Bioinorganic chemistry of molybdenum and tungsten enzymes: A structural–functional modeling approach

New solvated Mo(VI) complexes of isatin based asymmetric bisthiocarbohydrazones as potent bioactive agent: synthesis, DFT-molecular docking studies, biological activity evaluation and crystal structures

New solvated Mo(VI) complexes were isolated from the reaction of [MoO2(acac)2] with asymmetric isatin bisthiocarbohydrazone ligands. The ligands were obtained from the reaction of isatin monothiocarbohydrazone with 3,5-dibromo salicylaldehyde (L1), 3,5-dichloro salicylaldehyde (L2) and 3-chloro-5-bromo salicylaldehyde (L3), respectively. In the complexes, the ligands serve as ONS donors and coordinate to the [MoO2]2+ nucleus. The bonding sites are azomethine nitrogen atom, phenolic oxygen atom and thiol sulfur atom. The sixth coordination site is completed by an oxygen atom from an ethanol solvent. The ethanol-coordinated Mo(VI) complexes, C1-C3, [MoO2L(EtOH)] (L: L1-L3), were characterized using elemental analysis, IR and 1H NMR spectroscopies, and conductivity measurements. By crystallizing ethanol-solvated solid complexes from an EtOH/DMSO mixture, DMSO-solvated complexes (C4-C6) suitable for X-ray crystallography were obtained. Crystal structure analysis supports the proposed complex structures and geometries, but the ethanol in the sixth coordination site has been replaced by DMSO. When the anticarcinogenic effects of the ligands and complexes (C1-C3) on the C6 cell line were examined, it was found that the complexes showed higher activity than the ligands. The C3 complex appears to have the best anti-cancer activity compared to doxorubicin. Additionally, all compounds were determined to have high total antioxidant capacity. Data obtained from theoretical studies (DFT and docking) support experimental studies.

The Effect of Selenium-Based Ligands on Tungsten Acetylene Complexes

Bioinspired tungsten acetylene complexes containing pyridine-2-selenolato (PySe) or 6-methyl-pyridine-2-selenolato (6-MePySe) ligands were synthesized. 77Se NMR spectroscopy allowed for an assessment of the resonance structures in the pyridine-2-selenolato ligands and the rationalization of chemoselectivity observed in regard to 1,2 migratory insertion of HC≡CH. [W(CO)(C2H2)(CHCH-PySe)(PySe)] is formed exclusively via insertion of HC≡CH into the W-N bond, while the use of bulkier 6-MePySe allows for the isolation of [W(CO)(C2H2)(6-MePySe)2], which only partially reacts with excess HC≡CH to give [W(CO)(C2H2)(CHCH-6-MePySe)(6-MePySe)]. Oxidation of [W(CO)(C2H2)(6-MePySe)2] with pyridine-N-oxide gave the tungsten(IV) complex [WO(C2H2)(6-MePySe)2]. Complexes [W(CO)(C2H2)(6-MePySe)2] and [WO(C2H2)(6-MePySe)2] react with trimethyl phosphine to carbyne complex [W(CO)(CCH2PMe3)(PMe3)2(6-MePySe)]Cl and alkylidene complex [WO(CHCHPMe3)(PMe3)2(6-MePySe)]Cl, respectively. The addition of substituted alkynes to [W(CO)3(PySe)2] via thermal decarbonylation gave complexes [W(CO)(MeC≡CMe)(PySe)2] and [W(CO)(HC≡Ct-Bu)(PySe)2], respectively. The here presented complexes are relevant for the modeling of the active site of acetylene hydratase from Pelobacter acetylenicus, in which a tungsten atom is enclosed in a sulfur-rich coordination sphere. A recently published theoretical study concluded that the exchange of sulfur for selenium would increase the activity of the enzyme. Our findings contrast this claim as comparative analysis concludes negligible structural and electronic differences between the selenium-based and previously published sulfur-based complexes.

