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)

Abstract

The treatment of MCl₂·nH₂O (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 [{Ba₂(OOCC₆H₄SSC₆H₄COO)₂(H₂O)₂}·0.5H₂O]_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) (H₂-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 MgCl₂·6H₂O. 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 °C. Thermogravimetry on complexes 1-3 indicates the loss of coordinated and lattice water/solvent molecules below 200 °C (for complex 2) or 350 °C (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 the DTBB ligands (which are hepta- and octadentate) coordinate to the metal.