Platinum complexes reduction

Silyl(pinacol)borane (88) also adds to terminal alkenes in the presence of a coordinate unsaturated platinum complex (Scheme 1-31) [132]. The reaction selectively provides 1,2-adducts (97) for vinylarenes, but aliphatic alkenes are accompanied by some 1,1-adducts (98). The formation of two products can be rationalized by the mechanism proceeding through the insertion of alkene into the B-Pt bond giving 99 or 100. The reductive elimination of 97 occurs very smoothly, but a fast P-hydride elimination from the secondary alkyl-platinum species (100) leads to isomerization to the terminal carbon. 

Since it is known that the tetranuclear mixed-valent platinum-blue complexes such as 19 and 57 undergo disproportionation and reduction by water as Eqs. (1)—(3) and (7)—(9) show (106, 113), all the species appearing in Eqs. (1)—(3) and (7)—(9) are present in the solution. However, only one or several of the four species in the solution may in fact be resposible for the catalytic olefin oxidation. To clarify this point, the effect of the Pt oxidation state in the platinum complexes was compared. The results are summarized in Table VII, which... 

Volter (66) reported that a part of Pt(IV) (which is present as a non-stoichiometric oxychlorinated complex over his alumina support after calcination) is reduced reversibly below 550°C to a Pt complex related to soluble platinum. Above this temperature, reduction to the metallic state is complete. He attributed direct alkane- cycloalkane cyclization to this platinum complex (24, 66). 

Complexes involving oxime ligands display a variety of reactivity modes that lead to unusual types of chemical compounds. As far as the oxime chemistry of platinum is concerned, these complexes are involved in facile deprotonation of the OH group with formation of oximato complexes, reduction of Pt(IV) species, Pt(II)-assisted reactions with coordinated allene," alkylation by ketones, oxime-ligand-supported stabilization of Pt(III)—Pt(III) compounds, oxidative conversion into rare nitrosoalkane platinum(II) species, and coupling with organocyanamides. ... 

The inefficiency of the platinum/hydrogen reduction system and the dangers involved with the combination of molecular oxygen and molecular hydrogen led to a search for alternatives for the reduction of the manganese porphyrin. It was, for example, found that a rhodium complex in combination with formate ions could be used as a reductant and, at the same time, as a phase-transfer catalyst in a biphasic system, with the formate ions dissolved in the aqueous layer and the manganese porphyrin and the alkene substrate in the organic layer [28]. 

In many other cases, detailed examination of platinum(IV) substitution reactions has shown that the mechanisms involve oxidation-reduction steps. These redox reactions can be collected into two classes according to whether a bielectronic or a monoelectronic redox species reacts with the platinum complex (i.e. complementary and non-complementary redox reactions, respectively). 

Attempts have been made to mimic proposed steps in catalysis at a platinum metal surface using well-characterized binuclear platinum complexes. A series of such complexes, stabilized by bridging bis(diphenyl-phosphino)methane ligands, has been prepared and structurally characterized. Included are diplati-num(I) complexes with Pt-Pt bonds, complexes with bridging hydride, carbonyl or methylene groups, and binuclear methylplatinum complexes. Reactions of these complexes have been studied and new binuclear oxidative addition and reductive elimination reactions, and a new catalyst for the water gas shift reaction have been discovered. 

Recently we observed eel of the binuclear platinum complex tetra-kis(diphosphonato)diplatinate(II) (Pt (pop) ) (37). This anion has attracted much attention due to its intense green luminescence in room temperature solution (38-40) (excited state of this complex undergoes oxidative (42) and reductive quenching (41). From the quenching experiments the redox potentials were estimated to be E° = -1.4 V vs. SCE for the reduction and E° 1 V for the oxidation of Pt2(pop) - (41). The potential difference of 2.4 V almost matches the energy of the phosphorescing triplet ( 2.5 eV) of Pt -(pop) . Consequently, it should be possible to observe eel of this... 

Oxidative addition of the Si-aryl carbon bond in the silacyclobutene ring to Pt gives the optically active intermediate Pt-complex. Further coordination of (+)-l-methyl-l-(l-naphthyl)-2,3-benzosilacyclobut-2-ene to the complex and cr-bond metathesis will provide the cyclic dimer Pt-complex. Reductive elimination from the intermediate platinum complex gives cyclic polymers and oligomers. Preference of cr-bond metathesis over reductive elimination gives polymers of higher molecular weight. The presence of EtsSiH in the system results in the formation of linear products via cr-bond metathesis. 

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. 

Unfortunately, the redox potential of the Pt4 + /3+ couple is not known in literature. Although some stable Ptm compounds have been isolated and characterized (37), the oxidation state III is reached usually only in unstable intermediates of photoaquation reactions (38-40) and on titania surfaces as detected by time resolved diffuse reflectance spectroscopy (41). To estimate the potential of the reductive surface center one has to recall that the injection of an electron into the conduction band of titania (TH) occurs at pH = 7, as confirmed by photocurrent measurements. Therefore, the redox potential of the surface Pt4 + /3+ couple should be equal or more negative than —0.28 V, i.e., the flatband potential of 4.0% H2[PtClal/ TH at pH = 7. From these results a potential energy diagram can be constructed as summarized in Scheme 2 for 4.0% H2[PtCl6]/TH at pH = 7. It includes the experimentally obtained positions of valence and conduction band edges, estimated redox potentials of the excited state of the surface platinum complex and other relevant potentials taken from literature. An important remark which should be made here is concerned with the error of the estimated potentials. Usually they are measured in simplified systems - for instance in the absence of titania - while adsorption at the surface, presence of various redox couples and other parameters can influence their values. Therefore the presented data may be connected with a rather large error. 

Platinum-lead complexes undergo a cleavage of Pt—Pb bond with halogens and halogen acids506,507 510. These reactions are believed to occur through an electrophilic attack on Pt(II) leading to oxidative addition with the formation of a hexa-coordinated Pt(IV) complex. Reductive elimination of a plumbane results in the observed products (equation 193). 

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