Breaking Records With Steric Congestion

When chemist Robert Pascal and colleagues at Tulane University designed a macrobicyclic in,in-bisphosphine earlier this year, they noted the unusual closeness of the molecule’s two bridgehead phosphines (Org. Lett. 2013, DOI: 10.1021/ol400728m). They’ve now taken things a step further, synthesizing an in,in-bis(hydrosilane), in which two silicon-hydrogen bonds are pointed directly at each other. The molecule is so sterically crowded that the central nonbonded contact distance between the two silane hydrogens is only about 1.56 Å, the shortest for any crystallographically characterized compound (J. Am. Chem. Soc. 2013, DOI:10.1021/ja407398w). The previous record holder is a cage pentacyclodecane, with nonbonded hydrogens separated by a distance of 1.617 Å. The value of this project, Pascal tells C&EN, “is to explore the limits of stable molecular structures, to provide points of calibration for modern computational methods, and to explore methods for the synthesis of ‘unnatural’ rather than natural products.”