Slow Magnetic Relaxation in One-Dimensional Azido-Bridged Co^II Complexes

Crystal structures and magnetic properties of three one-dimensional (1D) azido-bridged cobalt(II) chains with different amide ligands (L), [Co₂(N₃)₄(DMF)₃] (1), [Co₄(N₃)₈(DEF)₅] (2), and [Co₂(N₃)₄(DIPF)₂] (3) (DMF = N,N-dimethylformamide, DEF = N,N-diethylformamide, and DIPF = N,N-diisopropylformamide), are reported to investigate the influence of L on their structures and magnetic properties. Single-crystal X-ray crystallographic analysis revealed that, although 1-3 all consist of cobalt chains bridged by end-on (EO) azides, the coordination geometry of the Co^II ions and the repeating units of the 1D structures are quite different. As the size of L increases, the ratio of L to Co^II decreases from 6:4 in 1 to 5:4 and 4:4 in 2 and 3, respectively. In 1, two [CoN₅O₁] and two [CoN₄O₂] distorted octahedra form the {[CoN₅O₁][CoN₄O₂]₂[CoN₅O₁]} tetramers (denoted as Co₄A), which are linked to each other by sharing the N-N edge to form the chain. Similarly, the chain structure of 3 is constructed from a similar tetramer unit {[CoN₅][CoN₄O₂]₂[CoN₅]} (denoted as Co₄B), where half of the Co^II centers are in the [CoN₅] trigonal bipyramid because of the larger steric effect of the DIPF ligand, while, for compound 2 of the medium-sized amide, it has the transition structure between those of 1 and 3. The chain is composed of two different repeating units: Co₄A unit similar to that in 1 and Co₄B unit similar to that in 3. Because of their similar structures, compounds 1-3 exhibit analogical magnetic properties. Direct-current magnetic measurements demonstrated that all compounds show intrachain ferromagnetic coupling through the EO azides and interchain anti-ferromagnetic interactions. Alternating-current data revealed the slow magnetic relaxation in the anti-ferromagnetic ordered phases. While compound 1 exhibits spin glass behavior, compounds 2 and 3 behave as the single-chain magnets. This difference might come from the interference of the anti-ferromagnetic ordering on the magnetic dynamic of the magnetic chain.