Computational study of acetylene hydration by bio-inspired group six catalyst models

Vinyl-pyrazole as a biomimetic acetaldehyde surrogate

Inspired by the enzyme acetylene hydratase, we investigated the reactivity of acetylene with tungsten(II) pyrazole complexes. Our research revealed that the complex [WBr2(pz-NHCCH3)(CO)3] (pz = 3,5-dimethyl-pyrazolate) facilitates the stochiometric reaction between pzH and acetylene to give N-vinyl-pz. This vinyl compound readily hydrolyzes to acetaldehyde, mirroring the product of acetylene hydration in the enzymatic process. The formation of the vinyl compound likely involves a reactive intermediate complex where acetylene acts as a two-electron donor, in contrast to isolable acetylene complexes that are inert to nucleophilic attack by water. Results suggest an alternative mechanism for the enzyme, including vinylation of a neighboring amino acid by acetylene in the active site prior to hydration.

Isolation and characterization of a bis(dithiolene)-supported tungsten-acetylenic complex as a model for acetylene hydratase

Acetylene hydratase is currently the only known mononuclear tungstoenzyme that does not catalyze a net redox reaction. The conversion of acetylene to acetaldehyde is proposed to occur at a W(IV) active site through first-sphere coordination of the acetylene substrate. To date, a handful of tungsten complexes have been shown to bind acetylene, but many lack the bis(dithiolene) motif of the native enzyme. The model compound, [W(O)(mnt)2]2-, where mnt2- is 1,2-dicyano-1,2-dithiolate, was previously reported to bind an electrophilic acetylene substrate, dimethyl acetylenedicarboxylate, and characterized by FT-IR, UV-vis, potentiometry, and mass spectrometry (Yadav, J; Das, S. K.; Sarkar, S., J. Am. Chem. Soc., 1997, 119, 4316-4317). By slightly changing the electrophilic acetylene substrate, an acetylenic-bis(dithiolene)‑tungsten(IV) complex has been isolated and characterized by FT-IR, UV-vis, NMR, X-ray diffraction, and X-ray absorption spectroscopy. Activation parameters for complex formation were also determined and suggest coordination-sphere reorganization is a limiting factor in the model complex reactivity.

Synthesis and Reactivity of a Bioinspired Molybdenum(IV) Acetylene Complex

The isolation of a molybdenum(IV) acetylene (C₂H₂) complex containing two bioinspired 6-methylpyridine-2-thiolate ligands is reported. The synthesis can be performed either by oxidation of a molybdenum(II) C₂H₂ complex or by substitution of a coordinated PMe₃ by C₂H₂ on a molybdenum(IV) center. Both C₂H₂ complexes were characterized by spectroscopic means as well as by single-crystal X-ray diffraction. Furthermore, the reactivity of the coordinated C₂H₂ was investigated with regard to acetylene hydratase, one of two enzymes that accept C₂H₂ 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 C₂H₂ with the soft nucleophile PMe₃ was observed to give a cationic ethenyl complex. A comparison with the tungsten analogues revealed less tightly bound C₂H₂ in the molybdenum variant, which, however, shows a higher reactivity toward nucleophiles.

Unraveling the Way Acetaldehyde is Formed from Acetylene: A Study Based on DFT

Acetylene hydratase (AH) of Pelobacter acetylenicus is a tungsten (W)-containing iron-sulfur enzyme that catalyzes the transformation of acetylene to acetaldehyde, the exact/true reaction mechanism of which is still in question. Scientists utilized different computational approaches to understand the reaction mechanism of acetylene hydration. Some identified it as a multistep (4-16) process that starts with the displacement of a water molecule present at the active site of AH with acetylene. However, some said that there is no need to displace water with acetylene at the active site of AH. As the reaction mechanism for the conversion of acetylene to acetaldehyde is still controversial and needs to be investigated further, DFT studies were performed on the model complexes derived from the native protein X-ray crystal structure of AH. Based on the computational results, here we are proposing the nucleophilic reaction mechanism where the water (Wat1424) molecule is coordinated to the W center and Asp13 is assumed to be in an anionic form. The Wat1424 molecule is activated by W and then donates one of its protons to the anionic Asp13, forming the W-bound hydroxide and protonated Asp13. The W-bound hydroxide then attacks the C1 atom of acetylene together with the transfer of a proton from Asp13 to its C2 atom, resulting in the formation of a vinyl alcohol intermediate complex. The energy barrier associated with this step is 14.4 kcal/mol. The final, rate-limiting, step corresponds to the tautomerization of the vinyl alcohol intermediate to acetaldehyde via intermolecular assistance of two water molecules, associated with an energy barrier of 18.9 kcal/mol. Also, the influence of the metal on the hydration of acetylene is studied when W is replaced with Mo.

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.

A density functional theory exploration on the Zn catalyst for acetylene hydration

The acetylene hydration method to produce acetaldehyde has been widely used for over 130 years; however, a detailed molecular-level understanding of the reaction mechanism is still lacking. In the present work, we systematically investigated the mechanisms of such reactions on ZnCl₂, Zn(OH) Cl, and Zn(OH)₂ catalysts through density functional theory (DFT) methods. The Fukui function, condensed Fukui function, and Hirshfeld charges enabled us to predict the active sites of the catalysts and acquire electron transfer information. From these data, we found that catalysts bearing hydroxyl groups exhibited relatively low adsorption performances compared with catalysts without this functionality. The calculations demonstrated that the three studied catalysts had three distinct reaction paths. For the Zn(OH)Cl and Zn(OH)₂ catalysts, the reaction took place through a one-shift H₂O molecule transfer route, avoiding higher energy barrier pathways. Interestingly, we found that the energy required for breaking the O-H bond in water determined the activation energy of the studied catalytic reactions. The activation barrier increased in the order Zn(OH)Cl ≈ Zn(OH)₂ < ZnCl₂. This trend suggests that Zn(OH)Cl and Zn(OH)₂ are promising catalysts for the hydration of acetylene. Graphical abstract.

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.