The almost quantitative formation of the triangular trinuclear copper derivative [Cu-3(mu(3)-OH)(mu-pz)(3)(MeCOO)(2)(Hpz)] (1) (Hpz = pyrazole), has been simply achieved by adding Hpz to an ethanol solution of Cu(MeCOO)(2).H2O. An X-ray molecular structure determination shows that 1 is completely unsymmetric and that trinuclear units result assembled in an extended bidimensional network formed through acetate bridges and hydrogen bonds. EPR and magnetic measurements are consistent with the presence of a single unpaired electron. Theoretical density functional calculations carried out for S = 1/2 provide a thorough description of the electronic structure of 1, allowing a detailed assignment of its UV-vis absorption spectrum. Compound 1 reacts with MeONa, yielding [Cu-3(mu(3)-OH)(mu-pz)(3)(MeCOO)(MeO)(Hpz)] (2) and [Cu-3(mu(3)-OH)(mu-pz)(3)(MeO)(2)(Hpz)] (3) through the substitution of one and two acetate ions, respectively, with MeO- ion(s). The spontaneous self-assembly of the triangular trinuclear Cu-3 moiety seems to occur only with pyrazole as can be inferred by the results obtained in the reactions of copper(II) acetate with some substituted pyrazoles leading to the formation of mononuclear [Cu(MeCOO)(2)(L)(2)] (4-8) and dinuclear [Cu(MeCOO)(2)(L)](2) (9-11) (L = substituted pyrazole) compounds. Also the presence of acetate ions seems to play a leading role in determining the formation of the trinuclear triangular arrangement, as indicated by the formation of a mononuclear derivative, [Cu(CF3COO)(2)(HpZ)](2) (compound 12), in the reaction of copper(II) trifluoroacetate with pyrazole. Compounds 1-3, as well as some other mono- and dinuclear copper(II)-substituted pyrazole complexes, have been tested as catalyst precursors in cyclopropanation reaction, observing the formation of products in a syn:anti ratio opposite that normally reported.
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