Protolytic Cleavage of Hg–C Bonds Induced by 1-Methyl-1,3-dihydro-2H-benzimidazole-2-selone: Synthesis and Structural Characterization of Mercury Complexes

Abstract

Multinuclear (¹H, ⁷⁷Se, and ¹⁹⁹Hg) NMR spectroscopy demonstrates that 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone, H(sebenzim^Me), a structural analogue of the selenoamino acid, selenoneine, binds rapidly and reversibly to the mercury centers of HgX₂ (X = Cl, Br, I), while X-ray diffraction studies provide evidence for the existence of adducts of composition [H(sebenzim^Me)]_xHgX₂ (X = Cl, x = 2, 3, 4; X = I, x = 2) in the solid state. H(sebenzim^Me) also reacts with methylmercury halides, but the reaction is accompanied by elimination of methane resulting from protolytic cleavage of the Hg–C bond, an observation that is of relevance to the report that selenoneine demethylates CysHgMe, thereby providing a mechanism for mercury detoxification. Interestingly, the structures of [H(sebenzim^Me)]_xHgX₂ exhibit a variety of different hydrogen bonding patterns resulting from the ability of the N–H groups to form hydrogen bonds with chlorine, iodine, and selenium.

Introduction

The toxicological properties of mercury¹ have been attributed to both its thiophilicity¹⁻⁴ and its selenophilicity.⁴⁻⁶ With respect to the latter, selenium is an important component of antioxidants,^7,9 and the interaction between Hg(II) and selenium compounds may reduce the bioavailability of selenium via the formation of insoluble mercury selenide species.^4,5,9 Furthermore, mercury may bind to the active sites of selenoenzymes and thereby inhibit their functions.^4,6 For example, selenium is a component of a variety of enzymes that incorporate the amino acids selenocysteine and selenomethionine (Figure 1), as illustrated by glutathione peroxidases, thioredoxin reductases, glycine reductases, formate dehydrogenases, and selenoprotein P.^4,5,7,10 Other examples of selenium-containing biomolecules include the amino acid derivatives selenoneine^11,12 and Se-methylselenoneine^12,13 (Figure 1), of which the latter was identified in human urine and blood.

Figure 1.

Selenium-containing derivatives of amino acids.

It has recently been shown that selenoamino acids (namely l-selenocysteine, l-selenoglutathione, d,l-selenopenicillamine, and l-selenomethionine) complex readily to methylmercury species¹⁴ and that cleavage of the Hg–C bond may be achieved under physiologically relevant conditions to yield mercury selenide via (MeHg)₂Se.¹⁵ Insoluble mercury selenide particles have also been observed in the brains of humans exposed to methylmercury species, and these particles are considered to be much less toxic than mobile, soluble methylmercury species such as CysHgMe.¹⁶ This observation provides evidence of the neuroprotective effects of selenium with respect to the prevention of mercury-induced damage to the central nervous system. Additionally, recent in vitro studies have shown that selenoneine may assist cells in removal of CysHgMe.^11e However, the interactions between mercury and selenium in biological systems are complex, and animal studies have produced contradictory results. For example, it has been observed that co-administration of diphenyl diselenide compounds with methylmercury chloride partially ameliorated methylmercury-induced oxidative damage to proteins in the livers and brains of intoxicated mice;¹⁷ on the other hand, rats simultaneously dosed with methylmercury chloride and diphenyl diselenide were shown to suffer more severe neurological symptoms, such as motor deficits and weight loss, than rats dosed with methylmercury chloride alone.¹⁸

A detailed understanding of the impact of mercury on the biochemical roles of selenium would, therefore, benefit considerably from the development of the chemistry of mercury in a coordination environment that features selenium. Therefore, we describe here the reactivity of 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone (Figure 2), H(sebenzim^Me),¹⁹ a structural analogue of selenoneine, towards mercury, including the protolytic cleavage of mercury–methyl bonds.

Figure 2.

Structurally characterized imidazole-2-selones.

Results and Discussion

1-R-imidazole-2-thiones, H(mim^R),²⁰⁻²² of which the methyl derivative is the well-known antithyroid drug, methimazole (tapazole),^23,24 are a widely studied class of molecules that can bind to a variety of metals,²⁵⁻²⁷ including mercury.²⁷ However, in contrast to the numerous studies pertaining to 1-R-imidazole-2-thiones, there are few corresponding investigations of 1-R-imidazole-2-selones, H(seim^R).²⁸⁻³² For example, only H(seim^Me)^28,29 and H(seim^Mes),²⁹ and the benzannulated derivatives, H(sebenzim^Me)^30,31a and H(sebenzim^But),^31a have been synthesized and structurally characterized (Figure 2). Moreover, there are very few examples of structurally characterized metal complexes that feature 1-R-imidazole-2-selone ligands.^31a,33−35 It is, therefore, appropriate to develop the chemistry of this class of ligands with respect to mercury. In this regard, we recently reported an improved synthesis of H(sebenzim^Me),^31a which has thereby allowed us to investigate the ability of this compound both to coordinate to mercury centers and to cleave mercury–carbon bonds.

Interaction of H(sebenzim^Me) with HgCl₂, HgBr₂, and HgI₂

Evidence for the ability of the imidazole-2-selone, H(sebenzim^Me), to coordinate to the mercury centers of HgX₂ (X = Cl, Br, I) in solution (Scheme 1) is provided by a combination of ¹H, ⁷⁷Se{¹H}, and ¹⁹⁹Hg{¹H} NMR spectroscopies. For example, the ¹⁹⁹Hg (Table 1) chemical shift changes progressively upon addition of H(sebenzim^Me) to a solution of HgCl₂ in DMSO-d₆. Correspondingly, the ⁷⁷Se (Table 1) and ¹H (Table 2 and Figure 3) chemical shifts associated with H(sebenzim^Me) also progressively shift upon addition to HgCl₂. In addition to providing evidence for coordination of H(sebenzim^Me) to mercury, the observation of a single resonance in both the ⁷⁷Se{¹H} and ¹⁹⁹Hg{¹H} NMR spectra for each concentration ratio, and also a single set of resonances in the ¹H NMR spectra, indicates that the coordination is reversible and that the process is facile on the NMR time scale at room temperature. Furthermore, low temperature (−40 °C) spectra in DMF-d₇ likewise show single resonances, thereby demonstrating that the exchange is still rapid at this temperature (data not shown).

Scheme 1.

