Stable Lewis acid chelate of a bis(imido) tungsten compound and implications for alpha-olefin dimerisation catalysis: a DFT study.

Dimerisation of alpha-olefins by a catalyst system comprising of a bis(imido) tungsten compound and a Lewis acid component has been investigated by density functional theory employing a catalyst model that closely mimics the real system together with ethylene chosen as a prototypical substrate. This study disclosed that the Lewis acid preferably forms four-membered chelates by bridging across tungsten-imido linkages. Complexation at one imido group is predicted as the favourable, stable mode of Lewis acid association onto a bis(imido) tungsten(VI) compound. The computational analysis revealed that the cocatalyst affects the energetics of the several steps to a different extent. Formation of the five-membered tungstanacycle and the further growth is moderately influenced, this is in contrast to its profound influence on metallacycle degradation. AlClMe2 chelation of both tungsten-imido groups is seen as being most effective in smoothing the energetics of the dimer-generating route. The predicted energy profile of the most accessible pathway, however, is at odds with observed catalytic abilities. This study indicates that the removal of one imido ligand by the Lewis acid is a viable process, thereby suggesting that mono(imido) tungsten compounds are effective in dimerisation catalysis of the reported catalyst system.