Binuclear Copper(II) Chelates of Amide-Based Cyclophanes

A chelating cyclophane has been synthesized by cyclocondensation of two ethylenediaminetetraacetic (EDTA) units with two p-phenylenediamine units: the resulting cyclophane is 2,9,18,25-tetraoxo-4,7,20,23-tetrakis(carboxymethyl)-1,4,7,10,17,20,23,26- octaaza[10.10]paracyciophane, abbreviated as (bis-edtapdn)H₄. Cyclocondensation of two EDTA and two 1,5-diaminonaphthalene units has given the naphthalenophane, 2,9,22,29-tetraoxo4,7,24,27-tetrakis(carboxymethyl)-i,4,7,10,2i,24,27,30- octaaza[10.10](1,5)naphthalenophane, (bis-edtanap)H₄-Studies of electronic and EPR spectra have been carried out on the binuclear Cu²⁺ complexes of these new ligands and of related chelating cyclophanes, 2,9,25,32-tetraoxo-4,7,27,30-tetrakis(carboxymethyl)-L4,7,10,24,-27,30,33- octaaza[10,1.10.1]paracyclophane, abbreviated as (bis-edtabpm)H4, and 2,9,25,32-tetraoxo-4,7,27,30-tetrakis(carboxymethyl)-1,4,7,10,24,27,30,33- octaaza-17,40-dioxa[ 10.1.10.1]paracyclophane, abbreviated as (bisedtabpe)H₄. Common features of these chelating cyclophanes are as follows: (1) amino, amide, and pendant carboxymethyl donor groups are substituents in the cyclophane ring, and (2) the amide groups are directly bound to the aromatic groups. These ligands formed neutral binuclear Cu²⁺ chelates [Cu₂L]⁰ that are water-insoluble. In alkaline solutions, these Cu²⁺ complexes were converted to anionic chelates [Cu₂(LH_-4)]^-4 in which deprotonated amide nitrogens coordinated Cu²⁺ ions. These anionic metal chelates of (bis-edtapdn)H₄, (bis-edtabpm)H₄, and (bis-edtabpe)H₄ exhibited three π-π* transition bands in the spectral range 240-340 nm, in contrast to the uncoordinated cyclophanes, which showed a single band in this spectral range. The unusual π-π* transition spectra of the [Cu₂(LH_-4)]^4- complexes originate from the combined effect of metal-ligand charge transfer and proximity of the π systems. The absorption and emission spectra of (bis-edtanap)H₄ were also influenced by coordination with copper. The EPR spectrum of [Cu₂(bis-edtanapH_-4)]^4- in a methanol glass matrix showed a hyperfine structure due to the spin exchange between two Cu²⁺ ions. These unusual spectral and magnetic properties arise from the strong coordination between Cu²⁺ ions and deprotonated amide nitrogens that are bound to the π systems.