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.

Direct, one-pot oxidation of benzyl alcohols to benzoic acid with H ₂ O ₂ catalyzed by Fe( iii ) complexes.

Reaction of [NiCl₂(PnH)₄] (1) (PnH=6‐tert‐butyl‐pyridazine‐3‐thione) with NiCl₂ affords the binuclear paddlewheel (PW) complex [Ni₂(Pn)₄] (2). Diamagnetic complex 2 is the first example of a PW complex capable of reversibly binding and releasing NH₃. The NH₃ ligand in [Ni₂(Pn)₄(NH₃)] (2⋅NH₃) enforces major spectroscopic and magnetic susceptibility changes, thus displaying vapochromic properties (λ_max(2)=532 nm, λ_max(2⋅NH₃)=518 nm) and magnetochemical switching (2: S=0; 2⋅NH₃: S=1). Upon repeated adsorption/desorption cycles of NH₃ the PW core remains intact. Compound 2 can be embedded into thin polyurethane films (2^P) under retention of its sensing abilities. Therefore, 2 qualifies as reversible optical probe for ammonia. The magnetochemical switching of 2 and 2⋅NH₃ was studied in detail by SQUID measurements showing that in 2⋅NH₃, solely the Ni atom coordinated the NH₃ molecule is responsible for the paramagnetic behavior.

Two enantiomerically pure chiral dioxidomolybdenum(VI) complexes (1 and 2) of the type [MoO₂L] (L = dianionic, tetradentate ONNO‐ligand) were synthesized and investigated in enantioselective oxidation reactions. The solid‐state structures of complex 1 and 2, determined via single‐crystal X‐ray diffraction analysis revealed two fundamentally different coordination geometries: a C1‐symmetric cis‐β isomer (Λ‐1), and a C2‐symmetric cis‐α isomer (Δ‐2). In both cases, only one of the two possible helical enantiomers (Λ‐ or Δ‐helix) was formed. The complexes were examined as precatalysts in the epoxidation of the challenging prochiral substrate trans‐stilbene, using either tert‐butyl hydroperoxide (TBHP) or cumyl hydroperoxide (CHP) as oxidants. The asymmetric cis‐β complex 1 was found to be significantly more active in the epoxidation than its cis‐α counterpart 2, asymmetric induction was, however, negligible for both complexes. The complexes were also tested in catalytic enantioselective sulfoxidation reactions where chiral induction could be achieved, albeit small. The observed putative molybdenum oxido‐peroxido intermediate 1‐O₂ could be identified as an important precomplex before formation of the active catalyst in sulfoxidation.

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).