The non-formation of macrocyclic tetraamides by coordinated ligand reactions

A recent paper has described the synthesis of macrocyclic tetraamide complexes by the reaction of a diamine (ethylenediamine or propylenediamine) with malonic, succinic or glutaric acids in methanol solution in the presence of a metal(II) salt at room temperature. We present evidence that macrocycle formation does not occur under these conditions and only mixed-ligand complexes are formed. The blue crystalline complex obtained with malonic acid, 1,3-diaminopropane and copper(II) chloride is shown by X-ray crystallography to be [Cu(mal)(pn)Cl₂]^2- pnH²⁺₂ (mal = malonate; pn = 1,3-diaminopropane). The structure comprises discrete [Cu(mal)(pn)Cl₂]^2- anions and pnH²⁺₂ cations. Within the anionic complex the geometry at each copper(II) is tetragonal with two long axial bonds to the chloride ligands, Cu-Cl(1) = 2.862(2) and Cu-Cl(2) = 3.042(2) Å. The malonato ring has a boat conformation with Cu-O = 1.966(3) Å and the 1,3-diaminopropane ring a chair conformation with Cu-N = 1.993(4) Å. A crystallographic mirror plane bisects the two rings through the copper and chloride ligands.