Crystal structure of poly[bis(μ-2-amino-4,5-dicyanoimidazolato-κ2 N 1:N 3)-trans-bis(N,N′-dimethylformamide-κO)cadmium]

The title compound, [Cd(C5H2N5)2(C3H7NO)2]n, is a two-dimensional coordination polymer extending parallel to (100). Notably, both the primary amino group and the cyano groups are involved in hydrogen-bonding interactions with DMF ligands to direct the assembly and stabilize the crystal packing.

In the title structure, [Cd(C 5 H 2 N 5 ) 2 (C 3 H 7 NO) 2 ] n or [Cd(adci) 2 (DMF) 2 ] n , the Cd 2+ ion is located on a twofold rotation axis and is six-coordinated in a CdN 4 O 2 manner by four imidazole N atoms of four symmetry-related 2-amino-4,5dicyanoimidazolate (adci) anions in the equatorial plane and by two O atoms of symmetry-related N,N-dimethylformamide (DMF) ligands in axial positions. The adci À anions bridge adjacent Cd 2+ ions [shortest CdÁ Á ÁCd separation = 6.733 (3) Å ] into a layered coordination polymer extending parallel to (001). The primary amino group and the non-coordinating cyano groups of adci À anions are involved in hydrogen-bonding interactions with DMF ligands to stabilize the crystal structure.

Chemical context
Porous materials such as metal-organic frameworks (MOFs) combining advantages of both organic and inorganic components have emerged as a unique class of crystalline solid-state materials today due to their potential applications in gas adsorption and separation (Collins & Zhou, 2007), catalysis (Gu et al., 2012) and analytical chemistry (Mondal et al., 2013). As a branch of MOFs, zeolitic imidazolate frameworks (ZIFs), which are topologically related to inorganic zeolites, commonly reveal high thermal and chemical stability (Eddaoudi et al., 2015). Bridging N-donor ligands such as 2-substituted 4,5-dicyanoimidazole (dci) molecules are often used to synthesize ZIFs (Sava et al., 2009;Mondal et al., 2014). In addition, the cyano group of dci can generate carboxylate- (Orcajo et al., 2014) or tetrazole-based (Xiong et al., 2002) ligands by in-situ ligand reactions.

Structural commentary
Complex (I) is a mononuclear cadmium coordination polymer, in which the central Cd 2+ ion exhibits a tetragonally distorted octahedral coordination environment (Fig. 1). The asymmetric unit of (I) comprises one Cd 2+ ion located on a twofold rotation axis, one 2-amino-4,5-dicyanoimidazolate ion and one DMF ligand, both in general positions. The Cd 2+ ion has an N 4 O 2 coordination set defined by four N atoms of four symmetry-related adci À anions in the equatorial plane and by two oxygen atoms of two symmetry-related DMF ligands in axial positions. The Cd-N bond lengths [2.339 (4) and 2.353 (4) Å ] and Cd-O bond length [2.322 (4) Å ] fall in normal ranges (Groom & Allen, 2014). Each adci À anion bridges two adjacent Cd 2+ ions in a bis-monodentate mode through two imidazole N atoms whereas the DMF molecules serve as terminal ligands. Thus, four Cd 2+ ions and four bridging adci À ligands generate a square motif aligned parallel to (001), as shown in Fig. 2. The CdÁ Á ÁCd distance along the edge of the square is 6.733 (3) Å , which is similar to previously reported structures Wang et al., 2010).

Supramolecular features
Complex (I) possesses various hydrogen-bonding interactions ( Table 1). The amino group and the non-coordinating cyano N atoms are involved in hydrogen-bonding interactions with DMF ligands to stabilize the crystal structure. In the 2D metalorganic network, intermolecular N1-H1AÁ Á ÁO1 hydrogen bonds between the primary amine group of adci À and the O atoms of an DMF ligand as well as C7-H7CÁ Á ÁN5 interactions between the methyl C atoms of DMF and the noncoordinating N atoms of the cyano group of an adci À anion play a crucial role in directing and stabilizing the assembly of the supramolecular structure (Kim et al., 2015;Sava et al., 2009), as shown in Fig. 3a. The layers are packed together by weak C7-H7BÁ Á ÁN4 interactions, involving the methyl C atom of DMF and another N atom of a cyano group (Fig. 3b). The lengths of these three hydrogen bonds fall in or approach the range (3.2-4.0 Å ) of weak hydrogen-bonding interactions (Desiraju, 1996;Steed & Atwood, 2000).

Figure 1
The coordination sphere around Cd 2+ in the structure of (I), with displacement ellipsoids drawn at the 30% probability level. H atoms bonded to C and N atoms have been omitted for clarity. [Symmetry code:

Figure 2
The two-dimensional network in the structure of (I), viewed perpendicular to the ab plane.

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 )