Oxidant-Dependent Switch of a Molybdenum(VI) Tetrazolate Complex from a Homogeneous to a Self-Separating Catalyst for Olefin Epoxidation

Although several decades have passed since the introduction
of
homogeneous molybdenum catalysts for the bulk industrial production
of epoxides from light olefins, the development of recyclable catalytic
systems to produce more complex epoxides remains a challenge. In this
work, we present a strategy for preparing a self-separating catalyst
by exploiting reaction-induced precipitation, starting from the molybdenum­(VI)
tetrazolate complex [MoO­(O2)­(pto)2] (Hpto =
5-(2-pyridyl-1-oxide)­tetrazole), which was synthesized via a one-pot
approach and crystallographically characterized. High epoxide selectivities
(96–100%) were achieved at high conversions (88–100%)
under mild conditions (70 °C) in all the studied reactions, from
that of the model substrate cis-cyclooctene to the
epoxidation of biobased dl-limonene and fatty acid methyl esters.
The catalytic reaction is homogeneous using tert-butyl
hydroperoxide as the oxidant, whereas with hydrogen peroxide, a transformation
to a self-separating catalyst takes place, which combines the high
catalytic activity of a homogeneous catalyst with the easy recovery
and reuse of a heterogeneous catalyst.

尽管用于轻质烯烃规模化工业生产环氧烷烃（epoxides）的均相钼催化剂（homogeneous molybdenum catalysts）问世已数十年，但开发可回收催化体系以制备更复杂环氧烷烃仍是一项挑战。本研究提出一种利用反应诱导沉淀制备自分离催化剂的策略，以通过一锅法合成并经晶体学表征的钼(VI)四唑配合物[MoO(O₂)(pto)₂]（Hpto = 5-(2-吡啶基-1-氧化物)四唑）为起始原料。在所研究的所有反应中，从模型底物顺式环辛烯（cis-cyclooctene）的环氧化反应，到生物基dl-柠檬烯（dl-limonene）与脂肪酸甲酯（fatty acid methyl esters）的环氧化反应，均在温和条件（70℃）下实现了高转化率（88%~100%）与高环氧烷烃选择性（96%~100%）。当以叔丁基过氧化氢（tert-butyl hydroperoxide）为氧化剂时，该催化反应为均相体系；而以过氧化氢（hydrogen peroxide）为氧化剂时，则会转化为自分离催化剂，兼具均相催化剂的高催化活性与多相催化剂（heterogeneous catalyst）易回收、可重复使用的优势。