Metal Binding Arrests Umbrella Inversion

Metal coordination chemistry has been used to bestow controllable switching properties on an oscillating nitrogen ring, a result that could find application in nanoscale devices (Chem. Commun., DOI: 10.1039/b712447c). Nitrogen's lone pair of electrons rapidly fluctuates in many nitrogen-containing compounds, interconverting a compound between two enantiomeric forms in a process called pyramidal or umbrella inversion. Now, U.K.-based researchers led by Michael Shipman of the University of Warwick and James H. R. Tucker of the University of Birmingham demonstrate that inversion of the nitrogen atom in the three-membered aziridine ring of a pyridine-aziridine compound can be arrested by coordinating the two nitrogen atoms with a palladium salt as shown. Adding a bisphosphine ligand to scavenge the palladium frees the aziridine nitrogen's lone pair so the ring can resume its flip-flop motion. The researchers used NMR spectroscopy to quantify the umbrella inversion and to observe multiple cycles of on-off switching.