Anomalous Magnetic Properties of Molecular Oxygen Adsorbed in Microporous Metal-Organic Solids

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Molecular oxygen forms 02-02 dimer in the coordination space of CPL-1 [Cu2(pzdc)2(pyz)]. It is found that the arrangement of 02 is associated not only with the interaction between the guest molecules and the host compound but also with the intermolecular interaction. The ab initio calculation of 02-02 dimer has pointed out that the magnetic interaction contributes to the formation of the H-structure alignment with singlet ground state (S = 0) while the most stable alignment of the quartet state (S = 2) is known to be different. Therefore we predicted the field-induced rearrangement in condensed 02 system in general.In this project, we have investigated the relation between the arrangement and the magnetic properties of 02 adsorbed in the coordination space by means of x-ray powder diffraction and magnetic measurements. In the case of CPL-1, the magnetization process and the temperature dependence of susceptibility can be interpreted by considering the excited state with different molecular arrangements. Similar magnetic properties have been observed in 02 adsorbed in Cu-CHD [Cu-1,4-cyclohexanedicarboxylic acid]. Though the MEM/Rietvelt analysis to determine the arrangement of adsorbed 02 is now in progress, the observed magnetic properties can be explained by same model. In CPL-p1 [Cu(dhba)2(4,4' -bpy)] and CID-4 [Cd(bpndc) (bpy)], the magnetization processes of adsorbed 02 show the metamagnetic transitions which depends on the adsorption. Though the arrangement of 02 is not clear, it is clear that the coordination spaces in these compounds are larger than that of CPL-1 or Cu-CHD. It is naturally considered that the existence of a few adsorption sites causes the adsorption dependence.Up to now the direct observation of field-induced rearrangement has not been succeeded. For CPL-1, the critical field is too high to carry out the XRD experiment. For Cu-CHD with lower critical field, the observation of field-induced rearrangement by XRD measurement is expected.