catena-Poly[[tetra-μ-formato-κ8 O:O′-dicopper(II)]-μ-hexamethylenetetramine-κ2 N 1:N 5]

In the title polymeric compound, [Cu2(HCO2)4(C6H12N4)]n, the CuII atom is five-coordinated in a square-pyramidal geometry that is defined by four O atoms from four formate ligands and one N atom from a hexamethylenetetramine ligand. The two CuII atoms are separated by 2.6850 (7) Å, and together with the four formate ligands they form a paddle-wheel unit. The hexamine ligand uses only two of its four N atoms to link Cu2 cluster units, affording a zigzag chain running along the b-axis direction. The hexamine ligand lies on a mirror plane.


Comment
The design and synthesis of metal-organic complexes or coordination polymers is a rapidly developing field in coordination and supramolecular chemistry during the past decades. Hexamethylenetetramine (hmt), also known as hexamine or urotropine, can be considered as one such simple heterocyclic compound with a cagelike structure which,owing to its high solubility in water and polar organic solvents, has found a broad variety of applications (Dreyfors et al., 1989). With regard to coordination chemistry, hmt is a versatile ligand capable of adopting different coordination modes that span from the terminal monodentate to bridging bi-, tri-and tetradentate modes. The well known [Cu 2 (carboxylate) 4 ] units with four bridging carboxylate ligands in the familiar η 1 :η 1 :µ coordination mode have accessible apical coordination sites and are ideally suited to serve as a metal-based linear spacer (Konar et al. 2003;Chiari et al., 1988;Wu et al., 2004;Sun et al., 2009). To date, the title copper(II) carboxylate complex, represents an exception, as only few formate copper(II) complexes have been studied.
The crystal structure of the title complex, which is isostructural with its copper analog (Wang et al., 2002), is built of hexamethylenetetramine molecules and paddle-wheel dicopper units, both of which occupy special positions. The structure of the centrosymmetric [Cu 2 (HCO 2 ) 4 ] moiety is shown in Fig. 1. The coordination geometry of the Cu atom may be described as a square pyramid, formed by four formatee O atoms and the N atom of the hexamine . The four basal Cu -O distances fall in the range from 1.963 (2) to 1.978 (2) Å. The central hmt has mirror symmetry, and therefore there is only one independent Cu II atom in the asymmetric unit. The Cu-Cu distance within the [Cu 2 (HCO 2 ) 4 ] unit is 2.6850 (7) Å indicating a strong interaction. The axial Cu(1)-N(1) distance is 2.212 (2) Å. The hexamine ligand uses only two of its four N atoms to link adjacent paddle-wheel Cu 2 -cluster units, to afford a zigzag chain running along the b axis of the unit cell (Fig. 2).

Experimental
The title compound was synthesized by the following method. Copper(II) formate tetrahydrate (0.015 g, 0.1 mmol) was dissolved in 20 ml me thanol to obtain solution A. Hexamine (0.007 g, 0.05 mmol) was dissolved in 10 ml methanol to obtain solution B. Solution B was layered carefully on solution A, and the tube was sealed and stored in room temperature. Green block crystals were obtained after two weeks. Analysis calculated for C 5 H 8 CuN 2 O 4 : C 26.85, H 3.60, N 12.52%; found: C 26.66, H 3.82, N 12.65%.

Refinement
All non-hydrogen atoms were refined anisotropically. The H atoms of formate were positioned geometrically and allowed to ride on their parent atoms, with C-H = 0.95 Å and U iso (H) = 1.2U eq (C). The H atoms of hexamethylenetetramine the were placed in geometrically idealized positions and refined as riding atoms, with C-H(CH 2 ) = 0.99 Å and U iso (H) = 1.2U eq (C).

Figure 2
The zigzag chain in the crystal packing structure of [Cu 2 (HCO 2 ) 4 (C 6 H 12 N 4 )] n along b the axis.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 R-factors(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (