Cleavage of Hg–C Bonds of Organomercurials Induced by ImOHSe via Two Distinct Pathways

Thermal Decomposition of Metacinnabar (β-HgS) during Monoethylene Glycol Regeneration in Natural Gas Processing

Elemental mercury and mercury (Hg)-bearing particles may be present in gas and condensate from specific geologic reservoirs and be coproduced with them. In this study, we found that over 70% of the Hg mass in field monoethylene glycol (MEG) is present as 100-200 nm particulate β-HgS, and it is therefore important to understand the decomposition behavior of β-HgS in MEG to determine the partitioning of mercury species in liquid natural gas (LNG) plants. Thermal decomposition studies in MEG and MEG-water solutions showed that β-HgS decomposition to elemental mercury started at around 100 °C, which is significantly lower than the 200 °C required for β-HgS decomposition in an inert gas. Density functional theory calculations supported the experimental observations that β-HgS has a lower decomposition temperature in solvents than its counterpart without a solvent because the solvent interactions decrease the Hg-S bond strength. Thermal decomposition studies at 130 °C showed that increased water content and decreased β-HgS particle size significantly increased the decomposition rate, while some common additives in field MEG did not have a significant effect. Experiment results suggest the decomposition pathway of β-HgS in MEG/water includes dissolution to form dissolved Hg(II) ions, followed by reduction to form elemental mercury by reaction with MEG. This study highlights the strong effect of solvent on the thermal decomposition mechanism of β-HgS, improving our understanding of the fate and species of Hg in petrochemical processing.

A greener approach towards the synthesis of N-heterocyclic thiones and selones using the mechanochemical technique

This manuscript describes the synthesis of N-heterocyclic thiones and selones of a variety of imidazolium salts involving an eco-friendly and solventless ball-milling technique. The products have been isolated in almost quantitative yield, involving a minimum quantity of solvents only for the isolation of products by column chromatography, and in some cases for purification purposes. Both mono- and bisimidazolium salts afforded N-heterocyclic thiones and selones. The methodology is found to be superior in terms of reaction time, yield and energy efficiency as compared to conventional solution-state reactions.

Exploiting the κ2 -Fashioned Coordination of [Se2 ]-Donor Ligand L3 Se for Facile Hg-C Bond Cleavage of Mercury Alkyls and Cytoprotection against Methylmercury-Induced Toxicity

The Hg-C bond of MeHgCl, a ubiquitous environmental toxicant, is notoriously inert and exceedingly difficult to cleave. The cleavage of the Hg-C bond of MeHgCl at low temperature, therefore, is of significant importance for human health. Among various bis(imidazole)-2-selones L_n Se (n=1-4, or 6), the three-spacer L₃ Se shows extraordinarily high reactivity in the degradation of various mercury alkyls including MeHgCl because of its unique ability to coordinate through κ² -fashion, in which both the Se atoms simultaneously attack the Hg center of mercury alkyls for facile Hg-C bond cleavage. It has the highest softness (σ) parameter and the lowest HOMO(L_n Se)-LUMO(MeHgX) energy gap and, thus, L₃ Se is the most reactive among L_n Se towards MeHgX (X=Cl or I). L₃ Se is highly efficient, more than L₁ Se, in restoring the activity of antioxidant enzyme glutathione reductase (GR) that is completely inhibited by MeHgCl; 80 % GR activity is recovered by L₃ Se relative to 50 % by L₁ Se. It shows an excellent cytoprotective effect in liver cells against MeHgCl-induced oxidative stress by protecting vital antioxidant enzymes from inhibition caused by MeHgCl and, thus, does not allow to increase the intracellular reactive oxygen species (ROS) levels. Furthermore, it protects the mitochondrial membrane potential (ΔΨ_m ) from perturbation by MeHgCl. Major Hg-responsive genes analyses demonstrate that L₃ Se plays a significant role in MeHg⁺ detoxification in liver cells.

Coordination of 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone to zinc and cadmium: Monotonic and non-monotonic bond length variations for [H(sebenzimMe)]2MCl2 complexes (M = Zn, Cd, Hg)

