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 Tm(Ph)]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 Tm(Ph)]Li and LiSCH2C(O)N(H)Ph] (Tm(Ph)] = tris(2-mercapto-1-phenylimidazolyl)hydroborato ligand). Structural characterization of Tm(Ph)]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 Tm(Ph)]ZnSCH2C(O)N(H)Ph is subject to alkylation, a reaction that is analogous to the function of the Ada DNA repair protein. Specifically, Tm(Ph)]ZnSCH2C(O)N(H)Ph reacts with MeI to yield PhN(H)C(O)CH2SMe and Tm(Ph)]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 degrees C for the reaction of Tm(Ph)]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 Tm(Ph)]ZnSCH2C(O)N(H)Ph. Correspondingly, mechanisms that involve direct alkylation of coordinated thiolate are predicted to be characterized by kH/kD < or = 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.