Untapped talents: insight into the ecological significance of methanotrophs and its prospects

The deep ocean is a rich reservoir of unique organisms with great potential for bioprospecting, ecosystem services, and the discovery of novel materials. These organisms thrive in harsh environments characterized by high hydrostatic pressure, low temperature, and limited nutrients. Hydrothermal vents and cold seeps, prominent features of the deep ocean, provide a habitat for microorganisms involved in the production and filtration of methane, a potent greenhouse gas. Methanotrophs, comprising archaea and bacteria, play a crucial role in these processes. This review examines the intricate relationship between the roles, responses, and niche specialization of methanotrophs in the deep ocean ecosystem. Our findings reveal that different types of methanotrophs dominate specific zones depending on prevailing conditions. Type I methanotrophs thrive in oxygen-rich zones, while Type II methanotrophs display adaptability to diverse conditions. Verrumicrobiota and NC10 flourish in hypoxic and extreme environments. In addition to their essential role in methane regulation, methanotrophs contribute to various ecosystem functions. They participate in the degradation of foreign compounds and play a crucial role in cycling biogeochemical elements like metals, sulfur, and nitrogen. Methanotrophs also serve as a significant energy source for the oceanic food chain and drive chemosynthesis in the deep ocean. Moreover, their presence offers promising prospects for biotechnological applications, including the production of valuable compounds such as polyhydroxyalkanoates, methanobactin, exopolysaccharides, ecotines, methanol, putrescine, and biofuels. In conclusion, this review highlights the multifaceted roles of methanotrophs in the deep ocean ecosystem, underscoring their ecological significance and their potential for advancements in biotechnology. A comprehensive understanding of their niche specialization and responses will contribute to harnessing their full potential in various domains.

New bis[MoO2] and [MoO(O2)] compounds: An artificial enzyme with peroxidase activity against o-phenylenediamine and dopamine

In this study, two binuclear dioxido- and oxidoperoxido molybdenum (VI) complexes, [{Mo^VIO₂}₂(L)(H₂O)₂] 1 and [{Mo^VIO(O₂)}₂(L)(H₂O)₂] 2, were synthesized. Complex 1 was obtained through a 1:2 reaction of ligand I with MoO₂(acac)₂, while complex 2 was synthesized in situ by reacting MoO₃ with H₂O₂ in a 1:2 ratio. The structures and characteristics of the complexes were examined employing several techniques such as elemental (CHN) analysis, spectroscopy (FT-IR, UV-Vis, ¹H, and ¹³CNMR), and thermal study (TGA). SC-XRD analysis of complex 1a revealed that the molybdenum central atom adopts an octahedral geometry and is bonded to phenolic oxygen, enolate oxygen, and azomethine nitrogen atoms. Powder X-ray diffraction was used to determine the purity of the bulk material, and the results were compared to single crystal data. Computational calculations were performed using density functional theory (DFT) at the B3LYP/6-31G(d, p) level of theory for the ligand and the LANL2DZ level of theory for the complexes, yielding geometry optimized structures that were then employed in frequency and NMR-calculations. These theoretical findings were compared to the experimental results and showed a good correlation. Furthermore, the complexes exhibited peroxidase-like activity in the presence of hydrogen peroxide as evidenced by the oxidation of o-phenylenediamine and dopamine.

Dioxygen Activation by a Bioinspired Tungsten(IV) Complex

An increasing number of discovered tungstoenzymes raises interest in the biomimetic chemistry of tungsten complexes in oxidation states +IV, +V, and +VI. Bioinspired (sulfur-rich) tungsten(VI) dioxido complexes are relatively prevalent in literature. Still, their energetically demanding reduction directly correlates with a small number of known tungsten(IV) oxido complexes, whose chemistry is not well explored. In this paper, a reduction of the [WO₂(6-MePyS)₂] (6-MePyS = 6-methylpyridine-2-thiolate) complex with PMe₃ to a phosphine-stabilized tungsten(IV) oxido complex [WO(6-MePyS)₂(PMe₃)₂] is described. This tungsten(IV) complex partially releases one PMe₃ ligand in solution, creating a vacant coordination site capable of activating dioxygen to form [WO₂(6-MePyS)₂] and OPMe₃. Therefore, [WO₂(6-MePyS)₂] can be used as a catalyst for the aerobic oxidation of PMe₃, rendering this complex a rare example of a tungsten system utilizing dioxygen in homogeneous catalysis. Additionally, the investigation of the reactivity of the tungsten(IV) oxido complex with acetylene, substrate of a tungstoenzyme acetylene hydratase (AH), revealed the formation of the tungsten(IV) acetylene adduct. Although this adduct was previously reported as an oxidation product of the tungsten(II) acetylene carbonyl complex, here it is obtained via substitution at the sulfur-rich tungsten(IV) center, mimicking the initial step of the first shell mechanism for AH as suggested by computational studies.

