Reduction vs. Addition: The Reaction of an Aluminyl Anion with 1,3,5,7‐Cyclooctatetraene

Uneasy lies the head that wears a crown—The initial product from the reaction of a two‐coordinate aluminyl anion with 1,3,5,7‐cyclooctatetraene (COT) contains the reduced COT‐ligand with pronounced aromatic character. Addition of 18‐crown‐6 to K[Al(NON^Ar)(COT)] promotes isomerization of the carbocycle to afford the 9‐aluminabicyclo[4.2.1]nona‐2,4,7‐triene anion.

Abstract

The potassium aluminyl complex K[Al(NON^Ar)] (NON=NON^Ar=[O(SiMe₂NAr)₂]²⁻, Ar=2,6‐iPr₂C₆H₃) reacts with 1,3,5,7‐cyclooctatetraene (COT) to give K[Al(NON^Ar)(COT)]. The COT‐ligand is present in the asymmetric unit as a planar μ₂‐η²:η⁸‐bridge between Al and K, with additional K⋅⋅⋅π‐aryl interactions to neighboring molecules that generate a helical chain. DFT calculations indicate significant aromatic character, consistent with reduction to [COT]²⁻. Addition of 18‐crown‐6 causes a rearrangement of the C₈‐carbocycle to form the isomeric 9‐aluminabicyclo[4.2.1]nona‐2,4,7‐triene anion.

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