Table 1. ¹⁹⁹Hg and ⁷⁷Se Chemical Shift Values for HgCl₂/H(sebenzim^Me) in DMSO-d₆.

[H(sebenzimMe)]/[HgCl2]	199Hg δ (ppm)	77Se δ (ppm)
0	–1450	N/A
1	–1201	12
2	–1061	15
3	–1013	33
4	–1010	43
∞a	N/A	83

^aValue for H(sebenzim^Me).

Table 2. ¹H (N-CH₃) NMR Chemical Shift Values for HgX₂/H(sebenzim^Me) in DMSO-d₆.

	1H δ (ppm)
[H(sebenzimMe)]/[HgX2]	HgCl2	HgBr2	HgI2
1	3.96	3.96	3.96
2	3.87	3.89	3.90
3	3.83	3.85	3.86
4	3.81	3.82	3.83
5	3.79	3.81	3.81
6	3.78	3.79	3.80
7	3.78	3.79	3.79
8	3.77	3.78	3.79
9	3.77	3.78	3.78
10	3.76	3.77	3.78
11	3.76	3.77	3.77
∞a	3.75	3.75	3.75

^aValue for H(sebenzim^Me).

Figure 3.

Variation of ¹H NMR chemical shift of the methyl group of H(sebenzim^Me) in the presence of HgX₂ as a function of the molar ratio. Data plotted are to three significant figures.

Although the fluxionality prevents identification of the precise solution composition (Scheme 1), the tetrakis, tris, and bis complexes, [H(sebenzim^Me)]₄HgCl₂, [H(sebenzim^Me)]₃HgCl₂, and [H(sebenzim^Me)]₂HgCl₂,^31a may be obtained by crystallization from a solution that contains the respective number of equivalents of H(sebenzim^Me).

The molecular structures of [H(sebenzim^Me)]₃HgCl₂ and [H(sebenzim^Me)]₄HgCl₂ have been determined by X-ray diffraction, as illustrated in Figures 4 and 5, respectively. Of these, the latter compound is particularly important because there are no structurally characterized mononuclear mercury compounds with four dative L-type³⁶ selenium donors currently listed in the Cambridge Structural Database (CSD).^37,38 Furthermore, efforts to synthesize a tetrakis selone complex of mercury (other than for unsubstituted selenourea) have been reported to be unsuccessful.^35i,39 For example, treatment of HgCl₂ with 4 equiv of N,N-dimethylselenourea (DmSeU) was reported to yield only the bis complex, (DmSeU)₂HgCl ₂.³⁵ⁱ

Figure 4.

Molecular structure of [H(sebenzim^Me)]₃HgCl₂, which is more appropriately represented as the ion pair, {[H(sebenzim^Me)]₃HgCl}[Cl].

Figure 5.

Molecular structure of the cation {[H(sebenzim^Me)]₄Hg}²⁺ of {[H(sebenzim^Me)]₄Hg}[Cl]₂ (only one of the independent molecules is shown).

In addition to [H(sebenzim^Me)]₄HgCl₂ being of significance because its existence demonstrates that a mercury center can accommodate four selenium L-type donor ligands, the tris complex, [H(sebenzim^Me)]₃HgCl₂, is of interest because structurally characterized mercury compounds with three L-type selenium donors are also uncommon. Thus, compounds with a HgSe₃ motif are typically polynuclear selenide or selenolate derivatives; there are, nevertheless a few structurally characterized mononuclear compounds that contain mercury coordinated to three dative L-type selenium ligands, of which [(MeImSe)₃HgCl]Cl,^35h {[N(CH₂CH₂SePh)₃Hg(κ²-NO₃)}(NO₃),⁴⁰ and {[CpFe(CO)₂P(OPrⁱ)₂Se]₃Hg}(ClO₄)₂^41,42 are illustrative.

Comparison of the molecular structures of [H(sebenzim^Me)]₃HgCl₂ (Figure 4) and [H(sebenzim^Me)]₄HgCl₂ (Figure 5) with that of [H(sebenzim^Me)]₂HgCl₂^31a reveals interesting structural variations as a function of composition, as summarized in Figure 6. First, there is a progressive increase in the Hg–Cl distances in the sequence [H(sebenzim^Me)]₂HgCl₂ < [H(sebenzim^Me)]₃HgCl₂ < [H(sebenzim^Me)]₄HgCl₂, as summarized in Table 3. Thus, whereas the two Hg–Cl bond lengths in the bis complex [H(sebenzim^Me)]₂HgCl₂ [2.4942(7) and 2.5727(8) Å] are comparable to the mean value of 2.43 Å for structurally characterized four-coordinate mercury compounds listed in the CSD,⁴³ the shortest Hg···Cl distance in the tetrakis complex, [H(sebenzim^Me)]₄HgCl₂, is 3.913 Å, such that the compound may be better represented as {[H(sebenzim^Me)]₄Hg}[Cl]₂. The Hg–Cl distances in the tris complex, [H(sebenzim^Me)]₃HgCl₂, are intermediate between those of [H(sebenzim^Me)]₂HgCl₂ and [H(sebenzim^Me)]₄HgCl₂, with values of 2.7506(10) and 3.2397(9) Å. While the latter value is sufficiently large that it cannot be considered to correspond to a Hg–Cl covalent bond, the shorter distance of 2.7506(10) Å is only 0.32 Å longer than the CSD average (vide supra) and may therefore be viewed as corresponding to a weak covalent interaction, such that the compound can be formulated as {[H(sebenzim^Me)]₃HgCl}[Cl]. In accord with the long Hg–Cl bond distance, the coordination geometry of {[H(sebenzim^Me)]₃HgCl}⁺ deviates significantly from tetrahedral. Thus, the four-coordinate τ₄ index (Table 4)⁴⁴ of {[H(sebenzim^Me)]₃HgCl}⁺ (0.78) is close to that for an idealized trigonal monopyramid (0.85) in which chlorine occupies an axial position;⁴⁴ in the extreme that the axial chlorine is considered to serve the role of a counterion, the mercury would be described as approximately trigonal planar.

Figure 6.

Comparison of the mercury coordination environments of [H(sebenzim^Me)]₂HgCl₂ (top), [H(sebenzim^Me)]₃HgCl₂ (center), and [H(sebenzim^Me)]₂HgCl₂ (bottom).

