Sulfate-bridged dimeric trinuclear copper(II)–pyrazolate complex with three different terminal ligands

The crystal structure of the first trinuclear copper–pyrazolate (pz) complex with three different terminal ligands, including a strongly binding sulfate, is reported. The sulfate ion also acts as bridging ligand, leading to a dimeric structure, [Cu3(μ3-OH)(μ-4-Cl-pz)3(μ4-SO4)(DMF)(H2O)]2·4DMF·2H2O.

The reaction of CuSO 4 Á5H 2 O, 4-chloropyrazole (4-Cl-pzH) and triethylamine (Et 3 N) in dimethylformamide (DMF) produced crystals of diaquahexakis (-4chloropyrazolato-2  [Cu 3 (OH)(SO 4 )(C 3 H 2 ClN 2 ) 3 (C 3 H 7 NO)(H 2 O)] 2 Á4C 3 H 7 NOÁ2H 2 O. The centrosymmetric dimeric molecule consists of two trinuclear copperpyrazolate units bridged by two sulfate ions. The title compound provides the first example of a trinuclear copper-pyrazolate complex with three different terminal ligands on the Cu atoms, and also the first example of such complex with a strongly binding basal sulfate ion. Within each trinuclear unit, the Cu II atoms are bridged by -pyrazolate groups and a central 3 -OH group, and are coordinated by terminal sulfate, H 2 O and DMF ligands, respectively. Moreover, the sulfate O atoms coordinate at the apical position to the Cu atoms of the symmetry-related unit, providing square-pyramidal coordination geometry around each copper cation. The metal complex and solvent molecules are involved in O-HÁ Á ÁO hydrogen bonds, leading to a two-dimensional network parallel to (101).
A unique class of copper-pyrazolate complexes is defined by nanojars, based on a series of cyclic polymerization isomers, [cis-Cu II (-OH)(-pz)] n (pz = pyrazolate anion, n = 6-14, except 11), which incarcerate anions with large hydration energies (e.g., sulfate, phosphate, carbonate) with unprecedented strength (Fernando et al., 2012;Mezei, 2015;Ahmed, Szymczyna et al., 2016) and permits the extraction of such anions from water into aliphatic solvents (Ahmed, Calco et al., 2016). Nanojars are obtained by self-assembly from a copper salt, pyrazole and a base (needed both for deprotonating pyrazole and as a hydroxide ion source) in the presence of an anion with large hydration energy, via a trinuclear intermediate, which is isolable and can be converted into nanojars by adding a base . Use of a strong base, such as sodium or tetrabutylammonium hydroxide, allows the preparation of nanojar solutions in different organic solvents. In contrast, a weak base, such as triethylamine, can only be employed as hydroxide source (Et 3 N + H 2 O $ Et 3 NH + + HO À ) if the nanojar product is precipitated out of the solution by dilution with excess water, in which the nanojar is not soluble (Fernando et al., 2012). Isolation of the title compound provides further evidence that in a neat organic solvent, such as N,N-dimethylformamide, the selfassembly process using triethylamine halts at the trinuclear stage, due to the acidity of the conjugate acid (triethylammonium cation, pK a = 10.75 in H 2 O).

Supramolecular features
The dimeric metal complex participates in an intricate hydrogen-bond network with the solvent DMF and H 2 O molecules. Numerical details of the hydrogen bonding are given in

Synthesis and crystallization
Copper sulfate pentahydrate (1.000 g), 4-chloropyrazole (411 mg) and Et 3 N (1.2 mL) were dissolved in DMF (20 mL) yielding a deep-blue solution. Dark-blue prismatic crystals of the title compound were obtained upon slow evaporation of the solvent.

Diaquahexakis(µ-4-chloropyrazolato-κ 2 N:N′)bis(N,N-dimethylformamide)di-µ 3 -hydroxido-bis(µ 4 -sulfatoκ 4 O:O′:O′′:O′′)hexacopper(II) N,N-dimethylformamide tetrasolvate dihydrate
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.