High Thermal and Chemical Stability in Pyrazolate Bridged Metal-Organic Frameworks with Exposed Metal Site

Reactions between the tritopic pyrazole-based ligand 1,3,5-tris(1H-pyrazol-4-yl)benzene (H3BTP) and transition metal acetate salts in DMF afford microporous pyrazolate-bridged metal-organic frameworks of the type M3 (BTP) 2·xsolvent (M 1/4 Ni (1), Cu, (2), Zn (3), Co (4)). Ab-initio X-ray powder diffraction methods were employed in determining the crystal structures of these compounds, revealing 1 and 2 to exhibit an expanded sodalite-like framework with accessible metal cation sites, while 3 and 4 possess tetragonal frameworks with hydrophobic surfaces and narrower channel diameters. Compounds 1-4 can be desolvated without loss of crystallinity by heating under dynamic vacuum, giving rise to microporous solids with BET surface areas of 1650, 1860, 930 and 1027 m2 g-1, respectively. Thermogravimetric analyses and powder X-ray diffraction measurements demonstrate the exceptional thermal and chemical stability of these frameworks. In particular, 3 is stable to heating in air up to at least 510 °C, while 1 is stable to heating in air to 430 °C, as well as to treatment with boiling aqueous solutions of pH 2 to 14 for two weeks. Unexpectedly, 2 and 3 are converted into new crystalline metal-organic frameworks upon heating in boiling water. With the combination of stability under extreme conditions, high surface area, and exposed metal sites, it is anticipated that 1 may open the way to testing metal-organic frameworks for catalytic processes that currently employ zeolites.