One-Dimensional and Two-Dimensional Coordination Polymers from Self-Assembling of Trinuclear Triangular Cu(II) Secondary Building Units

The reactions of pyrazole (Hpz) with some copper(II) carboxylates in the presence of water yield trinuclear copper
derivatives characterized by the triangular core [Cu3(μ3-OH)(μ-pz)3(RCOO)2] (R = H, C2H5, C3H7). Copper(II) formate
gives [Cu3(μ3-OH)(μ-pz)3(HCOO)2(Hpz)2] (1), whereas copper propionate and butyrate afford [Cu3(μ3-OH)(μ-pz)3(C2H5COO)2(EtOH)] (2) and [Cu3(μ3-OH)(μ-pz)3(C3H7COO)2(MeOH)(H2O)] (3), respectively, both containing solvent
molecules coordinated to copper atoms. Magnetic susceptibilities are consistent with a single unpaired electron for
each trinuclear unit of 1−3, and EPR measurements indicate that higher spin states, generated by exchange
coupling between copper atoms, may be populated at room temperature. Density-functional calculations provide
the description of the electronic structures of 1−3, allowing, at the same time, the assignment of their UV−vis
absorption spectra. X-ray molecular structure determinations show that triangular trinuclear units of 1 are connected
to each other through single formate bridges, forming one-dimensional (1D) zigzag coordination polymers, whereas
in 2 and 3, two oxygen atoms of two carboxylate ions doubly bridge two copper atoms of different triangles, thus
generating hexanuclear units. Moreover, in 2, two other propionate ions link together two hexanuclear units yielding
a 12-membered cycle and giving rise to 1D coordination polymers. The supramolecular assemblies of 1−3 are
compared to that of the previously reported trinuclear triangular copper(II) derivative [Cu3(μ3-OH)(μ-pz)3(CH3COO)2(Hpz)] (A), where a two-dimensional (2D) coordination polymer is present. The reactions of 3,5-dimethylpyrazole
(Hpz*) with copper(II) carboxylates in the same conditions yield 1:2 Cu(RCOO)2/Hpz* adducts.