Bicyclic Diphosphines Flip Inside Out

In chemistry that contortionists can relate to, a research team led by Michael Stollenz and John A. Gladysz of Texas A&M University has identified a rare case of isomerism in which a macromolecule turns itself inside out (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201100893). By careful study of ³¹P NMR spectra, the researchers discovered that the alkyl chains of a cagelike bicyclic diphosphine, made up of two phosphorus atoms linked by three (CH₂)₁₄ chains, can reversibly slide between one another to invert the molecule, flipping over the phosphorus atoms and interconverting three configurational isomers. The researchers designate these isomers as in/in, out/out, and in/out, depending on whether each phosphorus atom’s lone pair of electrons is facing into or out of the molecular cage. This type of isomerization might be a general mechanism for encapsulating atoms or molecules that has been overlooked, Gla dysz says. It could be useful for controlled sequestration, transport, and delivery of guest molecules that bind to the phosphorus lone pairs, he suggests.