Structures and H2 Adsorption Properties of Porous Scandium Metal-Organic Frameworks

Two new three‐dimensional ScIII metal–organic frameworks {[Sc3O(L1)3(H2O)3]⋅Cl0.5(OH)0.5(DMF)4(H2O)3}∞ (1) (H2L1=1,4‐benzene‐dicarboxylic acid) and {[Sc3O(L2)2(H2O)3](OH)(H2O)5(DMF)}∞ (2) (H3L2=1,3,5‐tris(4‐carboxyphenyl)benzene) have been synthesised and characterised. The structures of both 1 and 2 incorporate the trinuclear trigonal planar [Sc3(O)(O2CR)6] building block featuring three ScIII centres joined by a central μ3‐O2− donor. Each ScIII centre is further bound by four oxygen donors from four different bridging carboxylate anions, and a molecule of water located trans to the μ3‐O2− donor completes the six coordination at the metal centre. Frameworks 1 and 2 show high thermal stability with retention of crystallinity up to 350 °C. The desolvated materials 1 a and 2 a, in which the solvent has been removed from the pores but with water or hydroxide remaining coordinated to ScIII, show BET surface areas based upon N2 uptake of 634 and 1233 m2 g−1, respectively, and pore volumes calculated from the maximum N2 adsorption of 0.25 cm3 g−1 and 0.62 cm3 g−1, respectively. At 20 bar and 78 K, the H2 isotherms for desolvated 1 a and 2 a confirm 2.48 and 1.99 wt % total H2 uptake, respectively. The isosteric heats of adsorption were estimated to be 5.25 and 2.59 kJ mol−1 at zero surface coverage for 1 a and 2 a, respectively. Treatment of 2 with acetone followed by thermal desolvation in vacuo generated free metal coordination sites in a new material 2 b. Framework 2 b shows an enhanced BET surface area of 1511 m2 g−1 and a pore volume of 0.76 cm3 g−1, with improved H2 uptake capacity and a higher heat of H2 adsorption. At 20 bar, H2 capacity increases from 1.99 wt % in 2 a to 2.64 wt % for 2 b, and the H2 adsorption enthalpy rises markedly from 2.59 to 6.90 kJ mol−1. It's ScIIIandalous: {[Sc3O(L1)3(H2O)3]⋅Cl0.5OH0.5(DMF)4(H2O)3}∞ (1) (H2L1=1,4‐benzenedicarboxylic acid) and {[Sc3O(L2)2(H2O)3]OH(H2O)5(DMF)}∞ (2) (H3L2=1,3,5‐tris(4‐carboxyphenyl)benzene) incorporate the trinuclear trigonal planar [Sc3(O)(O2CR)6] building block. After appropriate thermal treatment on the acetone‐exchanged sample 2, the generation of free metal coordination sites has been achieved to give an increase in the BET surface area in 2 b.