Structures and H 2 Adsorption Properties of Porous Scandium Metal–Organic Frameworks

Two new three‐dimensional Sc III metal–organic frameworks {[Sc 3 O(L 1 ) 3 (H 2 O) 3 ] ⋅ Cl 0.5 (OH) 0.5 (DMF) 4 (H 2 O) 3 } ∞ ( 1 ) (H 2 L 1 =1,4‐benzene‐dicarboxylic acid) and {[Sc 3 O(L 2 ) 2 (H 2 O) 3 ](OH)(H 2 O) 5 (DMF)} ∞ ( 2 ) (H 3 L 2 =1,3,5‐tris(4‐carboxyphenyl)benzene) have been synthesised and characterised. The structures of both 1 and 2 incorporate the trinuclear trigonal planar [Sc 3 (O)(O 2 CR) 6 ] building block featuring three Sc III centres joined by a central μ 3 ‐O 2− donor. Each Sc III centre is further bound by four oxygen donors from four different bridging carboxylate anions, and a molecule of water located trans to the μ 3 ‐O 2− 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 Sc III , show BET surface areas based upon N 2 uptake of 634 and 1233 m 2  g −1 , respectively, and pore volumes calculated from the maximum N 2 adsorption of 0.25 cm 3  g −1 and 0.62 cm 3  g −1 , respectively. At 20 bar and 78 K, the H 2 isotherms for desolvated 1 a and 2 a confirm 2.48 and 1.99 wt % total H 2 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 m 2  g −1 and a pore volume of 0.76 cm 3  g −1 , with improved H 2 uptake capacity and a higher heat of H 2 adsorption. At 20 bar, H 2 capacity increases from 1.99 wt % in 2 a to 2.64 wt % for 2 b , and the H 2 adsorption enthalpy rises markedly from 2.59 to 6.90 kJ mol −1 .