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.

Inspired by the proposed inner-sphere mechanism of the tungstoenzyme acetylene hydratase, we have designed tungsten acetylene complexes and investigated their reactivity. Here, we report the first intermolecular nucleophilic attack on a tungsten-bound acetylene (C 2 H 2 ) in bioinspired complexes employing 6-methylpyridine-2-thiolate ligands. By using PMe 3 as a nucleophile, we isolated cationic carbyne and alkenyl 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 [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 .

Two novel iminophenolate ligands with amidopropyl side chains (HL2 and HL3) on the imine functionality have been synthesized in order to prepare dioxidomolybdenum(VI) complexes of the general structure [MoO2L2] featuring pendant internal hydrogen bond donors. For reasons of comparison, a previously published complex featuring n-butyl side chains (L1) was included in the investigation. Three complexes (1–3) obtained using these ligands (HL1–HL3) were able to activate dioxygen in an in situ approach: The intermediate molybdenum(IV) species [MoO(PMe3)L2] is first generated by treatment with an excess of PMe3. Subsequent reaction with dioxygen leads to oxido peroxido complexes of the structure [MoO(O2)L2]. For the complex employing the ligand with the n-butyl side chain, the isolation of the oxidomolybdenum(IV) phosphino complex [MoO(PMe3)(L1)2] (4) was successful, whereas the respective Mo(IV) species employing the ligands with the amidopropyl side chains were found to be not stable enough to be isolated. The three oxido peroxido complexes of the structure [MoO(O2)L2] (9–11) were systematically compared to assess the influence of internal hydrogen bonds on the geometry as well as the catalytic activity in aerobic oxidation. All complexes were characterized by spectroscopic means. Furthermore, molecular structures were determined by single-crystal X-ray diffraction analyses of HL3, 1–3, 9–11 together with three polynuclear products {[MoO(L2)2]2(µ-O)} (7), {[MoO(L2)]4(µ-O)6} (8) and [C9H13N2O]4[Mo8O26]·6OPMe3 (12) which were obtained during the synthesis of reduced complexes of the type [MoO(PMe3)L2] (4–6).

The isolation of a molybdenum(IV) acetylene (C 2 H 2 ) complex containing two bioinspired 6-methylpyridine-2-thiolate ligands is reported. The synthesis can be performed either by oxidation of a molybdenum(II) C 2 H 2 complex or by substitution of a coordinated PMe 3 by C 2 H 2 on a molybdenum(IV) center. Both C 2 H 2 complexes were characterized by spectroscopic means as well as by single-crystal X-ray diffraction. Furthermore, the reactivity of the coordinated C 2 H 2 was investigated with regard to acetylene hydratase, one of two enzymes that accept C 2 H 2 as a substrate. While the reaction with water resulted in the vinylation of the pyridine-2-thiolate ligands, an intermolecular nucleophilic attack on the coordinated C 2 H 2 with the soft nucleophile PMe 3 was observed to give a cationic ethenyl complex. A comparison with the tungsten analogues revealed less tightly bound C 2 H 2 in the molybdenum variant, which, however, shows a higher reactivity toward nucleophiles.