Protonation platinum hydride complexes

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). 

Except for 1-ethynylcyclohexanol, it appears that the addition of protonic acid to triphenylphosphine platinum hydrides is unfavorable. Nevertheless, the existence of such complexes with triethylphosphine ligands is proved sufficiently since, in addition to the isolation of complexes with hydrochloric acid (10, 14), good evidence is presented for the intermediacy of triethylphosphine Pt(IV) hydrides with silanes and phosphines (15,16). 

Protonation of the dihydrido(methyl)platinum(iv) complex 998 forms cationic hydridoplatinum(ii) complex with the protonated Tp ligand 999 (Scheme 124). It is stabilized by addition of ethylene and MeCN to form the complex with these ligands 1000, while the complex without addition of such ligands tends to form a dinuclear complex with two bridging hydride ligands 1001. Addition of SiHEt3 to 999 forms the dihydrido(triethylsilyl)platinum(iv) complex 1002. ... 

The dimethylplatinum(ll) complex with a bidentate iV,iV,iV -trimethyl-A -(2-picolyl)ethylenediamine ligand undergoes methylation by Mel and protonation by several protic acids to produce cationic trimethylplatinum(iv) complex 1018 and methylplatinum(ll) complex 1019, respectively (Scheme 129). The protonation involves an intermediate cationic hydrido(dimethyl)platinum(lv) complex 1020. Scrambling of hydrogen atoms between hydride and methyl ligands during the reaction indicates an intermediate complex with methane as a ligand. 

Hydrides of Pt(II) are the most numerous of any transition metal hydride group. In addition to the presence of the hydride ligand, the complexes invariably have a coordinated phosphine, and synthetic routes to these compounds using both hydridic and protonic reagents have been reported (I). The pure complexes are usually both air stable and kinetically inert. The purpose of this chapter is to show the diversity of hydrides that can be obtained from protonation reactions on zero-valent and di-valent triphenylphosphine platinum compounds, and to rationalize the type and nature of the product formed from the character of the acid HX. 

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