Electronic spectra parameters as a tool for studying the structure of the catalytic center and the mechanism of activation of d 0-transition metal complexes in olefin polymerization

Abstract

The quantum-chemical model formerly designed is described, which explains the high reactivity of the d0-transition metal-carbon σ-bond in concerted reactions including polymerization of olefins. In this model, the energy of transition from the ground singlet state to the excited singlet or triplet state corresponding to the transfer of electron density from the metal-carbon bond to vacant d-atomic orbitals correlates with its relative elongation after which a change in the valence state of a metal begins. This change is caused by the difference in the geometries of valence s-, p-, and d-atomic orbitals having close energies; as a result, at a certain bond elongation, partial uncoupling of electrons involved in bonding takes place so that one of them becomes localized on d-atomic orbital. This process facilitates formation of the reactive state of the bond of the biradical type and leads to a reduction in the energy barrier of the insertion of an olefin molecule into this bond. Lower energies of this barrier correspond to lower values of ΔE(S0 → Sj) and ΔE(S0 → Tj). As shown at the example of zirconocenes Cp2ZrMe2 and Me2CCp2ZrMe2, alongside with a reduction in the energy barrier of olefin insertion into the Zr-Me bond of the cationic complex being formed, the characteristic absorption band in the spectrum of the corresponding neutral derivative shifts to the long-wave region. This band is attributed to the transfer of the electron density from the Zr-Me bond to the Zr atom. Analysis is performed of the causes for bathochromic shifts of the long-wave absorption band for adduct formation in the systems including metallocene and Al-containing cocatalyst.