Unexpected Reaction Promoted by NH+···O=Mo Hydrogen Bonds in Nonpolar Solvents

Single-Crystal Structures of Benzenehexathiol and Its Disulfide Forms

Oligothiols are useful as building blocks in the construction of disulfide-based macrocycles and polymers or as ligands for coordination polymers. Above all, benzenehexathiol (BHT) is a particularly important molecule, as it is used to construct conductive two-dimensional MOFs. Despite the desire to clarify its structure and isolate it to high purity, the chemical instability of BHT has hampered single-crystal X-ray structure analysis of intact BHT. In addition, the synthesis of discrete disulfide molecules of BHT has not been reported. Here, we succeed in obtaining the single crystals of intact BHT, which is analyzed by single crystal X-ray structure analysis. Furthermore, the structures of a group of molecules with intermolecular disulfide bonds (BHT·4im and BHT2·2TBA, im = imidazole, TBA = tetrabutylammonium cation) obtained by processing BHT in the presence of bases are determined.

The chemistry of dithietes, 1,2,5,6-tetrathiocins and higher oligomers

The synthesis and reactivity patterns of the strained dithiete ring are compared with their dimeric tetrathiocin counterparts and higher oligomers, highlighting: (i) their cycloaddition chemistry with organic dienophiles as a route to sulfur-containing heterocycles; (ii) their oxidative addition chemistry to low valent transition metal complexes to generate transition metal dithiolate complexes and; (iii) the base-catalysed isomerizations between different dithiete oligomers.

An Asymmetrically Substituted Aliphatic Bis-Dithiolene Mono-Oxido Molybdenum(IV) Complex With Ester and Alcohol Functions as Structural and Functional Active Site Model of Molybdoenzymes

A Mo^IV mono-oxido bis-dithiolene complex, [MoO(mohdt)₂]²⁻ (mohdt = 1-methoxy-1-oxo-4-hydroxy-but-2-ene-2,3-bis-thiolate) was synthesized as a structural and functional model for molybdenum oxidoreductase enzymes of the DMSO reductase family. It was comprehensively characterized by inter alia various spectroscopic methods and employed as an oxygen atom transfer (OAT) catalyst. The ligand precursor of mohdt was readily prepared by a three-step synthesis starting from dimethyl-but-2-ynedioate. Crystallographic and ¹³C-NMR data support the rationale that by asymmetric substitution the electronic structure of the ene-dithio moiety can be fine-tuned. The Mo^IVO bis-dithiolene complex was obtained by in situ reaction of the de-protected ligand with the metal precursor complex trans-[MoO₂(CN)₄]⁴⁻. The catalytic oxygen atom transfer mediated by the complex was investigated by the model OAT reaction from DMSO to triphenylphosphine with the substrate transformation being monitored by ³¹P NMR spectroscopy. [MoO(mohdt)₂]²⁻ was found to be catalytically active reaching 93% conversion, albeit with a rather low reaction rate (reaction time 56 h). The observed overall catalytic activity is comparable to those of related complexes with aromatic dithiolene ligands despite the novel ligand being aliphatic in nature and originally perceived to perform more swiftly. The respective results are rationalized with respect to a potential intermolecular interaction between the hydroxyl and ester functions together with the electron-withdrawing functional groups of the dithiolene ligands of the molybdenum mono-oxido complex and equilibrium between the active monomeric Mo^IVO and Mo^VIO₂ and the unreactive dimeric Mo2VO₃ species.

Remote Charge Effects on the Oxygen-Atom-Transfer Reactivity and Their Relationship to Molybdenum Enzymes

