Investigating the Case of Titanium(IV) Carboxyphenolate Photoactive Coordination Polymers

The reactivity of 2,5-dihydroxyterephthalic acid (H4DOBDC) with titanium(IV) precursors was thoroughly investigated for the synthesis of metal-organic frameworks under solvothermal conditions. Four crystalline phases were isolated whose structures were studied by a combination of single-crystal or powder X-ray diffraction and solid-state NMR. The strong coordination ability of the phenolate moieties was found to favor the formation of isolated TiO6 octahedra bearing solely organic ligands in the resulting structures, unless hydrothermal conditions and precondensed inorganic precursors are used. It is worth noting that these solids strongly absorb visible light, as a consequence of the ligand-to-metal charge transfer (LMCT) arising from Ti-phenolate bonds. Preliminary photocatalytic tests suggest that one compound, namely, MIL-167, presents a higher activity for hydrogen evolution than the titanium carboxylate MIL-125-NH2 but that such an effect cannot be directly correlated with its improved light absorption feature.