Platinum complexes carbon dioxide reactions

Reactions of carbon subsulphide and of elementary phosphorus, sulphur and selenium with complexes of the platinum metals Sulphur dioxide insertion reactions of transition metal alkyls and related complexes... 

The characterization and crystal structure of the dimer [Pt2( -dppm)3] (dppm = bis(diphenyl-phosphino)methane), first reported as a deep red complex in 1978, was described by Manojlovic-Muir et al. in 1986.11 The structure, the first of its type, is made up of two parallel and almost eclipsed trigonal-planar platinum moieties bridged by three diphosphine ligands. The Pf Pt separation is 3.0225(3) A, too long to be considered a bond.11 [Pt2(//-dppm)3] catalyzes the hydrogenation/reduction of carbon dioxide with dimethylamine to give dimethylformamide12 (Equation (1)) and the reverse reaction.13... 

The slow step is deduced from the reluctance of Complex XXII to react with water or OH- in model reactions to regenerate XV under mild conditions. This is probably because the single positive charge in Complex XXII is spread over two platinum centers and the carbonyl group is therefore not sufficiently activated towards nucleophilic attack. In contrast the dication [Pt2(CO)2(/i-dppm)2]2+, XIII, reacts readily with water to give XXII and carbon dioxide. 

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

The ion-pair approach to the design of photosensitizers for electron transfer processes has been followed also in the case of [Co" (sep)] -oxalate system. In a deoxygenated solution, the excitation in the IPCT band of [Co" (sep)] +-HC204 causes the reduction of [Co" (sep)] + to [Co"(sep)] " and the oxidation of oxalate to carbon dioxide. The [Co"(sep)] + complex is a sufficiently strong reductant to reduce H+ to H2 at moderately acidic pH values. Thus, when the photoreaction is carried out in the presence of colloidal platinum catalyst, such a reaction indeed occurs, and H2 evolves from the solution in addition to carbon dioxide. Under such conditions, the overall reaction is the oxidation of oxalate, which plays the role of sacrificial agent, combined with the reduction of water to yield carbon dioxide and dihydrogen, according to Eq. 8. 

As many carbonate complexes are synthesized usually in aqueous solution under fairly alkaline conditions, the possibility of contamination by hydroxy species is often a problem. To circumvent this, the use of bicarbonate ion (via saturation of sodium carbonate solution with COj) rather than the carbonate ion can often avoid the precipitation of these contaminants. Many other synthetic methods use carbon dioxide as their starting point. Transition metal hydroxo complexes are, in general, capable of reacting with CO2 to produce the corresponding carbonate complex. The rate of CO2 uptake, which depends upon the nucleophilicity of the OH entity, proceeds by a mechanism that can be regarded as hydroxide addition across the unsaturated C02. There are few non-aqueous routes to carbonate complexes but one reaction (3), illustrative of a synthetic pathway of great potential, is that used to prepare platinum and copper complexes. Ruthenium and osmium carbonate complexes result from the oxidation of coordinated carbon monoxide by dioxygen insertion (4). ... 

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