Design of a Magnetic Bistability Molecular System Constructed by H-Bonding and π···π-Stacking Interactions

Crystal structures and magnetic properties were determined for two novel polymorphs of the complex [H2DABCO][Ni(mnt)2] [(H2DABCO)2+ = diprotonated 1,4-diazabicyclo[2.2.2]octane; mnt2- = maleonitriledithiolate]. For each
polymorph, anions form a layered structure in which two kinds of dimers were observed. The adjacent anionic
sheets are held together by cations via H-bonding interactions between protons of cations and CN groups of
anions. Two polymorphs possess spin bistability; namely, upon cooling, a magnetic transition happens at around
120 K with about 1 K hysteresis on heating for the α phase and at 112 K with about 10 K hysteresis for the β
phase. Above the transition, the magnetic behaviors of two polymorphs can be approximately interpreted by a
singlet−triplet model of an antiferromagnetically coupled S = 1/2 dimer, which is supported by the crystal structures
and spin dimer analyses based on extended Hückel molecular orbital calculations.