Triphosphatetrahedrane opens the door to new p-block-based materials

Chemists at MIT have reported the first synthesis of triphosphatetrahedrane (J. Am. Chem. Soc. 2021, DOI: 10.1021/jacs.1c07959). The new molecule looks similar to tetrahedrane—a hydrocarbon with four carbon atoms arranged in the shape of a tetrahedron—but with phosphorus atoms replacing three out of four carbons in the corners. Due to the molecule’s relatively low calculated ring strain, triphosphatetrahedrane could be easier to synthesize compared with other mixed carbon-phosphorus tetrahedranes.

The synthesis involved a tag team of two different niobium complexes: the first complex generates a carbon-centered radical by abstracting a halogen atom from trihalomethane, and the other stabilizes a triangle of phosphorus atoms. Then the carbon radical and the P₃ triangle combine to form the tetrahedrane.

The team found that triphosphatetrahedrane stayed intact in tetrahydrofuran solutions at room temperature and decomposes after a few days. To stabilize the compound and characterize it using X-ray crystallography, the researchers bound the molecule to an iron complex, which yielded purple crystals.

Previously, the same group had isolated an elusive phosphatetrahedrane with only one phosphorus atom and three tert-butyl substituents for stabilization. “Incorporating further phosphorus atoms successfully alleviated the strain without needing to add the bulky substituents,” says Martin-Louis Riu, lead author of the paper and a graduate student in the lab of Christopher Cummins. Phosphorus is a heavy p-block element with diffuse frontier electron orbitals, which enable tight bonding angles without much strain, Riu explains.

Furthermore, the team observed that concentrated triphosphatetrahedrane solutions generate an unknown black precipitate, probably a polymerized compound. If confirmed, this phosphorus derivative could find applications in materials science. Also, the C–H vertex of the tetrahedrane is ripe for functionalization, Riu says, and thus holds promise for tailoring new compounds’ properties.