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.