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

Article abstract of DOI:10.1021/ic000223q

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)Na[Zn(SC₆H₄COO)₂]·H₂O (2), (NEt₄)₂Na[Co(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 {Co(III)(S intersection set O)₃}Na(μ-H₂O)₂Na{Co(III)(S intersection set O)₃} core, and that of the pentanuclear [Na₃{Co(SC₆H₄COO)₃}₂(MeOH)₆]^3- anion in 4 as two dinuclear [{Co(III)(S intersection set 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 Co(III)S₃O₃ core. The selection of 2,2-diphenyl-2-mercaptoacetic acid as ligand with reductive properties has afforded the first mononuclear complex containing a Co(II)S₂O₂ 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)₂[Zn{Ph₂C(S)COO}₂] (8) and (NEt₄)₂[Co{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 Co(II)S₂O₂ 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.