Table 3. Selected Bond Length Data for {[H(sebenzim^Me)]_xHg} Compounds.

	d (Å)
compound	Hg–X	Hg–Se
[H(sebenzimMe)]2HgCl2	2.4942(7), 2.5727(8)	2.5732(5), 2.6090(5)
[H(sebenzimMe)]3HgCl2	2.7506(10), 3.2397(9)	2.5690(4), 2.5864(4), 2.6730(4)
[H(sebenzimMe)]4HgCl2a	–	2.6203(6), 2.6327(6), 2.7025(6), 27284(7), 2.6260(6), 2.6365(6), 2.6959(6), 2.7267(7)
[H(sebenzimMe)]2HgI2 (monoclinic)	2.7280(3), 2.7463(4)	2.6850(4), 2.6980(4)
[H(sebenzimMe)]2HgI2 (orthorhombic)	2.7791(7), 2.8041(7)	2.6149(10), 2.6396(10)
[H(sebenzimMe)2HgI	2.7497(4)	2.5466(6), 2.5748(6), 3.0904(6), 3.3215(6)
[H(sebenzimMe)2]2Hg	–	2.6230(12), 2.6230(12), 2.6320(12), 2.6367(13)

^aValues for two crystallographically independent molecules.

Table 4. Four-Coordinate τ₄ Indices for {[H(sebenzim^Me)]_xHg} Derivatives.

compound	τ4
[H(sebenzimMe)]2HgCl2a	0.94
[H(sebenzimMe)]3HgCl2	0.78
[H(sebenzimMe)]4HgCl2	0.88
[H(sebenzimMe)]2HgI2 (monoclinic)	0.88
[H(sebenzimMe)]2HgI2 (orthorhombic)	0.94
[H(sebenzimMe)2]2Hg	0.88

^aData taken from ref (31a).

By comparison to the large variation in Hg–Cl interactions within [H(sebenzim^Me)]_xHgCl₂, the average Hg–Se bond lengths exhibit little variation, increasing only slightly as a function of x, i.e., bis (2.591 Å) < tris (2.611 Å) < tetrakis (2.671 Å). These Hg–Se bond lengths are comparable to the mean value of 2.643 Å for compounds listed in the CSD,³⁷ but are longer than those in compounds such as Hg(SePh)₂ [2.480 Å]⁴⁵ and [Tm^But]HgSePh [2.524 Å],⁴⁶ which feature normal covalent bonds. The Hg–Se bond lengths in [H(sebenzim^Me)]_xHgCl₂ are, nevertheless, comparable to the values in [Tse^Mes]HgI [2.674 Å]⁴⁷ and (PrⁱImSe)₂HgCl₂ [2.584 Å],³⁵ⁱ which feature Hg←Se dative covalent bonds.³⁶ The latter type of interaction is recognized to be highly flexible,⁴⁸ as indicated by the fact that the Hg–Se bonds within [Hg₂(SePh₂)₄][ClO₄]₂ range from 2.65 to 2.92 Å.⁴⁹ As such, the variation in Hg–Se bond length within the series of [H(sebenzim^Me)]_xHgCl₂ complexes may be rationalized by the dative nature of the interactions.

A common feature of all [H(sebenzim^Me)]_xHgCl₂ structures is that each chloride, regardless of whether it is attached covalently to the mercury center, participates in hydrogen bonding interactions with the imidazole N–H moieties. There is, nevertheless, an interesting difference with respect to the nature of the hydrogen bonding interactions. Specifically, each chlorine that is covalently bound to mercury participates in an intramolecular N–H···Cl interaction,⁵⁰⁻⁵² whereas each outer-sphere chloride anion participates in a N–H···Cl···H–N interaction⁵³ that serves to link together two H(sebenzim^Me) moieties, as summarized in Figure 7.

Figure 7.

Intramolecular (top and middle) versus intermolecular (bottom) N–H···Cl hydrogen bonding interactions in [H(sebenzim^Me)]_xHgCl₂ complexes.

Thus, whereas [H(sebenzim^Me)]₂HgCl₂ exhibits only intramolecular N–H···Cl interactions and is a discrete mononuclear species,^31a,54 [H(sebenzim^Me)]₃HgCl₂ and [H(sebenzim^Me)]₄HgCl₂ also exhibit intermolecular N–H···Cl interactions. Specifically, [H(sebenzim^Me)]₃HgCl₂ exhibits an intramolecular N–H···Cl interaction and intermolecular N–H···Cl···H–N interactions that bridge two molecules, thereby creating a dimeric structure (Figure 8), while [H(sebenzim^Me)]₄HgCl₂ exhibits an intramolecular N–H···Cl···H–N interaction and intermolecular N–H···Cl···H–N interactions that result in a polymeric array (Figure 9). The various hydrogen bonding N···Cl distances in [H(sebenzim^Me)]_xHgCl₂ are in the range 3.031(7)–3.227(2) Å and are comparable to the values for other compounds with N–H···Cl interactions listed in the CSD [d_av(N···Cl) = 3.181 Å].³⁷ Furthermore, the N–H···Cl···H–N interactions that link together pairs of molecules are characterized by N···Cl···N angles in the range 100.1–119.7°, which are comparable to the average value of 99.9° for compounds listed in the CSD that feature N–H···Cl···H–N interactions wherein the chloride ion is not covalently bonded to any other atoms.³⁷

Figure 8.

Intermolecular H–N···Cl···H–N hydrogen bonding serves to link together two {[H(sebenzim^Me)]₃HgCl}⁺ moieties.

Figure 9.

Intermolecular H–N···Cl···H–N hydrogen bonding creates a chain of {[H(sebenzim^Me)]₄Hg}[Cl]}⁺ moieties, bridged by Cl^– ions.

The various hydrogen bonding networks in [H(sebenzim^Me)]_xHgCl₂ may be described by the graph set notations⁵⁵ that are summarized in Table 5. For example, the hydrogen-bonded dimer of [H(sebenzim^Me)]₃HgCl₂ forms a 20-membered ring that is described by the unitary graph set DDS(6) and the binary graph set R₄²(20).

