Dimetallic complexes of macrocycles with two rigid dibenzofuran units as receptors for detection of anionic substrates

Abstract

The hexaazamacrocycles [28](DBF)₂N₆ {cyclo[bis(4,6-dimethyldibenzo[b,d]furaniminoethyleneiminoethylene]} and [32](DBF)₂N₆ {cyclo[bis(4,6-dimethyldibenzo[b,d]furaniminopropyleneiminopropylene]} form stable dinuclear copper(II) complexes suitable to behave as receptors for several anionic substrates. These two receptors were used to study the binding interactions with several substrates, such as imidazole (Him) and some carboxylates [benzoate (bz⁻), oxalate (ox²⁻), malonate (mal²⁻), phthalate (ph²⁻), isophthalate (iph²⁻), and terephthalate (tph²⁻)] by spectrophotometric titrations and EPR spectroscopy in MeOH (or H₂O):DMSO (1 : 1 v/v) solution. The largest association constant was found for ox²⁻ with Cu₂[32](DBF)₂N₆⁴⁺, whereas for the aromatic dicarboxylate anions the binding constants follow the trend ph²⁻ > iph²⁻ > tph²⁻, i.e. decrease with the increase of the distance of the two binding sites of the substrate. On the other hand, the large blue shift of 68 nm observed by addition of Him to Cu₂[32](DBF)₂N₆⁴⁺ points out for the formation of the bridged Cu⋯im⋯Cu cascade complex, indicating this receptor as a potential sensor for the detection and determination of imidazole in solution. The X-band EPR spectra of the Cu₂[28](DBF)₂N₆⁴⁺ and Cu₂[32](DBF)₂N₆]⁴⁺ complexes and the cascade complexes with the substrates, performed in H₂O:DMSO (1 : 1 v/v) at 5 to 15 K, showed that the Cu⋯Cu distance is slightly larger than the one found in crystal state and that this distance increases when the substrate is accommodated between the two copper centres. The crystal structure of [Cu₂[28](DBF)₂N₆(ph)₂]·CH₃OH was determined by X-ray diffraction and revealed the two copper centres bridged by two ph²⁻ anions at a Cu⋯Cu distance of 5.419(1) Å. Each copper centre is surrounded by three carboxylate oxygen atoms from two phthalate anions and three contiguous nitrogen atoms of the macrocycle in a pseudo octahedral coordination environment.