Magnetic coupling of guest metallocene molecules with SURMOF-2 host matrix

Abstract

Metal-organic frameworks (MOFs) are crystalline and porous, molecular solids consisting of metal nodes and organic ligands. An interesting example is the MOF-2 system, where Cu2+ ions form antiferromagnetically coupled dimers to yield so-called paddlewheels. In the case of surface-anchored MOF-2 (SURMOF-2) systems the Cu2+ ions are connected via carboxylate and OH groups in a zipperlike fashion. This unusual coupling of the spin-1/2 ions within the resulting one-dimensional chains stabilizes a low-temperature ferromagnetic (FM) phase. In this study, the magnetic properties of two SURMOF-2 systems (Cu(bdc) and Cu(bpdc) with bdc=1, 4-benzendicarboxylic acid, bpdc=4,4-biphenyldicarboxylic acid) were investigated using x-ray magnetic circular dichroism both in the absorption and in the scattering geometry. The presence of the FM phase in these SURMOF-2 systems is confirmed. Taking advantage of the element sensitivity of this technique. it was established that the magnetic signal originates from Cu2+ ions. After loading of SURMOF-2 with metallocene molecules, no remarkable changes of the magnetic properties of the host matrix were observed. However, the magnetic behavior of the guest molecules, as it turned out, is rather different. In the case of nickelocene loading, a polarization effect was found, resulting in ferromagnetic ordering of the guest molecules. However, in the case of manganocene derivatives, the polarization effect is not observed and these molecules remained in their paramagnetic state. The details of these effects are discussed.