Synthesis and Reactivity of (η1-Alkynyl)diorganoplatinum(IV)Species, Including Structural Studies of PtIMe(p-Tol)(CCSiMe3)(dmpe) [dmpe = 1,2-bis(dimethylphosphino)ethane] and the Platinum(II) Reagent PtPh2(dmpe)

Synthetic routes to diaryl(alkynyl)platinum(IV) and methyl(aryl)(alkynyl)platinum(IV) motifs are presented, together with studies of selectivity in carbon-carbon coupling by reductive elimination from platinum(IV) centers, and convenient synthetic routes to the platinum(II) reagents PtMe2(dmpe) (1), Pt(p-Tol)2(dmpe) (2), PtPh2(dmpe) (3), (dmpe = 1,2-bis(dimethylphosphino)ethane], PtMe(p-Tol)(dmpe) (4), and PtMe(p-Tol)(COD) (COD = 1,5-cyclooctadiene). The hypervalent iodine(III) reagent IPh(CCSiMe3)(OTf) (OTf- = CF3SO3-) oxidizes the Pt(II) complexes PtR1R2(dmpe) to Pt(IV) complexes that are stable at low temperatures. Those complexes adopt configurations in which the triflate and η1-alkynyl ligands are cis to each other, Pt(OTf)R1R2(CtCSiMe3)(dmpe) (1a, 4a), with the alkynyl group trans to phosphorus, and form equilibria with triflate-free complexes (1c, 4c), assumed to be solvento cations. The mixtures of dimethylplatinum(IV) species (1a and 1c) and of methyl(p-tolyl)platinum(IV) species (4a and 4c) react with sodium iodide, acetonitrile, and pyridine to form the cis-configured isomers of the complexes PtIR1R2(CCSiMe3)(dmpe) (1b, 4b) and [PtR1R2(CCSiMe3)(dmpe)(L)](OTf) (L ) NCMe: 1d, 4d; L ) py: 1e, 4e). The diarylplatinum(II) complexes 2 and 3 are oxidized primarily to species with the triflate and alkynyl groups trans to each other, Pt(OTf)Ar2(CtCSiMe3)(dmpe) (2a, 3a). The trans-isomers of diaryl(triflato)platinum(IV) complexes do not take part in ligand substitution reactions with sodium iodide, acetonitrile, or pyridine, but minor Pt(IV) complexes in equilibrium with 2a and 3a do react with iodide to form PtIAr2(CtCSiMe3)(dmpe) (2b, 3b). Structural studies of PtPh2(dmpe) (3) and PtIMe(p-Tol)(CCSiMe3)(dmpe) (4b) reveal square-planar and octahedral geometries, respectively. On warming above -10 °C in acetone, the Pt(IV) triflate stages of all of the systems decompose by reductive elimination via methyl-methyl, methyl-aryl, or aryl-aryl coupling. Similarly, decomposition of PtIMe2- (CCSiMe3)(dmpe) (1b) under mild conditions in acetone results in Me-Me coupling. Decomposition of the isolated iodo complexes (2b, 3b, 4b) at temperatures above 50 °C reveals involvement of the alkynyl group in reductive elimination processes, giving minor quantities of Me3SiCtCI and ~1:1 ratios for the dominant products Me3SiCC-Ar and Ar-Ar (2b, 3b), and Me3SiCC-(p-Tol) and Me-(p-Tol) for 4b.