Formate Dehydrogenase Mimics as Catalysts for Carbon Dioxide Reduction

Formate dehydrogenases (FDH) reversibly catalyze the interconversion of CO₂ to formate. They belong to the family of molybdenum and tungsten-dependent oxidoreductases. For several decades, scientists have been synthesizing structural and functional model complexes inspired by these enzymes. These studies not only allow for finding certain efficient catalysts but also in some cases to better understand the functioning of the enzymes. However, FDH models for catalytic CO₂ reduction are less studied compared to the oxygen atom transfer (OAT) reaction. Herein, we present recent results of structural and functional models of FDH.

Inspired by Nature-Functional Analogues of Molybdenum and Tungsten-Dependent Oxidoreductases

Throughout the previous ten years many scientists took inspiration from natural molybdenum and tungsten-dependent oxidoreductases to build functional active site analogues. These studies not only led to an ever more detailed mechanistic understanding of the biological template, but also paved the way to atypical selectivity and activity, such as catalytic hydrogen evolution. This review is aimed at representing the last decade’s progress in the research of and with molybdenum and tungsten functional model compounds. The portrayed systems, organized according to their ability to facilitate typical and artificial enzyme reactions, comprise complexes with non-innocent dithiolene ligands, resembling molybdopterin, as well as entirely non-natural nitrogen, oxygen, and/or sulfur bearing chelating donor ligands. All model compounds receive individual attention, highlighting the specific novelty that each provides for our understanding of the enzymatic mechanisms, such as oxygen atom transfer and proton-coupled electron transfer, or that each presents for exploiting new and useful catalytic capability. Overall, a shift in the application of these model compounds towards uncommon reactions is noted, the latter are comprehensively discussed.

Iron and Zinc Porphyrin Linked MoO(dithiolene) Complexes in Relevance to Electron Transfer between Mo-cofactor and Cytochrome b5 in Sulfite Oxidase

Oxo-molybdenum (dithiolene) complex covalently linked individually to iron and zinc porphyrin have been synthesized to show an electron transfer between the two metal centres in relevance to electron transfer from...

Carbon Dioxide Reduction: A Bioinspired Catalysis Approach

While developed in a number of directions, bioinspired catalysis has been explored only very recently for CO₂ reduction, a challenging reaction of prime importance in the context of the energetic transition to be built up. This approach is particularly relevant because nature teaches us that CO₂ reduction is possible, with low overpotentials, high rates, and large selectivity, and gives us unique clues to design and discover new interesting molecular catalysts. Indeed, on the basis of our relatively advanced understanding of the structures and mechanisms of the active sites of fascinating metalloenzymes such as formate dehydrogenases (FDHs) and CO dehydrogenases (CODHs), it is possible to design original, active, selective, and stable molecular catalysts using the bioinspired approach. These metalloenzymes use fascinating metal centers: in FDHs, a Mo(W) mononuclear ion is coordinated by four sulfur atoms provided by a specific organic ligand, molybdopterin (MPT), containing a pyranopterin heterocycle (composed of a pyran ring fused with a pterin unit) and two sulfhydryl groups for metal chelation; in CODHs, catalytic activity depends on either a unique nickel-iron-sulfur cluster or a dinuclear Mo-Cu complex in which the Mo ion is chelated by an MPT ligand. As a consequence, the novel class of catalysts, designed by bioinspiration, consists of mononuclear Mo, W, and Ni and as well as dinuclear Mo-Cu and Ni-Fe complexes in which the metal ions are coordinated by sulfur ligands, more specifically, dithiolene chelates mimicking the natural MPT cofactor. In general, their activity is evaluated in electrochemical systems (cyclic voltammetry and bulk electrolysis) or in photochemical systems (in the presence of a photosensitizer and a sacrificial electron donor) in solution. This research is multidisciplinary because it implies detailed biochemical, functional, and structural characterization of the inspiring enzymes together with synthetic organic and organometallic chemistry and molecular catalysis studies. The most important achievements in this direction, starting from the first report of a catalytically active biomimetic bis-dithiolene-Mo complex in 2015, are discussed in this Account, highlighting the challenging issues associated with synthesis of such sophisticated ligands and molecular catalysts as well as the complexity of reaction mechanisms. While the very first active biomimetic catalysts require further improvement, in terms of performance, they set the stage in which molecular chemistry and enzymology can synergistically cooperate for a better understanding of why nature has selected these sites and for developing highly active catalysts.

