Cyclic organic and main-group radicals in which the unpaired electron is delocalized in an extended p-system tend to be thermally and chemically stable, allowing isolation and storage as pure species for extended periods. These planar p-radicals also often exhibit an interesting type of pairwise co-facial interaction, dubbed “pancake bonding”. The exacting geometric requirements and the typical enthalpies of pancake bonds are similar to those of hydrogen bonds. Like H-bonds, pancake bonds can be employed to direct crystal packing and supramolecular architectures.<br/>Recently, we have discovered a series of thiazyl-radical coordination complexes that exhibit pancake bonds in the solid state, but that also undergo crystal-to-crystal phase transitions that rupture the pancake bonds and thus switch the magnetic properties. This has led us to further explore the parallels between H-bonding and pancake bonding at a number of levels, including the design of a thiazyl radical (HbimDTDA) that is capable of both.<br/>Current progress on reversible crystal-to-crystal switching involving rupture of pancake bonds will be presented, with a specific focus on the HbimDTDA system in which a balance between pancake bonding and specific sigma-hole interactions appears to be a primary factor defining the interplay between the diamagnetic and paramagnetic states. The HbimDTDA system exhibits thermal hysteresis of the magnetic susceptibility, associated with the reversible (cyclable) single-crystal-to-single-crystal phase transition. The cooperativity necessary for hysteresis is imparted by a dense network of short intermolecular contacts, including H-bonding.

Magnetic bistability: pancake bonding vs. sigma-hole bonding

2019 Spring National Meeting

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