Nadia C. Mösch-Zanetti

The development of catalytic reactions based on earth-abundant first-row transition metals that use chemicals from renewable feedstocks aligns with current principles of sustainable chemistry. Here, we employ oxovanadium(IV) salen-type complexes ([VO(Ln)], n = 1–5: 1–5) as catalysts for the selective epoxidation of biodiesel-derived methyl oleate, where Ln are tetradentate salen-type ligands with different diamine linkers, namely ethylenediamine (1), 1,3-diaminopropane (2 and 5), diaminomaleonitrile (3), and 1,2-diaminocyclohexane (4). The 1,3-diaminopropane system was examined with both unsubstituted (p-H) (2) and substituted (p-OMe) (5) salicylaldehyde-aromatic rings. DFT analysis revealed the influence of the ligand on the electronics of the V═O moiety, with [VO(L3)] exhibiting the most electron-deficient vanadium center. Notably, this complex also proved to be the most efficient epoxidation catalyst under optimized conditions (1 mol % catalyst, 3.5 equiv oxidant-TBHP, no added solvent). UV–Vis spectroscopy monitoring of the reaction between the complexes 1–5 with excess oxidant (pseudo-first order conditions) highlighted pronounced ligand-dependent differences in reactivity. Linear kinetics were observed only for [VO(L2)] and [VO(L5)], both containing a 1,3-diaminopropane bridge. In contrast, compounds with saturated two-carbon bridges [VO(L1)] and [VO(L4)] reacted slowly with the oxidant, displaying an induction period. Finally, [VO(L3)] does not react with the oxidant under the same conditions, suggesting an alternative epoxidation mechanism via a V(IV) center in the initial stage of catalysis. These results demonstrate that vanadium salen-type complexes, although structurally similar, enable epoxidation through either the commonly proposed V(V)-peroxide pathway or a V(IV) Lewis-acidic center, depending on the nature of the moiety bridging the two imine nitrogens.

The aqueous chemistry of molybdenum and tungsten complexes is relevant due to their occurrence in metalloenzymes. However, water-stable and -soluble model complexes in biologically relevant higher oxidation states are rare. Metallocenes of the type [Cp₂M­(OH₂)]²⁺ (M = Mo, W) exhibit such properties despite the nonbiomimetic cyclopentadienyl (Cp) ligand. Therefore, the aqueous acid–base properties of these bis-aqua tungstocenes and molybdocenes were investigated, as their +IV oxidation states and coordinated H₂O render them ideal candidates. The precursors [Cp₂MoCl₂] (1a), [Cp₂Mo­(μ-OH)₂MoCp₂]­(pTsO)₂ (1b), [Cp₂Mo­(pTsO)₂] (3) and their tungsten analogues [Cp₂WCl₂] (2a), [Cp₂W­(μ-OH)₂WCp₂]­(pTsO)₂ (2b), and [Cp₂W­(pTsO)₂] (4) were studied via aqueous potentiometric titrations. Molybdocene acted as a triprotic acid, with deprotonation occurring from both mono- and dimeric species, while tungstocene reacted as a diprotic acid exclusively in its monomeric form. Tungsten complexes exhibit higher acidity with 1–1.5 lower pK _a values than molybdenum, which is of general interest for the understanding of tungstoenzymes. The substitution of chlorides by tosylates allowed the isolation of 3 and 4, which proved to be less suitable precursors for aqueous chemistry, as they were more difficult to hydrolyze, leading to partial degradation upon hydrolysis. This indicates that simply the presence of a Cp₂M motif is not sufficient for the formation of the aqueous species.

