Four dioxidomolybdenum­(VI) complexes of the general structure [MoO2L2] employing the S,N-bidentate ligands pyrimidine-2-thiolate (PymS, 1), pyridine-2-thiolate (PyS, 2), 4-methylpyridine-2-thiolate (4-MePyS, 3) and 6-methylpyridine-2-thiolate (6-MePyS, 4) were synthesized and characterized by spectroscopic means and single-crystal X-ray diffraction analysis (2–4). Complexes 1–4 were reacted with PPh3 and PMe3, respectively, to investigate their oxygen atom transfer (OAT) reactivity and catalytic applicability. Reduction with PPh3 leads to symmetric molybdenum­(V) dimers of the general structure [Mo2O3L4] (6–9). Kinetic studies showed that the OAT from [MoO2L2] to PPh3 is 5 times faster for the PymS system than for the PyS and 4-MePyS systems. The reaction of complexes 1–3 with PMe3 gives stable molybdenum­(IV) complexes of the structure [MoOL2(PMe3)2] (10–12), while reduction of [MoO2(6-MePyS)2] (4) yields [MoO­(6-MePyS)2(PMe3)] (13) with only one PMe3 coordinated to the metal center. The activity of complexes 1–4 in catalytic OAT reactions involving Me2SO and Ph2SO as oxygen donors and PPh3 as an oxygen acceptor has been investigated to assess the influence of the varied ligand frameworks on the OAT reaction rates. It was found that [MoO2(PymS)2] (1) and [MoO2(6-MePyS)2] (4) are similarly efficient catalysts, while complexes 2 and 3 are only moderately active. In the catalytic oxidation of PMe3 with Me2SO, complex 4 is the only efficient catalyst. Complexes 1–4 were also found to catalytically reduce NO3 – with PPh3, although their reactivity is inhibited by further reduced species such as NO, as exemplified by the formation of the nitrosyl complex [Mo­(NO)­(PymS)3] (14), which was identified by single-crystal X-ray diffraction analysis. Computed ΔG ⧧ values for the very first step of the OAT were found to be lower for complexes 1 and 4 than for 2 and 3, explaining the difference in catalytic reactivity between the two pairs and revealing the requirement for an electron-deficient ligand system.

Upon replacement of molybdenum by tungsten in DMSO reductase isolated from the Rhodobacteraceae family, the derived enzyme catalyzes DMSO reduction faster. To better understand this behavior, we synthesized two tungsten(VI) dioxido complexes [W VI O 2 L 2 ] with pyridine- (PyS) and pyrimidine-2-thiolate (PymS) ligands, isostructural to analogous molybdenum complexes we reported recently. Higher oxygen atom transfer (OAT) catalytic activity was observed with [WO 2 (PyS) 2 ] compared to the Mo species, independent of whether PMe 3 or PPh 3 was used as the oxygen acceptor. [W VI O 2 L 2 ] complexes undergo reduction with an excess of PMe 3 , yielding the tungsten(IV) oxido species [WOL 2 (PMe 3 ) 2 ], while with PPh 3 , no reactions are observed. Although OAT reactions from DMSO to phosphines are known for tungsten complexes, [WOL 2 (PMe 3 ) 2 ] are the first fully characterized phosphine-stabilized intermediates. By following the reaction of these reduced species with excess DMSO via UV–vis spectroscopy, we observed that tungsten compounds directly react to W VI O 2 complexes while the Mo analogues first form μ-oxo Mo(V) dimers [Mo 2 O 3 L 4 ]. Density functional theory calculations confirm that the oxygen atom abstraction from W VI O 2 is an endergonic process contrasting the respective reaction with molybdenum. Here, we suggest that depending on the sacrificial oxygen acceptor, the tungsten complex may participate in catalysis either via a redox reaction or as an electrophile.

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 [MoO 2 L 2 ] (L = pyrimidine-2-thiolate or 6methylpyridine-2-thiolate) were found to be active homogeneous catalysts for the reduction of ClO 4 − to ClO 3 − in CH 2 Cl 2 using PPh 3 as sacrificial oxygen acceptor. The subsequent stepwise reduction of ClO 3 − 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 PPh 3 to OPPh 3 via 1 H 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 ClO 4 − to ClO 3 − with [MoOL 2 ], which is generated after the reaction of [MoO 2 L 2 ] with PPh 3 , 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 [MoO 2 L 2 ] to PPh 3 .

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