Protolytic cleavage of Hg-C bonds induced by 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone: synthesis and structural characterization of mercury complexes.

Multinuclear ((1)H, (77)Se, and (199)Hg) 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 mercury[1] have been attributed to both its thiophilicity[1−4] 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 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 species[14] 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(sebenzimMe),[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(mimR),[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(seimR).[28−32] For example, only H(seimMe)[28,29] and H(seimMes),[29] and the benzannulated derivatives, H(sebenzimMe)[30,31a] and H(sebenzimBu),[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(sebenzimMe),[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(sebenzimMe) with HgCl2, HgBr2, and HgI2

Evidence for the ability of the imidazole-2-selone, H(sebenzimMe), 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(sebenzimMe) 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(sebenzimMe) also progressively shift upon addition to HgCl2. In addition to providing evidence for coordination of H(sebenzimMe) 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(sebenzimMe) 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

Value for H(sebenzimMe).

Table 2

1H (N-CH3) NMR Chemical Shift Values for HgX2/H(sebenzimMe) 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

Value for H(sebenzimMe).

Figure 3

Variation of 1H NMR chemical shift of the methyl group of H(sebenzimMe) 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(sebenzimMe)]4HgCl2, [H(sebenzimMe)]3HgCl2, and [H(sebenzimMe)]2HgCl2,[31a] may be obtained by crystallization from a solution that contains the respective number of equivalents of H(sebenzimMe).

The molecular structures of [H(sebenzimMe)]3HgCl2 and [H(sebenzimMe)]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-type[36] 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(sebenzimMe)]3HgCl2, which is more appropriately represented as the ion pair, {[H(sebenzimMe)]3HgCl}[Cl].

Figure 5

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

In addition to [H(sebenzimMe)]4HgCl2 being of significance because its existence demonstrates that a mercury center can accommodate four selenium L-type donor ligands, the tris complex, [H(sebenzimMe)]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(sebenzimMe)]3HgCl2 (Figure 4) and [H(sebenzimMe)]4HgCl2 (Figure 5) with that of [H(sebenzimMe)]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(sebenzimMe)]2HgCl2 < [H(sebenzimMe)]3HgCl2 < [H(sebenzimMe)]4HgCl2, as summarized in Table 3. Thus, whereas the two Hg–Cl bond lengths in the bis complex [H(sebenzimMe)]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(sebenzimMe)]4HgCl2, is 3.913 Å, such that the compound may be better represented as {[H(sebenzimMe)]4Hg}[Cl]2. The Hg–Cl distances in the tris complex, [H(sebenzimMe)]3HgCl2, are intermediate between those of [H(sebenzimMe)]2HgCl2 and [H(sebenzimMe)]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(sebenzimMe)]3HgCl}[Cl]. In accord with the long Hg–Cl bond distance, the coordination geometry of {[H(sebenzimMe)]3HgCl}+ deviates significantly from tetrahedral. Thus, the four-coordinate τ4 index (Table 4)[44] of {[H(sebenzimMe)]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(sebenzimMe)]2HgCl2 (top), [H(sebenzimMe)]3HgCl2 (center), and [H(sebenzimMe)]2HgCl2 (bottom).

Table 3

Selected Bond Length Data for {[H(sebenzimMe)]Hg} 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)

Values for two crystallographically independent molecules.

Table 4

Four-Coordinate τ4 Indices for {[H(sebenzimMe)]Hg} 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

Data taken from ref (31a).

By comparison to the large variation in Hg–Cl interactions within [H(sebenzimMe)]HgCl2, 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 [TmBu]HgSePh [2.524 Å],[46] which feature normal covalent bonds. The Hg–Se bond lengths in [H(sebenzimMe)]HgCl2 are, nevertheless, comparable to the values in [TseMes]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(sebenzimMe)]HgCl2 complexes may be rationalized by the dative nature of the interactions.

A common feature of all [H(sebenzimMe)]HgCl2 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 interaction[53] that serves to link together two H(sebenzimMe) moieties, as summarized in Figure 7.

Figure 7

Intramolecular (top and middle) versus intermolecular (bottom) N–H···Cl hydrogen bonding interactions in [H(sebenzimMe)]HgCl2 complexes.

