KINETIC-STUDY OF BETA-HYDRIDE ELIMINATION FROM MONOALKYL SOLVENTO COMPLEXES OF PLATINUM(II)

Electrophilic attack by a proton on cis-[Pt(PEt3)2(R)2] (R = C2H5, C2D5, n-C3H7, n-C4H9) complexes in protic solvents and subsequent fast isomerization of the cis-alkyl(solvento) intermediates produce the solvento species trans-[Pt(PEt3)2(R)(S)]+ (S = solvent). These latter undergo a facile thermal decomposition, yielding trans-[Pt(PEt3)2(H)(S)]+ and olefins under very mild conditions. The kinetics were followed by means of a variety of experimental techniques, including H-2 and P-31 NMR, GLC, and conventional spectrophotometry. Results in methanol at 30-degrees-C: R = C2H5, k = 0.19 X 10(-3) s-1, DELTAH(double dagger) = 115 +/- 3 kJ mol-1, DELTAS(double dagger) = +62 +/- 6 J K-1 mol-1; R = C2D5, k = 0.075 X 10(-3) s-1, DELTAH(double dagger) = 113 +/- 2 kJ mol-1, DELTAS(double dagger) = +49 +/- 6 J K-1 mol-1; R = n-C3H7, k = 45 X 10(-3) s-1, DELTAH(double dagger) = 95 +/- 2 kJ mol-1, DELTAS(double dagger) = +24 +/- 5 J K-1 mol-1; R = n-C4H9, k = 62 X 10(-3) s-1, DELTAH(double dagger) = 89 +/- 2 kJ mol-1, DELTAS(double dagger) = +5 +/- 6 J K-1 mol-1. Changes in the nature of the alcoholic solvents do not produce significant variations either in the rates or in the distribution of the olefin products. When chloride ion is added, the thermolysis is retarded because of the formation of the unreactive trans-[Pt(PEt3)2(R)Cl]. Values of the solvolytic equilibrium constants have been calculated from mass-law retardation plots. Other cationic species such as trans-[Pt(PEt3)2(R)(L)]+ (L = pyridine, amines, phosphines, etc.) were found to be unreactive. The mechanism is discussed within the framework of previous results on beta-hydride elimination of trans and cis monoalkyl complexes or olefin insertion into trans hydride complexes. A unified picture can be drawn which involves the easy loss of the solvent molecule and the formation of 3-coordinate 14-electron species as key intermediates.