Intramolecular coupling of η2-iminoacyls on zirconium bis(aryloxides) and calix[4]arenes: Revised mechanism by DFT calculations and car-parrinello molecular dynamics simulations

The intramolecular coupling of the two iminoacyl units in the bis(eta(2)-iminoacyl) complexes [calix[4](OMe)(2)(O)(2)-Zr(eta(2)-MeCNBUt)(2)] and [(2,6-(BU2C6H3O)-C-t)(2)-Zr(eta(2)-MeCNPh)(2)] to generate the corresponding enediamido species has been investigated by means of both static and dynamic density functional calculations. For both systems we have characterized the stationary points of the potential energy surfaces for the coupling reaction and evaluated the overall energy profile. Car-Parrinello molecular dynamics simulations have been performed to gain insight into the detailed mechanistic features of the iminoacyl coupling and surprisingly show that this reaction occurs through an asynchronous reaction mechanism in which a decoordinated iminoacyl attacks the residual coordinated iminoacyl C-N double bond. A transition state search, performed without any symmetry constraint, led to structures consistent with the proposed mechanism 23.8 and 21.2 kcal mol(-1) above the starting his(eta(2)-iminoacyl) reagents, in good agreement with experimental activation enthalpies. On the basis of the proposed reaction mechanism we are also able to reproduce the activation energy of the coupling reaction with electron-withdrawing substituents on the iminoacyl moiety. Inspection of the electronic structure changes along the proposed reaction pathway suggests that the iminoacyl coupling process can be described as an intramolecular attack of the decoordinated iminoacyl carbon lone-pair at the coordinated iminoacyl pi* orbital, thus reconciling the iminoacyl coupling reaction mechanism with the generally accepted pattern of acyl and iminoacyl reactivity.