Thus, whereas [H(sebenzimMe)]2HgCl2 exhibits only intramolecular N–H···Cl interactions and is a discrete mononuclear species,[31a,54] [H(sebenzimMe)]3HgCl2 and [H(sebenzimMe)]4HgCl2 also exhibit intermolecular N–H···Cl interactions. Specifically, [H(sebenzimMe)]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(sebenzimMe)]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(sebenzimMe)]HgCl2 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 [dav(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(sebenzimMe)]3HgCl}+ moieties.

Figure 9

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

The various hydrogen bonding networks in [H(sebenzimMe)]HgCl2 may be described by the graph set notations[55] that are summarized in Table 5. For example, the hydrogen-bonded dimer of [H(sebenzimMe)]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(sebenzimMe)]HgCl2 and [H(sebenzimMe)]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)R21(10)D21(3)R21(10)D22(11)D22(11)D22(11)D22(11)D22(11)D22(11)D22(11)D22(11)
[H(sebenzimMe)]2HgI2 (monoclinic)	R22(8)R22(8)	C(12)C(12)C44(24)
[H(sebenzimMe)]2HgI2 (orthorhombic)	S(6)C(6)	–

Coordination of H(sebenzimMe) 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(sebenzimMe)]2HgCl2 [1.862(3) and 1.864(3) Å],[31a] [H(sebenzimMe)]3HgCl2 [1.868(3), 1.859(3), and 1.857(3) Å], and [H(sebenzimMe)]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(sebenzimMe) [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(sebenzimMe) 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(sebenzimMe) signals (Table 2 and Figure 3). Interestingly, the 77Se NMR chemical shift of the H(sebenzimMe) 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(sebenzimMe) and HgX2 move upfield from the value of pure H(sebenzimMe) by values of 68 ppm (X = Cl), 54 ppm (X = Br), and 35 ppm (X = I). Despite the reversibility of coordination of H(sebenzimMe), the bis complex, [H(sebenzimMe)]2HgI2, may, nevertheless, be isolated from reactions performed in either acetonitrile or benzene.

Interestingly, the crystals of [H(sebenzimMe)]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(sebenzimMe) 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(sebenzimMe)]2HgI2 obtained from acetonitrile (Figure 10), with a τ4 index of 0.94, is close to tetrahedral (τ4 = 1.00), monoclinic [H(sebenzimMe)]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(sebenzimMe)]2HgI2 (2.792 Å) is longer than that of the monoclinic version (2.737 Å), the average Hg–Se bond length of orthorhombic [H(sebenzimMe)]2HgI2 (2.627 Å) is shorter than that of the monoclinic version (2.692 Å). Similarly to HgCl2, coordination of H(sebenzimMe) to HgI2 is accompanied by only small increases in the lengths of the C–Se bonds. Thus, the C–Se bond lengths in [H(sebenzimMe)]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(sebenzimMe)]HgCl2, which range from 1.851(9) to 1.896(11) Å.

Figure 10

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

Figure 11

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

The most striking differences in the structures of orthorhombic and monoclinic [H(sebenzimMe)]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(sebenzimMe)]2HgCl2 (vide supra), as illustrated in Figure 14.

Figure 12

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

Figure 13

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

Figure 14

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

For example, whereas [H(sebenzimMe)]2HgCl2 is observed to have two intramolecular N–H···Cl interactions, the orthorhombic form of [H(sebenzimMe)]2HgI2 possesses one intramolecular and one intermolecular N–H···I interaction,[57] thereby creating a hydrogen-bonded helical chain of [H(sebenzimMe)]2HgI2 molecules (Figure 12).[58] In contrast to [H(sebenzimMe)]2HgCl2 and orthorhombic [H(sebenzimMe)]2HgI2, however, the monoclinic form of [H(sebenzimMe)]2HgI2 possesses no intramolecular or intermolecular N–H···I interactions. Rather, the N–H groups of the H(sebenzimMe) 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(seimR) derivatives in the absence of metal coordination (Figure 13).[29,59] Interestingly, H(sebenzimMe) 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(sebenzimMe)]2HgI2 [3.486(7) and 3.589(7) Å] are substantially longer than the analogous N···Cl interactions in [H(sebenzimMe)]2HgCl2. Thus, while the mean N···Cl distance in [H(sebenzimMe)]2HgCl2 is 3.182 Å, the mean N···I distance in orthorhombic [H(sebenzimMe)]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(sebenzimMe) is an analogue of selenoneine, we have also investigated the reactivity of H(sebenzimMe) towards methylmercury halides. Significantly, we have observed that H(sebenzimMe) 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(sebenzimMe) reacts with MeHgI at 100 °C to liberate CH4 (as observed by 1H NMR spectroscopy) and afford [H(sebenzimMe)2]HgI (Scheme 2). The importance of this observation is underscored by the fact that selenoneine, of which H(sebenzimMe) is a structural analogue, has recently been shown to achieve demethylation of CysHgMe.[11e]

Scheme 2

The molecular structure of [H(sebenzimMe)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) value[63] 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(sebenzimMe)2]HgI.

