[Fe3(HCOO)6]: A Permanent Porous Diamond Framework Displaying H2/N2 Adsorption, Guest Inclusion, and Guest‐Dependent Magnetism

Abstract

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 FeFe 4 tetrahedral nodes, is prepared successfully by using a solution‐chemistry method. The as‐prepared compound, [Fe 3(HCOO)6](CH 3 OH)1.5(H 2 O)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 °C. The flexibility of the framework and the amphiphilic nature of the surface of the pores consisting of both C–H and O 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 nonpolar guests of different sizes. A series of guest‐inclusion compounds, [Fe 3(HCOO)6](I 2)0.84 (3‐iodine), [Fe 3(HCOO)6](C 4 H 8 O) (4‐THF), [Fe 3(HCOO)6](C 4 H 4 O) (5‐furan), [Fe 3(HCOO)6](C 6 H 6) (6‐benzene), [Fe 3(HCOO)6](CH 3 CN) (7‐acetonitrile), and [Fe 3(HCOO)6]((CH 3)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.