Equilibrium studies on the protonation and Cu(II) complexation by an hexaaza macrocycle containing p-xylyl spacers.: The crystal structure of the hexaprotonated ligand and the kinetics of decomposition of the Cu(II) complexes

The protonation constants of the ligand 3,6,9,16,19,22-hexaazatricyclo[22.2.2.2(11.14)]triaconta-1 (26),11(12),13,24,27, 29-hexaene (L, BPXD) have been determined in water at 25 degreesC by potentiometric procedures and indicate a lower basicity than that corresponding to the analogue ligand with m-xylyl spacers between the diethylemtriamine subunits (BMXD). The crystal structure of a salt of H6L6+ containing Br- and NO3- as anions has been solved by X-ray diffraction procedures and reveals that the macrocycle adopts an almost planar configuration. The protonated amine groups are involved in a complex network of hydrogen bonds with the anions and water molecules, with two anions being placed close to the cavity of the macrocycle. Despite the lower basicity of BPXD, the stability of the mononuclear CuL2+ and HCuL3+ complexes is several log units higher than that corresponding to the analogous BMXD complexes, which suggests the possibility of a different coordination mode for both closely related ligands. Actually, the whole set of stability constants for the mono and binuclear Cu(II)-L complexes can be rationalised by considering that the macrocycle acts as tetradentate in the mononuclear species. In contrast, the kinetic parameters for decomposition of the mono and binuclear Cu(II)-L complexes in acid solutions are only slightly different from those previously determined for the BMXD complexes. (C) 2001 Elsevier Science B.V. All rights reserved.