Mechanism of interaction of DNA bases with dichloro‐[1‐alkyl‐2‐(naphthylazo)imidazole]palladium(II) complexes: A cytosine case

Abstract

AbstractInteraction of cytosine (C) with dichloro‐[1‐alkyl‐2‐(α‐naphthylazo)imidazole]‐palladium(II) [Pd(α‐NaiR)Cl2, 1] and dichloro‐[1‐alkyl‐2‐(β‐naphthylazo)imidazole]‐palladium(II) [Pd(β‐NaiR)Cl2, 2] complexes {where alkyl R = Me (a), Et (b) or Bz (c)} in acetonitrile (MeCN)‐water (50% v/v) medium to yield [{1‐alkyl‐2‐(α‐naphthylazo)‐imidazole}bis(cytosine)]palladium(II)dichloride (3a, 3b, 3c) and [{1‐alkyl‐2‐(β‐naphthylazo)‐imidazole}bis(cytosine)]palladium(II)dichloride (4a, 4b, 4c) was studied. The products were characterized by microanalytical data and spectroscopic techniques (FT‐IR, UV–vis, and NMR). The reaction kinetics show first‐order dependence of the rate on each of the concentration of Pd(II) complex and C. External addition of Cl− ion (LiCl) did not influence this nucleophilic substitution rate process and has proved the cleavage of first PdCl bond is the rate‐determining step. Thermodynamic parameters standard enthalpy of activation (Δ‡Ho) and standard entropy of activation (Δ‡So) were determined from variable temperature kinetic studies. The negative values of Δ‡So indicate that the reaction proceeds through an associative inner sphere mechanism. The magnitude of the second‐order rate constant k2 increases in the following order: Pd(NaiEt)Cl2 (b) < Pd(NaiMe)Cl2 (a) < Pd(NaiBz)Cl2 (c) as well as Pd(α‐NaiR)Cl2 (1) < Pd(β‐NaiR)Cl2 (2), which corroborates with the experimental Δ‡Ho values. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 463–472, 2009