Contrasting the Mechanism of H 2 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 commonly found stabilized with potassium cations from the reaction of Al-halogen precursors and alkali compounds. These systems are very reactive toward the activation of σ-bonds and in reactions with electrophiles. Various research groups have detected that the potassium atoms play a stabilization role via electrostatic and cation ⋯ π interactions with nearby (aromatic)-carbocyclic rings from both the ligand and from the reaction with unsaturated substrates. Since stabilizing K⋯H bonds are witnessed in the activation of this class of molecules, we aim to unveil the role of these metals in the activation of the smaller and less polarizable H ₂ molecule, together with a comprehensive characterization of the reaction mechanism. In this work, the activation of H ₂ utilizing a NON-xanthene-Al dimer, [K{Al(NON)}] ₂ (D) and monomeric, [Al(NON)] ^- (M) complexes are studied using density functional theory and high-level coupled-cluster theory to reveal the potential role of K ⁺ atoms during the activation of this gas. Furthermore, we aim to reveal whether D is more reactive than M (or vice versa), or if complicity between the two monomer units exits within the D complex toward the activation of H ₂ . The results suggest that activation energies using the dimeric and monomeric complexes were found to be very close (around 33 kcal mol ^-1 ). However, a partition of activation energies unveiled that the nature of the energy barriers for the monomeric and dimeric complexes are inherently different. The former is dominated by a more substantial distortion of the reactants (and increased interaction energies between them). Interestingly, during the oxidative addition, the distortion of the Al complex is minimal, while H ₂ distorts the most, usually over 0.77 Δ E d i s t ≠ . Overall, it is found here that electrostatic and induction energies between the complexes and H ₂ are the main stabilizing components up to the respective transition states. The results suggest that the K ⁺ atoms act as stabilizers of the dimeric structure, and their cooperative 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.
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