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

By reaction of the trinuclear triangular copper(II) complex [Cu-3(mu(3)-OH)(mu-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 [{Cu-3(mu(3)-OH)(mu-pz)(3)(Hpz)(2)}(2)(mu-MeCOO)(2)](Cl)(2)center dot 2H(2)O, 2, the mononuclear [CuCl2(Hpz)(4)], 3, the heptanuclear neutral [{Cu-3(mu(3)-OH)(mu-pz)(3)(Cl)(2)(Hpz)(2)(H2O)}(2){CuCl2(Hpz)(2)}], 4, and the hexanuclear neutral [{Cu-3(mu(3)-OH)(mu-pz)(3)(Cl)(Hpz)(3)}(2)(mu-Cl)(2)]center dot H2O, 5, complexes were obtained. Compounds 2, 4, and 5 all maintain the [Cu-3(mu(3)-OH)(mu-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 [Cu-3(mu(3)-OH)(mu-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 A, leading to a free space corresponding to ca. 9% of the total crystal volume.