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><sub>2</sub><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, [Mo<sub>2</sub>O<sub>6</sub>(<i>trethbz</i>)<sub>2</sub>]·H<sub>2</sub>O (<b>1</b>), [Mo<sub>4</sub>O<sub>12</sub>(<i>trPhCO</i><sub>2</sub><i>H</i>)<sub>2</sub>]·0.5H<sub>2</sub>O (<b>2a</b>), [Mo<sub>4</sub>O<sub>12</sub>(<i>trPhCO</i><sub>2</sub><i>H</i>)<sub>2</sub>]·H<sub>2</sub>O (<b>2b</b>), and [Mo<sub>8</sub>O<sub>25</sub>(<i>trhis</i>)<sub>2</sub>(<i>trhisH</i>)<sub>2</sub>]·2H<sub>2</sub>O (<b>3</b>), were synthesized and characterized. The monofunctional <i>tr</i>-ligand resulted in the formation of a zigzag chain [Mo<sub>2</sub>O<sub>6</sub>(<i>trethbz</i>)<sub>2</sub>] built up from <i>cis-</i>{MoO<sub>4</sub>N<sub>2</sub>} octahedra united through common μ<sub>2</sub>-O vertices. Employing the heterodonor ligand with <i>tr/–CO</i><sub>2</sub><i>H</i> functions afforded either layer or ribbon structures of corner- or edge-sharing {MoO<sub>5</sub>N} polyhedra (<b>2a</b> or <b>2b</b>) stapled by <i>tr</i>-links in axial positions, whereas −CO<sub>2</sub>H 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><sup><i>+</i></sup>) and a negatively charged −CO<sub>2</sub><sup>–</sup> 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 μ<sub>2</sub>-CO<sub>2</sub><sup>–</sup> units, which occur in protonated bidentate or zwitterionic tetradentate forms (<i>trhisH</i><sup><i>+</i></sup> and <i>trhis</i>, respectively). This leads to a new zigzag subtopological motif (<b>3</b>) of negatively charged polyoxomolybdate {Mo<sub>8</sub>O<sub>25</sub>}<sub><i>n</i></sub><sup>2<i>n</i>–</sup> consisting of corner- and edge-sharing <i>cis-</i>{MoO<sub>4</sub>N<sub>2</sub>} and {MoO<sub>6</sub>} 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.