Steric constraints imposed on facultative ligands. Complexes of triethylenetetramine and a homologue

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Abstract
Although five-membered rings are known to be the most stable for aliphatic bidentate diamine ligands, multidentate ligands with linked consecutive five-membered rings are not necessarily the most stable in all of their facultative dispositions. Evidence for steric constraints in triethylenetetramine (1,4,7,10-tetra-azadecane) complexes of the transition metals is reviewed, and some complexes are described of a previously unexplored ligand, 1,4,8,11-tetra-azaundecane, which is a homologue of triethylenetetramine. Comparison of the chemistry of the metal complexes of this new ligand with that of the complexes of triethylenetetramine brings the evidence for steric constraints in the latter system into clear perspective. A cis configuration is strongly favoured by triethylenetetramine in its octahedral complexes, while the new ligand tends to favour a trans configuration. The origin of such steric constraints is discussed mainly in terms of conformational considerations. Other origins of steric constraints in multidentate ligands are considered briefly, with examples. Some details of the chemistry of complexes of this new ligand with cobalt(III), chromium(III), rhodium(III), copper(II), and nickel(II) are presented. The new compounds reported are trans-[Co ligand Cl2]Cl, cis-[Co ligand Cl2]Cl, cis-[Co ligand C2O4]2C2O4, cis-[Cr ligand Cl2]Cl, cis-[Cr ligand Cl2]ClO4, trans-[Rh ligand Cl2]NO3, Cu ligand X2(where X = Cl, Br, ClO4), and Ni ligand X2(where X = Cl, Br, I, ClO4).

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