Metal (Mn2+, Co2+ and Cd2+) ions induced differentiation in mixed-ligands MOFs constructed from H3btb and H2bdc: Highly porous Structures, new topology nets and two-step gate-opening behaviors

Abstract

Mix-ligands strategy is highly efficient for the rational construction of MOFs with particular structure motifs and high porosity, yet challenging and requiring matching in hierarchical coordination assembly and synergetic crystallization. The solvothermal reactions of 1,3,5-tris[4-carboxyphenyl]benzene (H3btb) and 1,4-benzenedicarboxylic acid (H2bdc) with different 3d metal ions, generated highly porous mixed-ligands MOFs of [(CH3)2NH2]2[Mn3(btb)2 (bdc)]·12DMF·3MeOH (Mn-btb-bdc) having anionic 3D frameworks constructed from linear [Mn3(COO)8] nodes, [Co4(btb)2(bdc)(H2O)3]·15DMF·7H2O (Co-btb-bdc) processing neutral frameworks constituted by linear [Co3(COO)6(H2O)4] and paddle-wheel [Co2(COO)4(H2O)2] nodes, as well as one single-ligand MOFs of [(CH3)2NH2][Cd(btb)(H2O)]·8DMF (Cd-btb-2fold) exhibiting 2-fold interpenetrated ths net frameworks due to the unsuccessful introduce of the coexisted bdc ligand. Two new topology net of 2,3,8-connected net with point symbol of (42·6)2(44·66·812·105·12)(6) and 2,3,4,6-connected net with Point Symbol of (4·82)4(42·811·102)2(84·122)(82) were found for Mn-btb-bdc and Co-btb-bdc. All these three MOFs are highly porous with void fractions of 60.9%, 52.2% and 61.4%, stemming from 3D-penetrated, 3D-penetrated and 1D channels, respectively. Such structural differentiations are attributed to the diversified electronic structure and coordination preference of different 3d metal ions, as well as the solvent template and temperature effects. Two-steps gate-opening behaviour in gas sorption isotherms was found in the flexible Mn-btb-bdc MOFs.