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

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‘)L3], and bispyrrolyl species, [Zr(pyr‘)2L2], 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 η5 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 σ bond to the metal. The slippage from a π to a σ-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 σ 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 α 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 η3 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(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 (Ea < 7 kcal mol-1) corroborates the experimental observation
of fluxionality in solution, at room temperature.