Cobalt(III) complexes of stereospecific linear NSNN tetradentate ligands. Synthesis and characterization of ternary amino acid complexes containing tetradentate ligands with terminal amine donors

Abstract

The preparation and characterization of cis-β-[Co(gee)(gly)] + (geeH N-{2-[(2-aminoethyl)thio]ethyl}-2-aminoacetamide) and cis-β-[Co(ege)(AA)] + (egeH N-(2-aminoethyl)-2-[(2-aminoethyl)thio]acetamide; AAgly, L-ala, L-leu, L-ile) complexes as PF 6 − or mixed Cl −/PF 6 − salts are described. 1H and 13C NMR spectra of the new cobalt complexes demonstrate that only one geometrical isomer (with respect to the bidentate chelate) is obtained with very high stereoselectivity. X-ray structural studies revealed that orientation of the amino acidato chelate is the β 1 configuration; that is, the isomer with the carboxylate donor of the amino acid coordinated trans to the amido function to the tetradentate backbone ligand. β 1-[Co(gee)(gly)]PF 6·H 2O ( I) crystallized in the monoclinic space group P2 1/ a with a=11.748(2), b=11.810(2), c=12.341(3) Å, β=101.43(2)° and Z=4 and was refined to R=0.042. β 1-[Co(ege)(gly)]PF 6 ( II) crystallized in the monoclinic space group P2 1/ n with a=11.467(2), b=8.173(1), c=17.421(3) Å, β=106.69(2)° and Z=4 and was refined to R=0.052. The complexes, β-[Co(L)Cl 2] (Lgee or ege), were found to be convenient precursors for the hydrolysis of the peptide bond of glygly to give β 1-[Co(L)(gly)] + under mild conditions.