Theoretical investigation of bis(imido)chromium (VI) cations as polymerization catalysts

Direct insertion of ethylene into the chromium-carbon bond in singly charge d bis(imido)chromium(VI) cations has been investigated for n-alkyl and benz yl as starting polymer chains. Frontside coordination of ethylene takes pla ce without activation to give a stable complex. Subsequent insertion into t he Cr-alkyl bond requires a free energy of activation of 15 kcal/mol and an even higher barrier in the case of a benzyl ligand. Near-the transition st ate, the reaction coordinate is dominated by inversion of the metal complex . Ethylene coordination in the backside mode requires considerable activati on, and the additional increase in free energy through loss of entropy make s direct coordination unlikely. An indirect route to the beta -agostic back side pi -complex is found by inversion of the corresponding frontside: ethy lene-chromium complex. This rearrangement takes place via a transition stat e that has comparable or slightly lower free energy than that of direct ins ertion starting from the frontside pi -complex. However, once the backside ethylene-chromium complex is formed, subsequent insertion into the Cr-C sig ma -bond takes place with a low reaction barrier. Starting from frontside c oordination to the chromium complex, ethylene is found to add to a chromium -nitrogen bond in a [2+2] cycloaddition, to produce the corresponding azach romacyclic compound. The associated free energy of activation is low, at 3- 6 kcal/mol for the model systems investigated, and suggests that the bis(im ido)chromiumbenzyl species may be susceptible to chemical modification.