Interpenetrated (8,3)-c and (10,3)-b metal-organic frameworks based on {Fe^III₃} and {Fe^III₂Co^II} pivalate spin clusters

Two new magnetic cluster-based 3D coordination polymers consisting of μ₃-oxo-centered cationic homometallic [Fe^III ₃O(O₂CCMe₃)₆]⁺ or neutral heterometallic [Fe^III₂Co^IIO(O ₂CCMe₃)₆] coordination clusters bridged by different N,N′-donor ligands into three-dimensional networks of {[Fe ₃O(O₂CCMe₃)₆(4,4′-bpy) _1.5](OH)·0.75(CH₂Cl₂)·x(H ₂O)}_n (1, x ≤ 8) and {[Fe₂CoO(O ₂CCMe₃)₆(bpe)_0.5(pyz)]}_n (2) (where 4,4′-bpy = 4,4′-bipyridine; bpe = 1,2′-bis(4- pyridyl)ethylene; pyz = pyrazine) have been prepared under solvothermal conditions. Single-crystal X-ray diffraction studies reveal the existence of a 6-fold interpenetrated network with rare (8,3)-c (etc) topology for 1 and a 3-fold interpenetrated network with (10,3)-b (ths) topology for 2. The interpenetration effectively results in very low BET surface areas, and the compounds have to be regarded as nonporous. Magnetic studies of 1 and 2 point to both ferro- and antiferromagnetic intra- and intercluster exchange interactions between the isotropic Fe^III and the strongly anisotropic Co ^II spin centers. ⁵⁷Fe Mössbauer spectroscopy confirms the uniform ferric (+III) valence state in both 1 and 2, and low-temperature data for 1 point toward distinct hyperfine fields for the Fe sites.