Amelioration of enteric dysbiosis by polyoxotungstates in mice gut

Here we show that Preyssler-type polyoxotungstates (Preyssler-type POTs, [NaP₅W₃₀O₁₁₀]^-14) complexed with peptides can prevent the dysbiotic expansion of anaerobic bacteria of the Enterobacteriaceae family. In a dextran sulfate sodium (DSS)-induced colitis model, symptom remission of C57BL/6 J mice with colitis is achieved by orally treated with POT complexes. Ten days of daily administration of POT complexes reduces 5% body weight loss and the mRNA levels of proinflammatory markers (77% reduction for Il6, 73% reduction for Tnf, 91% reduction for Cxcl1) in the caecum and proximal colon. Bacterial population analysis reveals that these Enterobacteriaceae population in the caecal content decline by one order of magnitude after administration of POT complexes. POT complexes exert anti-inflammatory effects indirectly on the host immune system by inhibition of malignant expansion of anaerobic Enterobacteriaceae during gut inflammation. Furthermore, POTs show negligible effect on bacterial growth in vitro, healthy mice and their microbiota composition under homeostatic conditions. Rationally designed POT complexes will provide distinctive approach to improve enteric bacteria dysbiosis-associated gut inflammation by balancing bacterial communities.

Combining Structural with Functional Model Properties in Iron Synthetic Analogue Complexes for the Active Site in Rabbit Lipoxygenase

Iron complexes that model the structural and functional properties of the active iron site in rabbit lipoxygenase are described. The ligand sphere of the mononuclear pseudo-octahedral cis-(carboxylato)(hydroxo)iron(III) complex, which is completed by a tetraazamacrocyclic ligand, reproduces the first coordination shell of the active site in the enzyme. In addition, two corresponding iron(II) complexes are presented that differ in the coordination of a water molecule. In their structural and electronic properties, both the (hydroxo)iron(III) and the (aqua)iron(II) complex reflect well the only two essential states found in the enzymatic mechanism of peroxidation of polyunsaturated fatty acids. Furthermore, the ferric complex is shown to undergo hydrogen atom abstraction reactions with O-H and C-H bonds of suitable substrates, and the bond dissociation free energy of the coordinated water ligand of the ferrous complex is determined to be 72.4 kcal·mol^-1. Theoretical investigations of the reactivity support a concerted proton-coupled electron transfer mechanism in close analogy to the initial step in the enzymatic mechanism. The propensity of the (hydroxo)iron(III) complex to undergo H atom abstraction reactions is the basis for its catalytic function in the aerobic peroxidation of 2,4,6-tri(tert-butyl)phenol and its role as a radical initiator in the reaction of dihydroanthracene with oxygen.

Soft Scorpionate Hydridotris(2-mercapto-1-methylimidazolyl) borate) Tungsten-Oxido and -Sulfido Complexes as Acetylene Hydratase Models

