Complexation of copper(II) with acyclic amide ligands: structure of an unexpected rearrangement product resulting from an intramolecular reaction of an amine with an amide

The copper(II) coordination chemistry of an acyclic diamide ligand, H2L1, has been investigated [H2L1=2,6-bis(1-propanecarboxamido-3-amino)pyridine]. Purple compounds, Cu(L1+H)NO3 and Cu(L1′)OAc, are formed when H2L1 is reacted with copper(II) nitrate and copper(II) acetate, respectively. The latter complex was characterised by X-ray crystallography which revealed that an unexpected intramolecular dehydration reaction, between one of the amide groups in H2L1 and an amine group from the same ‘arm’, had resulted in the formation of an amidine group. The copper(II) ion is square pyramidal, with the four basal plane nitrogen donor atoms provided by the ligand (L1′)− and the apical site occupied by an oxygen donor from an acetate ion. The two ligand types, (L1+H)− and (L1′)−, can be distinguished by cyclic voltammetry: as anticipated the Cu(L1+H)NO3 complex (Epa=+0.98 V, in H2O vs. SCE), with two coordinated amide nitrogen atoms, is more easily oxidised than the Cu(L1′)OAc complex (Epa=+1.13 V, in H2O vs. SCE), in which there is only one coordinated amide nitrogen atom.