Crystal structure of a CuII complex with a bridging ligand: poly[[pentakis[μ2-1,1′-(butane-1,4-diyl)bis(1H-imidazole)-κ2 N 3:N 3′]dicopper(II)] tetranitrate tetrahydrate]

A novel two-dimensional→ three-dimensional CuII coordination polymer based on the 1,1′-(1,4-butane-1,4-diyl)bis(1H-imidazole) ligand, containing one crystallographically unique CuII centre has been synthesized under hydrothermal conditions.


Chemical context
In the past decade, entangled systems of metal-organic frameworks (MOFs) have attracted great attention because of their undisputed aesthetic topological structures, fascinating properties and applications, such as molecular machines and sensor devices, and potential biological applications (Carlucci et al., 2003a;Bu et al., 2004;Batten & Robson, 1998;Perry et al., 2007;Yang et al., 2012;Baburin et al., 2005;Blatov et al., 2004). Currently, many chemists are making great contributions to this field, and a number of compounds with entangled framework structures have been synthesized and characterized, which are based on N-donor ligands due to their diversity in coordination modes and their versatile conformations (Murphy et al., 2005;Wu et al., 2011a;Yang et al., 2008;Zhang et al., 2013). However, the controlled synthesis of crystals with entangled framework structures is still a significant challenge, although many entangled coordination compounds of this sort have already been obtained (Carlucci et al., 2003b;Batten, 2001;Wu et al., 2011b). According to previous literature, the construction of MOFs mainly depends on the nature of the organic ligands, metal ions, the temperature, the pH value, and so on (James, 2003;Chen et al., 2010;Ma et al., 2004).
Recently, 1,1 0 -(1,4-butanediyl)bis(imidazole) and carboxylate ligands have frequently been employed in the construction of coordination compounds due to their flexible character, and coordination compounds displaying different structural motifs have been reported (Wen et al., 2005;Chen et al., 2009;Dong et ISSN 1600-5368 al., 2007. However, the syntheses of complexes based on inorganic ions have been scarcely been reported. It is interesting to note that the Cu II complexes based on inorganic counter-ions and the biim ligand, [Cu(biim) (Ma et al., 2004). In (II), (III) and (IV), the Cu II cations are bridged by biim ligands, forming infinite 4 4 networks that contain 44-membered rings. It is worth mentioning that no interpenetration occurs in (II) and (III), while in (IV), two 4 4 networks are interpenetrated in a parallel fashion, forming a two-dimensional !two-dimensional sheet. In the present work, we describe the synthesis and structure of one such entangled Cu II complex, the title compound (I), [Cu 2 (C 10 H 14 N 4 ) 5 ](NO 3 ) 4 Á4H 2 O, which exhibits a novel twodimensional!three-dimensional polymeric structure, and which was prepared under hydrothermal conditions instead of at room temperature.

Structural commentary
The structure of compound, (I) (Fig. 1), contains one Cu II , two and one half biim ligands, two nitrate ions and two water molecules per asymmetric unit. The Cu II cation is five-coordinated and exhibits a distorted CuN 5 square-pyramidal coordination geometry from the biim ligands (Table 1). The cis basal N-Cu-N bond angles range from 88.42 (15) to 90.72 (15) , and the apical bond angles from 92.02 (14) to 101.23 (15) .

Topological features
The Cu II cations are linked by biim ligands, giving a 4 4 layer; the layers are further bridged by biim ligands at nearly vertical directions, generating a double sheet with a thickness of 14.61 Å (Fig. 2). The sheet exhibits Cu 4 (biim) 4 windows built up from four Cu II atoms and four biim ligands with dimensions of 14.11 Â 14.07 Å 2 . From a topological viewpoint, the sheet reveals a 5-connected topology, in which the Cu atom acts as a 5-connected node and the biim ligand is regarded as a linker. Considering the composition, the Schlä fli symbol of the twodimensional network can be defined as 4 8 .6 2 (Fig. 3).

Figure 2
The two-dimensional double layer with large windows in (I). examples of two-dimensional!three-dimensional entangled structures have been observed: the networks in these are mainly focused on 4 4 and 6 3 topologies. Two-dimensional! three-dimensional entangled frameworks with 4 8 .6 2 topology have scarcely been reported. It should be pointed out that although the starting materials used for syntheses of (I) and the related compound (III) are the same, their complex structures are entirely different (Ma et al., 2004). The structure of (III) can be symbolized as a 4 4 net, and has no interpenetration. Although it is hard to propose definitive reasons as to why compounds (I) and (III) adopt different configurations, it can be speculated that pH values and temperature may exert an important influence on the resulting architectures.

Synthesis and crystallization
A mixture of biim (0.057 g, 0.3 mmol), Cu(NO 3 ) 2 Á3H 2 O (0.048 g, 0.2 mmol) and water (15 ml) was mixed and stirred at room temperature for 10 min. The mixture was adjusted with 1 M HNO 3 to pH ' 5 and then sealed in a 25 ml Teflon-lined autoclave and heated at 443 K for three days. Then the mixture was cooled to room temperature, and black-blue crystals of (I) were obtained in 56% yield based on Cu II . Elemental analysis, found: C 42.85, N 24.14, H 5.56%; calculated for C 25 H 39 CuN 12 O 8 (M r = 699.22): C 42.94, N 24.04, H 5.62%.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms bonded to C atoms were positioned geometrically and refined as riding atoms,with C-H distances of 0.93 (aromatic) or 0.96 Å (CH 2 ) with U iso (H) = 1.2U eq (C). H atoms bonded to O atoms were located from difference maps, refined with O-H = 0.84 (1) and HÁ Á ÁH The topology of the two-dimensional layer in (I).

Poly[[pentakis[µ 2 -1,1′-(butane-1,4-diyl)bis(1H-imidazole)-κ 2 N 3 :N 3′ ]dicopper(II)] tetranitrate tetrahydrate]
Crystal data Special details Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(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