The Different Supramolecular Arrangements of the Triangular [Cu₃(μ₃-OH)(μ-pz)₃]²⁺(pz = Pyrazolate) Secondary Building Units. Synthesis of a Coordination Polymer with Permanent Hexagonal Channels

Abstract

By reaction of the trinuclear triangular copper(II) complex [Cu3(μ3-OH)(μ-pz)3(MeCOO)2(Hpz)] (Hpz = pyrazole), 1b, with aqueous HCl, four different crystalline species were formed and recovered through fractional crystallization. In order, the hexanuclear dicationic [{Cu3(μ3-OH)(μ-pz)3(Hpz)2}2(μ-MeCOO)2](Cl)2·2H2O, 2, the mononuclear [CuCl2(Hpz)4], 3, the heptanuclear neutral [{Cu3(μ3-OH)(μ-pz)3(Cl)2(Hpz)2(H2O)}2{CuCl2(Hpz)2}], 4, and the hexanuclear neutral [{Cu3(μ3-OH)(μ-pz)3(Cl)(Hpz)3}2(μ-Cl)2]·H2O, 5, complexes were obtained. Compounds 2, 4, and 5 all maintain the [Cu3(μ3-OH)(μ-pz)3]2+ core; nevertheless, they exhibit relevant differences in their molecular structures and supramolecular arrangements. In compound 2 a hexanuclear cluster, based on two monodentate acetate groups bridging two [Cu3(μ3-OH)(μ-pz)3(Hpz)2] units and clearly reminiscent of the structure of 1b, was observed, while the sequential replacement of the acetates by chloride ions generated 4 and 5. Although these two compounds were formed according to the same stoichiometry, they are characterized by very different molecular and supramolecular structures. The hexanuclear species 5 arranged, through hydrogen bonds, into a 3D, nonporous metal−organic framework (MOF), while the heptanuclear species 4 self-assembled through Cu−Cl bridges, giving rise to a 3D MOF having permanent hexagonal, star-shaped, parallel channels. The internal free diameter of these channels is about 4 Å, leading to a free space corresponding to ca. 9% of the total crystal volume.