Trends in Exchange Coupling for Trimethylenemethane-Type Bis(semiquinone) Biradicals and Correlation of Magnetic Exchange with Mixed Valency for Cross-Conjugated Systems

A magnetostructural correlation (conformational electron spin exchange modulation) within an
isostructural series of biradical complexes is presented. X-ray crystal structures, variable-temperature
electron paramagnetic resonance spectroscopy, zero-field splitting parameters, and variable-temperature
magnetic susceptibility measurements were used to evaluate molecular conformation and electron spin
exchange coupling in this series of molecules. Our combined results indicate that the ferromagnetic portion
of the exchange couplings occurs via the cross-conjugated π-systems, while the antiferromagnetic portion
occurs through space and is equivalent to incipient bond formation. Thus, molecular conformation controls
the relative amounts of ferro- and antiferromagnetic contributions to exchange coupling. In fact, the exchange
parameter correlates with average semiquinone ring torsion angles via a Karplus−Conroy-type relation.
Because of the natural connection between electron spin exchange coupling and electronic coupling related
to electron transfer, we also correlate the exchange parameters in the biradical complexes to mixed valency
in the corresponding quinone−semiquinone radical anions. Our results suggest that delocalization in the
cross-conjugated, mixed-valent radical anions is proportional to the ferromagnetic contribution to the
exchange coupling in the biradical oxidation states.