  Multiferroicity and magnetodielectric coupling in metal-organic-frameworks
(MOFs) is rare and so far restricted mainly to formate-based systems with
perovskite structure. In the course of this work we designed a tetragonal
framework [Co(C16H15N5O2)], exhibiting spin-chains of Co2+ ions, which are
bridged by an organic linker containing a dipolar nitrobenzene moiety. This
compound shows relaxor-like ferroelectricity at 100 K, which is followed by the
onset of complex magnetic order at 15 K, indicative of weak ferromagnetism. The
clear anomaly of the dielectric constant at the magnetic ordering transition
indicates magnetodielectric coupling, which is also confirmed by magnetic-field
dependent dielectric measurements. Weak ferromagnetism and magnetodielectric
coupling, both probably result from a significant Dzyaloshinskii-Moriya
interaction, which cants the spin structure and locally breaks inversion
symmetry. We document that the introduction of dipolar nitrobenzene as building
block in the crystal structure paths the way to design new multiferroic and
magnetodielectric MOFs.