Table 5. Hydrogen Bonding Networks for [H(sebenzim^Me)]_xHgCl₂ and [H(sebenzim^Me)]₂HgI₂ Derivatives.

	unitary network	binary network
[H(sebenzimMe)]2HgCl2	S(6)S(6)	–
[H(sebenzimMe)]3HgCl2	DDS(6)	R42(20)
[H(sebenzimMe)]4HgCl2	DDDDDDDD	D21(3)R2(10)D21(3)R2(10)D22(11)D2(11)D22(11)D2(11)D22(11)D2(11)D22(11)D2(11)
[H(sebenzimMe)]2HgI2 (monoclinic)	R22(8)R2(8)	C(12)C(12)C44(24)
[H(sebenzimMe)]2HgI2 (orthorhombic)	S(6)C(6)	–

Coordination of H(sebenzim^Me) to HgCl₂ is accompanied by only relatively small increases in the lengths of the C–Se bonds. Thus, the C–Se bond lengths of [H(sebenzim^Me)]₂HgCl₂ [1.862(3) and 1.864(3) Å],^31a [H(sebenzim^Me)]₃HgCl₂ [1.868(3), 1.859(3), and 1.857(3) Å], and [H(sebenzim^Me)]₄HgCl₂ [1.854(12), 1.896(11), 1.851(9), 1.851(9), 1.857(11), 1.854(11), 1.869(9), and 1.856(9) Å] are only slightly longer than that of free H(sebenzim^Me) [1.838(2) Å].^31a Despite these minor metrical changes, however, it is interesting to note that both the ¹³C (see Experimental Section and ref (31a)) and ⁷⁷Se NMR (Table 1) chemical shifts of the [CSe] moiety are sensitive towards the changes induced by coordination to mercury. Similar spectroscopic trends have been observed in related systems,³⁵ⁱ and also for thione counterparts.⁵⁶

NMR spectroscopic studies also demonstrate that H(sebenzim^Me) binds reversibly to HgBr₂ and HgI₂ in DMSO-d₆, and that the processes are facile on the NMR time scale, as indicated by the observation of single sets ¹H NMR chemical shifts for the H(sebenzim^Me) signals (Table 2 and Figure 3). Interestingly, the ⁷⁷Se NMR chemical shift of the H(sebenzim^Me) moiety is more sensitive towards coordination of HgCl₂ than to coordination of either HgBr₂ or HgI₂. For example, the ⁷⁷Se NMR chemical shifts of 2:1 mixtures of H(sebenzim^Me) and HgX₂ move upfield from the value of pure H(sebenzim^Me) by values of 68 ppm (X = Cl), 54 ppm (X = Br), and 35 ppm (X = I). Despite the reversibility of coordination of H(sebenzim^Me), the bis complex, [H(sebenzim^Me)]₂HgI₂, may, nevertheless, be isolated from reactions performed in either acetonitrile or benzene.

Interestingly, the crystals of [H(sebenzim^Me)]₂HgI₂ obtained from the two different reaction solvents are not isomorphous, and the molecules adopt different geometries, as illustrated in Figures 10 and 11. Specifically, the H(sebenzim^Me) ligands are oriented in different directions relative to both each other and the iodide ligands. Accompanying these variations in conformation are differences in the mercury coordination environments. For example, whereas the orthorhombic form of [H(sebenzim^Me)]₂HgI₂ obtained from acetonitrile (Figure 10), with a τ₄ index of 0.94, is close to tetrahedral (τ₄ = 1.00), monoclinic [H(sebenzim^Me)]₂HgI₂ obtained from benzene (Figure 11), with a τ₄ index of 0.88, is distorted towards trigonal monopyramidal (τ₄ = 0.85). In addition to these angular variations, there are small differences in Hg–Se and Hg–I bond lengths. Thus, while the average Hg–I bond length of orthorhombic [H(sebenzim^Me)]₂HgI₂ (2.792 Å) is longer than that of the monoclinic version (2.737 Å), the average Hg–Se bond length of orthorhombic [H(sebenzim^Me)]₂HgI₂ (2.627 Å) is shorter than that of the monoclinic version (2.692 Å). Similarly to HgCl₂, coordination of H(sebenzim^Me) to HgI₂ is accompanied by only small increases in the lengths of the C–Se bonds. Thus, the C–Se bond lengths in [H(sebenzim^Me)]₂HgI₂ [1.852(9) and 1.858(9) Å for the orthorhombic form and 1.871(3) and 1.863(3) Å for the monoclinic form] are comparable to those observed in [H(sebenzim^Me)]_xHgCl₂, which range from 1.851(9) to 1.896(11) Å.

Figure 10.

Molecular structure of orthorhombic [H(sebenzim^Me)]₂HgI₂ obtained from acetonitrile solution.

Figure 11.

Molecular structure of monoclinic [H(sebenzim^Me)]₂HgI₂ obtained from benzene solution.

The most striking differences in the structures of orthorhombic and monoclinic [H(sebenzim^Me)]₂HgI₂ do not, however, pertain to the mercury coordination environment. Rather, the differences are associated with the distinct hydrogen bonding motifs (Figures 12 and 13). Furthermore, these hydrogen bonding patterns are also different from that of the chloride counterpart, [H(sebenzim^Me)]₂HgCl₂ (vide supra), as illustrated in Figure 14.

Figure 12.

Hydrogen bonding network for orthorhombic [H(sebenzim^Me)]₂HgI₂ obtained from acetonitrile solution, illustrating intramolecular and intermolecular N–H···I interactions.

Figure 13.

Hydrogen bonding network for monoclinic [H(sebenzim^Me)]₂HgI₂ obtained from benzene solution, illustrating “head-to-head” N–H···Se interactions.

Figure 14.

Comparison of hydrogen bonding interactions in [H(sebenzim^Me)]₂HgCl₂ and [H(sebenzim^Me)]₂HgI₂.

For example, whereas [H(sebenzim^Me)]₂HgCl₂ is observed to have two intramolecular N–H···Cl interactions, the orthorhombic form of [H(sebenzim^Me)]₂HgI₂ possesses one intramolecular and one intermolecular N–H···I interaction,⁵⁷ thereby creating a hydrogen-bonded helical chain of [H(sebenzim^Me)]₂HgI₂ molecules (Figure 12).⁵⁸ In contrast to [H(sebenzim^Me)]₂HgCl₂ and orthorhombic [H(sebenzim^Me)]₂HgI₂, however, the monoclinic form of [H(sebenzim^Me)]₂HgI₂ possesses no intramolecular or intermolecular N–H···I interactions. Rather, the N–H groups of the H(sebenzim^Me) ligands participate in pairs of centrosymmetric intermolecular N–H···Se interactions that link adjacent molecules together in a manner similar to that observed for certain H(seim^R) derivatives in the absence of metal coordination (Figure 13).^29,59 Interestingly, H(sebenzim^Me) itself does not adopt this “head-to-head” motif, but rather adopts a polymeric “head-to-tail” structure.^31a As such, coordination of the selenium to a metal promotes centrosymmetric N–H···Se interactions in this system, with there being no comparable structures currently listed in the CSD. The existence of this motif is undoubtedly a consequence of the fact that iodide is, by comparison to chloride, a poor hydrogen bond acceptor,^60a such that N–H···Se interactions may compete with N–H···I interactions.

