Bistable Multifunctionality and Switchable Strong Ferromagnetic-to-Antiferromagnetic Coupling in a One-Dimensional Rhodium(I)–Semiquinonato Complex

We
present a comprehensive study of the synthesis, heat capacity,
crystal structures, UV–vis−NIR and mid-IR spectra, DFT
calculations, and magnetic and electrical properties of a one-dimensional
(1D) rhodium­(I)–semiquinonato complex, [Rh­(3,6-DBSQ-4,5-(MeO)2)­(CO)2]∞ (3), where
3,6-DBSQ-4,5-(MeO)2•– represents
3,6-di-tert-butyl-4,5-dimethoxy-1,2-benzosemiquinonato
radical anion. The compound 3 comprises neutral 1D chains
of complex molecules stacked in a staggered arrangement with short
Rh–Rh distances of 3.0796(4) and 3.1045(4) Å at 226 K
and exhibits unprecedented bistable multifunctionality with respect
to its magnetic and conductive properties in the temperature range
of 228–207 K. The observed bistability results from the thermal
hysteresis across a first-order phase transition, and the transition
accompanies the exchange of the interchain C–H···O
hydrogen-bond partners between the semiquinonato ligands. The strong
overlaps of the complex molecules lead to unusually strong ferromagnetic
interactions in the low-temperature (LT) phase. Furthermore, the magnetic
interactions in the 1D chain drastically change from strongly ferromagnetic
in the LT phase to antiferromagnetic in the room-temperature (RT)
phase with hysteresis. In addition, the compound 3 exhibits
long-range antiferromagnetic ordering between the ferromagnetic chains
and spontaneous magnetization because of spin canting (canted antiferromagnetism)
at a transition temperature TN of 14.2
K. The electrical conductivity of 3 at 300 K is 4.8 ×
10–4 S cm–1, which is relatively
high despite Rh not being in a mixed-valence state. The temperature
dependence of electrical resistivity also exhibits a clear hysteresis
across the first-order phase transition. Furthermore, the ferromagnetic
LT phase can be easily stabilized up to RT by the application of a
relatively weak applied pressure of 1.4 kbar, which reflects the bistable
characteristics and demonstrates the simultaneous control of multifunctionality
through external perturbation.