Supramolecular Enforcement of Planar Tetracoordinate Halides.

Planar tetracoordinate halides have been widely explored by quantum computational approaches but remain rare as deliberately targeted experimental motifs. Here we show that these geometries can be accessed by supramolecular enforcement, and challenge its connection to the VSEPR model. Bowl-shaped calix[4]pyrrolato indium units preorganize four lithium ions into a confined Li4 pocket that traps Cl-, Br-, and I- in strongly flattened to ideal planar tetracoordinate environments. Computational analysis of the parent tetrahedral [Li4X]3 + units reveals small planarization energies that can be overcome by a charge-attenuating host of appropriate shape. The experimental series shows that halide planarity is highly sensitive to subtle supramolecular effects, such as calix[4]pyrrolato doming and crystal packing, experimentally confirming the generally low angular deformation energies. The results establish supramolecular enforcement as a rational design principle for planar tetracoordinate heavier halides and show that such motifs are accessible outcomes of engineered electrostatic cavities.