Activation of the Hg–C Bond of Methylmercury by [S2]-Donor Ligands

Abstract

Here we report that [S2]-donor ligands BmmOH, BmmMe, and BmeMe bind rapidly and reversibly to the mercury centers of organomercurials, RHgX, and facilitate the cleavage of Hg-C bonds of RHgX to produce stable tetracoordinated Hg(II) complexes and R2Hg. Significantly, the rate of cleavage of Hg-C bonds depends critically on the X group of RHgX (X = BF4-, Cl-, I-) and the [S2]-donor ligands used to induce the Hg-C bonds. For instance, the initial rate of cleavage of the Hg-C bond of MeHgI induced by BmeMe is almost 2-fold higher than the initial rate obtained by BmmOH or BmmMe, indicating that the spacer between the two imidazole rings of [S2]-donor ligands plays a significant role here in the cleavage of Hg-C bonds. Surprisingly, we noticed that the initial rate of cleavage of the Hg-C bond of MeHgI induced by BmeMe (or BmmMe) is almost 10-fold and 100-fold faster than the cleavage of Hg-C bonds of MeHgCl and [MeHg]BF4 respectively, under identical reaction conditions, suggesting that the Hg-C bond of [MeHg]BF4 is highly inert at room temperature (21 °C). We also show here that the nature of the final stable cleaved products, i.e. Hg(II) complexes, depends on the X group of RHgX and the [S2]-donor ligands. For instance, the reaction of BmmMe with MeHgCl (1:1 molar ratio) afforded the formation of the 16-membered metallacyclic dinuclear mercury compound (BmmMe)2Hg2Cl4, in which the two Cl atoms are located inside the ring, whereas due to the large size of the I atom, a similar reaction with MeHgI yielded polymeric [(BmmMe)2HgI2]m·(MeHgI)n. However, the treatment of BmmMe with ionic [RHg]BF4 led to the formation of the tetrathione-coordinated mononuclear mercury compound [(BmmMe)2Hg](BF4)2, where BF4- serves as a counteranion.