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

The preparation and characterization of cis-β-[Co(gee)(gly)]⁺ (geeHN-{2-[(2-aminoethyl)thio]ethyl}-2-aminoacetamide) and cis-β-[Co(ege)(AA)]⁺ (egeHN-(2-aminoethyl)-2-[(2-aminoethyl)thio]acetamide; AAgly, L-ala, L-leu, L-ile) complexes as PF₆^- or mixed Cl^-/PF₆^- 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 β₁ configuration; that is, the isomer with the carboxylate donor of the amino acid coordinated _trans to the amido function to the tetradentate backbone ligand. β₁-[Co(gee)(gly)]PF₆·H₂O (I) crystallized in the monoclinic space group P2₁/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. β₁-[Co(ege)(gly)]PF₆ (II) crystallized in the monoclinic space group P2₁/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)Cl₂] (Lgee or ege), were found to be convenient precursors for the hydrolysis of the peptide bond of glygly to give β₁-[Co(L)(gly)]⁺ under mild conditions.