Bonding Geometry of Pyrrolyl in Zirconium Complexes: Fluxionality between σ and π Coordination

Abstract

The bonding geometry of pyrrolyl ligands (pyr') in zirconium complexes was studied by means of DFT/Β3LYP calculations with a DZVP basis set. Monopyrrolyl complexes, [Zr-(pyr')L 3 ], and bispyrrolyl species, [Zr(pyr') 2 L 2 ], were addressed, with different pyrrolyl ligands, pyr' = pyrrolyl (pyr), 2,5-dimethylpyrrolyl (dmp), 2,4-dimethyl-3-ethylpyrrolyl (dmep), and ancillary ligands, L = chloride, methyl. Two coordination modes of pyr' were considered. The first is the π coordination of pyr', with the bonding to the metal established by means of the ring π orbitals. Slightly distorted η 5 coordination results, with the nitrogen tending to approach the metal. The second corresponds to a σ-pyr', with the nitrogen lone pair used to establish a a bond to the metal. The slippage from a π to a δ-pyr' produces an electronically poorer metal center and results, normally, in a less stable complex, although this effect is partially compensated by a strengthening of the Zr-N in the a coordination mode, when compared with the one existing in a π-pyr'. The σ-pyr' coordination and the π-σ slippage were found to be disfavored by substituents in the a carbon atoms of pyr', confirming a known empirical rule. The interconversion between the two coordination modes of the pyr' ligand was studied in all the complexes and determined to correspond to a slippage process without any significant ring folding. In the corresponding transition states the Zr-C bond breaking process is completed for the two β carbon atoms, and the slipping pyr' ring coordinates in a flat η 3 mode. The activation energies calculated for the slippage process in the various species (<10 kcal mol - 1 ) suggest the possibility of fluxionality in solution, even at room temperature. The mechanism for the rac/meso isomerization of [Zr(π-dmep) 2 (CH 3 ) 2 ] was also studied and determined to proceed via consecutive slippage and rotation of the dmep ring, the rate-determining step corresponding to the first slippage of that ligand. The calculated activation energy (E a < 7 kcal mol - 1 ) corroborates the experimental observation of fluxionality in solution, at room temperature.