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 bee n determined in water at 25 degreesC by potentiometric procedures and indic ate a lower basicity than that corresponding to the analogue ligand with m- xylyl spacers between the diethylemtriamine subunits (BMXD). The crystal st ructure of a salt of H6L6+ containing Br- and NO3- as anions has been solve d 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, wit h two anions being placed close to the cavity of the macrocycle. Despite th e lower basicity of BPXD, the stability of the mononuclear CuL2+ and HCuL3 complexes is several log units higher than that corresponding to the analo gous BMXD complexes, which suggests the possibility of a different coordina tion mode for both closely related ligands. Actually, the whole set of stab ility constants for the mono and binuclear Cu(II)-L complexes can be ration alised by considering that the macrocycle acts as tetradentate in the monon uclear species. In contrast, the kinetic parameters for decomposition of th e mono and binuclear Cu(II)-L complexes in acid solutions are only slightly different from those previously determined for the BMXD complexes. (C) 200 1 Elsevier Science B.V. All rights reserved.