Hydride complexes of platinum

This method is not usually one of general use, but it was a method employed in the synthesis of the first hydride complex of platinum, PtHCl(PEt3)2. ... 

Hydride complexes of platinum have received considerable study since the preparation of PtHCl(PEt3)2- Spectroscopic studies by NMR techniques have been widely used because of the structural information which can be obtained from coupling constant data to Pt and other nuclei. Platinum is widely used as a heterogeneous catalyst, and vibrational studies on platinum hydride complexes have been useful for comparison of a hydrogen atom bonded to a single platinum with that bonded to a surface. Complexes of platinum have been used to catalyze hydrogenation, hydrosilylation and isomerization reactions with alkenes and alkynes, as well as H/D exchange reactions on alkanes. Hydride complexes are frequently proposed as intermediates in these reactions, and the pathways related to the known chemistry of hydride complexes. 

Section III.C A Hydrido(methyl)carbene Complex of Platinum(IV) (223) and Methyl(hydrido)platinum(IV) Complexes with Flexible Tridentate Nitrogen-Donor Ligands (224) are structurally related to the system shown in Scheme 13 and give additional information on how steric and electronic factors influence the stability of platinum(IV) methyl hydrides. 

Both hydride cind jr-allyl complexes of these metals are well known in the -f-2 oxidation state (60), but neither hydride nor r-allyl complexes of platinum(IV) are known. 

It has recently been shown (38a) that the triphenylphosphine complexes of platinum are in all probability complex hydrides of platinum(II), with general formula [(Ph3P)jPtTI2] where x - 2, 3, or 4. As these materials have always been used in the preparation of other platinum(O) complexes, it is now doubtful whether such complexes have in fact ever been obtained since it is possible that they are all hydrides. 

Solvato complexes of platinum(II) of the type fraws-[PtY(solvent)L2]+ (Y = hydride, alkyl, or aryl solvent = alcohol or ketone L = tertiary phosphine or arsine) have been known since 1961.1 They are obtained by halogen abstraction from the corresponding halo complexes tran.s-[PtXYL2] in the presence of the desired solvent.2 The methanol complex is also rapidly and quantitatively formed when trans-[PtH(N03)(PEt3)2] is dissolved in this solvent.2... 

Hydrosilylation is by far the most important route for obtaining monomers and other precursors to fluorinated polysiloxanes. Hydrosylilation80 is the addition of silicon hydride moiety across an unsaturated linkage using transition metal complexes of platinum or rhodium such as Speier s catalyst, hexachloroplatinic acid in isopro-... 

Hydride complexes of palladium and platinum are almost invariably stabilized by phosphine ligands and play an important role in catalytic processes such as hydrogenation. Examples are Pt(H)ClL2 and Pt(H)2L2, as well as hydrido alkyls and aryls, trans,-Pt(H)(R)L2. There are cis and trans isomers. A typical reaction is the insertion of alkenes and alkynes into the Pt—H bond 33... 

To prepare complexes with different phosphites it may be important to use either THF as reaction solvent, or if an alcohol solvent is desired, the alcohol having the same alkyl substituents as the phosphite must be used in order to avoid problems with transesterification of the phosphite ligand. The equilibrium between tetra- and penta-coordination (equation 410) is controlled by steric effects. Using similar synthetic procedures the alkyl diphenylphosphinite complexes of platinum [Pt PPh2(OR) 4](BPh4)2 (R = Me, Et) can also be prepared. Unexpectedly the reaction between Pt(l-a,4,5- j-CiHi3)(cod) and P(OR)3 (R = Me, Et, Ph) gives the hydride complex [PtH P(OR)3 4] . ... 

Formation of a siloxane network via hydrosilylation can also be initiated by a free-radical mechanism (300-302). A photochemical route makes use of photosensitizers such as peresters to generate radicals in the system. Unfor-timately, the reaction is quite sluggish. Several complexes of platinum such as (jj-cyclopentadienyl)trialkylplatinum(rV) compoimds have been found to be photoactive. The mixture of silicone polymer containing alkenyl functional groups with silicon hydride cross-linker materials and a catalytic amoimt of a cy-clopentadienylplatinum(IV) compound is stable in the dark. Under UV radiation, however, the platinum complex imdergoes rapid decomposition with release of platinum species that catalyze rapid hydrosilylation and network formation (303-308). Other UV-active hydrosilylation catalyst precursors include (acetylacetonate)Pt(CH3)3 (309), (acetylacetonate)2Pt (310-312), platinum tri-azene compounds (313,314), and other sytems (315,316). 

Other metals can catalyze Heck-type reactions, although none thus far match the versatility of palladium. Copper salts have been shown to mediate the arylation of olefins, however this reaction most probably differs from the Heck mechanistically. Likewise, complexes of platinum(II), cobalt(I), rhodium(I) and iridium(I) have all been employed in analogous arylation chemistry, although often with disappointing results. Perhaps the most useful alternative is the application of nickel catalysis. Unfortunately, due to the persistence of the nickel(II) hydride complex in the catalytic cycle, the employment of a stoichiometric reductant, such as zinc dust is necessary, however the nickel-catalyzed Heck reaction does offer one distinct advantage. Unlike its palladium counterpart, it is possible to use aliphatic halides. For example, cyclohexyl bromide (108) was coupled to styrene to yield product 110. 

Alkenes.—Deuterium exchange of propene with MeOD homogeneously catalysed by complexes of platinum, rhodium, and nickel can be monitored by microwave spectroscopy. The results show considerable incorporation of deuterium at C-2, a result which cannot be accommodated by the 7t-allyl-metal hydride mechanism for exchange/isomerization of olefins. However, the n- or /i -allyl-metal hydride mechanism for olefin isomerization has received some useful supporting evidence. The compound (54) can be generated... 

Paramagnetic transition-metal hydride complexes are very rare, and this 17-electron Ir(IV) complex is the first example of a stable paramagnetic hydride complex of the platinum-group metals. It was prepared by the reaction of (NH4)2 [IrClg] with an excess amount of P(i-Pr)3 in refluxing ethanol containing concentrated HCl. ... 

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