Methylation of (2-methylethanethiol-bis-3,5-dimethylpyrazolyl)methane zinc complexes and coordination of the resulting thioether: Relevance to zinc-containing alkyl transfer enzymes

A series of zinc complexes using a new tripodal, N2S, heteroscorpionate ligand (L3SH) that is isostructural and isoelectronic with the well-known N-3 trispyrazolylborates have been methylated in solution and the coordination properties of the resulting thioether examined. This system models the reactivity of zinc-containing enzymes involved in alkyl group transfers such as the DNA repair protein Ada from E. coli, or farnasyl transferase where it has been shown that the thioether resulting from alkyl group transfer remains in the coordination sphere of the zinc. The following complexes have been structurally characterized: [(L3S)ZnI] (1), [(L3SCH(3))ZnI2] (2), [(L3SCH(3))ZnI]BF4 (3), [(L3SCH(3))Zn-mu -bis-acetato-mu -hydroxo-Zn(L3SCH(3))]BF4 (5), [(L3SCH(3))ZnSPhF5]ClO4 (7), and [(L3SCH(3))(2)Zn](BF4)(2) (8). Complexes 3, 4, 5, 7. and 8 all display thioether coordination. Thus in the absence of superior anionic ligands, thioethers are reasonably good donors to zinc in either a tetrahedral or octahedral geometry. The methylation of the complex [(L3S)ZnSPhF5], which contains two different thiols, produces a single product, 7, where only the aliphatic thiol has been alkylated. This observation validates the suggestion that reactivity in enzymes with multiple zinc-bound thiols could be controlled by differences in thiol pK(a) (Hammes, B. S.; Warthen, C. R.; Crans, D.; Carrano, C. J. J. Biol. Inorg. Chern. 2000, 6, 82. Compound 7 is also of interest in that it resembles the metal ion-binding site of the blue copper protein, azurin.