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.