Nature-Inspired Homogeneous Catalytic Perchlorate Reduction Using Molybdenum Complexes

Inspired by the reactivity of (per)chlorate reducing molybdoenzymes and encouraged by the lack of molybdenum-containing functional models thereof, two molybdenum(VI) complexes of the type [MoO2L2] (L = pyrimidine-2-thiolate or 6-methylpyridine-2-thiolate) were found to be active homogeneous catalysts for the reduction of ClO4- to ClO3- in CH2Cl2 using PPh3 as sacrificial oxygen acceptor. The subsequent stepwise reduction of ClO3- to Cl- is facilitated by our catalysts, but it can also proceed with only PPh 3 without the aid of a catalyst. We followed the decrease in perchlorate concentration in the catalytic solutions not only indirectly by oxidation of PPh3 to OPPh3 via H-1 NMR spectroscopy but also directly by determining the perchlorate concentration at certain time points over 24 h with high-performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICPMS/MS). These experiments revealed the pyrimidine-2-thiolate system to be more efficient. The reduction of ClO4- to ClO3- with [MoOL2], which is generated after the reaction of [MoO2L2] with PPh3, was computed to be highly exergonic with low kinetic barriers for both catalysts. Thus, the rate-determining step of the overall catalytic reaction is the initial oxygen atom transfer from [MoO2L2] to PPh3.

Automated summary

Two molybdenum(VI) complexes act as catalysts for the reduction of (per)chlorate, with the pyrimidine-2-thiolate system showing higher efficiency. The reduction of (per)chlorate to chloride is facilitated by the catalysts, with the initial oxygen atom transfer from [MoO2L2] to PPh3 being the rate-determining step.