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]paracyclophane, abbreviated as (bis-edtapdn)H-4. Cyclocondensation of two EDTA and two 1,5-diaminonaphthalene units has given the naphthalenophane, 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, (bis-edtanap)H-4. 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)-1,4,7,10,24, 27,30,33-octaaza[10.1.10.1]paracyclophane, abbreviated as (bis-edtabpm)H-4, 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 (bis-edtabpe)H-4. 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+ chelales [Cu2L](0) that are water-insoluble. In alkaline solutions, these Cu2+ complexes were converted to anionic chelates [Cu-2(LH-4)](4-) in which deprotonated amide nitrogens coordinated Cu2+ ions. These anionic metal chelates of (bis-edtapdn)H-4, (bis-edtabpm)H-4, and (bis-edtabpe)H-4 exhibited three pi-pi* 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 pi-pi*: transition spectra of the [CU2(LH-4)](4-) complexes originate from the combined effect of metal-ligand charge transfer and proximity of the pi systems. The absorption and emission spectra of (bis-edtanap)H-4 were also influenced by coordination with copper. The EPR spectrum of [Cu-2(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 pi systems.