Platinum sols

Louie J, Hartwig JF (1996) A route to PdO from Pdll metallacycles in amination and cross-coupling chemistry. Angew Chem Int Ed Engl 35 2359-2361 Mucalo MR, Coouey RP (1989) F.T.I.R. spectra of carbon monoxide adsorbed on platinum sols. J Chem Soc Chem Commun 94—95 Munoz MP, Martin-Matute B, Fernandez-Rivas C, Cardenas DJ, Echavarren AM (2001) Palladacycles as precatalysts in Heck and cross-coupling reactions. Adv Synth Catal 343 338-342... 

One of the reasons for the loss in enantioselectivity is the washing out of the modifier fi om the platinum surface [5]. Using the chiral modifier as stabilizer of Pt-colloids prevents this problem. Therefore, the platinum sol stabilized by the protonated alkaloid DH-CIN prepared according to [7] was chosen as chiral model colloid in the enantioselective hydrogenation of ethyl pyravate. The alkaloid not only stabilizes the platiniun particles, it also induces enantioselectivity in catalytic hydrogenations [7]. 

The liquid phase hydrogenation of benzene on carrier-fixed ruthenium colloid catalysts suspended in an aqueous solution of sodium hydroxide proceeds with 59% cyclohexene selectivity at 50% benzene conversion. The catalysts are prepared by adsorbing a hydrophilic stabilized ruthenium metal colloid on lanthanum oxide. Protection of metal colloids with chiral molecules can lead to a new type of enan-tioselective catalyst combining good selectivity control with extraordinarily high activity in hydrogenation reactions. This concept has been applied for the first time in the form of platinum sols stabilized by the alkaloid dihydrocinchonidinel °°l (Fig. 7). 

This concept, leading to this new type of catalyst, is an extension to colloidal metals of the well-known heterogeneous enantioselective catalyst. Platinum sols stabilized by dihydrocinchonidine (Fig. 7) can be synthesized in different particle sizes by reduction of platinum tetrachloride with formie acid in the presence of different amounts of alkaloid. The resulting nanoparticles are enantioselective in the hydrogenation of ethyl 2-oxopropionate (ethyl pyruvate) optical yields are 75-80%... 

D. N. Furlong, A. Launikonis, W. H. F. Sasse, and J. V. Sanders. CoUoidal platinum sols -preparation, characterization and stabihty towards salt./. Chem. Soc.-Farad. Trans. 1,80 571, 1984... 

Chiral molecules on the surface of the metal colloid can induce enantioselective control. Following this concept a new type of enantioselective platinum sol catalyst stabilized by the alkaloid dihydrocinchonidine was designed [120, 121]. Chirally modified Pt catalyst precursors have been prepared in different particle sizes by the reduction of platinum tetrachloride with formic acid in the presence of different amounts of the chiral alkaloid. Optical yields up to 80% ee were obtained in the hydrogenation of ethyl pyruvate. This type of catalyst was demonstrated to be structure insensitive since turnover frequencies (ca. 1 s ) and enantiomeric excess are independent of the particle size. 

Boennemann and Braun found a very effective enantioselective colloidal catalyst. A platinum sol stabilized by DHCnd revealed very high enantioselectivity in the hydrogenation of ethyl pyruvate (EtPy) into ( )-(+)-ethyl lactate (i -EtLa) with an ee of 78% at 12°C and atmospheric pressure in a MflS i-homogeneous phase of a solution of AcOH + MeOH (Scheme 3.3.). 

The infrared spectra of CO on colloidal platinum, palladium, and ruthenium stabilized with either cellulose acetate or nitrocellulose have been reported. [23] Although the spectra show an interesting size dependence in the case of ruthenium in nitrocellulose, interpretation is difficult since the spectra differ markedly from those reported for CO on supported ruthenium. For the platinum sols, a preference for linear CO coordination is observed, as is the case for the supported metal, while for palladium the bridging mode is preferred exdusively. 

PVP stabilized rhodium or platinum sols (3.5 nm) have been grafted onto mer-capto-derivatized silica. [254] The S-H groups ligate strongly to the metal particles. In this case the grafted catalyts are one to two orders of magnitude more active for the hydrogenation of alkenes than the unsupported colloidal catalysts in the same solvent system, and 300 times more active than commercial Rh/C and Pt/C catalysts. 

A variety of inorganic redox reactions are catalyzed by platinum metal, [270] in several of which platinum sols have been observed to be active. Gold sols have also been shown to be active redox catalysts in, for example, the reaction between ferricyanide and thiosulfate ions (Eqn. 6.8). [271, 272]... 

The nature of the stabilizing layer around the platinum catsdyst particles has a significant effect on the efficiency of the catalyst. A comparison of radiolytically produced platinum/poly-iV-methylol acrylamide sols with platinum sol (from HjPtClg chemical reduction) stabilized by a range of other polymers showed that... 

Suspensions Prussian blue Platinum sols (Bredig)... 

Both in the case of ferments and platinum sols the catalytic power is revived, although somewhat slowly. The cause for the renewing is probably the oxidation of the poison by the hydrogen peroxide. This latter substance is a well-known antidote for hydrocyanic acid poisoning in the human body. As might be expected from these considerations the poisoning effect on platinum sols is much less if hydrogen pjeroxide is present when the poison is added. This is also true for enzymes. 

Experiments with platinum sols in hydrogen and oxygen atmospheres fully confirmed this point of view ... 

---