Discrete Open-Shell Tris(bipyridinium radical cationic) Inclusion Complexes in the Solid State

The
solid-state properties of organic radicals depend on radical–radical
interactions that are influenced by the superstructure of the crystalline
phase. Here, we report the synthesis and characterization of a substituted
tetracationic cyclophane, cyclobis­(paraquat-p-1,4-dimethoxy­phenylene),
which associates in its bisradical dicationic redox state with the
methyl viologen radical cation (MV•+) to give a 1:1 inclusion complex. The (super)­structures of the reduced
cyclophane and this 1:1 complex in the solid state deviate from the
analogous (super)­structures observed for the reduced state of cyclobis­(paraquat-p-phenylene) and that of its trisradical tricationic complex.
Titration experiments reveal that the methoxy substituents on the p-phenylene linkers do not influence binding of the cyclophane
toward small neutral guestssuch as dimethoxy­benzene
and tetrathiafulvalenewhereas binding of larger radical cationic
guests such as MV•+ by the reduced
cyclophane decreases 10-fold. X-ray diffraction analysis reveals that
the solid-state superstructure of the 1:1 complex constitutes a discrete
entity with weak intermolecular orbital overlap between neighboring
complexes. Transient nutation EPR experiments and DFT calculations
confirm that the complex has a doublet spin configuration in the ground
state as a result of the strong orbital overlap, while the quartet-state
spin configuration is higher in energy and inaccessible at ambient
temperature. Superconducting quantum interference device (SQUID) measurements
reveal that the trisradical tricationic complexes interact antiferromagnetically
and form a one-dimensional Heisenberg antiferromagnetic chain along
the a-axis of the crystal. These results offer insights
into the design and synthesis of organic magnetic materials based
on host–guest complexes.