CuII and ZnII coordination chemistry of pyrazole-containing polyamine receptors -: Influence of the hydrocarbon side chain length on the metal coordination

The synthesis of a new macrocyclic receptor (L-4) containing two 3,5-dimethylpyrazole units connected by dipropylenetriamine bridges is reported for the first time; pH-metric titrations indicate that L-4 shows six protonation steps in the pH range 2-11. In the absence of metal ions, the pyrazole moieties are not involved in acid-base processes in this pH range. Addition of Cull and Zn-II results in deprotonation of the pyrazole moieties which act as bis(monodentate) eta(1):eta(1) ligands. This induced deprotonation occurs at higher pH values than in the complexes of the analogous ligand containing diethylenetriamine bridges (L-1). The crystal structures of [Cu-2(H-2L4)](ClO4)(2) and [Zn-2(H-2L4)](ClO4)(2) obtained by treatment of Na[H-2L4] with either Cu(ClO4)(2).6H(2)O or Zn(ClO4)(2).6H(2)O in methanol followed by recrystallisation from water/methanol also show the deprotonation of the pyrazole moieties. The coordination geometry around each metal ion is square-pyramidal and involves all nitrogen atoms of the macrocycle. The crystal structure of [Zn-2(H-2L1)](ClO4)(2) shows full involvement of all the nitrogen atoms of the macrocycle in the coordination to the metal ions. The coordination geometry can be defined as midway between a square pyramid and an irregular trigonal bipyramid. Treatment of neutral L-4 in methanol with Cu(ClO4)(2).6H(2)O yields a blue complex which, after heating in boiling water, crystallises as a red compound. Elemental microanalyses, ESI-MS and FAB-MS data of both forms of this complex are consistent with the formula [Cu2L4](ClO4)(4).2H(2)O. Furthermore, these data along with the paramagnetic H-1 NMR spectrum of the red form of the compound suggest a structure in which the pyrazole rings are deprotonated while the central nitrogen atoms of the bridging chains are protonated and consequently uncoordinated. The change from the blue (square-pyramidal) to the red form (square-planar) can be ascribed to dissociation of the water molecules which occupy the axial positions in the Cull coordination spheres of the blue form. The variation of the magnetic susceptibility of the red complex [Cu2L4](ClO4)(4).2H(2)O and the complex [Cu-2(H-2L1)](ClO4)(2) with temperature has been studied in the 2-300 K temperature range. Fitting of the experimental data provides J values which are among the highest found for doubly pyrazolate-bridged dicopper(II) complexes {J = -299 cm(-1) for red-[Cu2L4] (ClO4)(4).2H(2)O and J = -286 cm(-1) for [Cu-2(H-2L4)]- (ClO4)(2)}. A trinuclear Cu-II complex of formula [Cu-3(H-2L4)] (ClO4)(4).2MeOH was also isolated after treatment of L-4 with Cu(ClO4)(2).6H(2)O solutions of higher concentration. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).