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 PPh3 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 1H 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.