As would be expected, the hydrogen bonding N···I interactions in orthorhombic [H(sebenzim^Me)]₂HgI₂ [3.486(7) and 3.589(7) Å] are substantially longer than the analogous N···Cl interactions in [H(sebenzim^Me)]₂HgCl₂. Thus, while the mean N···Cl distance in [H(sebenzim^Me)]₂HgCl₂ is 3.182 Å, the mean N···I distance in orthorhombic [H(sebenzim^Me)]₂HgI₂ is 3.541 Å. For reference, the mean N···Cl distance for compounds listed in the CSD with N–H···Cl interactions involving a terminal metal chloride is 3.332 Å,⁵² while the analogous N···I distance is 3.707 Å.⁶⁰

Interaction of 2-Seleno-1-methylbenzimidazole with Methylmercury Halides

In view of the fact that the protolytic cleavage of the Hg–C bond is a critical step in detoxification of organomercurials,^{27h,27i,61,62} and recognizing that H(sebenzim^Me) is an analogue of selenoneine, we have also investigated the reactivity of H(sebenzim^Me) towards methylmercury halides. Significantly, we have observed that H(sebenzim^Me) not only coordinates to the mercury center, as observed for HgX₂, but it is also capable of cleaving the Hg–C bonds of MeHgX. For example, H(sebenzim^Me) reacts with MeHgI at 100 °C to liberate CH₄ (as observed by ¹H NMR spectroscopy) and afford [H(sebenzim^Me)₂]HgI (Scheme 2). The importance of this observation is underscored by the fact that selenoneine, of which H(sebenzim^Me) is a structural analogue, has recently been shown to achieve demethylation of CysHgMe.^11e

Scheme 2.

The molecular structure of [H(sebenzim^Me)₂]HgI has been determined by X-ray diffraction, as illustrated in Figure 15, which demonstrates that it features mercury in an approximately trigonal planar environment, with a pyramidality (P) value⁶³ of only 0.2°. The bond angles at mercury, however, deviate from 120° [Se–Hg–Se = 140.91(2)°; Se–Hg–I = 114.87(2)° and 104.02(2)°], such that the geometry is distorted towards T-shaped, which is not uncommon for mercury.⁶⁴

Figure 15.

Molecular structure of the monomeric unit, [H(sebenzim^Me)₂]HgI.

The most interesting feature of [H(sebenzim^Me)₂]HgI, however, pertains to the fact that the H(sebenzim^Me) and (sebenzim^Me) moieties are linked by N–H···N hydrogen bonding interactions, with a N···N distance of 2.720(6) Å.^65,66 As such, the combined fragment, [H(sebenzim^Me)₂], may be viewed as an LX-type ligand.³⁶ In this regard, the two Hg–Se bond lengths present in [H(sebenzim^Me)₂]HgI [2.5466(6) and 2.5748(6) Å] are very similar.

While the primary coordination environment about mercury is trigonal planar, it is evident that there are additional intermolecular Hg···Se interactions [3.0904(6) and 3.3215(6) Å] that are substantially longer than those within [H(sebenzim^Me)₂]HgI [2.5466(6) and 2.5748(6) Å], and which serve to link together adjacent molecules, as illustrated in Figure 16. In this regard, the extended coordination geometry of mercury may be viewed as five-coordinate and, with a τ₅ index⁶⁷ of 0.51, is intermediate between the idealized values for square pyramidal (τ₅ = 0) and trigonal bipyramidal (τ₅ = 1) geometries.

Figure 16.

Extended structure of {[H(sebenzim^Me)₂]HgI}_x.

In view of the kinetic stability of two-coordinate RHgX complexes towards protolytic cleavage,⁶⁸ it is likely that the mechanism for formation of [H(sebenzim^Me)₂]HgI involves the initial formation of an adduct, [H(sebenzim^Me)]_xHg(Me)I, which undergoes either intramolecular protolytic cleavage of the Hg–Me bond, or cleavage in an intermolecular manner to afford a mercury–selenoimidazolyl species.

H(sebenzim^Me) is not only capable of cleaving the Hg–C bond of MeHgI, but also cleaves the Hg–C bond of MeHgCl, although the reaction follows a different course than that of MeHgI. Specifically, reaction of MeHgCl with H(sebenzim^Me) at 100 °C results in evolution of methane, as observed by ¹H NMR spectroscopy, and the formation of a mixture of [H(sebenzim^Me)]₄HgCl₂ (vide supra) and [H(sebenzim^Me)₂]₂Hg (Scheme 3). The latter compound can also be obtained via the cleavage of the Hg–Ph bonds of Ph₂Hg with H(sebenzim^Me), as illustrated in Scheme 4.

Scheme 3. Protolytic Cleavage of MeHgCl by H(sebenzim^Me).

Scheme 4. Protolytic Cleavage of Ph₂Hg by H(sebenzim^Me).

The formation of [H(sebenzim^Me)]₄HgCl₂ and [H(sebenzim^Me)₂]₂Hg upon treatment of MeHgCl with H(sebenzim^Me) is indicative of a ligand redistribution process. For example, one possibility is that incipient {[H(sebenzim^Me)₂]HgCl}, the counterpart of the above iodide derivative, could redistribute to give [H(sebenzim^Me)₂]₂Hg and HgCl₂, of which the latter would be trapped by H(sebenzim^Me) to afford [H(sebenzim^Me)]₄HgCl₂.^69,70

The molecular structure of [H(sebenzim^Me)₂]₂Hg has been determined by X-ray diffraction (Figure 17), which demonstrates that pairs of H(sebenzim^Me) and (sebenzim^Me) ligands are linked together via hydrogen bonding interactions to produce the combined LX-type ligand,⁷¹ [H(sebenzim^Me)₂], in a manner akin to that observed for [H(sebenzim^Me)₂]HgI. However, while the N···N distances within [H(sebenzim^Me)₂]₂Hg [2.724(14) and 2.732(14) Å] are comparable to that observed for [H(sebenzim^Me)₂]HgI [2.720(6) Å], the angles between the H(sebenzim^Me) and (sebenzim^Me) planes (76.6° and 76.5°) are distinctly larger than that in [H(sebenzim^Me)₂]HgI (47.3°). Thus, it is evident that the hydrogen-bonded [H(sebenzim^Me)₂] ligand is quite flexible with respect to the twist angles of the benzimidazole ring systems. The coordination geometry about mercury in [H(sebenzim^Me)₂]₂Hg is distorted tetrahedral (τ₄ = 0.88), with Hg–Se bond lengths in a narrow range of 2.6228(12)–2.6367(13) Å.