A series of WIV alkyne complexes with the sulfur-rich ligand hydridotris(2-mercapto-1-methylimidazolyl) borate) (TmMe ) are presented as bio-inspired models to elucidate the mechanism of the tungstoenzyme acetylene hydratase (AH). The mono- and/or bis-alkyne precursors were reacted with NaTmMe and the resulting complexes [W(CO)(C2 R2 )(TmMe )Br] (R=H 1, Me 2) oxidized to the target [WE(C2 R2 )(TmMe )Br] (E=O, R=H 4, Me 5; E=S, R=H 6, Me 7) using pyridine-N-oxide and methylthiirane. Halide abstraction with TlOTf in MeCN gave the cationic complexes [WE(C2 R2 )(MeCN)(TmMe )](OTf) (E=CO, R=H 10, Me 11; E=O, R=H 12, Me 13; E=S, R=H 14, Me 15). Without MeCN, dinuclear complexes [W2 O(μ-O)(C2 Me2 )2 (TmMe )2 ](OTf)2 (8) and [W2 (μ-S)2 (C2 Me2 )(TmMe )2 ](OTf)2 (9) could be isolated showing distinct differences between the oxido and sulfido system with the latter exhibiting only one molecule of C2 Me2 . This provides evidence that a fine balance of the softness at W is important for acetylene coordination. Upon dissolving complex 8 in acetonitrile complex 13 is reconstituted in contrast to 9. All complexes exhibit the desired stability toward water and the observed effective coordination of the scorpionate ligand avoids decomposition to disulfide, an often-occurring reaction in sulfur ligand chemistry. Hence, the data presented here point toward a mechanism with a direct coordination of acetylene in the active site and provide the basis for further model chemistry for acetylene hydratase.

Oxygen Atom Transfer Reactivity of Molybdenum(VI) Complexes Employing Pyrimidine- and Pyridine-2-thiolate Ligands

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.

Anion Binding and Oxidative Modification at the Molybdenum Cofactor of Formate Dehydrogenase from Rhodobacter capsulatus Studied by X-ray Absorption Spectroscopy

Formate dehydrogenase (FDH) enzymes are versatile catalysts for CO₂ conversion. The FDH from Rhodobacter capsulatus contains a molybdenum cofactor with the dithiolene functions of two pyranopterin guanine dinucleotide molecules, a conserved cysteine, and a sulfido group bound at Mo(VI). In this study, we focused on metal oxidation state and coordination changes in response to exposure to O₂, inhibitory anions, and redox agents using X-ray absorption spectroscopy (XAS) at the Mo K-edge. Differences in the oxidative modification of the bis-molybdopterin guanine dinucleotide (bis-MGD) cofactor relative to samples prepared aerobically without inhibitor, such as variations in the relative numbers of sulfido (Mo═S) and oxo (Mo═O) bonds, were observed in the presence of azide (N₃^-) or cyanate (OCN^-). Azide provided best protection against O₂, resulting in a quantitatively sulfurated cofactor with a displaced cysteine ligand and optimized formate oxidation activity. Replacement of the cysteine ligand by a formate (HCO₂^-) ligand at the molybdenum in active enzyme is compatible with our XAS data. Cyanide (CN^-) inactivated the enzyme by replacing the sulfido ligand at Mo(VI) with an oxo ligand. Evidence that the sulfido group may become protonated upon molybdenum reduction was obtained. Our results emphasize the role of coordination flexibility at the molybdenum center during inhibitory and catalytic processes of FDH enzymes.

Deoxydehydration of polyols catalyzed by a molybdenum dioxo-complex supported by a dianionic ONO pincer ligand

Deoxydehydration (DODH) is the net reduction of diols and polyols to alkenes or dienes and water. Molybdenum cis-dioxo bis-phenolate ONO complexes were synthesized and have been shown to be active for DODH. Catalysts were screened for activity at 150-190 °C, and appreciable yields of up to 59% were obtained. PPh₃, Na₂SO₃, Zn, C, 3-octanol and 2-propanol were screened as reductants. Additionally, the reactivities of a variety of diols were screened. With (R,R)-(+)-hydrobenzoin as substrate, DODH occurs via a mechanism where reduction of the Mo catalyst is a result of diol oxidation to form two equivalents of aldehyde. These reactions result in complete conversion and near quantitative yields of trans-stilbene and benzaldehyde.

