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, (S)-4-(1-phenylpropyl)-1,2,4-triazole (trethbz), 4-(1,2,4-triazol-4-yl)­benzoic
acid (trPhCO2H), and
3-(1H-imidazol-4-yl)-2-(1,2,4-triazol-4-yl)­propionic
acid (trhis), were prepared to evaluate their coordination
behavior in the development of molybdenum­(VI) oxide organic hybrids.
Four compounds, [Mo2O6(trethbz)2]·H2O (1), [Mo4O12(trPhCO2H)2]·0.5H2O (2a), [Mo4O12(trPhCO2H)2]·H2O (2b), and
[Mo8O25(trhis)2(trhisH)2]·2H2O (3), were synthesized and characterized. The monofunctional tr-ligand resulted in the formation of a zigzag chain [Mo2O6(trethbz)2] built
up from cis-{MoO4N2} octahedra
united through common μ2-O vertices. Employing the
heterodonor ligand with tr/–CO2H functions afforded either layer or ribbon structures
of corner- or edge-sharing {MoO5N} polyhedra (2a or 2b) stapled by tr-links in axial
positions, whereas −CO2H groups remained uncoordinated.
The presence of the im-heterocycle as an extra function
in trhis facilitated formation of zwitterionic molecules
with a protonated imidazolium group (imH+) and a negatively charged −CO2– group, whereas the tr-fragment
was left neutral. Under the acidic hydrothermal conditions used, the
organic ligand binds to molybdenum atoms either through [N–N]-tr or through both [N–N]-tr and
μ2-CO2– units, which
occur in protonated bidentate or zwitterionic tetradentate forms (trhisH+ and trhis, respectively). This leads to a new zigzag subtopological motif
(3) of negatively charged polyoxomolybdate {Mo8O25}n2n– consisting of corner- and edge-sharing cis-{MoO4N2} and {MoO6} octahedra, while
the tetradentate zwitterrionic trhis 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 (cis-cyclooctene
and trans-β-methylstyrene) with tert-butylhydroperoxide. The best-performing catalyst (1) was explored for the epoxidation of other olefins, including biomass-derived
methyl oleate, methyl linoleate, and prochiral dl-limonene.

本研究制备了三种含有1,2,4-三唑基供体基团的有机配体，包括(S)-4-(1-苯基丙基)-1,2,4-三唑（trethbz）、4-(1,2,4-三唑-4-基)苯甲酸（trPhCO2H）和3-(1H-咪唑-4-基)-2-(1,2,4-三唑-4-基)丙酸（trhis），以评估其在钼(VI)氧化物有机杂化材料发展中的配位行为。合成了四种化合物，分别为[Mo2O6(trethbz)2]·H2O (1)、[Mo4O12(trPhCO2H)2]·0.5H2O (2a)、[Mo4O12(trPhCO2H)2]·H2O (2b)和[Mo8O25(trhis)2(trhisH)2]·2H2O (3)，并对其进行了表征。单官能团tr-配体导致形成由顺式-{MoO4N2}八面体通过共同的μ2-O顶点连接而成的之字形链[Mo2O6(trethbz)2]。采用具有tr/–CO2H功能的异供体配体，可形成层状或带状结构，其中{MoO5N}多面体通过轴向位置的tr-连接键合，而–CO2H基团保持未配位。trhis中的咪唑杂环作为额外功能，促进了带正电的咪唑基（imH+）和带负电的–CO2–基团组成的两性离子的形成，而tr片段保持中性。在所用的酸性水热条件下，有机配体通过[N–N]-tr或同时通过[N–N]-tr和μ2-CO2–单元与钼原子结合，这些单元存在于质子化的双齿配位或两性离子的四齿配位形式（trhisH+和trhis）。这导致了一种新的之字形亚拓扑结构（3），由带负电的多氧钼酸盐{Mo8O25}n2n–组成，包括角共享和边共享的顺式-{MoO4N2}和{MoO6}八面体，而四齿两性离子的trhis物种将这些链连接成二维网络。化合物的电子光谱显示了与半导体行为一致的光学带隙。这些化合物作为环氧化催化剂通过非手性和前手性烯烃（顺式环辛烯和反式β-甲基苯乙烯）与叔丁基过氧化氢的模型反应进行了研究。表现最佳的催化剂（1）被用于其他烯烃的环氧化，包括生物质衍生的甲基油酸、甲基亚油酸和前手性dl-柠檬烯。