6533b839fe1ef96bd12a6d5f

RESEARCH PRODUCT

CuIIand ZnIICoordination Chemistry of Pyrazole‐Containing Polyamine Receptors − Influence of the Hydrocarbon Side Chain Length on the Metal Coordination

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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 Cu I I and Zn I I results in deprotonation of the pyrazole moieties which act as bis(monodentate) η 1 :η 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 - 2 L 4 )](ClO 4 ) 2 and [Zn 2 (H - 2 L 4 )](ClO 4 ) 2 obtained by treatment of Na 2 [H - 2 L 4 ) with either Cu(ClO 4 ) 2 .6H 2 O or Zn(ClO 4 ) 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 - 2 L 1 )](ClO 4 ) 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(ClO 4 ) 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 [Cu 2 L 4 ](ClO 4 ) 4 .2H 2 O. Furthermore, these data along with the paramagnetic 1 H 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 Cu I I coordination spheres of the blue form. The variation of the magnetic susceptibility of the red complex [Cu 2 L 4 ]-(ClO 4 ) 4 .2H 2 O and the complex [Cu 2 (H - 2 L 4 )](ClO 4 ) 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(n) complexes {J = -299 cm - 1 for red-[Cu 2 L 4 ](ClO 4 ) 4 .2H 2 O and J = -286 cm - 1 for [Cu 2 (H - 2 L 4 )]-(ClO 4 ) 2 }. A trinuclear Cull complex of formula [Cu 3 -(H - 2 L 4 )](ClO 4 ) 4 .2MeOH was also isolated after treatment of L 4 with Cu(ClO 4 ) 2 .6H 2 O solutions of higher concentration.