Coordination polymers of 5,5′-dithiobis(2-nitrobenzoic acid): Synthesis, structure and topology

In this work we report on new coordination assemblies of the 5,5′-dithiobis(2-nitrobenzoic acid) acid (H₂L) with Ce(III), La(III), Pr(III), Tb(III)) and Cd(II) metal ions. Two different secondary building units (SBUs) of di-nuclear metal clusters are observed in [Ce₂(L)₃(DMA)₃(H₂O)₄]_n·DMA (1; L = doubly deprotonated anion of H₂L, DMA = N,N-dimethylacetamide), the structure of which represents a 2D coordination polymer. The reactions of H₂L with La- and Pr-metal salts yielded two isomorphous structures of [Ln(L)(DMF)(H₂O)(HCOO)]_n, Ln = La (2), Pr (3) (DMF = N,N-dimethylformamide), characterized by coordination networking in two dimensions as well. In similar reaction conditions complexation of the organic ligand with Tb ions resulted in the formation of 1D polymeric chains of [Tb(L)(DMA)₂(NO₃)]_n·H₂O (4). The conformation of the ligand in these structures is characterized by a ≥90° dihedral angle about the central S-S bond. In the reaction of Cd ions with H₂L in the presence of 4,4′-bipyridyl (bpy) a 2D coordination network of [Cd(L)(bpy)(H₂O)]_n (5) was obtained, revealing an acute C-S-S-C torsion angle of 83.3° about the central bond. Similar reaction in the absence of bpy yields yet a different product, [Cd(L₁₂)(DMF)]_n (6) (L₁₂ = dissociated 5-thio-2-nitrobenzoate moiety). Its formation involves breaking of the S-S bond, and coordination of the two separate thio-2-nitrobenzoate parts to the Cd ion through their S-sites. Compounds 1-6 have been analyzed by X-ray diffraction techniques and infrared spectroscopy, throwing light on the preferred molecular shape of this ligand and its coordination patterns with the various metal ions.