Manganese(II), Iron(II), and Mixed-Metal Metal-Organic Frameworks Based on Chains with Mixed Carboxylate and Azide Bridges: Magnetic Coupling and Slow Relaxation.

Mn(II) and Fe(II) compounds derived from azide and the zwitterionic l-carboxylatomethylpyridinium-4-carbox)date ligand are isomorphous three-dimensional metal--organic frameworks (MOFs) with the sra net, in which the metal ions are connected into anionic chains by mixed (μ-l, l-azide)bis(μ-carboxylate) triple bridges and the chains are cross-linked by the cationic backbones of the zwitterionic ligands. The Mn(ll) MOFs display typical one-dimensional antiferromagnetic behavior. In contrast, with one more d electron per metal center, the Fe(n) counterpart shows intrachain ferromagnetic interactions and slow relaxation of magnetization attributable to the single-chain components. The activation energies for magnetization reversal in the infinite- and finite-chain regimes are Δτ1, = 154 K and Δτ2 = 124 K, respectively. Taking advantage of the isomorphism between the Mn(II) and Fe(II) MOFs, we have prepared a series of mixedmetal Mn1-xIIFexII MOFs with x = 0.41, 0.63, and 0.76, which intrinsically feature random isotropic/anisotropic sites and competing antiferromagnetic--ferromagnetic interactions. The materials show a gradual antiferromagnetic-to-ferromagnetic evolution in overall behaviors as the Fe(II) content increases, and the Fe-rich materials show complex relaxation processes that may arise for mixed SCM and spin-glass mechanisms. A general trend is that the activation energy and the blocking temperature increase with the Fe(II) content, emphasizing the importance of anisotropy for slow relaxation of magnetization. [ABSTRACT FROM AUTHOR]