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)

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Abstract

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 PtR₂(dmpe) [R = Me (1), p-Tol (2), Ph (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) [1: R¹ = R² = Me; 2: R¹ = R² = p-Tol; 3: R¹ = R² = Ph; 4: R¹ = Me, R² = p-Tol; dmpe = 1,2-bis(dimethylphosphino)ethane] to Pt(IV) complexes that are stable at low temperatures. Those complexes resulting from oxidation of 1 and 4 adopt configurations in which the triflate and η¹-alkynyl ligands are cis to each other, Pt(OTf)R¹R²(CCSiMe₃)(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(CCSiMe₃)(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(CCSiMe₃)(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 Me₃SiCCI 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.

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