Reactions of 2-mercaptobenzoic acid with divalent alkaline earth metal ions:: Synthesis, spectral studies, and single-crystal X-ray structures of calcium, strontium, and barium complexes of 2,2′-dithiobis(benzoic acid)

The treatment of MCl2. nH(2)O (M = Ca, Sr, or Ba) with 2-mercaptobenzoic acid (H-2-MBA) in a 1:2 ratio in an EtOH/H2O/NH3 mixture leads to the formation of extended polymeric solids [{Ca(OOCC6H4SSC6H4COO)(H2O)(2)}.0.5(C2H5OH)](n) (1), [{Sr(OOCC6H4SSC6H4COO)(H2O)(2)}.0.5(C2H5OH)](n) (2), and [{Ba-2(OOCC6H4SSC6H4COO)(2)- (H2O)(2)}.0.5H(2)O](n) (3), respectively. In all of the cases, under the reaction conditions employed, the H-2-MBA ligand undergoes thiol oxidation to form 2,2'-dithiobis(benzoic acid) (H-2-DTBB). While the DTBB forms a 1:1 complex with heavier alkaline earth metals (1-3), only an ammonium salt, [(HOOCC6H4SSC6H4COOH)-(HOOCC6H4SSC6H4COONH4)] (4), was obtained as the final product in the reaction of H-2-MBA with MgCl2. 6H(2)O. Compounds 1-4 have been characterized with the aid of elemental analysis, thermal analysis, and infrared spectroscopic studies. All of the products are found to be thermally stable and do not melt on heating to 250 degreesC. Thermogravimetry on complexes 1-3 indicates the loss of coordinated and lattice water/solvent molecules below 200 degreesC (for complex 2) or 350 degreesC (for complexes 1 and 3). The solid-state structures of all of the derivatives 1-4 have been established by single-crystal X-ray diffraction studies, The calcium and strontium coordination polymers 1 and 2 are isomorphous. The DTBB ligands in 1 and 2 are hexadentate, and the compounds have a channel structure in which solvent ethanol molecules are included. In compound 3, barium ion forms a complex 3-dimensional coordination polymer where both the carboxylate and the sulfur centers of die DTBB ligands (which are hepta- and octadentate) coordinate to the metal.