Planar Biphenyl-Bridged Biradicals as Building Blocks for the Design of Quantum Magnets

We have synthesized and investigated a new biphenyl-4,4′-bis­(nitronyl
nitroxide) radical with intermediately strong antiferromagnetic interactions.
This organic biradical belongs to a family of materials that can be
used as a building block for the design of new quantum magnets. For
quantum magnetism, special attention has been paid to coupled S = 1/2 dimer compounds, which when
placed in a magnetic field, can be used as model systems for interacting
boson gases. Short contacts between the oxygen atoms of the nitronyl
nitroxide units and the hydrogen atoms of the benzene rings stabilize
a surprisingly planar geometry of the biphenyl spacer and are responsible
for a small magnetic interdimer coupling. The strength of the antiferromagnetic
intradimer coupling constant J/kB = −14.0 ± 0.9 K, fitting the experimental
SQUID-data using an isolated-dimer model. The deviations from the
isolated-dimer model are attributed to a small interdimer coupling J′/kB, on the order of
1 K, consistent with the crystal structure.