Figure 17.

Molecular structure of [H(sebenzim^Me)₂]₂Hg.

Conclusions

In summary, 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone, H(sebenzim^Me), is a structural analogue of selenoneine and coordinates reversibly to the metal centers of HgX₂ (X = Cl, Br, I). Furthermore, H(sebenzim^Me) is also capable of cleaving the Hg–C bond of methylmercury halides, thereby mimicking the role of selenoneine in demethylating CysHgMe. X-ray diffraction studies demonstrate that while two equivalents of H(sebenzim^Me) simply coordinate to mercury centers of HgX₂ (X = Cl, I), the third and fourth equivalents result in displacement of the chloride ligands. Thus, [H(sebenzim^Me)]₃HgCl₂ and [H(sebenzim^Me)]₄HgCl₂ are better represented as ion pairs, namely {[H(sebenzim^Me)]₃HgCl}[Cl] and {[H(sebenzim^Me)]₄Hg}[Cl]₂, of which the latter is the first example of a structurally characterized tetrahedral mercury compound that features four L-type selenium donors. A common feature of all [H(sebenzim^Me)]_xHgCl₂ structures is that each chloride, regardless of whether it is attached covalently to the mercury center or serves as a counterion, participates in hydrogen bonding interactions with the imidazole N–H moieties. The nature of the network, however, depends critically on the number of H(sebenzim^Me) donors. For example, whereas [H(sebenzim^Me)]₂HgCl₂ exhibits only intramolecular N–H···Cl interactions and is a discrete mononuclear species, [H(sebenzim^Me)]₃HgCl₂ exhibits an intramolecular N–H···Cl interaction and intermolecular N–H···Cl···H–N interactions that bridge two molecules, resulting in a dimeric structure, while [H(sebenzim^Me)]₄HgCl₂ exhibits an intramolecular N–H···Cl···H–N interaction and intermolecular N–H···Cl···H–N interactions that result in a polymeric array. This investigation demonstrates that not only is H(sebenzim^Me) a good ligand for mercury, capable of displacing halide ligands, but is also capable of protolytically cleaving mercury–carbon bonds, a result that is of relevance to the role of selenium compounds in the detoxification of mercury compounds.

Experimental Section

General Considerations

NMR spectra were measured on a Bruker Avance 500 DMX spectrometer. ¹H NMR spectra are reported in ppm relative to SiMe₄ (δ = 0) and were referenced internally with respect to the protio solvent impurity (δ 7.16 for C₆D₅H and 2.50 for DMSO-d₅).⁷²¹³C NMR spectra are reported in ppm relative to SiMe₄ (δ = 0) and were referenced internally with respect to the solvent (δ 128.06 for C₆D₆ and 39.52 for DMSO-d₆).⁷²⁷⁷Se NMR spectra are reported in ppm relative to neat Me₂Se (δ = 0) and were referenced using a solution of Ph₂Se₂ in C₆D₆ (δ = 460) as an external standard.⁷³¹⁹⁹Hg NMR spectra are reported in ppm relative to neat Me₂Hg (δ = 0) and were referenced using a 1.0 M solution of HgI₂ in DMSO-d₆ (δ = −3106) as an external standard.⁷⁴ Coupling constants are given in hertz. IR spectra were recorded as KBr pellets on a Nicolet iS10 FT-IR spectrometer (ThermoScientific), and the data are reported in reciprocal centimeters. 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone was obtained by a literature method,^31a and all other chemicals were purchased from Sigma-Aldrich.

CAUTION! All mercury compounds are toxic, and appropriate safety precautions must be taken in handling these compounds.

X-ray Structure Determinations

Single-crystal X-ray diffraction data were collected on a Bruker Apex II diffractometer, and crystal data, data collection, and refinement parameters are summarized in Table 6. The structures were solved using direct methods and standard difference map techniques, and were refined by full-matrix least-squares procedures on F² with SHELXTL (Version 2013/4).⁷⁵

Table 6. Crystal, Intensity Collection, and Refinement Data.

	[H(sebenzimMe)]3HgCl2·(MeCN)	[H(sebenzimMe)]4HgCl2	[H(sebenzimMe)]2HgI2	[H(sebenzimMe)]2HgI2	[H(sebenzimMe)2]HgI·0.5(benzene)	[H(sebenzimMe)2]2Hg
lattice	triclinic	monoclinic	orthorhombic	monoclinic	triclinic	triclinic
formula	C26H27Cl2HgN7Se3	C32H32Cl2HgN8Se4	C16H16HgI2N4Se2	C16H16HgI2N4Se2	C19H18HgIN4Se2	C32H30HgN8Se4
formula weight	945.91	1115.98	876.64	876.64	787.79	1043.07
space group	P1̅	P21	Pbcn	P21/c	P1̅	P1̅
a/Å	10.1199(8)	12.8918(14)	16.884(2)	14.0994(17)	8.0273(8)	8.7906(7)
b/Å	11.9549(10)	14.5673(15)	8.4266(11)	15.2939(19)	11.7704(12)	13.0704(10)
c/Å	14.3848(12)	19.000(2)	30.211(4)	10.1211(12)	11.7946(12)	15.0622(12)
α/°	74.8680(10)	90	90	90	88.8410(10)	104.4970(10)
β/°	86.9400(10)	94.939(2)	90	92.800(2)	88.2190(10)	98.2150(10)
γ/°	66.2060(10)	90	90	90	73.2710(10)	90.0560(10)
V/Å3	1534.5(2)	3554.9(7)	4298.3(10)	2179.9(5)	1066.65(19)	1657.1(2)
Z	2	4	8	4	2	2
temperature/K	150(2)	150(2)	130(2)	130(2)	130(2)	150(2)
radiation (λ)/Å	0.71073	0.71073	0.71073	0.71073	0.71073	0.71073
ρ(calcd)/g cm–3	2.047	2.085	2.709	2.671	2.453	2.090
μ(Mo Kα)/mm–1	8.777	8.612	13.429	13.240	12.086	9.074
θmax/deg	30.721	30.612	30.034	30.569	30.034	31.492
no. of data collected	25209	42569	128567	68656	30304	28237
no. of data	9472	21344	6281	6679	6221	10891
no. of parameters	365	888	235	235	251	418
R1 [I > 2σI)]	0.0301	0.0409	0.0502	0.0218	0.0323	0.0810
wR2 [I > 2σI]	0.0561	0.0617	0.1081	0.0516	0.0853	0.1831
R1 [all data]	0.0474	0.0684	0.0904	0.0251	0.0378	0.1123
wR2 [all data]	0.0608	0.0688	0.1216	0.0525	0.0878	0.1869
Rint [all data]	0.0345	0.0411	0.1616	0.0447	0.0464	0.1061
GOF	0.979	1.011	0.988	1.093	1.054	1.848

