Density Functional Study of the π Bond Activation at the PdSn Bond of the (H2PC2H4PH2)PdSnH2 Complex. Why Do the (H2PC2H4PH2)Pd and SnH2 Counterparts Mutually Rotate?

Abstract

The activation mechanism of the π bonds, the nonpolar C⋮C of C2H2 and the polar CO of HCHO and the C⋮N of HCN, at the PdSn bond of the model complex (H2PC2H4PH2)PdSnH2 is theoretically examined using a density functional method (B3LYP). For the nonpolar ethyne C⋮C π bond, the reaction proceeds by the homolytic mechanism supported by the rotation of the (H2PC2H4PH2)Pd group around the Pd−Sn axis. The charge transfers, the electron donation from the ethyne π orbital to the Sn p orbital, and the back-donation from Pd dπ orbital to the ethyne π* orbital successfully occur step by step to complete the π bond breaking during the reaction. Without the rotation of the (H2PC2H4PH2)Pd group, the first charge transfer from the ethyne π orbital to the Sn p orbital is too weak to break the π bond. On the other hand, for the strongly polarized formaldehyde CO π bond, the donation of lone pair electron on the CO oxygen to the Sn p orbital is so strong that both the nucleophilicity of the Pd atom and the electrophilicity of CO carbon are significantly enhanced through the Pd(dπ)−Sn(pπ) orbital, and the CO π bond is broken by the heterolytic mechanism with the electrophilic attack of the CO carbon to the Pd atom. In this mechanism, the rotation of the (H2PC2H4PH2)Pd group is not necessary. However, when the CO π bond approaches the Sn atom in a η2-fashion, the reaction proceeds by the homolytic mechanism with the rotation of the (H2PC2H4PH2)Pd group, which is similar to the case of ethyne. In the case of hydrogen cyanide, where the C⋮N π bond has to approach the Sn atom in a η2-fashion to break its π bond, only the homolytic pathway with the rotation of the (H2PC2H4PH2)Pd group exists. The first charge transfer from the substrate to the Sn p orbital plays a key role in determining the mechanism, and it is strengthened enough to break the π bond by the rotation of the (H2PC2H4PH2)Pd group in the homolytic mechanism. The potential energy surface of the activation reaction was quite smooth with a small energy barrier even in the homolytic mechanism with the rotation of the (H2PC2H4PH2)Pd group due to the successive stepwise process. The ligand effects on the activation activity are also discussed.