Aluminum squared

Abstract

Three-coordinate aluminum compounds are potent Lewis acids: They readily attract electron donors, binding them to form a saturated tetrahedral product. Ebner et al. now report that four-coordinate aluminum can act as a Lewis acid, too, so long as it's forced ahead of time into a square planar geometry. To attain this unusual bonding motif, they embedded an aluminum(III) ion within a macrocycle of four negatively charged pyrrole donors. In addition to binding tetrahydrofuran, this aluminum center accepted hydride and fluoride ligands.