Hanson Carly S, Donohoe Michael, Pratt Derek A
Department of Chemistry and Biomolecular Sciences, University of Ottawa, 10 Marie Curie Pvt., Ottawa K1N 6N5, Ontario, Canada.
J Org Chem. 2023 Dec 15;88(24):17420-17429. doi: 10.1021/acs.joc.3c02246. Epub 2023 Dec 5.
Molybdenum dithiocarbamates (MDTCs) are indispensable lubricant additives. Although their role as antiwear agents is well established, they have also been attributed antioxidant properties that are not understood. MDTCs do not inhibit autoxidation, but they markedly enhance the capacity of diphenylamines (DPAs)─ubiquitous radical-trapping antioxidants (RTAs)─to do so. We find this synergy to be evident not only at elevated temperatures (160 °C in -hexadecane) but also at moderate temperatures, where autoxidations can be continuously monitored and kinetics more easily interpreted (100 °C in squalane). Interestingly, the synergy disappeared in an unsaturated hydrocarbon (-hexadec-1-ene), where the RTA activity of the DPA is known to result from the diarylnitroxide derived therefrom. Autoxidations of squalane carried out in the presence of the diarylnitroxide─wherein it is a poor inhibitor─were much better inhibited in the presence of MDTC, suggesting that it converts the nitroxide to (a) more competent RTA(s). Indeed, preparative experiments revealed two species: DPA and a DPA dimer into which a single oxygen atom had been incorporated. This conversion is accelerated by the oxidation of MDTC to a dioxo molybdenum species. A mechanism is proposed to account for these observations, and the implications of our findings and their interpretation are discussed.
二硫代氨基甲酸钼（MDTCs）是不可或缺的润滑添加剂。尽管它们作为抗磨剂的作用已得到充分证实，但人们也认为它们具有尚未被理解的抗氧化性能。MDTCs本身并不抑制自氧化反应，但它们能显著增强二苯胺（DPAs）——普遍存在的自由基捕获抗氧化剂（RTAs）——的抑制能力。我们发现这种协同作用不仅在高温下（在正十六烷中为160°C）明显，在中等温度下也很明显，在中等温度下可以连续监测自氧化反应，且动力学更容易解释（在角鲨烷中为100°C）。有趣的是，在不饱和烃（1-十六烯）中这种协同作用消失了，已知在该不饱和烃中DPA的RTA活性源于由此衍生的二芳基氮氧化物。在二芳基氮氧化物存在下进行的角鲨烷自氧化反应（其中二芳基氮氧化物是一种较差的抑制剂），在MDTC存在下受到了更好的抑制，这表明MDTC将氮氧化物转化为了更有效的RTA。实际上，制备实验揭示了两种物质：DPA和一种掺入了单个氧原子的DPA二聚体。MDTC氧化为二氧代钼物种会加速这种转化。本文提出了一种机制来解释这些观察结果，并讨论了我们研究结果及其解释的意义。
