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

Abstract

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 T(N) 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.

Multifunctional One-Dimensional Rhodium(I)–Semiquinonato Complex: Substituent Effects on Crystal Structures and Solid-State Properties

“…The temperature dependence of the resistivity of 4 in the RT phase exhibits very small temperature dependence and an unusual decrease below a maximum at 245 K before reaching a minimum at 218 K. The unusual decrease observed in the resistivity can be ascribed to the crossover from a semiconductor with a small activation energy of E a = 31 meV (280−255 K) in the RT phase to a semiconductor with a large activation energy of E a = 199 meV (192−142 K) in the LT phase, as seen in Figure 11. A similar change in conductive properties from a small thermally activated behavior (E a = 88 meV) in the RT phase to a relatively large thermally activated behavior (E a = 386 meV) in 17 The UV−vis−NIR and mid-IR spectra were connected smoothly at approximately 4000−5500 cm −1 . a Measured as KBr-pressed disks at 298 K. sh = shoulder.…”

“…144°. The Rh1−Rh2 and Rh1−Rh2′ distances are 2.9662(8) and 3.2266(8) Å, respectively, whose average distance (3.096 Å) is almost the same as those of compounds 3 (3.092 Å) 17 and 4 (3.063 Å); however, the complex molecules in 5 are strongly dimerized in the 1D chain. Bond angles Rh2−Rh1−Rh2′ and Rh1−Rh2− Rh1* are both 179.…”

“…The Rh1−Rh2 and Rh1−Rh2* distances are 3.0943(6) and 3.0313(6) Å, respectively, which are 0.215 Å shorter than those of [Rh(3,6-DBSQ)(CO) 2 ] ∞ (1) (3.252(4) and 3.304(5) Å at 294−297 K) 11 on average and are almost the same as those of [Rh(3,6-DBSQ-4,5-(MeO) 2 )(CO) 2 ] ∞ (3) (3.0796(4) and 3.1045(4) Å at 226 K). 17 The bond angles of Rh2−Rh1−Rh2* and Rh1−Rh2−Rh1′ are both 172. 37(1)°, resulting in a slightly folded zigzag Rh−Rh chain structure.…”

“…= 3.603(5) Å), indicating that the adjacent 1D chains are connected two-dimensionally parallel to the ab plane by the weak C−H···O hydrogen bonds, as shown in Supporting Information Figures S1 and S2, which is in contrast to the three-dimensional hydrogen-bonded network observed in 3. 17,34 The three tert-butyl groups bonded to the C3, C22, and C25 atoms are disordered over two positions.…”

“…The broad absorption band observed in 3 has been attributed to the charge-transfer absorption from a filled 1D d band resulting from axial d z 2 orbitals to vacant semiquinonato π* levels. 17 The more this band expands from the mid-IR to the far-IR region, the more the energy of the electronic excitations decreases; thus, the electrical conductivity is expected to increase. Compound 4 is expected to exhibit a relatively high electrical conductivity despite Rh not being in a mixed-valence state, because the absorption band shows a similar large expanse in the mid-IR region, as observed for 3.…”