Stereodynamism in Pentacoordinate Al(Salen) Catalysts

Area of Science:

• Organometallic Chemistry
• Asymmetric Catalysis

Background:

• M(Salen) complexes are vital in asymmetric catalysis, offering broad applicability and high selectivity.
• Developing structural models for M(Salen) complex function is essential for continued innovation in catalysis.

Purpose of the Study:

• To investigate the solution-phase behavior of Aluminum(Salen) complexes using a three-dimensional structural approach.
• To complement experimental findings with solid-state 27Al Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) and computational studies.

Main Methods:

• Three-dimensional structural investigation of Al(Salen) complexes in solution.
• Solid-state 27Al MAS NMR spectroscopy.
• Computational investigations.

Main Results:

• Al(Salen)Cl complexes rapidly equilibrate between R,R-(M) and R,R-(P) species in solution via halide-catalyzed rearrangements.
• The backbone maintains a trans-diequatorial conformation, with axial configurations determined by shifts between square based pyramidal (sbp) and distorted trigonal bipyramidal (dtbp) coordination geometries.
• Orthogonal helical diastereomers with conserved point chirality were identified, leading to a revised model for pentacoordinate M(Salen) complexes using the Addison geometry parameter τ.

Conclusions:

• Dynamic metal coordination geometry and stereodynamism are key factors in pentacoordinate M(Salen) complexes.
• These findings necessitate a re-evaluation of current models for asymmetric catalysis induction.
• The observed behavior of Al(Salen) complexes holds potential for applications beyond asymmetric catalysis.