Activation of Molecular Hydrogen by Inter‐ and Intramolecular Al−N Lewis Pairs

Abstract

The field of frustrated Lewis pair chemistry offers many opportunities to activate molecular hydrogen, but AlÀ N systems have not been established yet. In this work, we describe several intermolecular classical AlÀ N Lewis pairs and an intramolecular ortho-ala-aminoarene for the activation of molecular hydrogen. Their ability was investigated using the isotope exchange reaction from HD to H 2 and D 2 . The herein studied intermolecular Lewis pairs were based on alkylalanes and N-methyldiphenylamine, while the intramolecular Lewis pair was (o-TMPÀ C 6 H 4 )AlH 2 ((2-(2,2,6,6-tetramethyl-piperidin-1-yl)phenyl)aluminium dihydride). The activation of molecular hydrogen was carried out in toluene under mild conditions and monitored by 1 H and 2 H NMR spectroscopy. Furthermore, the AlÀ N interaction has been probed by 27 Al NMR and crystallographic studies. Additionally, the crystal structure of pure Al i Bu 3 was determined. These studies may attribute the pronounced reactivities of these AlÀ N compounds to elongated AlÀ N bond lengths.

Amino−Organolithium Compounds and their Aggregation for the Synthesis of Amino−Organoaluminium Compounds

“…The organolithium compounds were synthesised by lithiumÀ halogen-exchange reactions at low temperature or by directed ortho-metallation with n-butyllithium based on literature procedures, [34,43] Scheme 2.…”

“…Similar AlÀ N compounds recently garnered particular interest for their potential to activate molecular hydrogen as Lewis pairs with typical FLP reactivity. [34] Subsequently, the above-mentioned organolithium compounds (mostly etherates) were reacted with dialkylaluminiumchlorides R 2 AlCl or freshly prepared AlH 3 , Scheme 3. First, Me 2 NC 6 H 4 AlMe 2 (9) was synthesised and crystallised from hexane/Et 2 O.…”

“…To investigate the influence of the steric demand of the amino group, the compounds (o-TMPÀ C 6 H 4 )AlR 2 (R = H, Me, Et, Np) (15-18) were synthesised. The synthesis of 15 has been reported previously, [34] while the others were synthesised analogously to above-mentioned compounds, see experimental section.…”

“…Also, we recently reported on the hydrogenation of metallic aluminium in the presence of piperidine [32] or TEDA (mechanochemical, TEDA = triethylenediamine) [33] and H 2 activation by inter-and intramolecular AlÀ N Lewis pairs. [34] Among the plenty of FLP (Frustrated Lewis Pair) literature, [35] the AlÀ N systems garnered far less attention. Nevertheless, there are noteworthy reports, e. g., by Uhl and co-workers, [36] and selected examples of reactions with chalcogen-containing double bonds (CO 2 , [37] ketones, [38] isocyanates [39] ), dehydrocoupling, [40] acryl and lac-tone polymerisation are among the few that have been reported.…”

“…These organolithium compounds were, for demonstration purposes, used to synthesise intramolecular AlÀ N Lewis pairs and provide alkyl derivatives of the known (o-TMPÀ C 6 H 4 )AlH 2 , which has been used for hydrogen [34] and alkyne [42] activation.…”