Contrasting the Mechanism of H2 Activation by Monomeric and Potassium-Stabilized Dimeric Al(I) Complexes: Do Potassium Atoms Exert any Cooperative Effect?

Aluminyl anions are low valent, anionic and carbenoid aluminum species. Various research groups have detected that the potassium atoms play a stabilization role via electrostatic and cation … π interactions with nearby (aromatic)-carbocyclic rings. In this work, the activation of H 2 utilizing a NON-xanthene-Al dimer ( D ) and monomeric ( M ) catalysts are studied to reveal the potential role of K +  atoms during the activation of this gas.  We aim to reveal whether if D is more reactive than M (or vice versa), or if complicity between the two monomer units exits within the D catalyst . The results suggest that activation energies using the dimeric and monomeric catalysts were found to be very close (around 33 kcal mol -1 ). However, a partition of activation energies unveiled that the natures of the energy barriers are inherently different, the former being dominated by a more substantial distortion of the reactants. Interestingly, during the oxidative addition, the distortion of the Al complex is minimal, while H 2 distorts the most, usually over 0.77ΔE dist . Overall, it is found here that electrostatic and induction energies are the main stabilizing components of the interaction energy. The results suggest that the K + atoms act as stabilizers of the dimeric structure, and their catalytic role on the reaction mechanism may be negligible, acting as mere spectators in the activation of H 2 . Cooperation between the two monomers in D is lacking, and therefore the subsequent activation of H 2 is wholly disengaged.