Construction of Metal–Organic Frameworks Based on Two Neutral Tetradentate Ligands

The solvothermal reaction of two new neutral tetradentate ligands with different bivalent metal salts gave seven metal–organic frameworks (MOFs): [Co₂(L1) (<i>trans</i>-chdc)₂]·5H₂O (<b>1</b>), [Zn₂(L1)­(<i>trans</i>-chdc)­(NO₂)₂]·DMF (<b>2</b>), [Cd₂(L1)­(<i>trans</i>-chdc)₂]·4H₂O (<b>3</b>), [Zn₂(L1)­(1,4-bdc)₂]·(H₂O)₃ (<b>4</b>), [Cd₂(L1)­(1,4-bdc)₂]·DMF·(solvent)_<i>x</i> (<b>5</b>), [Co­(L2) (<i>trans</i>-chdc)­(H₂O)]·1.5H₂O (<b>6</b>), [Co­(L2) (1,4-bdc) (H₂O)] · 2H₂O (<b>7</b>), (L1 = 1,1′-oxybis­[3,5-diimidazole]-benzene, L2 = 1,1′-oxybis­[3,5-dipyridine]-benzene, <i>trans</i>-chdc = <i>trans</i>-1,4-cyclohexanedicarboxylic acid, 1,4-bdc = 1,4-benzenedicarboxylate). These MOFs were prepared to examine the effects of the core metal ion or organic ligand on the topology and interpenetration form. The results show that the imidazole ligand can rotate easily to coordinate to metal ions, while pyridine ligand exhibits the weaker coordinative abilities, which may influence the self-assembly. Compounds <b>1</b>, <b>3</b>, and <b>5</b> are three-dimensional (3D) frameworks with 2-fold interpenetrated forms, whereas complex <b>4</b> shows a 3-fold interpenetrated structure. Interestingly, compound <b>2</b> exhibits a 4-fold interpenetration. Compound <b>6</b> features a two-dimensional polymeric layer structure which exhibits a rare 2-fold interpenetrating 3D <b>hms</b> array if H-bonds are taken into account. For compound <b>7</b>, the dinuclear cobalt secondary building unit (SBU) assembles with mixed ligands L2 and 1,4-bdc to construct a 3D α-<b>Po</b> structure