Distortions of a flexible metal-organic framework from substituted pendant ligands

Four new variants of the 1,4-benzenedicarboxylate MIL-53 structure have been prepared for Co(II) under solvothermal conditions and their structures solved and refined from single-crystal X-ray data. All materials contain pendant pyridine-N-oxide ligands that bridge pairs of Co(II) atoms in the inorganic backbone of the structure via O. By the use of the ligands 3-bromopyridine-N-oxide, 4-methoxypyridine-N-oxide, isoquinoline-N-oxide and 4-phenylpyridine-N-oxide, materials are prepared with the same topology but distinct structures. These illustrate how the MIL-53 structure is able to distort to accommodate the bulk of the various substituents on the pyridine ring. The bulkiest pendant ligand, 4-phenylpyridine-N-oxide, results in a distortion of the diamond-shaped channels in an opposite sense to that seen previously in expanded forms of the parent MIL-53 structure. By comparison with published crystal structures for MIL-53 with various occluded guests, the structural distortions that take place to accommodate the pendant ligands are quantified and it is shown how a twisting of the 1,4-benzenedicarboxylate ligand, instead of a hinging about the μ(2)-carboxylate-metal connection, allows the new structures that are observed.