The reactions of 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone, H(sebenzimMe), towards the zinc and cadmium halides, MX2 (M = Zn, Cd; X = Cl, Br, I), afford the adducts, [H(sebenzimMe)]2MX2, which have been structurally characterized by X-ray diffraction. The halide ligands of each of these complexes participate in hydrogen bonding interactions with the imidazole N-H moieties, although the nature of the interactions depends on the halide. Specifically, the chloride and bromide derivatives, [H(sebenzimMe)]2ZnX2 and [H(sebenzimMe)]2CdX2 (X = Cl, Br), exhibit two intramolecular N-H•••X interactions, whereas the iodide derivatives, [H(sebenzimMe)]2ZnI2 and [H(sebenzimMe)]2CdI2, exhibit only one intramolecular N-H•••I interaction. Comparison of the M-Se and M-Cl bond lengths of the chloride series, [H(sebenzimMe)]2MCl2 (M = Zn, Cd, Hg), indicates that while the average M-Cl bond lengths progressively increase as the metal becomes heavier, the variation in M-Se bond length exhibits a non-monotonic trend, with the Cd-Se bond being the longest. These different trends provide an interesting subtlety concerned with use of covalent radii in predicting bond length differences. In addition to tetrahedral [H(sebenzimMe)]2CdCl2, [H(sebenzimMe)]3,CdCl2•[H(sebenzim)Me]4CdCl2, which features both five-coordinate and six-coordinate coordinate centers, has also been structurally characterized. Finally, the reaction between CdI2 and H(sebenzimMe) at elevated temperatures affords the 1-methylbenzimidazole complex, [H(sebenzimMe)]-[H(benzimMe)]CdI2, a transformation that is associated with cleavage of the C-Se bond.

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

Cytoprotective effects of imidazole-based [S1] and [S2]-donor ligands against mercury toxicity: a bioinorganic approach

Here we report the coordination behaviour of an imidazole-based [S1]-donor ligand, 1,3-dimethyl-imidazole-2(3H)-thione (L1), and [S2]-donor ligand, 3,3'-methylenebis(1-methyl-imidazole-2(3H)-thione) (L2) or 4,4'-(3,3'-methylenebis-(2-thioxo-2,3-dihydro-imidazole-3,1-diyl))dibutanoic acid (L3), with HgX2 (X = Cl, Br or I) in solution and the solid state. NMR, UV-Vis spectroscopic, and single crystal X-ray studies demonstrated that L1 or L2 coordinated rapidly and reversibly to the mercury center of HgX2 through the thione moiety. Treatment of L2 with HgCl2 or HgBr2 afforded 16-membered metallacycle k1-(L2)2Hg2Cl4 or k1-(L2)2Hg2Br4 where two Cl or Br atoms are located inside the ring. In contrast, treatment of L2 with HgI2 afforded a chain-like structure of k1-[L2Hgl2]n, possibly due to the large size of the iodine atom. Interestingly, [S1] and [S2]-donor ligands (L1, L2, and L3) showed an excellent efficacy to protect liver cells against HgCl2 induced toxicity and the strength of their efficacy is in the order of L3 > L2 > L1. 30% decrease of ROS production was observed when liver cells were co-treated with HgCl2 and L1 in comparison to those cells treated with HgCl2 only. In contrast, 45% and 60% decrease of ROS production was observed in the case of cells co-treated with HgCl2 and thiones L2 and L3, respectively, indicating that [S2]-donor ligands L2 and L3 have better cytoprotective effects against oxidative stress induced by HgCl2 than [S1]-donor ligand L1. Water-soluble ligand L3 with N-(CH2)3CO2H substituents showed a better cytoprotective effect against HgCl2 toxicity than L2 in liver cells.

Selenoprotein P as the major transporter for mercury in serum from methylmercury-poisoned rats

Selenium (Se) has been found to promote weight gain, decrease hepatic damage, but redistribute mercury (Hg) in brains and livers in methylmercury (MeHg)-poisoned rats. The aims of the present work were to examine the effects of Se on the levels of Hg in serum and the role of serum selenoproteins in binding with Hg in MeHg-poisoned rats. The concentration of Se, Hg and MeHg were studied using ICP-MS and CVAFS. The Hg- and Se-binding selenoproteins were separated and quantified using affinity chromatography with post-column isotope dilution analysis using both enriched ⁷⁸Se and ¹⁹⁹Hg. It was found that Se treatment reduced Hg levels in serum in MeHg-poisoned rats. Among the three separated selenoproteins, the amounts of SelP-bound Hg and Se increased to 73% and 93.6%, from 64.4% and 89.3% of the total Hg and Se, respectively after Se treatment, suggesting that SelP acts as a major transporter for Hg and pool for Se in serum. Over 90% of the total Hg was MeHg in serum, and the molar ratios of MeHg to Se as 1:4 and 1:9 in the formed MeHg-Se-SelP complex in the control and the Se treatment group, respectively. The elevated Se level binding with SelP facilitated the Hg extraction from tissues and organs, as well as its redistribution in brains and livers through blood circulation in the MeHg-poisoned rats. Together, our findings provide direct evidence that serum SelP is the major Hg transporter in MeHg-poisoned rats.