Various metal-organic frameworks (MOFs) have been prepared to obtain materials that show specific or multifunctional properties. Porous MOFs that contain free space where guest molecules can be accommodated are of particular interest because they can be applied in gas storage [1][2][3][4] and separation, [4][5][6] selective adsorption and separation of organic molecules, [1, 7] ion exchange, [8] catalysis, [9] sensor technology, [1,10] and for the fabrication of metal nanoparticles.[11]Secondary building units (SBUs) with a specific geometry have often been employed [12] for the modular construction of porous MOFs as they make the design and prediction of molecular architectures simple and easy. In particular, {M 2 -(CO 2 ) 4 }-type paddlewheel clusters that can be formed from the solvothermal reaction of M 2+ ions and the appropriate carboxylic acid are widely used for the construction of porous frameworks. Three-dimensional porous frameworks with various topologies (Pt 3 O 4 , boracites, NbO, and PtS nets) can be built from paddlewheel-type metal cluster SBUs and tri-or tetracarboxylates, [13][14][15][16] whereas pillared square-grid networks can be constructed from paddlewheel cluster SBUs and dicarboxylates in the presence of diamine ligands. [17] Porous MOFs with accessible metal sites (AMSs) should have a higher hydrogen storage capacity than those without AMSs, [14,18] although there are not yet enough experimental data to support this assumption. To determine the effect of AMSs in a MOF on H 2 adsorption, the H 2 uptakes should be compared for the same framework in the absence and presence of AMSs, or for two independent isostructural MOFs with and without AMSs. H 2 uptake has previously been measured under several different outgassing conditions. [13] Unfortunately, these experiments could not clearly demonstrate the effect of AMSs as the exact formula and structure at each stage were not known. Furthermore, even when coordinating solvent molecules are successfully removed with retention of the porous framework structure, the metal ion sometimes transforms its coordination geometry to the thermodynamically most stable form instead of keeping the AMSs. [4,19] Herein we report two porous MOFs with the same NbOtype net topology, namely [{Zn 2 (abtc)(dmf) 2 } 3 ]·4 H 2 O·10 dmf (1) and [{Cu 2 (abtc)(H 2 O) 2 } 3 ]·10 dmf·6 (1,4-dioxane) (2; H 4 abtc = 1,1'-azobenzene-3,3',5,5'-tetracarboxylic acid [20] ), and compare the gas adsorption data for the MOFs with and without AMSs.[21] Heating crystals of 1 and 2 under precisely controlled conditions allowed us to prepare [{Zn 2 -(abtc)(dmf) 2 } 3 ] (1 a; SNU-4) and [{Cu 2 (abtc)(dmf) 2 } 3 ] (2 a; SNU-5'), which have no AMSs, as well as [{Cu 2 (abtc)} 3 ] (2 b; SNU-5), which has AMSs. The framework structure of 1 a is the same as that of 1 and those of 2 a and 2 b are the same as that of 2, as evidenced by the PXRD patterns. Solid 1 a, 2 a, and 2 b exhibit higher adsorption capabilities for N 2 , CO 2 , CH 4 , and H 2 than other previously reported MOFs. In pa...

Various metal-organic frameworks (MOFs) have been prepared to obtain materials that show specific or multifunctional properties. Porous MOFs that contain free space where guest molecules can be accommodated are of particular interest because they can be applied in gas storage [1][2][3][4] and separation, [4][5][6] selective adsorption and separation of organic molecules, [1, 7] ion exchange, [8] catalysis, [9] sensor technology, [1,10] and for the fabrication of metal nanoparticles. [11] Secondary building units (SBUs) with a specific geometry have often been employed [12] for the modular construction of porous MOFs as they make the design and prediction of molecular architectures simple and easy. In particular, {M 2 -(CO 2 ) 4 }-type paddlewheel clusters that can be formed from the solvothermal reaction of M 2+ ions and the appropriate carboxylic acid are widely used for the construction of porous frameworks. Three-dimensional porous frameworks with various topologies (Pt 3 O 4 , boracites, NbO, and PtS nets) can be built from paddlewheel-type metal cluster SBUs and tri-or tetracarboxylates, [13][14][15][16] whereas pillared square-grid networks can be constructed from paddlewheel cluster SBUs and dicarboxylates in the presence of diamine ligands. [17] Porous MOFs with accessible metal sites (AMSs) should have a higher hydrogen storage capacity than those without AMSs, [14,18] although there are not yet enough experimental data to support this assumption. To determine the effect of AMSs in a MOF on H 2 adsorption, the H 2 uptakes should be compared for the same framework in the absence and presence of AMSs, or for two independent isostructural MOFs with and without AMSs. H 2 uptake has previously been measured under several different outgassing conditions. [13] Unfortunately, these experiments could not clearly demonstrate the effect of AMSs as the exact formula and structure at each stage were not known. Furthermore, even when coordinating solvent molecules are successfully removed with retention of the porous framework structure, the metal ion sometimes transforms its coordination geometry to the thermodynamically most stable form instead of keeping the AMSs. [4,19] Herein we report two porous MOFs with the same NbOtype net topology, namely [{Zn 2 (abtc)(dmf) 2 } 3 ]·4 H 2 O·10 dmf (1) and [{Cu 2 (abtc)(H 2 O) 2 } 3 ]·10 dmf·6 (1,4-dioxane) (2; H 4 abtc = 1,1'-azobenzene-3,3',5,5'-tetracarboxylic acid [20] ), and compare the gas adsorption data for the MOFs with and without AMSs. [21] Heating crystals of 1 and 2 under precisely controlled conditions allowed us to prepare [{Zn 2 -(abtc)(dmf) 2 } 3 ] (1 a; SNU-4) and [{Cu 2 (abtc)(dmf) 2 } 3 ] (2 a; SNU-5'), which have no AMSs, as well as [{Cu 2 (abtc)} 3 ] (2 b; SNU-5), which has AMSs. The framework structure of 1 a is the same as that of 1 and those of 2 a and 2 b are the same as that of 2, as evidenced by the PXRD patterns. Solid 1 a, 2 a, and 2 b exhibit higher adsorption capabilities for N 2 , CO 2 , CH 4 , and H 2 than other previously reported MOFs. ...