Contrasting the Mechanism of H₂Activation by Monomeric and Potassium‐Stabilized Dimeric Al^IComplexes: Do Potassium Atoms Exert any Cooperative Effect?

Abstract

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� � � p 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)] À (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 À 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 DE 6 ¼ dist . 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.

Hydrocarbon Soluble Alkali‐Metal‐Aluminium Hydride Surrog[ATES]

“…It has been well documented that attractive London Dispersion Forces (LDF) influence the stability, structure, and reactivity of organometallic compounds [28a,b] . Lately, these non‐covalent interactions have been found to be a factor towards the stabilization of theoretically calculated intermediates for bimetallic potassium‐aluminyl compounds, [29a,b] exhibiting interesting alkali‐metal‐mediated reactivity [30a,b] . A recent study by Power and co‐workers demonstrated the significance of LDF in the incomplete reduction of 1‐adamantol with alkali metals (Li, Na, and K) [31] .…”

Alkali metal influences in aluminyl complexes

“…9 In contrast, state-of-the-art calculations on the reaction of I with H 2 did not reveal any assisting role of the K + ion. 10 Although the first series of aluminyl anions have been made exclusively with K + counter cations, it is meanwhile clear that these aluminyls are also stable as free anions 11 or as contact ion-pairs with other alkali metal cations like Li + or Na + . 11 b ,12 Recently, complex II has been prepared with the full series of alkali metal counter cations Li–Cs.…”

Dynamic ion-pairing behaviour of “naked” aluminyls: Facile activation of H2

“…[2,8] However, the activation of H2 by monomeric aluminyls involving charge-separated species has not been explored experimentally yet. [19] To address this query and better understand the reactivity of the distinct monomeric aggregates of the metal anion 2, we have monitored their reactivity with H2 gas by in-situ 1 H NMR spectroscopy.…”