New Zn(II) complexes with N2S2 Schiff base ligands. Experimental and theoretical studies of the role of Zn(II) in disulfide thiolate-exchange

Abstract

Described are the synthesis and characterization of two, potentially tetradentate, N2S2 Schiff-base ligands, containing a disulfide bond, N,N′-bis(3-phenylprop-2-en-1-ylidene)-2,2′-disulfanediyldianiline (L1) and N,N′-bis(3,3-diphenylprop-2-en-1-ylidene)-2,2′-disulfanediyldianiline (L2), and their reaction with Zn2+. Surprisingly, both L1 and L2 undergo reductive disulfide bond scission upon reaction with Zn2+ in alcoholic media to give, under alcohol oxidation, the respective Zn(NS)2 complexes Zn(L3)2 (1) and Zn(L4)2 (2), where the L3 and L4 are the respective bidentate thiolate-imine anions. The ligands L1 and L2 and the complexes 1 and 2 have been characterized spectroscopically, and the crystal and molecular structures of the two complexes have been determined by single crystal X-ray diffraction. The coordination geometry around Zn(II) centers in both complexes is a distorted tetrahedron. In addition, DFT calculations (B3LYP/LANL2DZ/6-311++G(d,p)) support the structure of 1. Cyclic voltammetric studies demonstrate that Zn(II) shifts the reduction potential of the disulfide ligands L1 and L2 to less negative values thus making them more susceptible to reductive cleavage of the disulfide bond. The results of semi-empirical PM6 calculations offer key insight into the nature of the transition state for this reaction.