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

The hexaazamacrocycles [28](DBF)(2)N-6 {cyclo[bis(4,6-dimethyldibenzo[b,d] furaniminoethyl-eneiminoethylene]} and [32](DBF)(2)N-6 {cyclo[bis(4,6-dimethyldibenzo[b,d] furaniminopropyl-eneiminopropylene]} 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 (ox(2-)), malonate (mal(2-)), phthalate (ph(2-)), isophthalate (iph(2-)), and terephthalate (tph(2-))] by spectrophotometric titrations and EPR spectroscopy in MeOH (or H2O): DMSO (1 : 1 v/v) solution. The largest association constant was found for ox(2-) with Cu-2[32](DBF)(2)N-6(4+), whereas for the aromatic dicarboxylate anions the binding constants follow the trend ph(2-) > iph(2-) > tph(2-), 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 Cu-2[32](DBF)(2)N-6(4+) points out for the formation of the bridged Cu center dot center dot center dot im center dot center dot center dot 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 Cu-2[28](DBF)(2)N-6(4+) and Cu-2[32](DBF)(2)N-6](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 center dot center dot center dot 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 [Cu-2[28](DBF)(2)N-6(ph)(2)]center dot CH3OH was determined by X-ray diffraction and revealed the two copper centres bridged by two ph(2)-anions at a Cu center dot center dot center dot Cu distance of 5.419(1) angstrom. 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.