Isoreticular metal-organic framework series based on Ni8-hydroxo clusters and pyrazolate ligands : control of hydrophobicity

Nowadays, many research groups are trying to synthesize new Metal-Organic Frameworks (MOFs) with very large surface areas and pore volumes, to obtain highly porous compounds suitable for the adsorption of a huge amount of gases, vapours or small/middle size molecules. On top of this, renewed interest is addressed towards novel porous structures showing high stability in conditions mimicking operative ones (which means in the presence of humidity) that would result in their effective employment in practical applications. In this communication we report on the synthesis, structural and textural characterization of an isoreticular MOF series based on octanuclear hydroxo nickel(II) cubic clusters, with a special focus on the possibility to modulate the hydrophobicty of these materials in order to make them potentially useful for applications involving the adsorption of harmful molecules in environmental conditions. Herein, each Ni8 cluster acts as a twelve connecting node by employing mixed pyrazolato/carboxylato and bispyrazolato linear linkers of increasing length. In this way, highly porous 3D frameworks of [Ni8(OH)4(OH2)(L)6]n general formulation are obtained in good yields and high purity as polycrystaline powders. As expected, the length of the linkers has an impact on the pore dimenison, making possible to finely tune pores size and void volume (Figure 1). Moreover, the different functionalization of the organic spacers strongly influences polarity and framework stability of the resulting MOFs. Noteworthy, the incorporation of trifluoromethyl residues in the end members of the series gives rise to a significant increase of hydrophobicity of the porous structure. As a consequence, water accessibility to the framework is precluded, which is beneficial for the capture of harmful volatile organic compounds under ambient humid conditions.