The most interesting feature of [H(sebenzimMe)2]HgI, however, pertains to the fact that the H(sebenzimMe) and (sebenzimMe) 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(sebenzimMe)2], may be viewed as an LX-type ligand.[36] In this regard, the two Hg–Se bond lengths present in [H(sebenzimMe)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(sebenzimMe)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 index[67] 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(sebenzimMe)2]HgI}.

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(sebenzimMe)2]HgI involves the initial formation of an adduct, [H(sebenzimMe)]Hg(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(sebenzimMe) 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(sebenzimMe) at 100 °C results in evolution of methane, as observed by 1H NMR spectroscopy, and the formation of a mixture of [H(sebenzimMe)]4HgCl2 (vide supra) and [H(sebenzimMe)2]2Hg (Scheme 3). The latter compound can also be obtained via the cleavage of the Hg–Ph bonds of Ph2Hg with H(sebenzimMe), as illustrated in Scheme 4.

Scheme 3

Protolytic Cleavage of MeHgCl by H(sebenzimMe)

Scheme 4

Protolytic Cleavage of Ph2Hg by H(sebenzimMe)

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

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

Conclusions

In summary, 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone, H(sebenzimMe), is a structural analogue of selenoneine and coordinates reversibly to the metal centers of HgX2 (X = Cl, Br, I). Furthermore, H(sebenzimMe) 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(sebenzimMe) simply coordinate to mercury centers of HgX2 (X = Cl, I), the third and fourth equivalents result in displacement of the chloride ligands. Thus, [H(sebenzimMe)]3HgCl2 and [H(sebenzimMe)]4HgCl2 are better represented as ion pairs, namely {[H(sebenzimMe)]3HgCl}[Cl] and {[H(sebenzimMe)]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(sebenzimMe)]HgCl2 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(sebenzimMe) donors. For example, whereas [H(sebenzimMe)]2HgCl2 exhibits only intramolecular N–H···Cl interactions and is a discrete mononuclear species, [H(sebenzimMe)]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(sebenzimMe)]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(sebenzimMe) 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).[72]13C 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).[72]77Se 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.[73]199Hg 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.

.

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(sebenzimMe)]2HgI2

A suspension of H(sebenzimMe) (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(sebenzimMe)]2HgI2 (54 mg, 56% yield) were deposited and isolated by decanting the solution. Crystals of [H(sebenzimMe)]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(sebenzimMe)]3HgCl2

A solution of HgCl2 (17 mg, 0.06 mmol) in CH3CN (1 mL) was added to a solution of H(sebenzimMe) (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(sebenzimMe)]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(sebenzimMe)]4HgCl2

A suspension of H(sebenzimMe) (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(sebenzimMe)]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(sebenzimMe)2]2Hg

A suspension of H(sebenzimMe) (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(sebenzimMe)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(sebenzimMe) towards MeHgI: Formation of [H(sebenzimMe)2]HgI

A suspension of H(sebenzimMe) (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(sebenzimMe)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, JSe–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(sebenzimMe) towards MeHgCl: Formation of [H(sebenzimMe)2]2Hg and [H(sebenzimMe)]4HgCl2

A suspension of H(sebenzimMe) (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(sebenzimMe)2]2Hg and large, yellow blocks of [H(sebenzimMe)]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(sebenzimMe)

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(sebenzimMe) (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(sebenzimMe)

A solution of HgCl2 (17.0 mg, 0.063 mmol) in DMSO-d6 (0.6 mL) was treated with four aliquots of H(sebenzimMe) (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(sebenzimMe)

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(sebenzimMe) in DMSO-d6 (0.5 mmol in 1.00 mL) and was monitored by 77Se{1H} NMR spectroscopy.