Structural and spectroscopic studies of binuclear Cu2+ and Co2+ complexes with an amide-based naphthalenophane

Binuclear Cu2+ and Co2+ complexes with a chelating naphthalenophane were characterized by single-crystal X-ray analyses, electronic absorption spectroscopy and luminescence spectroscopy: the naphthalenophane is 2,9,22,29-tetraoxo-4,7,24,27-tetrakis(carboxymethyl)-1,4,7,10,21,24,27,30-octaaza[10.10](1,5)naphthalenophane (abbreviated as LH4). The Cu2+ complex crystallized as [Cu2L]0 from acidic solution and [Cu2(LH−4)]4− from basic solution: the coordination geometry around each metal ion in [Cu2L]0 is a square pyramid with an amide oxygen atom, two amino nitrogen atoms and two carboxylate oxygen atoms; the amide nitrogen atoms in [Cu2(LH−4)]4− are deprotonated and construct a square planar coordination geometry together with amino nitrogen atoms around each metal ion. The formation of the two structures is due to a change in the coordination linkage of the amide groups. [Cu2(LH−4)]4− shows strong metal–ligand charge transfer bands caused by the coordination of the amide nitrogen atoms that are directly bonded to the naphthyl groups. In the Co2+ complex, [Co2L(H2O)2]0, each metal ion has a seven-coordination geometry; the emission and excitation bands in the luminescence spectra showed a red shift upon metal complexation. In all metal complexes studied, the naphthyl groups are distorted from the planar structure, as a result of metal complexation that causes contraction of the macrocyclic rings.