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 Ttrs = 229.1 K, and its magnetic properties drastically
change from antiferromagnetic coupling in the room-temperature (RT)
phase to strong ferromagnetic coupling in the low-temperature (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 TN = 10.9 K. Furthermore, this
compound exhibits an interesting crossover from a semiconductor with
a small activation energy (Ea = 31 meV)
in the RT phase to a semiconductor with a large activation energy
(Ea = 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.