| Abstract |
The bonding geometry of pyrrolyl ligands (pyr ) in zirconium complexes was studied by means of DFT/B3LYP 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 pi coordination of pyr , with the bonding to the metal established by means of the ring pi orbitals. Slightly distorted eta(5) coordination results, with the nitrogen tending to approach the metal. The second corresponds to a sigma-pyr , with the nitrogen lone pair used to establish a sigma bond to the metal. The slippage from a pi to a sigma-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 pi-pyr . The sigma-pyr coordination and the pi-sigma 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 carbon atoms, and the slipping pyr ring coordinates in a flat eta(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 raclmeso isomerization of [Zr(pi-dmep)(2)(CH3)(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. |