Synthesis of [H(sebenzim^Me)]₂HgI₂

A suspension of H(sebenzim^Me) (46 mg, 0.22 mmol) and HgI₂ (50 mg, 0.11 mmol) in C₆D₆ (2 mL) in an NMR tube equipped with a J. Young valve was heated overnight at 100 °C. Over this period, yellow, X-ray-quality crystals of [H(sebenzim^Me)]₂HgI₂ (54 mg, 56% yield) were deposited and isolated by decanting the solution. Crystals of [H(sebenzim^Me)]₂HgI₂ were also obtained from an acetonitrile solution. Anal. Calcd for C₁₆H₁₆I₂HgN₄Se₂: C, 21.9; H, 1.8; N, 6.4. Found: C, 22.0; H, 1.6; N, 6.4. ¹H NMR (DMSO-d₆): δ 3.90 [s, 6H of CH₃], 7.42 [m, 4H of C₆H₄], 7.51 [m, 2H of C₆H₄], 7.71 [m, 2H of C₆H₄], not observed [NH]. ¹³C{¹H} NMR (DMSO-d₆): δ 33.3 [CH₃], 111.8 [CH of C₆H₄], 112.1 [CH of C₆H₄], 124.4 [CH of C₆H₄], 125.0 [CH of C₆H₄], 131.8 [C of C₆H₄], 133.6 [C of C₆H₄], 152.7 [CSe]. ⁷⁷Se{¹H} NMR (DMSO-d₆): δ 48 ppm. ¹⁹⁹Hg{¹H} NMR (DMSO-d₆): not observed. IR data (KBr pellet, cm^–1): 3172 (m), 3114 (m), 3056 (m), 2986 (w), 2929 (w), 1619 (w), 1498 (m), 1486 (m), 1447 (vs), 1391 (w), 1364 (w), 1346 (s), 1333 (m), 1246 (w), 1226 (w), 1159 (w), 1132 (m), 1091 (m), 1008 (w), 902 (vw), 804 (w), 748 (vs), 727 (w), 664 (w).

Synthesis of [H(sebenzim^Me)]₃HgCl₂

A solution of HgCl₂ (17 mg, 0.06 mmol) in CH₃CN (1 mL) was added to a solution of H(sebenzim^Me) (40 mg, 0.19 mmol) in CHCl₃ (2 mL). The pale yellow solution was allowed to stand at room temperature for 4 days at room temperature, over which period colorless crystals were deposited as the solution evaporated. X-ray-quality crystals of [H(sebenzim^Me)]₃HgCl₂·(CH₃CN) were isolated by decanting the mother liquor and dried in vacuo (39 mg, 66% yield). Anal. Calcd for C₂₆H₂₇Cl₂HgN₇Se₃: C, 33.0; H, 2.9; N, 10.4. Found: C, 33.6; H, 2.3; N, 9.9. ¹H NMR (DMSO-d₆): δ 3.83 [s, 9H of CH₃], 7.35 [m, 6H of C₆H₄], 7.40 [m, 3H of C₆H₄], 7.62 [m, 3H of C₆H₄], 13.93 [br, N-H]. ¹³C{¹H} NMR (DMSO-d₆): δ 32.7 [CH₃], 111.3 [CH of C₆H₄], 111.4 [CH of C₆H₄], 123.9 [CH of C₆H₄], 124.5 [CH of C₆H₄], 131.6 [C of C₆H₄], 133.4 [C of C₆H₄], 154.6 [CSe]. ⁷⁷Se{¹H} NMR (DMSO-d₆): δ 35 ppm. ¹⁹⁹Hg{¹H} NMR (DMSO-d₆): δ −1020 ppm. IR data (KBr pellet, cm^–1): 3448 (w), 3032 (m), 2969 (m), 2918 (m), 2850 (m), 2804 (m), 2740 (m), 2693 (m), 2588 (w), 2514 (w), 1618 (w), 1502 (s), 1449 (vs), 1399 (m), 1360 (m), 1348 (s), 1334 (s), 1258 (m), 1242 (m), 1154 (w), 1132 (w), 1096 (s), 1008 (w), 805 (w), 740 (vs).

Synthesis of [H(sebenzim^Me)]₄HgCl₂

A suspension of H(sebenzim^Me) (85 mg, 0.40 mmol) and HgCl₂ (27 mg, 0.10 mmol) in CD₃CN (2 mL) in an NMR tube equipped with a J. Young valve was heated overnight at 100 °C. Over this period, pale yellow, X-ray-quality crystals of [H(sebenzim^Me)]₄HgCl₂ (94 mg, 84% yield) were deposited and isolated by decanting the solution. Anal. Calcd for C₃₂H₃₂Cl₂HgN₈Se₄: C, 34.4; H, 2.9; N, 10.0. Found: C, 34.7; H, 2.6; N, 10.0. ¹H NMR (DMSO-d₆): δ 3.80 [s, 12H of CH₃], 7.33 [m, 12H of C₆H₄], 7.56 [m, 4H of C₆H₄], 13.72 [br, N-H]. ¹³C{¹H} NMR (DMSO-d₆): δ 32.5 [CH₃], 111.0 [CH of C₆H₄], 111.1 [CH of C₆H₄], 123.6 [CH of C₆H₄], 124.2 [CH of C₆H₄], 131.7 [ring junction C of C₆H₄], 133.5 [ring junction C of C₆H₄], 156.6 [CSe]. ⁷⁷Se{¹H} NMR (DMSO-d₆): δ 44 ppm. ¹⁹⁹Hg{¹H} NMR (DMSO-d₆): δ −1012 ppm. IR data (KBR pellet, cm^–1): 3424 (w), 3032 (m), 2971 (m), 2919 (m), 2849 (m), 2727 (w), 2668 (w), 1618 (w), 1498 (m), 1447 (vs), 1390 (w), 1346 (s), 1333 (m), 1247 (w), 1156 (w), 1134 (w), 1097 (m), 1009 (w), 901 (vw), 804 (w), 756 (m), 747 (s).

