Study of the Origin of the Hindered Rotation of an Aryl Ring in Chromium Aminocarbene Complexes Bearing an Aromatic Ring Attached to the Carbene Carbon Atom

Abstract

The substituted chromium aminocarbene complexes cis-tetracarbonyl[(η2-N-allyl-N-allylamino)(4-X-phenyl)carbene]chromium(0) (X = MeO, Me, H, Cl, CO2Me, CF3) were prepared by the reaction of the corresponding N,N-diallylamides with Na2Cr(CO)5 and Me3SiCl. Using dynamic NMR, the rotational barrier of the aryl ring in these complexes was established. The obtained values are higher for the complexes bearing electron acceptors compared to those bearing donor substituents and correspond with the Hammett equation. This indicates that the restricted rotation of aryl ring in the chromium aminocarbene complexes is a balance between steric and conjugation effects; while the former increases the barrier, the latter decreases it by stabilizing the transition state of the rotation. This finding is compatible with the fact that in the related cis-tetracarbonyl[(N-alkyl-η2-N-allylamino)(phenyl)carbene]chromium(0) complexes (alkyl = Me, Et, i-Pr) the rotational barrier grows steadily with the growing steric demand of the substituent. The introduction of the methyl group to the ortho position of the aryl ring resulted in an increase of the rotational barrier above 76 kJ mol-1 even for the N-Me derivative.