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

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-l-ylidene)-2,2'-disulfanediyldianiline (L-1) and N,N'-bis(3,3-diphenylprop-2-en-1-ylidene)-2,2'-disulfanediyldianiline (L-2), and their reaction with Zn2+. Surprisingly, both L-1 and L-2 undergo reductive disulfide bond scission upon reaction with Zn2+ in alcoholic media to give, under alcohol oxidation, the respective Zn(NS)2 complexes Zn(L-3)(2) (1) and Zn(L-4)(2) (2), where the L-3 and L-4 are the respective bidentate thiolate-imine anions. The ligands L-1 and L-2 and the complexes I 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 I. Cyclic voltammetric studies demonstrate that Zn(II) shifts the reduction potential of the disulfide ligands L-1 and L-2 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. (C) 2014 Elsevier Ltd. All rights reserved.