium was complex, poorly understood and difficult to reproduce. 

The rich nucleophilic reactivity of square-planar platinum(II) and palladium(II) complexes is well established. One of the most documented examples is the stepwise oxidative addition of aUcyl halides to organoplatinum(II) [1] and organopalladium (II) [2,3] complexes via SN2-type substitution at the sp carbon center. Additionally, electron-rich Pt centers are subject to protonation at the metal to generate Pt hydrides as the first step in the protonolysis of many platinum-carbon bonds [4—7]. With a less reactive Lewis acid such as SO2, reversible adduct formation is observed [8], and this reaction has been used in the development of sensors [9-11],... 

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