Interaction of Hydrophobic Tungsten Cluster Complexes with a Phospholipid Bilayer

Nanoconstructions composed of lipid vesicles and inorganic units (nanoparticles, metal complexes) arouse much interest across materials science and nanotechnology as hybrid materials combining useful functionalities from both parts. Ideally, these units are to be embedded into the bilayer to keep the biophysical performance of lipid vesicles having inorganic moieties screened from the environment. This can be achieved by doping a lipid bilayer with cluster complexes of transition metals. In this work, we report the preparation of nanoparticles from trinuclear W₃S₄ cluster complexes and egg phosphatidylcholine. A systematic study of their properties was performed by the differential scanning calorimetry, NMR spectroscopy, dynamic light scattering, and transmission electron microscopy. Phospholipids and clusters have been found to spontaneously self-assemble into novel cluster-lipid hybrid materials. The behavior of clusters in the hydrophobic lipid environment is determined by the structure of the ligands and cluster-to-lipid ratio. Intact cluster complexes bearing compact hydrophobic ligands are embedded into the hydrophobic midplane of a lipid bilayer, whereas cluster complexes bearing larger ligands drive the aggregation of lipids and cluster complexes. Considering these differences, it could be possible to obtain different self-assembled associates such as cluster-doped liposomes or lipid-covered crystals. These cluster-lipid hybrids can be a platform for the design of new materials for nanotechnology.

Bioinspired Tungsten Complexes Employing a Thioether Scorpionate Ligand

The synthesis and characterization of a series of novel tungsten complexes employing the bioinspired, sulfur-rich scorpionate ligand [PhTt] (phenyltris((methylthio)methyl)borate) are reported. Starting from the previously published tungsten precursor [WBr₂(CO)₃(NCMe)₂], a salt metathesis reaction with 1 equiv of Cs[PhTt] led to the desired complex [WBr(CO)₃(PhTt)] (1), making it the first tungsten complex employing a poly(thioether)borate ligand. Surprisingly, the reaction of [WBr₂(CO)₃(NCMe)₂] with an excess of the ligand gave complex [W(CO)₂(η²-CH₂SMe)(PhTt)] (2) with a bidentate (methylthio)methanide ligand as the major product. Thereby, phenyldi((methylthio)methyl)borane is formed, which was isolated and characterized by NMR spectroscopy. The bromido ligand in [WBr(CO)₃(PhTt)] was further substituted by the S,N-bidentate methimazole in order to make the first coordination sphere more sulfur-rich forming [W(CO)₂(mt)(PhTt)] (3). Alkyne tungsten complexes employing the sulfur-rich scorpionate ligand were accessible by reaction of [WBr₂(CO)(C₂R₂)₂(NCMe)] (R = Me, Ph) with Cs[PhTt] forming [WBr(CO)(C₂R₂)₂(PhTt- S, S')] (R = Me 4, Ph 5), with the potentially tridentate ligand coordinated only via two sulfur atoms. In the case of 4, the higher flexibility of the bidentate coordination leads to the formation of two isomers with respect to the six-membered ring formed by the tungsten center and the two coordinated sulfur atoms of the ligand. All complexes 1-5 were characterized by single-crystal X-ray diffraction analysis.

Dioxygen Activation with Molybdenum Complexes Bearing Amide-Functionalized Iminophenolate Ligands

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).

Tungsten from typical magmatic hydrothermal systems in China and its environmental transport

Tungsten is of extraordinarily high concentrations in the geothermal waters discharging from several representative Tibetan magmatic hydrothermal systems (up to 1103 μg/L), which are also characterized by exceptionally high W/Mo molar ratios (up to 1182). The geochemical origins of the tungsten in these geothermal waters were investigated, with a comparison to those from Rehai, the sole magmatic hydrothermal system in Yunnan, which is another major part of the Yunnan-Sichuan-Tibet Geothermal Province of China. The results show that the lithology of reservoir host rocks is the primary factor controlling the tungsten concentrations of the geothermal waters, although the contribution of magmatic fluid input cannot be ruled out. In this study, the geothermal waters are generally rich in sulfide, and therefore the molybdenum in the reservoir fluids has been substantially precipitated as the form of molybdenite; in contrast, the reservoir fluids are well undersaturated with respect to tungstenite which is much more soluble than molybdenite. Thus the neutral/alkaline hot springs, i.e. the evolved reservoir fluids, have high W/Mo molar ratios as well. In the hot spring sediments, the distribution pattern of tungsten is quite different. The concentrations of tungsten are the highest in the sediments with high iron concentrations collected from the acid hot spring vents and outflow channels. The adsorption of aqueous tungsten onto iron-bearing minerals, like goethite or pyrite, is favorable at acid pH values and thereby responsible for the very high tungsten concentrations of these acid hot spring sediments. The proportions of thiotungstates in total tungsten are quite low for all the hot springs, as indicated by thermodynamic calculations, suggesting that thiolation of tungstate has little impacts on the environmental transport and fate of geothermal tungsten in the investigated hydrothermal areas. This is the first study to report the tungsten geochemistry of hot springs in mainland China.

