Poly[bis(N,N-dimethylformamide)tris(μ4-trans-stilbene-4,4′-dicarboxylato)tricadmium(II)]: a two-dimensional network with an unusual 36 topology

In the title compound, [Cd3(C16H10O4)3(C3H7NO)2]n or [Cd3(SDA)3(DMF)2]n (H2SDA is trans-stilbene-4,4′-dicarboxylic acid and DMF is dimethylformamide), the linear dicarboxylate ligand forms a two-dimensionally layered metal–organic network with the relatively uncommon 36 topology. The structure reveals trinuclear secondary building units and has an octahedral geometry at a central metal ion (occupying a symmetry site) and tetrahedral geometries at two surrounding symmetrically equivalent metal ions lying on a threefold axis. The six-connected planar trinuclear CdII centers, Cd3(O2CR)6, play a role as potential nodes in generation of the relatively uncommon 36 topology. The coordinated DMF unit is disordered around the threefold axis.

In the title compound, [Cd 3 (C 16 H 10 O 4 ) 3 (C 3 H 7 NO) 2 ] n or [Cd 3 (SDA) 3 (DMF) 2 ] n (H 2 SDA is trans-stilbene-4,4 0 -dicarboxylic acid and DMF is dimethylformamide), the linear dicarboxylate ligand forms a two-dimensionally layered metal-organic network with the relatively uncommon 3 6 topology. The structure reveals trinuclear secondary building units and has an octahedral geometry at a central metal ion (occupying a 3 symmetry site) and tetrahedral geometries at two surrounding symmetrically equivalent metal ions lying on a threefold axis. The six-connected planar trinuclear Cd II centers, Cd 3 (O 2 CR) 6 , play a role as potential nodes in generation of the relatively uncommon 3 6 topology. The coordinated DMF unit is disordered around the threefold axis.

Comment
The study of one, two or three dimensional metal-organic frameworks (MOFs) has attracted much attention in the past decade due to their various intriguing framework topologies but also for their potential applications in gas storage (Rosi et al., 2003), separation (Dybtsev et al., 2004) and catalysis (Seo et al., 2000) etc. Many factors play important role in the synthesis of MOFs such as the coordination geometry of metal ions (Chi et al.,2006), the structure of organic ligands , the solvent system (Eddaoudi et al., 2002),the counteranion (Luan et al., 2006), and the ratio of ligands to metal ions (Saalfrank et al., 2001). The simplest 2D sheets are those which comprise just one kind of regular polygon based upon hexagons, squares and triangles. Since three hexagons, four squares and six triangles meet at a node in a 2D network with angles of 120°, 90° and 60°,respectively, the corresponding Schläfli topology symbols are 6 3 , 4 4 and 3 6 , respectively (Hill et al., 2005). Although there were many examples of uninodal regularly tiled 2D metal-organic frameworks comprising linked squares or hexagons, however, a few examples comprising linked and tiled triangles have been reported only very recently (Edgar et al., 2001;Williams et al., 2005;Hawxwell et al., 2006;Dincâ & Long, 2005). Herein the formation of a two-dimensional metal-organic framework with an uncommon 3 6 tessellated topology, [Cd 3 (SDA) 3 (DMF) 2 ], (I), constructed from tri-nuclear cadmium SBUs (secondary building units) linked by a novel 4,4'-stilbenedicarboxylate ligand (Park et al., 2006) is reported.
The two-dimensional 3 6 tessellated network structure of 1 with the atomic numbering scheme is shown in Fig. 1 in which the coordinated DMF molecules are shown in only one of its three disordered components. The crystal structure of 1 is constructed from the tri-nuclear Cd 3 (O 2 CR) 6 SBUs cluster which contains two crystallographically equivalent four-coordinate terminal metal centers (Cd2) in which the O atom (O1S) of the DMF is axially coordinated and a six-coordinate central metal atom (Cd1). The coordination environment around the central Cd II atom, Cd1, in the trinuclear center is an octahedron with all six positions occupied by one carboxylate oxygen, O1, from each half unit of six SDA ligands ( Fig. 1) and that of the two symmetry equivalent neighbouring Cd II atoms, Cd2, is a tetrahedron with three coordination sites occupied by the other carboxylate oxygen, O2, from a half unit of three SDA ligands and the vacant site occupied by an oxygen atom, O1S in the DMF molecule.

Experimental
A mixture of Cd(NO 3 ) 2 .6H 2 O (0.122 g, 3.95 x 10 -4 mol) and H 2 SDA (0.106 g,3.95 x 10 -4 mol) was suspended in DMF (1.3 ml), placed in a sealed-glasstube,and heated at 90°C for 3 days. Upon cooling to room temperature, the pale-yellow crystalline was formed, collected by filtration, washed with DMF,and driedunder a reduced pressure at room temperature for 5 h to give the product (0.178 g, 78% All the non-hydrogen atoms were refined anisotropically, and hydrogen atoms were added to their geometrically ideal positions with distances C-H = 0.94 Å (aromatic H), C-H = 0.94 Å (attached to carboxylic C in DMF) and C-H = 0.97 Å (attached to methyl C in DMF). Coordinated DMF is disordered over three sites around the threefold axis. Even if oxygen O1S was refined with a unique position, the large displacement factor attained suggests some kind of unresolved splitting. Similarity restraints in distances and thermal parameters were used in order to attain a reasonable geometry of the (disordered) coordinated DMF.

Poly[bis(N,N-dimethylformamide)tris(µ 4 -trans-stilbene-4,4'-dicarboxylato)tricadmium(II)]
Crystal data [Cd 3 (C 16   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-supplementary materials sup-4 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. (