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

Here we report that [S₂]-donor ligands Bmm^OH, Bmm^Me, and Bme^Me 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 Bme^Me is almost 2-fold higher than the initial rate obtained by Bmm^OH or Bmm^Me, 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 Bme^Me (or Bmm^Me) 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 Bmm^Me with MeHgCl (1:1 molar ratio) afforded the formation of the 16-membered metallacyclic dinuclear mercury compound (Bmm^Me)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 [(Bmm^Me)₂HgI₂]_m·(MeHgI)_n. However, the treatment of Bmm^Me with ionic [RHg]BF₄ led to the formation of the tetrathione-coordinated mononuclear mercury compound [(Bmm^Me)₂Hg](BF₄)₂, where BF₄^- serves as a counteranion.