[Fe3(HCOO)6]:: A permanent porous diamond framework displaying H2/N2 adsorption, guest inclusion, and guest-dependent magnetism

The porous magnet [Fe-3(HCOO)(6)], the iron member of the [M-3(HCOO)(6)] family (where M = Mn, Fe, Co, Ni, etc.), based on a diamond framework consisting of Fe-centered FeFe4 tetrahedral nodes, is prepared successfully by using a solution-chemistry method. The as-prepared compound, [Fe-3(HCOO)(6)](CH3OH)(1.5)(H2O)(0.5) (1-parent), exhibits facile removal of its guests, methanol, and water, to give the desolvated framework [Fe-3(HCOO)(6)] (2-empty) that displays permanent porosity and thermal stability up to 270 degrees C. The flexibility of the framework and the amphiphilic nature of the surface of the pores consisting of both C-H and 0 arrays allows 2-empty to take up significant H-2 and N-2 at liquid-nitrogen temperatures and a wide spectrum of both polar and non-polar guests of different sizes. A series of guest-inclusion compounds, [Fe-3(HCOO)(6)](I-2)(0.64) (3-iodine), [Fe-3(HCOO)(6)](C4H8O) (4-THF), [Fe-3(HCOO)(6)](C4H4O) (5-furan), [Fe-3(HCOO)(6)](C6H6) (6-benzene), [Fe-3(HCOO)(6)](CH3CN) (7-acetonitrille), and [Fe-3(HCOO)(6)]((CH3)(2)CO) (8-acetone) are successfully prepared by vapor diffusion of the guest into the pores of 2-empty and their structures are characterized by using single-crystal X-ray crystallography. Zigzag molecular arrays of the guest are formed in the confined channels of the host framework, and the weak hydrogen-bonding provides the main host-guest interaction. All the compounds show 3D long-range magnetic ordering and guest-modulated Curie temperatures, coercive fields, and remnant magnetization as a consequence of the subtle rearrangement of the magnetic framework that conforms to the size of the guests and the difference in host-guest interactions. A possible magnetic structure of the framework is proposed to account for magnetic competition and geometrical frustration. The thermal and spectroscopic properties of the compounds are also reported.