STOICHIOMETRIC MODEL REACTIONS IN OLEFIN HYDROFORMYLATION BY PLATINUM TIN SYSTEMS

The three independent steps involved in the hydroformylation process, insertion of the olefin, insertion of carbon monoxide, and hydrogenolysis, have been investigated with use of platinum-tin catalysts and 1-pentene as olefin at low pressure and temperature in CH2Cl2. In the temperature range 198-308 K, the three reactions can be studied consecutively. All the intermediates observed were prepared and characterized separately. The complex trans-[PtH(SnCl3)(PPh3)2] was used as the initial compound for this sequence. The hydrido complex crystallized in the monoclinic space group C2/c, with a = 31.345 (5) angstrom, b = 12.716 (3) angstrom, c = 18.135 (3) angstrom, beta = 96.5 (2)-degrees, Z = 8, and R (R(w)) = 0.056 (0.060) for 3235 independent reflections having I > 2.56-sigma(I). The large Pt-Sn bond (2.601 (1) angstrom) distance correlates satisfactorily with the low 1J(Pt-Sn) value. The Pt-Sn bond is necessary for the insertion of 1-pentene in the hydrido-platinum complex and for the hydrogenolysis of the acyl compounds under these mild conditions. The insertion of 1-pentene was observed at 198 K, giving the cis-alkyl complex; CO insertion took place after isomerization to the trans-alkyl complex. The instability of Pt-Sn and Pt-C bonds in the trans-acyl complex favors easy decarbonylation or loss of SnCl2, so any other platinum complex without tin accepts SnCl2 from the acyl complex. The hydrogenolysis of trans-[Pt(SnCl3)(COC5H11)(PPh3)2] under 1.5 bar of H2-CO (1:1) did not yield n-hexanal quantitatively; only 12% of n-hexanal was formed. Thus, decarbonylation was the main process observed. From the reactions studied, it is possible to propose the following order of Pt-Sn bond stability: trans-[Pt(SnCl3)(COC5H11)(PPh3)2] < [Pt(SnCl3)(C5H11)(PPh3)2] < trans-[PtH-(SnCl3)(PPh3)2] < [PtH(SnCl3)(CO)(PPh3)2] < [Pt(mu-Cl)(SnCl3)(PPh3)]2 < [PtCl(SnCl3)(PPh3)]- < [PtCl2(SnCl3)2]2-. The insertion reactions studied with cis-[PtCl2(olefin)(PR3)] as an olefin carrier and the hydrido-platinum complexes trans-[PtHCl(PPh3)2], trans-[PtH(SnCl3)(PPh3)2], and [PtH(SnCl3)-(CO)(PPh3)2] as hydrogen carriers exclude the participation of intermolecular steps by reaction of two different platinum complexes under the experimental conditions described.