Tetrakis(2,2′-bipyridine)di-μ3-hydroxido-bis(μ-2-oxidobenzoato)tetracopper(II) dinitrate tetrahydrate

The tetranuclear title complex, [Cu4(C7H4O3)2(OH)2(C10H8N2)4](NO3)2·4H2O, has a crystallographically imposed centre of symmetry. The CuII atoms display a distorted square-pyramidal coordination geometry and are linked by two μ2-phenolate O atoms from the salicylate ligands and two μ3-hydroxo groups, forming a Cu4O4 core that adopts a ‘stepped-cubane’ geometry. In the crystal, the cations are linked by O—H⋯O hydrogen bonds to the nitrate anions, which are in turn connected via O—H⋯O interactions to centrosymmentric water tetramers.

The tetranuclear title complex, [Cu 4 (C 7 H 4 O 3 ) 2 (OH) 2 -(C 10 H 8 N 2 ) 4 ](NO 3 ) 2 Á4H 2 O, has a crystallographically imposed centre of symmetry. The Cu II atoms display a distorted squarepyramidal coordination geometry and are linked by two 2phenolate O atoms from the salicylate ligands and two 3hydroxo groups, forming a Cu 4 O 4 core that adopts a 'steppedcubane' geometry. In the crystal, the cations are linked by O-HÁ Á ÁO hydrogen bonds to the nitrate anions, which are in turn connected via O-HÁ Á ÁO interactions to centrosymmentric water tetramers.
The atom-numbering scheme of the title compound is shown in Fig. 1. The title complex has a crystallographically imposed centre of symmetry, and consists of a chair-like [Cu 4 (C 7 H 4 O 3 ) 2 (OH) 2 (bpy) 4 ]2 + dication (bpy = 2,2'-bipyridine), two nitrate anions, and four lattice water molecules. The coordination geometry around each copper(II) ion can be described as a five-coordinate distorted square pyramid. In the crystal packing, the nitrate counter-anions stabilize the crystal structure through water O-H···O nitrate hydrogen bonds and the complex molecules are linked into one-dimensional chains by intermolecular O-H···O bonding interactions involving the solvent water molecules and the nitrate counter-anions ( Fig.   2 and Table 1).

Experimental
A mixture of salicylic acid (0.05 mmol), copper nitrate trihydrate (0.05 mmol), 2,2'-bipyridine (0.05 mmol) and 10 ml H 2 O were put into a 23-ml Teflon lined reactor and heated at 418 K in oven for 48 h. After the autoclave was cooled during 24 h to room temperature, the solid was filtered off. The resulting filtrate was allowed to stand at room temperature, and slow evaporation for 3 weeks afforded block single crystals.

Refinement
H atoms bound to C atoms were positioned geometrically (C-H = 0.93 Å) and allowed to ride on their parent atoms with U iso (H) = 1.2U eq (C). The H atoms of the water molecules were located in Fourier difference maps and allowed to ride on their parent atoms with U iso (H) = 1.5U eq (O) Figures   Fig. 1. The structure of the title compound with displacement ellipsoids drawn at the 50% probability level. Unlabeled atoms are related to the labeled ones by the symmetry operation 1-x, 1-y, 1-z.

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.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.