Calix[4]pyrrole Aluminate: A Planar Tetracoordinate Aluminum(III) Anion and Its Unusual Lewis Acidity

Tetracoordinate aluminum­(III)
anions are generally tetrahedral
and non-Lewis acidic. Herein, we describe the first planar tetracoordinate
aluminum anion. The anion is isolated in its free form, representing
a new “anti-van’t Hoff/Le Bel” case. Despite
its negative charge, it remains Lewis acidic and permits the formation
of various unique adducts, e.g., a dianionic p-block metal hydride.
Turning an inert spectator anion into a Lewis acid by planarization
verifies theoretical predictions made 40 years ago, ultimately connects
“anti-van’t Hoff/Le Bel” structure with reactivity,
and pushes further the field of geometrically enhanced p-block element
reactivity.