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
1-carboxylatomethylpyridinium-4-carboxylate 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-1,1-azide)­bis­(μ-carboxylate) triple
bridges and the chains are cross-linked by the cationic backbones
of the zwitterionic ligands. The Mn­(II) MOFs display typical one-dimensional
antiferromagnetic behavior. In contrast, with one more d electron
per metal center, the Fe­(II) 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 mixed-metal MnII1–xFeIIx 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.