Triazolyl, Imidazolyl, and Carboxylic Acid Moieties in the Design of Molybdenum Trioxide Hybrids: Photophysical and Catalytic Behavior

Three organic ligands bearing 1,2,4-triazolyl donor moieties, (<i>S</i>)-4-(1-phenylpropyl)-1,2,4-triazole (<i>trethbz</i>), 4-(1,2,4-triazol-4-yl)­benzoic acid (<i>trPhCO</i>₂<i>H</i>), and 3-(1<i>H</i>-imidazol-4-yl)-2-(1,2,4-triazol-4-yl)­propionic acid (<i>trhis</i>), were prepared to evaluate their coordination behavior in the development of molybdenum­(VI) oxide organic hybrids. Four compounds, [Mo₂O₆(<i>trethbz</i>)₂]·H₂O (<b>1</b>), [Mo₄O₁₂(<i>trPhCO</i>₂<i>H</i>)₂]·0.5H₂O (<b>2a</b>), [Mo₄O₁₂(<i>trPhCO</i>₂<i>H</i>)₂]·H₂O (<b>2b</b>), and [Mo₈O₂₅(<i>trhis</i>)₂(<i>trhisH</i>)₂]·2H₂O (<b>3</b>), were synthesized and characterized. The monofunctional <i>tr</i>-ligand resulted in the formation of a zigzag chain [Mo₂O₆(<i>trethbz</i>)₂] built up from <i>cis-</i>{MoO₄N₂} octahedra united through common μ₂-O vertices. Employing the heterodonor ligand with <i>tr/–CO</i>₂<i>H</i> functions afforded either layer or ribbon structures of corner- or edge-sharing {MoO₅N} polyhedra (<b>2a</b> or <b>2b</b>) stapled by <i>tr</i>-links in axial positions, whereas −CO₂H groups remained uncoordinated. The presence of the <i>im-</i>heterocycle as an extra function in <i>trhis</i> facilitated formation of zwitterionic molecules with a protonated imidazolium group (<i>imH</i>^<i>+</i>) and a negatively charged −CO₂^– group, whereas the <i>tr-</i>fragment was left neutral. Under the acidic hydrothermal conditions used, the organic ligand binds to molybdenum atoms either through [N–N]-<i>tr</i> or through both [N–N]-<i>tr</i> and μ₂-CO₂^– units, which occur in protonated bidentate or zwitterionic tetradentate forms (<i>trhisH</i>^<i>+</i> and <i>trhis</i>, respectively). This leads to a new zigzag subtopological motif (<b>3</b>) of negatively charged polyoxomolybdate {Mo₈O₂₅}_<i>n</i>^2<i>n</i>– consisting of corner- and edge-sharing <i>cis-</i>{MoO₄N₂} and {MoO₆} octahedra, while the tetradentate zwitterrionic <i>trhis</i> species connect these chains into a 2D net. Electronic spectra of the compounds showed optical gaps consistent with semiconducting behavior. The compounds were investigated as epoxidation catalysts via the model reactions of achiral and prochiral olefins (<i>cis</i>-cyclooctene and <i>trans</i>-β-methylstyrene) with <i>tert</i>-butylhydroperoxide. The best-performing catalyst (<b>1</b>) was explored for the epoxidation of other olefins, including biomass-derived methyl oleate, methyl linoleate, and prochiral dl-limonene.