Activation and Photoinduced Release of Alkynes on a Biomimetic Tungsten Center: The Photochemical Behavior of the W-S-Phoz System

The synthesis and structural determination of four tungsten alkyne complexes coordinated by the bio-inspired S,N-donor ligand 2-(4',4'-dimethyloxazoline-2'-yl)thiophenolate (S-Phoz) is presented. A previously established protocol that involved the reaction of the respective alkyne with the bis-carbonyl precursor [W(CO)2 (S-Phoz)2 ] was used for the complexes [W(CO)(C2 R2 )(S-Phoz)2 ] (R=H, 1 a; Me, 1 b; Ph, 1 c). Oxidation with pyridine-N-oxide gave the corresponding W-oxo species [WO(C2 R2 )(S-Phoz)2 ] (R=H, 2 a; Me, 2 b; Ph, 2 c). All W-oxo-alkyne complexes (2 a, b, c) were found to be capable of alkyne release upon light irradiation to afford five-coordinate [WO(S-Phoz)2 ] (3). The photoinduced release of the alkyne ligand was studied in detail by in situ 1 H NMR measurements, which revealed correlation of the photodissociation rate constant (2 b>2 a>2 c) with the elongation of the alkyne C≡C bond in the molecular structures. Oxidation of [WO(S-Phoz)2 ] (3) with pyridine-N-oxide yielded [WO2 (S-Phoz)2 ] (4), which shows highly fluxional behavior in solution. Variable-temperature 1 H NMR spectroscopy revealed three isomeric forms with respect to the ligand arrangement versus each other. Furthermore, compound 4 rearranges to tetranuclear oxo compound [W4 O4 (μ-O)6 (S-Phoz)4 ] (5) and dinuclear [{WO(μ-O)(S-Phoz)}2 ] (6) over time. The latter two were identified by single-crystal X-ray diffraction analyses.

Structurally characterized one oxo–desoxo bridged Mo2–bis(dithiolene) complex and its interconversion to a discrete oxo or desoxo DMSOR model

The dissymmetric binuclear complex1acts as a precursor of the molybdoenzyme models of the dimethylsulfoxide reductase (DMSOR) class.

Comparative studies on the contribution of NHS hydrogen bonds in tungsten and molybdenum benzenedithiolate complexes

A series of monooxotungsten(iv) and dioxotungsten(vi) benzenedithiolates, (NEt₄)₂[W^IVO(1,2-S₂-3-RCONHC₆H₃)₂] (1-W; R = CH₃ (a), t-Bu (b), or CF₃ (c)) and (NEt₄)₂[W^VIO₂(1,2-S₂-3-RCONHC₆H₃)₂] (2-W), were synthesized and compared with the corresponding molybdenum analogues. Single crystals of trans-1b-W were successfully obtained, and the crystal structure was determined by X-ray analysis although 1b-Mo could not be crystallized. The NHS hydrogen bonds shifted the potential of the W(iv/v) redox couple to more positive values, and the strength of the hydrogen bond and the positive shift value were strongly correlated. The hydrogen bonds in both 1-W and 2-W were weaker than those in the corresponding molybdenum analogues; however, the effect of the hydrogen bonds on the redox potential was greater in 1-W.

Oxygen atom transfer between DMSO and benzoin catalyzed by cis-dioxidomolybdenum(vi) complexes of tetradentate Mannich bases

Dioxidomolybdenum(vi) complexes of tetradentate ONNO donor Mannich base ligands for the catalytic oxygen atom transfer between benzoin and DMSO are reported.