Platinum dihydrogen complexes

Platinum(II) Complexes with Hydrido and Dihydrogen Ligands 718... 

Scheme 10.2 Heterolytic cleavage of dihydrogen on platinum SPO complex.

In addition to catalyzing hydroformylation, the platinum SPO complexes are excellent hydrogenation catalysts for aldehydes (as already demonstrated by the side products of hydroformylation), in particular, in the absence of carbon monoxide. Further, in ibis process, the facile heterolytic splitting of dihydrogen may play a role. The hydrogenation of aldehydes requires the presence of carboxylic acids, and perhaps the release of alkoxides from platinum requires a more reactive proton donor than that available on the nearby SPO. For example, 4 hydrogenates 2-methylpropanal at 95 °C and 40 bar of H2 to give the alcohol, with a TOF of 9000 mol moN h (71). 

The reactions of [Pt2H2(//-H)(//-dppm)2]PFfi with alkynes serve to introduce a number of studies of the elimination of dihydrogen from this cationic complex. Addition of one of a range of neutral ligands causes H2 elimination and formation of a platinum(I) complex of the form [Pt2HL(/i-dppm)2]+ (108,109) ... 

Autocatalytic decomposition of [PtMe2(COD)] on platinum black, under dihydrogen has already been pointed out in Uquid solution [44]. Indeed, the platinum atoms present in the starting complex are incorporated into the surface of the solid platinum catalyst, thus becoming the reactive sites for further cycles of chemisorption and reaction (Scheme 1). 

For example Kurihara and Fendler [258] succeeded in forming colloid platinum particles, Ptin, inside the vesicle cavities. An analogous catalyst was proposed also by Maier and Shafirovich [164, 259-261]. The latter catalyst was prepared via sonification of the lipid in the solution of a platinum complex. During the formation of the vesicles platinum was reduced and the tiny particles of metal platinum were adsorbed onto the membranes. Electron microscopy has shown a size of 10-20 A for these particles. With the Ptin-catalyst the most suitable reductant proved to be a Rh(bpy)3+ complex generated photochemically in the inner cavity of the vesicle (see Fig. 8a). With this reductant the quantum yield for H2 evolution of 3% was achieved. Addition of the oxidant Fe(CN), in the bulk solution outside vesicles has practically no effect on the rate of dihydrogen evolution in the system. Note that the redox potential of the bulk solution remains positive during the H2 evolution in the vesicle inner cavities, i.e. the inner redox reaction does not depend on the redox potential of the environment. Thus redox processes in the inner cavities of the vesicles can proceed independently of the redox potential in the bulk solution. 

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. 

Fig. 7.12 INS spectra (BT4, NIST) of (a) ethyne on platinum black annealed at 300 K with 500 mbar of dihydrogen (b) ethyne on platinum black adsorbed at 120 K, no dihydrogen. Reproduced from [64] with permission from the American Institute of Physics. [Os3(n2-CO)(CO)9( i3-Ti -C2H2)] (c) experimental, (d) modelled with the Wilson GF method. Reproduced from [67] with permission from the PCCP Owner Societies, (e) [Co2(CO)6(li2-ri -C2H2)], experimental (INBeF, ILL). Reproduced from [66] with permission from the American Chemical Society. Note that the peak pattern of adsorbed ethyne (b) is similar to that of ethyne in the osmium complex (c). Note also the additional peaks near 500 and 1400 cm when adsorbed ethyne was treated with hydrogen (a).

The recent worldwide emphasis on the topics of energy conservation and energy production has directed increased attention on molecular based catalysts, in particular for photocatalytic water splitting. An example of the contributions that resonance Raman and SERS can potentially make to this broad area is found in the field of photocatalytic production of dihydrogen. These catalysts are typically based on a light harvesting moiety, for example a ruthenium complex and a catalytic centre, typically palladium-or platinum-based. A recurrent question in regard to the use of these catalysts is that of the decomposition of complexes to form potentially active nanoparticles. 

J. R. Fisher, A. J. Mills, S. Sumner, M. P. Brown, M. A. Thomson, R. J. Ihiddephatt, A. A. Frew, L. Manojovic-Muir, K. W. Muir, Reversible displacement of dihydrogen by carbon monoxide in binuclear platinum complexes. Characterization of binuclear carbonyl complexes of platinum(I), Organometalhcs 1 (1982) 1421-1429. 

---