Polymorphism in a Cobalt-Based Single-Ion Magnet Tuning Its Barrier to Magnetization Relaxation

A large barrier to magnetization
reversal, a signature of a good
single-molecule magnet (SMM), strongly depends on the structural environment
of a paramagnetic metal ion. In a crystalline state, where SMM properties
are usually measured, this environment is influenced by crystal packing,
which may be different for the same chemical compound, as in polymorphs.
Here we show that polymorphism can dramatically change the magnetic
behavior of an SMM even with a very rigid coordination geometry. For
a cobalt­(II) clathrochelate, it results in an increase of the effective
barrier from 109 to 180 cm–1, the latter value being
the largest one reported to date for cobalt-based SMMs. Our finding
thus highlights the importance of identifying possible polymorphic
phases in search of new, even more efficient SMMs.