Phosphangulene: A Molecule for All Chemists

Phosphangulene (1) is a hexacyclic triarylphosphine with a distinctive conical shape and other features that allow the compound to be viewed from diverse perspectives and to be embraced by chemists from different parts of the field as a molecule worthy of special attention. In recent work, phosphangulene and its derivatives have proven to be effective tools for probing general principles that govern molecular organization in solids. The phosphangulene family is particularly well-suited for these studies because systematic structural changes in the compounds are easy to introduce. In crystals of phosphangulene itself, molecules are stacked efficiently like hats, giving rise to an R3m structure that is polar and pyroelectric. Simple conversion of the compound into phosphangulene oxide (7a) or other chalcogenides blocks effective stacking and forces crystallization to produce alternative structures that have many suboptimal intermolecular interactions and vary little in energy as their geometries are altered. This leads to high levels of polymorphism, and phosphangulene oxide (7a) belongs to the elite set of compounds known to exist in five or more forms characterized by single-crystal X-ray diffraction. For similar reasons, phosphangulene chalcogenides form crystals with complex unit cells in which multiple inequivalent molecules are needed to optimize packing, and the compounds are also predisposed to form solvates and mixed crystals containing other molecules. For example, crystallization of a 1:1 mixture of phosphangulene and oxide 7a yielded needles composed of pure phosphangulene along with crystals of the oxide containing substantial amounts of phosphangulene. Phosphangulene has one known polymorph, and its crystallization rejects the oxide. In contrast, the oxide is highly polymorphic, and its crystallization is prone to errors in which molecules in the lattice are replaced by other compounds. Packing in crystals of the oxide appears to be so ineffective that the orientation and even the identity of the molecular components can be varied without imposing severe energetic penalties.Because substituted members of the phosphangulene family have awkward curved shapes that cannot be packed efficiently, they have emerged as highly effective partners for cocrystallizing fullerenes and for using concave-convex interactions to control how fullerenes can be organized in materials. This can be achieved without eliminating fullerene-fullerene contacts of the type needed to ensure conductivity. In addition, phosphangulene has created unlimited opportunities for making complex structures with large curved aromatic surfaces based on a new strategy in which the central atom of phosphorus is used to form covalent bonds with other elements or to introduce coordinative interactions with metals. In these ways, recent work has put phosphangulene in the spotlight as a compound of unusually broad interest and shown that it can appropriately be called a molecule for all chemists.