The First Structurally Authenticated Organomercury(1+) Thioether Complexes − Mercury−Carbon Bond Activation Related to the Mechanism of the Bacterial Enzyme Organomercurial Lyase

The new compounds [MeHg([9]aneS3)](BF4) (1), [MeHg([12]aneS3)](BF4) (2), and [(MeHg)2([14]aneS4)](BF4)2 (3) have been prepared and their crystal structures determined. In 1, the thioether acts as a tridentate ligand [Hg−S 2.611(2)−2.768(2) Å] and thus the metal atom is tetrahedrally coordinated, which is rare in organomercury chemistry. Temperature‐dependent 1H and 13C NMR spectra showed that this coordination is retained in acetonitrile solution. In crystalline 2 and 3, linear‐coordinated HgII occurs with Hg−S bond lengths of 2.441(4) and 2.425(2) Å. [MeHg([9]aneS3)]+ was found to be stable towards ligand substitution by CF3SO3− in dimethyl sulfoxide, whereas the thioether was partly displaced by CF3CO2− and completely by CH3CO2−. Protonolysis by the very strong Brønsted acid CF3SO3H in [D3]nitromethane transformed the methanido ligand into methane. The degree of Hg−C bond cleavage was ca. 25% for the four‐coordinate [MeHg([9]aneS3)]+ after 1 h, whereas no reaction was observed for two‐coordinate [MeHg(SEt2)]+ or MeHgCl under similar conditions even after 24 h. The product Hg2+ was trapped as [Hg([9]aneS3)2]2+, which is a six‐coordinate complex, as shown by a crystal structure analysis of [Hg([9]aneS3)2](BF4)2·2 CH3CN. Quantum chemical calculations [MP2/SDD + SDD ECP Hg, 6‐31+G(d,p) other elements] confirmed that hydrogen transfer activation barriers are significantly lower for “high”‐coordinate (CN > 2) complexes. Hg−C bond activation by the enzyme organomercurial lyase is possibly also based on multiple (cysteinyl) sulfur ligation. We propose a hypothetical reaction mechanism that involves an OH‐containing amino acid side chain or a water molecule simultaneously serving as a proton acceptor (from Cys−SH) and donor (to R− of RHg+). This mechanism is supported by quantum chemical calculations on a model system. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)