The synthesis and isolation of the stereospecific ligand, N-(2-aminoethyl)-2-[(2-aminoethyl)thio]acetamide (egeH). Comparison of the crystal structure of [Co(ege)(NO₂)₂] ·4H₂O with related systems

A convenient synthetic scheme that utilizes readily available starting materials and leads to the isolation of the linear, stereospecific NSNN tetradentate ligand, N-(2-aminoethyl)-2-[(2-aminoethyl)thio]acetamide (egeH, 6), as its dihydrobromide salt is reported. Ligand 6 deprotonates and forms octahedral complexes with cobalt(III) of only one geometric isomer, for example, [Co(ege)(NO₂)₂]·4H₂O (7). Compound 7 crystallized in the monoclinic space group I2/c, a = 25.83(2), b = 7.896(6), c = 15.703(8) Å, β = 90.36(5)°, V = 3202(3) ų, Z = 8. Refinement was based on 3105 reflections and converged at R = 0.038, R_w = 0.047. The structural determination reveals that the geometry of the backbone ligand is cis-β. In addition, the structures of [Co(egpy)(NO₂)₂] (8) (where egpyH = N-(2-aminoethyl)-2-[(2-pyridylmethyl)thio]acetamide) and [Co(eee)(NO₂)₂]Br (9) (where eee = 1,8-diamino-3,6-dithiaoctane), which also contain stereospecific ligands, were also determined. Compound 8 crystallized in the monoclinic space group P2₁/c, a = 7.628(3), b = 12.347(4), c = 16.433(8) Å, β = 108.39(3)°, V = 1470(1) ų, Z = 4. Refinement was based on 2249 reflections and converged at R = 0.034, R_w = 0.042. Compound 9 crystallized in the monoclinic space group P2₁/a, a = 12.719(9), b = 13.855(4), c = 8.033(6) Å, β = 111.32(4)δ, V = 1319(1) Å ³, Z = 4. Refinement was based on 2300 reflections and converged at R = 0.029, R_w = 0.035. The tetradentate ligand in the former adopts the cis-β conformation, while for the latter, the expected cis-α geometry is observed. The structural parameters are compared and the origins of the attendant stereospecificities are discussed in terms of the nature of the internal donor atoms of the tetradentate ligands.