Unprecedented stabilization of cobalt(II) in a tetrahedral S2O2 environment: the use of a redox-noninnocent ligand

The reaction of Zn(II) and Co(II) with thiosalicylic acid, o-HSC6H4COOH, and its methyl ester has led to the following complexes: Zn(SC6H4COO)] (1), (NEt4)NaZn(SC6H4COO)2].H2O (2), (NEt4)2NaCo(SC6H4COO)3].2H2O (3), (NEt4)3Na3(Co(SC6H4COO)3)2].6MeOH (4), Zn(SC6H4COOMe)2] (5), and Co(SC6H4COOMe)n], n = 2 (6), 3 (7). These ligands have not allowed stabilization of Co(II) in a sulfur-oxygen coordination environment. The structures of complexes 2-4 and 7 have been determined crystallographically. Those of 2-4 show significant similarities such as the behavior of the -SC6H4COO- anion as chelating ligand and the involvement of sodium ions as a structural element. Thus, the structure of the Na(Zn(SC6H4COO)2)(H2O)]- anion in complex 2 can be described as infinite chains of consecutive Zn(SC6H4COO)2]2- metalloligands linked by Na(H2O)]+ centers, that of the Na(Co(SC6H4COO)3(H2O)2)]2(4-) anion in 3 as a centrosymmetric tetranuclear Co2Na2 dimer with a (CoIII(Ssymbol: see text]O)3)Na(mu-H2O)2Na(CoIII(Ssymbol: see text]O)3) core, and that of the pentanuclear Na3(Co(SC6H4COO)3)2(MeOH)6]3- anion in 4 as two dinuclear (CoIII(Ssymbol: see text]O)3)Na(MeOH)3] fragments linked to a central sodium ion, which appears to be the first structurally characterized example of a NaS6 site. The use of the o-HSC6H4COOMe ligand allowed the synthesis of Co(SC6H4COOMe)2] (6) but not its full structural characterization. Instead, Co(SC6H4COOMe)3] (7) was obtained and structurally characterized. It consists of mononuclear molecules containing an octahedral CoIIIS3O3 core. The selection of 2,2-diphenyl-2-mercaptoacetic acid as ligand with reductive properties has afforded the first mononuclear complex containing a CoIIS2O2 core and thus an unprecedented model for Co(II)-substituted metalloproteins containing tetrahedral MS2O2 active sites. The synthesis and full structural characterization of the isostructural complexes (NEt4)2Zn(Ph2C(S)COO)2] (8) and (NEt4)2Co(Ph2C(S)COO)2] (9) show that they consist of discrete M(Ph2C(S)COO)2]2- anions, with a distorted tetrahedral coordination about the metal. In addition, the stability conferred by the ligand on the CoIIS2O2 core has allowed its characterization in solution by paramagnetic 1D and 2D 1H NMR studies. The longitudinal relaxation times of the hyperfine-shifted resonances and NOESY spectra have led to the assignment of all resonances of the cobalt complex and confirmed that it maintains its tetrahedral geometry in solution. Magnetic measurements (2-300 K) for complex 9 and 9.2H2O are in good agreement with distorted tetrahedral and octahedral environments, respectively.