A quantum chemical study of the molecular structure of active centers and growth in ethylene polymerization in the catalytic system LFeCl2/AlMe3 (L=2,6-bis-iminopyridyl)

摘要

Density functional theory with hybrid exchange-correlation functional B3P86 is used to calculate the molecular structures of neutral Fe(II) complexes formed in the LFeCl2/AlMe3 system (L = tridentate bis(imine)pyridyl ligand). A simplified model of the LFeCl2 complex is used in calculations, where L is replaced by three NH3 ligands. Parameters of geometric and electronic structures of the complexes (NH3)(3)FeMe(mu-Me)AlMe3 (I) and (NH3)(3)FeMe(mu-Me)(2)AlMe2 (IIA and IIB), which are the structures where the Fe-Me and Fe-g-Me groups are in one or two perpendicular planes, respectively, were determined. Complexes II, which were earlier identified using H-1 NMR spectroscopy, are more stable than complex I. Complex IIB is strongly polarized (the distances r(Fe-mu-Me) and r(Al-mu-Me) are 3.70 and 2.08 Angstrom, respectively) and coordinatively unsaturated due to the transfer of the methyl group from (NH3)(3)FeMe2 onto AlMe3. It has significant electron density deficit in the coordination sphere of the transition metal [(NH3)(3)FeMe](Q) (Q = +0.80e). The energetic profile of the reaction of ethylene addition to the Fe-Me bond for the complexes (NH3)(3)FeMe2, IIA and IIB, was calculated. It was shown that, compared to (NH3)(3)FeMe2, a drastic decrease in the activation energy of ethylene addition is observed in the case of IIB (from 135 to 66 kJ/mol). The reason for the more efficient activation of the complexes LFeMe2 by a weak Lewis acid (AlMe3) and for the increased reactivity of the metal-alkyl bond in complex IIB compared to the zirconocene complex CP2ZrMe2 is discussed.