Supercritical CO2 utilization for the crystallization of 2D metal-organic frameworks using tert-butylpyridine additive

Recently, the study of the synthesis of metal-organic frameworks (MOFs) in the presence of supercritical CO2 (scCO(2)) has become a burgeoning direction in materials science. MOFs crystallization in scCO(2) is often carried out in the presence of other solvents, such as ionic or organic liquids, to increase reagents solubility in the reaction media. In this work, a totally new and cleaner way is followed, utilizing small amounts of an ancillary ligand to increase metal solubility, which result in a more efficient use of scCO(2). The precipitation of 2D MOFs, based in the combination of different trifluoroacetates of transition metals (M(tfa)(2), M-II = Cu-II or Zn-II), and a bidentate pyridine derivative (dpy), was studied. MOFs of the type [M(tfa)(2)(dpy)(x)](proportional to) were precipitated using scCO(2) as the solvent medium and the ligand tert-butylpyridine (t-bpy) as additive. The targeted 2D MOFs were precipitated after a short reaction time (ca. 3 h) operating under relatively soft supercritical conditions (20 MPa and 333 K). This new synthetic approach is thought as a generic method that can be used to modify metal precursors in systems in which the reaction is too slow or incomplete in neat scCO(2). Moreover, the increase of metal centre solubility allowed the in situ deposition of MOF crystals grafted on modified substrates.