Multifunctional one-dimensional rhodium(I)-semiquinonato complex: Substituent effects on crystal structures and solid-state properties

Two new one-dimensional (1D) rhodium(I)-semiquinonato complexes formulated as [Rh(3,6-DBSQ-4,5-PDO)(CO)₂]_- (4; 3,6-DBSQ-4,5-PDO^•- = 3,6-di-tert-butyl-4,5-(1,3-propanedioxy)-1,2-benzosemiquinonato) and [Rh(3,6-DBSQ-4,5-(N,N′-DEN))(CO)₂]_- (5; 3,6-DBSQ-4,5-(N,N′-DEN)^•- = 3,6-di-tert-butyl-4,5-(N,N′-diethylenediamine)-1,2-benzosemiquinonato) were synthesized to explore the nature of the unusual structural phase transition and magnetic and conductive properties recently reported for [Rh(3,6-DBSQ-4,5-(MeO)₂)(CO)₂]_- (3; 3,6-DBSQ-4,5-(MeO)₂^•- = 3,6-di-tert-butyl-4,5-dimethoxy-1,2-benzosemiquinonato). Their crystal structures and magnetic and conductive properties were investigated. Compounds 4 and 5 comprise neutral 1D chains of complex molecules stacked in a staggered arrangement with fairly short average Rh-Rh distances of 3.06 Å for 4 and 3.10 Å for 5. These distances are similar to those for 3 (3.09 Å); however, the molecules of 5 are strongly dimerized in the 1D chain. Compound 4 undergoes a first-order phase transition at T_trs = 229.1 K, and its magnetic properties drastically change from antiferromagnetic coupling in the roomerature (RT) phase to strong ferromagnetic coupling in the lowerature (LT) phase. In addition, compound 4 exhibits a long-range ordering of net magnetic moments originating from the imperfect cancellation of antiferromagnetically coupled spins between the ferromagnetic 1D chains at T_N = 10.9 K. Furthermore, this compound exhibits an interesting crossover from a semiconductor with a small activation energy (E_a = 31 meV) in the RT phase to a semiconductor with a large activation energy (E_a = 199 meV) in the LT phase. These behaviors are commonly observed for 3. Alternating current susceptibility measurements of 4, however, revealed a frequency-dependent phenomenon below 5.2 K, which was not observed for 3, thus indicating a slow spin relaxation process that possibly arises from the movements of domain walls. In contrast, compound 5, which possesses a strongly dimerized structure in its 1D chain, shows no sign of strong ferromagnetic interactions and is an insulator, with a resistivity greater than 7 × 10⁷ ω cm.