ELECTROCHEMICAL REDUCTION OF COMPLEXES CONTAINING THE [MO3S(S2)3]4+ CORE IN AQUEOUS-MEDIA AND THE STRUCTURE OF BIS(TRIETHYLAMMONIUM) TRIS(2-MERCAPTOBENZOATO)TRIS(MU-DISULFIDO)(MU-3-THIO)-TRIANGULO-TRIMOLYBDATE(IV)

The electrochemical reduction of complexes containing the Mo3S(S2)3 core with 2-mercaptosuccinic acid (H3msa), 2,3-dimercaptosuccinic acid (H4dsa), 3,4-dihydroxybenzoic acid (H3dba), and 2-mercaptobenzoic acid (H2mba) was investigated by polarography and cyclic voltammetry in aqueous media. A Langmuir type adsorption and an irreversible reduction of the adsorbed complexes on the surface of the hanging-mercury electrode was detected by CV measurements. The difference polarography studies revealed a strong pH dependence of the peak potential E(p). However, E(p) was not affected by the nature of the peripheral ligands or by the overall charge of the complex. In acidic media, E(p) increased linearly with 56-60 mV/pH unit whereas in alkaline solutions the increase was only 28-30 mV/pH unit (20-25-degrees-C). These results indicated the cleavage of the mu-S2 bridges to mu-S bridges and H2S or HS-, respectively. The structure of the mba complex has been characterized by X-ray diffraction analysis: space group P2(1)/c, Z = 4, a = 12.086 (2) angstrom, b = 27.099 (3) angstrom, c = 14.526 (2) angstrom, and beta = 93.63 (1)-degrees. In the crystal, two [Mo3S7(mba)3]2- moieties form a dimer. The COO group of one ligand is associated with the three axial sulfur atoms of a second complex and vice versa. The average O-S distance of 2.87 angstrom indicated a weak binding, which is discussed in terms of an anionic binding site of the Mo3S7 core. The observation that only one isomer is formed (all ligands are bound with S cis to the mu-3-S atom) is discussed as a consequence of the trans effect of the mu-3-S atom. In aqueous solution, [Mo3S7(H2dsa)3]2- could be deprotonated to [Mo3S7(dsa)3]8-. The pK values of 3.3, 3.98, 4.89, 6.77, 7.64, and 11.7 (25-degrees-C, 0.1 M KNO3) indicated the presence of five noncoordinated -COOH groups and one noncoordinated -SH group. Thus, in this isomer, two ligands are bound to Mo by the two S atoms and one ligand is bound by one S and one O atom.