Theoretical Study of Cp2Zr-, (MeO)2Zr-, and M(PH3)-Mediated Coupling Reactions of Acetylenes (M = Ni, Pt). Significant Differences between Early- and Late-Transition-Metal Complexes

Abstract

Theoretical calculations with DFT, MP2 to MP4(SDQ), and CCSD(T) methods clearly display that the Cp2Zr-mediated coupling reaction of two acetylene molecules easily takes place through a nonsymmetrical transition state with nearly no barrier and significantly large exothermicity but the M(PH3)-mediated reaction (M = Ni, Pt) takes place through a symmetrical transition state with either a considerably large activation barrier for M = Pt or a moderately large activation barrier for M = Ni. The charge-transfer (CT) interaction between the d orbital of the transition-metal center and the π*−π* bonding couple of two acetylene molecules plays an important role for the C−C bond formation in the M(PH3)-mediated coupling reaction, which needs a symmetrical transition state structure. On the other hand, a CT interaction between the dπ−π* back-donation orbital of Cp2Zr(C2H2) and the π* orbital of the second acetylene molecule is strongly formed in the Cp2Zr-mediated coupling reaction, which leads to a nonsymmetrical transition state structure. These differences are reasonably interpreted by the dπ−π* back-donation of Cp2Zr(C2H2) being much stronger than that of M(PH3)(C2H2). This is because the early-transition-metal element has d orbitals at higher energy than does the late-transition-metal element. The Ni(PH3)-mediated coupling reaction takes place with a smaller activation barrier than does the Pt(PH3)-mediated reaction. This result is interpreted by noting that the 3d transition-metal element has d orbitals at higher energy than does the 5d transition-metal element. Also, (MeO)2Zr-mediated coupling reaction proceeds with nearly no barrier, in spite of the fact that the d orbital is at lower energy in (MeO)2Zr(C2H2) than that in Cp2Zr(C2H2). This is because the MeO group is flexible and gives rise to little steric repulsion between ligands and substrates.