Synthesis of [H(sebenzim^Me)₂]₂Hg

A suspension of H(sebenzim^Me) (40 mg, 0.19 mmol) and Ph₂Hg (17 mg, 0.05 mmol) in CD₃CN (0.7 mL) in an NMR tube equipped with a J. Young valve was heated overnight at 100 °C. Over this period, very pale yellow, X-ray-quality crystals of [H(sebenzim^Me)₂]₂Hg (32 mg, 65% yield) were deposited and isolated by decanting the solution. Anal. Calcd for C₃₂H₃₀HgN₈Se₄: C, 36.9; H, 2.9; N, 10.7. Found: C, 36.3; H, 2.9; N, 10.4. ¹H NMR (DMSO-d₆): δ 3.72 [s, 12H of CH₃], 7.17 [m, 8H of C₆H₄], 7.31 [m, 4H of C₆H₄], 7.40 [m, 4H of C₆H₄], not observed [NH]. ¹³C{¹H} NMR (DMSO-d₆): δ 32.0 [CH₃], 109.7 [CH of C₆H₄], 113.4 [CH of C₆H₄], 121.9 [CH of C₆H₄], 122.2 [CH of C₆H₄], 136.4 [C of C₆H₄], 156.0 [CSe]. ⁷⁷Se{¹H} NMR (DMSO-d₆): δ 74 ppm. ¹⁹⁹Hg{¹H} NMR (DMSO-d₆): not observed. IR data (KBr pellet, cm^–1): 3450 (vw), 3054 (w), 2932 (w), 2461 (w), 1904 (w), 1619 (w), 1514 (m), 1466 (vs), 1432 (vs), 1392 (s), 1359 (s), 1332 (vs), 1277 (vs), 1236 (m), 1150 (w), 1113 (w), 1086 (s), 1007 (m), 912 (w), 838 (vw), 806 (w), 736 (vs), 728 (vs), 662 (vw).

Reactivity of H(sebenzim^Me) towards MeHgI: Formation of [H(sebenzim^Me)₂]HgI

A suspension of H(sebenzim^Me) (64 mg, 0.30 mmol) and MeHgI (52 mg, 0.15 mmol) in C₆D₆ (2 mL) in an NMR tube equipped with a J. Young valve was heated overnight at 100 °C. Over this period, pale yellow, X-ray-quality crystals of [H(sebenzim^Me)₂]HgI·0.5(benzene) (67 mg, 56% yield) were deposited and isolated by decanting the solution. Anal. Calcd for C₁₉H₁₈HgIN₄Se₂: C, 29.0; H, 2.3; N, 7.1. Found: C, 29.1; H, 2.4; N, 7.1. ¹H NMR (DMSO-d₆): δ 3.75 [s, 6H of CH₃], 7.21 [m, 4H of C₆H₄], 7.41 [m, 2H of C₆H₄], 7.45 [m, 2H of C₆H₄], not observed [NH]. ¹³C{¹H} NMR (DMSO-d₆): δ 32.7 [CH₃], 110.6 [CH of C₆H₄], 113.8 [CH of C₆H₄], 122.9 [CH of C₆H₄], 123.2 [CH of C₆H₄], 134.6 [C of C₆H₄], 136.6 [C of C₆H₄], 151.2 [CSe, J_Se–C = 180]. ⁷⁷Se{¹H} NMR (DMSO-d₆): δ 48 ppm. ¹⁹⁹Hg{¹H} NMR (DMSO-d₆): not observed. IR data (KBr pellet, cm^–1): 3453 (vw), 3053 (w), 2932 (w), 2387 (w), 1901 (w), 1872 (w), 1863 (w), 1610 (w), 1523 (w), 1466 (vs), 1432 (vs), 1395 (s), 1336 (vs), 1277 (vs), 1236 (m), 1156 (w), 1112 (w), 1087 (s), 1007 (m), 910 (w), 846 (vw), 807 (w), 743 (vs), 736 (vs), 661 (vw).

Reactivity of H(sebenzim^Me) towards MeHgCl: Formation of [H(sebenzim^Me)₂]₂Hg and [H(sebenzim^Me)]₄HgCl₂

A suspension of H(sebenzim^Me) (85 mg, 0.40 mmol) and MeHgCl (25 mg, 0.10 mmol) in CD₃CN (2 mL) in an NMR tube equipped with a J. Young valve was heated overnight at 100 °C. Over this period, colorless plates of [H(sebenzim^Me)₂]₂Hg and large, yellow blocks of [H(sebenzim^Me)]₄HgCl₂ were deposited and were isolated by decanting the solution. The crystals were separated manually under a microscope for purposes of performing X-ray diffraction experiments.

¹H NMR Spectroscopic Study of the Titration of HgX₂ (X = Cl, Br, I) with H(sebenzim^Me)

A solution of HgX₂ (X = Cl, Br, I; 0.05 mmol) in DMSO-d₆ (0.6 mL) was treated with aliquots (40 μL) of a solution of H(sebenzim^Me) (126.7 mg, 0.6 mmol) in DMSO-d₆ (0.48 mL) and monitored by ¹H NMR spectroscopy. The data obtained are presented in Table 2.

⁷⁷Se{¹H} and ¹⁹⁹Hg{¹H} NMR Spectroscopic Study of the Titration of HgCl₂ with H(sebenzim^Me)

A solution of HgCl₂ (17.0 mg, 0.063 mmol) in DMSO-d₆ (0.6 mL) was treated with four aliquots of H(sebenzim^Me) (13.2 mg, 0.063 mmol) and monitored by ⁷⁷Se{¹H} and ¹⁹⁹Hg{¹H} NMR spectroscopy. The results of this titration are presented in Table 1.

⁷⁷Se{¹H} NMR Spectroscopic Study of HgX₂ (X = Cl, Br, I) with H(sebenzim^Me)

A solution of HgX₂ (X = Cl, Br, I) in DMSO-d₆ (0.05 mmol in 0.6 mL) was treated with 200 μL of a solution of H(sebenzim^Me) in DMSO-d₆ (0.5 mmol in 1.00 mL) and was monitored by ⁷⁷Se{¹H} NMR spectroscopy.

Supporting Information Available

Crystallographic data for all structurally characterized compounds (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

The authors declare no competing financial interest.

Funding Statement

National Institutes of Health, United States