Intramolecular NH···S Hydrogen Bonding in the Zinc Thiolate Complex [Tm^Ph]ZnSCH₂C(O)NHPh: A Mechanistic Investigation of Thiolate Alkylation as Probed by Kinetics Studies and by Kinetic Isotope Effects

The zinc thiolate complex [TmPh]ZnSCH2C(O)N(H)Ph, which features a tetrahedral [ZnS4] motif analogous to that of the Ada DNA repair protein, may be obtained by the reaction of Zn(NO3)2 with [TmPh]Li and Li[SCH2C(O)N(H)Ph] ([TmPh] = tris(2-mercapto-1-phenylimidazolyl)hydroborato ligand). Structural characterization of [TmPh]ZnSCH2C(O)N(H)Ph by X-ray diffraction demonstrates that the molecule exhibits an intramolecular NH···S hydrogen bond between the amide N−H group and thiolate sulfur atom, a structure that is reproduced by density functional theory (DFT) calculations. The thiolate ligand of [TmPh]ZnSCH2C(O)N(H)Ph is subject to alkylation, a reaction that is analogous to the function of the Ada DNA repair protein. Specifically, [TmPh]ZnSCH2C(O)N(H)Ph reacts with MeI to yield PhN(H)C(O)CH2SMe and [TmPh]ZnI, a reaction which is characterized by second-order kinetics that is consistent with either (i) an associative mechanism or (ii) a stepwise dissociative mechanism in which the alkylation step is rate determining. Although the kinetics studies are incapable of distinguishing between these possibilities, a small normal kinetic isotope effect of kH/kD = 1.16(1) at 0 °C for the reaction of [TmPh]ZnSCH2C(O)N(H*)Ph (H* = H, D) with MeI is suggestive of a dissociative mechanism on the basis of DFT calculations. In particular, DFT calculations demonstrate that a normal kinetic isotope effect requires thiolate dissociation because it results in the formation of [PhN(H)C(O)CH2S]- which, as an anion, exhibits a stronger NH···S hydrogen bonding interaction than that in [TmPh]ZnSCH2C(O)N(H)Ph. Correspondingly, mechanisms that involve direct alkylation of coordinated thiolate are predicted to be characterized by kH/kD ≤ 1 because the reaction involves a reduction of the negative charge on sulfur and hence a weakening of the NH···S hydrogen bonding interaction.