Cascade dicopper architectures of a dibenzodioxatetraazamacrocycle

Abstract

The dibenzodioxatetraazamacrocycle [26] pbz 2N 4O 2 was characterised by single crystal X-ray diffraction and the protonation constants of this compound and the stability constants of its copper(II) and lead(II) complexes were determined by potentiometry in water at 298.2 K in 0.10 mol dm −3 in KNO 3. Mono- and dinuclear complexes were found for both metal ions, the dinuclear complexes being the main species in the 5–7.5 pH range for copper(II) and 7.5–8.5 for lead(II). As expected the values of the stability constants for the copper(II) complexes are lower than those for related macrocycles containing only nitrogen atoms. The presence of mono- and dinuclear copper complexes was also confirmed by electrospray ionization mass spectrometry. These results suggest that the symmetric macrocyclic cavity of [26] pbz 2N 4O 2 has enough space for the coordination of two metal ions. Additionally, NMR spectroscopy showed that the dinuclear complex of lead(II) has high symmetry. The equilibrium constants of the dinuclear copper(II) complexes and dicarboxylate anions (oxalate, malonate and succinate) were also determined in 0.10 mol dm −3 aqueous KNO 3 solution. Only species containing one anion, Cu 2H h LA (2+ h) , were found, strongly suggesting that the anion bridges the two copper(II) ions. The binding constants of the cascade species formed by [Cu 2[26] pbz 2N 4O 2(H 2O) x ] 4+ with dicarboxylate anions decrease with the increase in length of the alkyl chain of the anion, a fact which was attributed to a higher conformational energy necessary for the rearrangement of the macrocycle to accommodate the larger anions bridging the two copper(II) centres. The variation of the magnetic susceptibility with temperature of [Cu 2(H 2[26] pbz 2N 4O 2)(oxa) 3] · 4H 2O and [Cu 2([26] pbz 2N 4O 2)(suc)Cl 2] were measured and the two complexes showed different behaviour.