Bonding and fluxionality in group-4 metal complexes with pyrrolyl ligands

摘要

The bonding of pyrrolyl ligands(pyr')in group-4 metal complexes,[M(pyr')Cl_3],is studied by means of DFT/B3LYP calculations with a VDZP basis set.Two metals(M=Ti,Hf)and two pyrrolyl ligands are addressed,pyr'=pyrrolyl(pyr),and 2,5-dimethylpyrrolyl(dmp).The study allowed the characterization of the pi-pyr'complexes with a eta~5 coordination of this ligand,the a complexes with the pyr'bonding established by the nitrogen lone pair,and the transition states for the interconver-sion between the two isomers.A comparative analysis of the pyr'bonding to the metal in all the species is provided,as well as a detailed study of the mechanism for the interconversion between the two pyr'coordination modes.This is a slippage process without significant folding of the pyr'ring and the activation energies obtained(E_a < 9 kcal mol~(-1))suggest the possible fluxionality between the two coordination modes,in solution.Calculated pyr'-M bond enthalpies show an increase in the bond strength going down the group,allowing a rationalization of the differences in stability of the two coordination modes observed for the various metals and,consequently,of the associated ring slippage process.