Oxidative addition platinum hydride complexes

It has been shown that NHC palladium (or nickel or platinum) hydride complexes can be formed by the oxidative addition of imidazolium cations of ionic liquids to pal-ladium(O) or Ni(0) or Pt(0) species [13]. It has been shovm that it is not only the C2 of the imidazolium ring that can bond to metals, but also the other ring carbons [14]. 

The proposed mechanism starts with a methyl group abstraction on platinum complex 416 with the borane reagent in the presence of diyne 414 (Scheme 105). The square-planar cationic diyne-platinum(n) complex 417 is converted to the octahedral platinum(rv) hydride intermediate 418 through oxidative addition of the hydrosilane. This complex decomposes rapidly with methane release to form another tetracoordinated platinum(n) species 419, followed by platinasilylation of the triple bond. The resulting vinylplatinum 420 undergoes an intramolecular carboplatination to... 

There are now a number of quite stable Pt(IV) alkyl hydride complexes known and the synthesis and characterization of many of these complexes were covered in a 2001 review on platinum(IV) hydride chemistry (69). These six-coordinate Pt(IV) complexes have one feature in common a ligand set wherein none of the ligands can easily dissociate from the metal. Thus it would appear that prevention of access to a five-coordinate Pt(IV) species contributes to the stability of Pt(IV) alkyl hydrides. The availability of Pt(IV) alkyl hydrides has recently allowed detailed studies of C-H reductive elimination from Pt(IV) to be carried out. These studies, as described below, also provide important insight into the mechanism of oxidative addition of C-H bonds to Pt(II). 

The question of which pathway is preferred was very recently addressed for several diimine-chelated platinum complexes (93). It was convincingly shown for dimethyl complexes chelated by a variety of diimines that the metal is the kinetic site of protonation. In the system under investigation, acetonitrile was used as the trapping ligand L (see Fig. 1) which reacted with the methane complex B to form the elimination product C and also reacted with the five-coordinate alkyl hydride species D to form the stable six-coordinate complex E (93). An increase in the concentration of acetonitrile led to increased yields of the methyl (hydrido)platinum(IV) complex E relative to the platinum(II) product C. It was concluded that the equilibration between the species D and B and the irreversible and associative1 reactions of these species with acetonitrile occur at comparable rates such that the kinetic product of the protonation is more efficiently trapped at higher acetonitrile concentrations. Thus, in these systems protonation occurs preferentially at platinum and, by the principle of microscopic reversibility, this indicates that C-H activation with these systems occurs preferentially via oxidative addition (93). 

If the mechanism is dissociative (Fig. 2), then oxidative addition occurs (step 3) to give a platinum(IV) hydride. This can then lose HC1 (step 4) and gain DC1 (step 5), to and from the solvent. The plati-num(IV) deuteride can then revert to a 7r-complex with deuterium in the aromatic ring (step 6), and this deuterated benzene is lost (step 7) with regeneration of the catalyst. 

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. 

Similar bis(silyl) chelating platinum complexes have been obtained by the oxidative addition of disilanes H3SiSiH3 to the dihydride complex (dcpe)PtH2. A rapid addition of the disilane to the hydride afforded selectively complex 85 whereas a slow addition afforded a mixture of complexes 85 and 87, presumably through complex 86 via an a-silyl shift. X-ray determination indicates that the Pt2Si4 ring core adopts a chair conformation in 85 (Scheme 39)227. 

A mechanism for catalysis by platinum compounds was proposed in 1965 by Chalk58) and has since been supported by increasing knowledge about silyl-metal systems and by the direct detection of Pt-Si211) and Rh-Si61,18s) complexes in the reaction mixtures. The suggested mechanism requires olefin coordination to the Pt(II) species (in the case of H2PtCl6 formed by reduction by the silicon hydride), oxidative addition of the silane, formation of an intermediate in which silicon and alkyl are both bonded to the platinum center, and reductive elimination of alkylsilane, probably assisted by coordination of more olefin ... 

Platinum(IV) hydride chemistry has been reviewed recently. These are usually formed by oxidative addition to a Pt species and are rather unstable. The reaction shown in equation (5), for example, is readily reversible, because the product easily loses HCl. Hydrido complexes are often involved as intermediates in reactions in which the first step is oxidative addition of an H-X species to Pt. ... 

In their zerovalent compounds, all three metals (Ni, Pd, Pt) undergo oxidative addition of alkyl, aryl, and acyl halides. For palladium, in particular, such reactions are key steps in a wide range of catalytic reactions. Palladium(II) and platinum(II) complexes also add C—X bonds to generate Pd(IV) and Pt(IV) species. Since C—C or C—H bond formation by reductive elimination often occurs readily, a common reaction sequence involves C—X addition followed by coupling of two alkyl groups, or an alkyl and a hydride ligand. 

Prior to 1982, Crabtree s report of the reaction of cyclopentane with a solvated IrH2(PPh3)2+ species to give a cyclopentadienyl-iridium product stood as the only well characterized example of a reaction of an alkane with a homogeneous transition metal, in contrast to the widespread reactivity of arenes [2]. Based upon the instability of the platinum methyl hydride complex Pt(PPh3)2(CH3)H, it was believed that alkane oxidative addition might not be a thermodynamically feasible process, and consequently few attempts were made to attempt such a reaction [3]. It was not until the discovery of the formation of stable alkane oxidative addition products in 1982 that it was realized that reactions of hydrocarbons were in fact feasible. 

Another interesting catalytic transformation involving alkynes is the hydro phosphinylation, which affords alkenylphosphine oxides [62]. The formation of hydride-phosphinito compounds is one of the key steps of the reaction. These species are formed by oxidative addition of the P-H bond of diphenylphosphine oxide to platinum(O) and palladium(O) complexes, which act as catalytic precursors. In this context, it should be mentioned that a novel method to prepare hydride-phosphinito compounds has been recently reported. The new strategy starts from 133 and involves the oxidative addition of the P-H bond of... 

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