CO INSERTION IN 4-COORDINATE CIS-METHYL(CARBONYL)PLATINUM-DIPHOSPHINECOMPOUNDS - AN IONIC MECHANISM FOR PLATINUM-DIPHOSPHINE-CATALYZED HYDROFORMYLATION

The reactions of the square-planar methylplatinum-cis-diphosphine comp lexes, Pt(Me)(Cl){(S,S)-BDPP}, 1, and Pt(Me)(SnCl3){(S,S)-BDPP}, 2, (B DPP = (2S,4S)-2,4-bis(diphenylphosphino)pentane) with CO, and a mixtur e of CO/H-2 = 1/1 have been studied in CD2Cl2 solutions by variable te mperature high-pressure NMR spectroscopy to establish the role of Pt-S nCl3 bond in the CO insertion and hydrogenolysis steps of the Pt-dipho sphine-catalyzed olefin hydroformylation reaction. At low temperatures (193-213 K), the formation of the unprecedented four-coordinate ionic cis-methyl(carbonyl)platinum-diphosphine compound, [Pt(Me)(CO){(S,S)- BDPP}](+)X(-), where X = Cl, 3a, and X = SnCl3, 3b, has been observed whether CO is applied neat or in a mixture with hydrogen. The formatio n of compounds 3a and 3b involve equilibrium reactions which are facil itated by increasing the CO pressure; however, while the formation of 3b is a fast (magnitudes faster than that of 3a at identical condition s) and quantitative reaction, the formation of compound 3a remains a s low reaction that does not reach completion in the range of 1-50 bar o f CO pressure. CO insertion in compounds 3a and 3b takes place only at temperatures close to ambient, yielding covalent acetyl compound Pt(C OMe)(Cl){(S,S)-BDPP}, 4, and ionic acetyl compound [Pt(COMe)(CO){(S,S) -BDPP}](SnCl3-)-Sn-+, 5, respectively. At room temperature, 5 but not 4 reacts further in the presence of hydrogen, resulting in the formati on of acetaldehyde and an unidendified Pt-diphosphine compound. Thus, it appears that the hydroformylation by Pt-BDPP complexes proceeds via an ionic reaction mechanism.