Mechanistic studies on the formation of Pt(II) hydroformylation catalysts in imidazolium-based ionic liquids

The kinetics of the formation of the active species cis-[Pt-II(PPh3)(2)Cl(SnCl3)] and cis-[Pt-II(PPh3)(2)(SnCl3)(2)] from the hydroformylation catalyst precursor cis-[Pt-II(PPh3)(2)Cl-2] in the presence of SnCl2, was studied in two different imidazolium-based ionic liquids. A large range of different chlorostannate melts consisting of 1-butyl-3-methyl-imidazolium cations and [SnxCly]((-y + 2x)) anions with varying molar fraction of SnCl2, were prepared and characterized by H-1 and Sn-119 NMR. The observed chemical shifts point to major changes in the composition of the anionic species within the melt. The second ionic liquid employed, viz., 1-butyl-3-methyl-imidazolium-bis(trifluormethylsulfonyl)amide was prepared in a colorless quality that enabled its application in kinetic studies. The concentration and temperature dependence of the substitution of Cl- by [SnCl3](-) to yield cis-(Pt-II(PPh3)(2)Cl(SnCl3)], could be studied in detail. Theoretical (DFT) calculations were employed to model the reaction progress and to resolve the role of the ionic liquid in the activation of the catalyst. The available results are presented and a plausible mechanism for the formation of the catalytically active species is suggested. (c) 2005 Elsevier B.V. All rights reserved.