We report the syntheses, crystal structures, and characterization of the novel cis-dioxomolybdenum(VI) complexes [Tpm*Mo^VIO₂Cl](MoO₂Cl₃) (1) and [Tpm*Mo^VIO₂Cl](ClO₄) (2), which are supported by the charge-neutral tris(3,5-dimethyl-1-pyrazolyl)methane (Tpm*) ligand. A comparison between isostructural [Tpm*Mo^VIO₂Cl]⁺ and Tp*Mo^VIO₂Cl [Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate] reveals the effects of one unit of overall charge difference on their spectroscopic and electrochemical properties, geometric and electronic structures, and O-atom-transfer (OAT) reactivities, providing new insight into pyranopterin molybdoenzyme OAT reactivity. Computational studies of these molecules indicate that the delocalized positive charge lowers the lowest unoccupied molecular orbital (LUMO) energy of cationic [Tpm*MoO₂Cl]⁺ relative to Tp*MoO₂Cl. Despite their virtually identical geometric structures revealed by crystal structures, the Mo^VI/Mo^V redox potential of 2 is increased by 350 mV relative to that of Tp*Mo^VIO₂Cl. This LUMO stabilization also contributes to an increased effective electrophilicity of [Tpm*MoO₂Cl]⁺ relative to that of Tp*MoO₂Cl, resulting in a more favorable resonant interaction between the molydenum complex LUMO and the highest occupied molecular orbital (HOMO) of the PPh₃ substrate. This leads to a greater thermodynamic driving force, an earlier transition state, and a lowered activation barrier for the orbitally controlled first step of the OAT reaction in the Tpm* system relative to the Tp* system. An Eyring plot analysis shows that this initial step yields an O≡Mo^IV-OPPh₃ intermediate via an associative transition state, and the reaction is ∼500-fold faster for 2 than for Tp*MoO₂Cl. The second step of the OAT reaction entails solvolysis of the O≡Mo^IV-OPPh₃ intermediate to afford the solvent-substituted Mo^IV product and is 750-fold faster for the Tpm* system at -15 °C compared to the Tp* system. The observed rate enhancement for the second step is ascribed to a switch of the reaction mechanism from a dissociative pathway for the Tp* system to an alternative associative pathway for the Tpm* system. This is due to a more Lewis acidic Mo^IV center in the Tpm* system.

Acid-facilitated product release from a Mo(IV) center: relevance to oxygen atom transfer reactivity of molybdenum oxotransferases

We report that pyridinium ions (HPyr⁺) accelerate the conversion of [Tp*Mo^IVOCl(OPMe₃)] (1) to [Tp*Mo^IVOCl(NCCH₃)] (2) by 10³-fold, affording 2 in near-quantitative yield; Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate. This novel reactivity and the mechanism of this reaction were investigated in detail. The formation of 2 followed pseudo-first-order kinetics, with the observed pseudo-first-order rate constant (k _obs) linearly correlated with [HPyr⁺]. An Eyring plot revealed that this HPyr⁺-facilitated reaction has a small positive value of ∆S ^‡ indicative of a dissociative interchange (I_d) mechanism, different from the slower associative interchange (I_a) mechanism in the absence of HPyr⁺ marked with a negative ∆S ^‡. Interestingly, log(k _obs) was found to be linearly correlated to the acidity of substituted pyridinium ions. This novel reactivity is further investigated using combined DFT and ab initio coupled cluster methods. Different reaction pathways, including I_d, I_a, and possible alternative routes in the absence or presence of HPyr⁺, were considered, and enthalpy and free energies were calculated for each pathway. Our computational results further underscored that the I_d route is energetically favored in the presence of HPyr⁺, in contrast with the preferred I_a-NNO pathway in the absence of HPyr⁺. Our computational results also revealed molecular-level details for the HPyr⁺-facilitated I_d route. Specifically, HPyr⁺ initially becomes hydrogen-bonded to the oxygen atom of the Mo(IV)-OPMe₃ moiety, which lowers the activation barrier for the Mo-OPMe₃ bond cleavage in a rate-limiting step to dissociate the OPMe₃ product. The implications of our results were discussed in the context of molybdoenzymes, particularly the reductive half-reaction of sulfite oxidase.

Snapshot of Oxidation of Thiolate by Diiodine: Stabilization of Intermediate by NH···S Hydrogen Bonds

Ordinary thiolate (RS^-) reacts with diiodine (I₂) to afford an intermediate sulfenyl iodide (RSI) by releasing I^-; RSI is readily converted to disulfide (RSSR) by a disproportionation reaction. In the case of thiolate Ar¹S^- containing very bulky acylamino groups forming NH···S hydrogen bonds, the crystal of the intermediate, [Ar¹S-I-I]^-, was obtained under usual conditions, and the structure was determined by X-ray diffraction analysis. The results show that the intramolecular NH···S hydrogen bonds stabilized the intermediate [Ar¹S-I-I]^-, consistent with theoretical calculations.

Synthesis, characterization and oxygen atom transfer reactivity of a pair of Mo(iv)O- and Mo(vi)O2-enedithiolate complexes – a look at both ends of the catalytic transformation

A novel pair of mono-oxo and di-oxo bis-dithiolene molybdenum complexes were synthesized, characterized and catalytically investigated as models for a molybdenum dependent oxidoreductase.