A structural evidence for the preferential coordination of the primary amide group versus the unionised carboxyl group: synthesis,molecular and crystal structure,and properties of [Cu(HADA)2], a new copper(II) bis-chelate (H2ADA=N-(2-carbamoylmethyl)iminodiacetic acid)

Bis(N-carboxymethyl-N-(2-carbamoylmethyl)glycinato)copper(II) has been synthesised by reaction of Cu₂(CO₃)(OH)₂ and N-(2-carbamoylmethyl)-iminodiacetic acid (H₂ADA) in water. The new compound Cu(HADA)₂] (1) has been characterised by thermal, spectral, magnetic and X-ray diffraction methods. Its crystal structure was solved to a final value of R1=0.036. The crystal consists in a 3D hydrogen bonded network builds up by centro-symmetric molecules where the Cu(II) atom exhibits an elongated octahedral coordination. The ligand HADA⁻ adopts a fac-NO + O(apical amido) tridentate chelating conformation Cu1–N1(amino)=2.063(2), Cu1–O31(carboxylato)=1.917(2), Cu1–O11(amido)=2.380(2) Å] and has a free N-carboxymethyl arm. A structural comparison between 1 and bis(N,N-bis(carboxymethyl)glycinato)copper(II) dihydrate (compound 2) reveals that the amide primary group is preferred to the unionised carboxyl group to form a Cu(II)–N, O-glycinamide-like chelate ring instead of a Cu(II)–N, O-glycine-like one.