Theoretical Study of the Structure, Bonding Nature, and Reductive Elimination Reaction of Pd(XH₃)(<i>η</i>³-C₃H₅)(PH₃) (X = C, Si, Ge, Sn). Hypervalent Behavior of Group 14 Elements

Abstract

The structure and bonding nature of Pd(XH3)(η3-C3H5)(PH3) (R-X; X = C, Si, Ge, Sn) and its C−X reductive elimination were investigated with MP2-MP4(SDQ) and CCSD(T) methods. The C−C reductive elimination is considerably exothermic (27.7 kcal/mol) and needs a significantly large activation energy (23.0 kcal/mol), where CCSD(T) values are given hereafter. This considerably large exothermicity can be easily interpreted in terms of the strong C−C bond and the weak Pd−CH3 bond. On the other hand, the C−Si, C−Ge, and C−Sn reductive eliminations easily occur with a moderate activation barrier (12−13 kcal/mol) and a moderate reaction energy; the exothermicities are 6.0 and 1.6 kcal/mol for the C−Si and C−Ge reductive eliminations, respectively, and the endothermicity of the C−Sn reductive elimination is 6.0 kcal/mol. These moderate reaction energies of C−Si, C−Ge, and C−Sn reductive eliminations are interpreted in terms of the decreasing orders of bond energy E(C−C) > E(C−Si) > E(C−Ge) > E(C−Sn) and E(Pd−SiH3) > E(Pd−GeH3) > E(Pd−SnH3) ≫ E(Pd−CH3). The moderate activation barriers of C−Si, C−Ge, and C−Sn reductive eliminations are reflected in their transition state structures, in which SiH3, GeH3, and SnH3 groups can interact with the allyl carbon atom, keeping the Pd−SiH3, Pd−GeH3, and Pd−SnH3 bonds intact. These features result from the hypervalency of these elements. In the C−C reductive elimination, the Pd−CH3 bond considerably weakens but the allyl−CH3 bond is not completely formed at the TS, which is consistent with no hypervalency of the C atom. The η1-allyl form, Pd(XH3)(η1-C3H5)(PH3), is much less stable than R-X by 7−8 kcal/mol. Intrinsic reaction coordinate calculations clearly show that the C−C reductive elimination occurs not through the η1-allyl form but directly from Pd(CH3)(η3-C3H5)(PH3) if PH3 does not exist in excess. If excess PH3 exists in the reaction medium, the C−X reductive elimination via Pd(XH3)(η1-C3H5)(PH3)2 is not excluded. The (η3-C3H5)−XH3 (X = C, Sn) reductive elimination requires a larger activation energy than the CH3−XH3 reductive elimination, because the Pd−(η3-C3H5) bond is stronger than the Pd−CH3 bond.