Madeleine A. Ehweiner

Understanding acetylene coordination to group VI metals is crucial for elucidating the long-standing mechanistic ambiguities of tungstoenzyme acetylene hydratase (AH), for which several contrasting pathways have been proposed. Motivated by a recently hypothesized mechanism (Biochemistry2025, 64, 10, 2154–2172.) involving one-electron oxidation of the tungsten center, we explore the reactivity of the acetylene ligand in Mo– and W–AH models bearing 4,6-dimethylpyrimidine-2-thiolate (PymS) ligands, chosen to mimic the sulfur-rich environment of the AH active site. To trigger the redox chemistry and provide protons, half an equivalent of trimethylamine-N-oxide dihydrate (TMAO·2H2O) was added to the [MoO(C2H2)(PymS)2], yielding two distinct vinylated products: 4,6-dimethyl-1-vinylpyrimidine-2(1H)-thione and 4,6-dimethyl-2-(vinylthio)pyrimidine, which could be interpreted as acetaldehyde surrogates. In this transformation, TMAO serves as an oxygen atom donor, accepting electrons from Mo(IV), and triggers the formation of the μ-oxido Mo(V) dimer, which destabilizes the coordinated C2H2 and promotes the reaction with the neighboring ligand. The presence of a Mo(V) species was detected by EPR spectroscopy, supporting a radical nature of the reaction. In contrast, the C2H2 ligand in the tungsten analogue does not undergo vinylation, likely due to its ligand environment not stabilizing the +V oxidation state. These observations may be extended to the biochemistry of the AH active site, where the W(V) oxidation state would be more readily accessible. Finally, our data provide experimental evidence for potential redox events during biological acetylene hydration and additionally suggest that acetylene coordination is essential to the process.

The Mo nitrogenase is more efficient for N₂ reduction than the alternative nitrogenases for reasons that are not fully understood. A set of related hypotheses center on the Mo ion facilitating electron delocalization, which could aid in substrate activation and/or provide enhanced stability during turnover, among other possibilities. To understand how the incorporation of Mo or other 'heterometals' influences the electronic structures of Fe-S clusters, we prepared and characterized an isostructural and isoelectronic series of [MFe₃S₄] clusters (M = V, Cr, Mo, W) that includes the first example of a [CrFe₃S₄] cluster. Comparison of the clusters' structural, spectroscopic, and computed properties reveals that the Mo- and W-containing clusters have greater M-Fe delocalization than those containing only 3d metals. Although the [CrFe₃S₄] cluster adopts the same ground spin state as the other clusters (S = 3/2), the smaller Cr atom engages in substantially attenuated─though not fully obliterated─M-Fe delocalization. Moreover, electrochemical and Mössbauer spectroscopic studies reveal that the clusters with more substantial M-Fe delocalization have more electron-rich Fe sites. Such trends, if they apply to the nitrogenase cofactors, could account for the greater efficiency of the Mo nitrogenase in biological nitrogen fixation.

Poly(phenylene methylene) (PPM) is a multifunctional polymer that is also active as an anticorrosion fluorescent coating material. Although this polymer was synthesized already more than 100 years ago, a versatile synthetic route to obtain soluble high molar mass polymers based on PPM has yet to be achieved. In this article, the influence of bifunctional bis-chloromethyl durene (BCMD) as a branching agent in the synthesis of PPM is reported. The progress of the reaction was followed by gel permeation chromatography (GPC) and NMR analysis. PPM-based copolymers with the highest molar mass reported so far for this class of materials (up to M_n of 205,300 g mol⁻¹) were isolated. The versatile approach of using BCMD was confirmed by employing different catalysts. Interestingly, thermal and optical characterization established that the branching process does not affect the thermoplastic behavior and the fluorescence of the material, thus opening up PPM-based compounds with high molar mass for applications.

The synthesis and structural determination of two isomers of the molybdenum(II) complex (η²-but-2-yne)carbonylbis[2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)benzenethiolato-κ² N,S]molybdenum(II), [Mo(C₁₁H₁₂NOS)₂(C₄H₆)(CO)] or Mo(CO)(C₂Me₂)(S-Phoz)₂, are presented. The N,N-cis–S,S-trans isomer 1 shows quite different bond lengths to the metal atom [Mo—N = 2.4715 (10) versus 2.3404 (11) Å; Mo—S = 2.4673 (3) versus 2.3665 (3) Å]. In the N,N-trans–S,S-cis isomer 2, which is isotypic with the corresponding W complex, the Mo—N bond lengths [2.236 (2) and 2.203 (2) Å], as well as the Mo—S bond lengths [2.5254 (8) and 2.5297 (8) Å], are almost the same.

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 (C₂H₂) in bioinspired complexes employing 6-methylpyridine-2-thiolate ligands. By using PMe₃ as a nucleophile, we isolated cationic carbyne and alkenyl complexes.

The structure of the title hydrated molecular salt, C₁₀H₈NS⁺·Cl⁻·H₂O, obtained by the reaction of sodium quinoline-8-thiolate Na(Quin-8-S) with CH₂Cl₂ and an aqueous solution of [Bu₄N]Cl, contains π-stacked cations [plane-to-plane separation = 3.338 (4)–3.356 (4) Å] and features chains built by alternating Cl⁻ anions and H₂O molecules connected by O—H...O hydrogen bonds. The cation shows whole-molecule disorder over two flipped orientations in a 0.853 (3):0.147 (3) ratio.

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).