Cobalt(III) Complexes of Stereospecific Linear NSNN Tetradentate Ligands. 1. Synthesis of Ligands that Adopt the Cis-β; Coordination Geometry

The syntheses of four linear tetradentate ligands containing a sulfur atom and an amide nitrogen atom as internal donor atoms and either amine or pyridine terminal donor groups are reported. The ligands include geeH (N-[2-((2-aminoethyl)thio)ethyl]-2-aminoacetamide), pygeH (N′-((2-pyridyl)methyl)-2-((2-aminoethyl)thio)acetamide), egeH (N-(2-aminoethyl)-2-((2-aminoethyl)thio)acetamide), and egpyH (N-(2-aminoethyl)-2-[((2-pyridyl)methyl)thio]acetamide). Co(III) complexes of these ligands were prepared, and chromatographic and spectroscopic evidence is presented which indicates that only one geometric isomer is formed. A single-crystal X-ray diffraction study of Co(gee)(NO₂)₂·H₂O (C₆H₁₄CoN₅S·H₂O) was undertaken to establish its stereochemistry. The complex crystallized in space group C2/c with a = 20.832 (6) Å, b = 7.823 (3) Å, c = 15.570 (4)Å, β = 95.19 (2)°, and Z = 8. The structure was solved by conventional Patterson and Fourier methods for 2786 independent reflections having [formula-omitted] and I > 3σ(I). Full-matrix least-squares refinement led to a final R value of 0.033. The tetradentate gee ligand adopts the cis-β coordination geometry about the cobalt atom. Arguments based upon ligand structural similarities, spectroscopic techniques, and interconversion reactions of the complexes are presented which suggest that all four ligands bind to Co(III) stereospecifically in the cis-β geometry for all complexes prepared in this study.