The bidendate ligands H Pn ^Me (4‐methyl‐6‐ tert ‐butyl‐3( 2H )‐thiopyridazine) and H Pn ^{t Bu} (4,6‐di‐ tert ‐butyl‐3(2 H )‐thiopyridazine) were coordinated to Ni(II) for the formation of paddlewheel (PW) complexes [Ni ₂ ( Pn ^R ) ₄ ]. Starting from [NiCl ₂ (H Pn ^Me ) ₄ ] ( 1 ^Me ), the poorly soluble hexanuclear cluster [Ni ₆ ( Pn ^Me ) ₁₂ ] ( 2 ^Me′ ) forms which is capable of binding NH ₃ reversibly. The obtained monomeric PW complex [Ni ₂ ( Pn ^Me ) ₄ (NH ₃ )] ( 3 ^Me ) shows vapochromism and magnetochemical switching. The third donor site in the pyridazine heterocycle Pn ^Me is found to be a key factor for selective NH ₃ uptake due to the ideal position of these additional nitrogen atoms in the second coordination sphere to form hydrogen bonds to NH ₃ . With excess NH ₃ the PW core of 3 ^Me collapses, and the octahedral bis–ammine complex [Ni( Pn ^Me ) ₂ (NH ₃ ) ₂ ] ( 4 ^Me ) is formed. Upon loss of NH ₃ , 3 ^Me is reformed. All three compounds 2 ^{Me ′} , 3 ^Me and 4 ^Me have a very different color in solution or suspension, which allows for basic sensing properties. Quantum mechanical calculations on complexes clarify the preference for the N site over sulfur in coordinating small molecules. The influence of hydrogen bonding on this preference is quantitatively emphasized, providing confirmation of the high selectivity of these systems for NH ₃ . In contrast to H Pn ^Me , with the bulkier ligand H Pn ^{t Bu} , two H Pn ^{t Bu} molecules coordinate to NiCl ₂ , giving the tetrahedral complex [NiCl ₂ (H Pn ^{t Bu} ) ₂ ] ( 5 ^{t Bu} ) while a PW complex was not observed.

Molybdenum trioxide (MoO₃) is widely utilized as an interfacial layer in organic electronic devices due to its high work function and favorable energy level alignment with organic semiconductors. While its role in facilitating hole injection has been extensively studied, the microscopic mechanisms underlying charge transfer at MoO₃/organic interfaces remain elusive. Here, we investigate the interaction between 2H-phthalocyanine (2H-Pc) and ultrathin MoO₃ films grown on Pd(100) as a model system to explore the microscopic origin of charge transfer from the organic layer to the oxide substrate. Using a combination of scanning tunneling microscopy/spectroscopy, X-ray photoemission spectroscopy, near-edge X-ray absorption fine structure, work function measurements, and density functional theory, we find clear evidence for integer charge transfer from the molecules into the substrate, resulting in positively charged molecules in both upright and flat adsorption geometries. The electronic signatures of charging are accompanied by distinct SOMO–SUMO gaps, with upright molecules exhibiting a small gap (∼0.4 eV), while flat-lying molecules show a significantly larger gap (∼1.5 eV) owing to reduced electronic screening. These findings provide atomically resolved insight into charge transfer and highlight how adsorption geometry and local dielectric environment govern the electronic structure of hybrid interfaces.

A bioinspired tungsten(iv) complex reduces molecular oxygen one electron at a time: spin density of the superoxide intermediate [WO(η¹-O₂)(PyS)₂(PMe₃)] is the stepping stone in the reduction of dioxygen.

Hydrosilylation of ketones using manganese complexes has emerged as an efficient and safer alternative to traditional reduction methods. Reported manganese(I) systems are typically monomeric Mn(I) carbonyl bromide catalysts, while dimeric systems have remained largely unexplored. Here, three manganese complexes of the dimeric type [{Mn(6‐R‐PyS)(CO)₃}₂] (R = H 1, CH₃ 2, CF₃ 3) featuring thiopyridine ligands and Mn₂S₂ cores are found to be catalysts in the visible light‐induced hydrosilylation of ketones at room temperature. Notably, optimal reactivity was achieved when the irradiation wavelength was set at 427 nm. Furthermore, a catalyst loading of 0.1 mol% was sufficient to achieve full conversion within 90 min over a wide scope of acetophenones and aliphatic ketones. However, the reactivity was drastically reduced when the sterically demanding, electron withdrawing CF₃ group was introduced to the ligand. An induction time of 10 min was observed that can be attributed to the formation of the active species, after which the reaction was found to proceed without irradiation. Further investigations into the mechanism revealed that upon irradiation of solutions of the complexes, CO is released forming an undefined paramagnetic species. In conclusion, for fast catalysis with the dinuclear Mn(I) complexes sterically unhindered ligands and activation with light are required. Furthermore, the catalytic reaction is amenable to continuous flow chemistry as exemplified by the hydrosilylation of acetophenone with 0.1 mol% of catalyst 1. When carried out in a photoflow reactor, the reaction is completed after 14.2‐min resistant time.

