Cationic Aluminium Complexes as Catalysts for Imine Hydrogenation

Area of Science:

• Organometallic Chemistry
• Catalysis
• Lewis Acid Chemistry

Background:

• Cationic Lewis acids are crucial catalysts but often require stabilizing ligands to control reactivity.
• Frustrated Lewis Pairs (FLPs) offer unique reactivity by preventing Lewis acid-base adduct formation.
• Developing efficient catalysts for imine hydrogenation remains an important goal in synthetic chemistry.

Purpose of the Study:

• To synthesize and characterize novel cationic aluminum complexes with varying β-diketiminate (BDI) ligands.
• To investigate the catalytic activity of these complexes in imine hydrogenation and other reactions.
• To elucidate the mechanism of catalysis using DFT calculations.

Main Methods:

• Synthesis of cationic aluminum complexes with BDI ligands and B(C6F5)4- counterions.
• Evaluation of Lewis acidity using the Gutmann-Beckett test.
• Catalytic testing in imine hydrogenation and the Tishchenko reaction.
• Computational analysis using Density Functional Theory (DFT).

Main Results:

• A series of cationic aluminum complexes with tunable steric and electronic properties were prepared.
• The Gutmann-Beckett acceptor numbers indicated varying Lewis acidities, influenced by BDI substituents.
• The (tBu,DIPP)AlMe+ complex demonstrated optimal catalytic activity in imine hydrogenation and Tishchenko reactions.
• DFT calculations revealed dual catalytic cycles involving Al-imine and Al-amine interactions, with the latter being energetically favored.

Conclusions:

• Steric and electronic tuning of BDI ligands is critical for achieving high catalytic activity in cationic aluminum complexes.
• The (tBu,DIPP)AlMe+ complex represents a highly effective catalyst for imine hydrogenation and the Tishchenko reaction.
• The catalytic mechanism involves a synergistic interplay between Lewis acid-stabilized imine/amine activation and autocatalytic pathways.