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.