One very unique feature of oxidorhenium(V) complexes is their dual catalytic activity in both reduction of stable oxyanions like perchlorate ClO₄ ⁻ and nitrate NO₃ ⁻ as well as epoxidation of olefins. In our ongoing research efforts, we were interested to study how an electron-withdrawing ligand would affect both these catalytic reactions. Hence, we synthesized the novel bidentate dimethyloxazoline-dichlorophenol ligand HL1 and synthesized oxidorhenium(V) complex [ReOCl(L1)₂] (1). Then, catalytic experiments were conducted showing that non-redox epoxidation activity is indeed enhanced, but redox catalysis via oxygen atom transfer (OAT) activity was reduced for ClO₄ ⁻ and NO₃ ⁻ reductions. From one nitrate reduction experiment, a small amount of the singly-oxidized dioxidorhenium(VI) complex [ReO₂(L1)] (2) could be isolated, confirming the successful reduction sequence of nitrate to nitrite NO₂ ⁻ (2e⁻ reduction) to NO (1e⁻ reduction). Furthermore, ligand L1 displayed a richer than usually observed coordination chemistry, allowing for the isolation of complexes [ReOCl₂(SMe₂)(L1)] (trans-3a), [ReOCl₂(OPPh₃)(L1)] (3b) and [ReCl₃(OPPh₃)(L1)] (3c). Complexes 1 and 3a-b were tested in cyclooctene epoxidation, 1 was additionally investigated as an oxyanion reduction catalyst of perchlorate and nitrate. All compounds HL1, 1, 2 and 3a–c could be characterized by single-crystal X-ray diffraction, besides other routine analyses.

Nature chooses a high-valent tungsten center at the active site of the enzyme acetylene hydratase to facilitate acetylene hydration to acetaldehyde. However, the reactions of tungsten-coordinated acetylene are still not well understood, which prevents the development of sustainable bioinspired alkyne hydration catalysts. Here we report the reactivity of two bioinspired tungsten complexes with the acetylene ligand acting as a four-: [W(CO)(C₂H₂)(PymS)₂] (1) and a two-electron donor: [WO(C₂H₂)(PymS)₂] (3), with PMe₃ as a nucleophile to simulate the enzyme's reactivity (PymS = 4-(trifluoromethyl)-6-methylpyrimidine-2-thiolate). In dichloromethane, compound 1 was found to react to the cationic carbyne [W≡CCH₂PMe₃(CO)(PMe₃)₂(PymS)]Cl (2-Cl) while 3 reacts to the vinyl compound [WO(CH═CHPMe₃)(PMe₃)₃(PymS)]Cl (4-Cl). The formation of the latter follows the common rules applied to η²-alkyne complexes, whereas the carbyne formation was not expected due to the challenging 1,2-H shift. To understand these differences in behavior between seemingly similar acetylene complexes, stepwise addition of the nucleophile in various solvents was investigated by synthetic, spectroscopic, and computational approaches. In this manuscript, we describe that only a four-electron donor acetylene complex can react to the carbyne over the η¹-vinyl intermediate and that 1,2-H shift can be assisted by an H-transfer reagent (in this case, the decoordinated PymS ligand). Furthermore, to favor the attack of PMe₃ at W coordinated acetylene, the metal center needs to be electron-poor and crowded enough to prevent nucleophile coordination. Finally, the intricate role of the anionic PymS ligand in the vicinity of the first coordination sphere models the potential involvement of amino acid residues during acetylene transformations in AH.

Frustrated Lewis Pairs (FLP) have found exciting access in catalysis and small molecule activation. Here, we describe the potential of two vanadium(V) oxido complexes with monoanionic bidentate N,O donors, [VOCl(L^CF3)₂] (1 a), [VOCl(L^H)₂] (1 b) as bases in FLPs. Upon reaction of 1 a with the Lewis acid B(C₆F₅)₃ the FLP [VOCl(L^CF3)₂]/B(C₆F₅)₃ (1 a/B(C₆F₅)₃) was observed. Addition of Ph₃SiH led to the reduction of V^V to V^IV under formation of the adduct [V{OB(C₆F₅)₃}(L^CF3)₂] (2 a) while FLP reactivity was not observed. Dissolving 2 a in CH₃CN leads to the V^IV oxido complex, [VO(L^CF3)₂] (3 a). In contrast, when 1 b was reacted with B(C₆F₅)₃, a V^IV oxido complex [VO(L^H)₂] (3 b) was obtained. Upon addition of B(C₆F₅)₃ to 3 b no reaction leading to the expected adduct such as 2 a could be observed. Only few crystals deriving from a disproportion reaction could be identified as an adduct dimer [V{OB(C₆F₅)₃}L^H(μ‐O)]₂. While FLP reactivity could not be found yet with the described systems, the redox labile nature and versatility of the metal complexes are demonstrated.