Density Functional Study of the Mecahnism of C ident to C, O-H, and N-H Bond Activation at the Pd=X (X = Sn, Si, C) Bonds of the (PH_3)_2Pd=XH_2 Complexes

摘要

The mechanism of the activation of the C=C .7r bond of ethyne, the 0:.- H bond of water, and the N-H bond of ammonia at the Pd=X (X = Sn, Si, C) bonds of the model (PH3)2Pd= XH2 complexes 1 to produce the products (PH3)2Pd(CH=CH)XH2 '(Pd-X) 4, (PH3)2Pd(H)X- (OH)H2 7, (PH3)2Pd(H)X(NH2)H2 9, respectively, has been theoretically investigated using a density functional method (B3I.. YP). The reaction mechanism significantly depends on both substrate and atom X of the' complex I. In the activation of the C=C .7r bond of ethyne at the Pd=Sn bond, a weak rJ2-interaction orthe C=C .7r bond not with Pd but with Sn initiates the reaction. The rotation of the (PH3)2Pd group around the Pd-Sn axis makes the reaction facile so that a small energy barrier of only 6.2kcal/mol is required. Although the reaction at the Pd=Sibond also takes the same mechanism) the reaction is downhill without energy barrier. On the other hand, when X is C, the reaction proceeds by a completely different mechanism without the coordination of the C=C .7r bond to the C atom. In the activation of the 0- H bond of water at the Pd=Sn bond, after the coordination of the H2O oxygen to the Snatom, the O-H bond is heterolytically broken and the hydrogen is transferred to the Pd atom as a proton with an energy barrier of 16.9 kcal/mol. The same heterolytic cleavage takes place at the Pd=Si bond, requiring a smaner energy barrier, while at the Pd=C bond the homolytic cleavage takes place without the coordination of the H2O oxygen to the C atom. In the case of NH3, independent orthe atom X, heterolytic cleavage of the N- H bond occurs after coordination of the NH3 nitrogen to the X atom and the transfer ora hydrogen as a proton to the Pdatom. The large energy barrier of 33.2 kcal/mol required in the reaction system with X =Sn for which no activation activity has been experimentally observed was reduced to half in the reaction system with X= C, suggesting an activation activity. The factors that determine the activation mechanism are discussed.