catena-Poly[[tetraaquanickel(II)]-μ3-benzene-1,3,5-tricarboxylato-3′:1:2-κ4 O 1:O 3,O 3′:O 5-[tetraaquanickel(II)]-μ2-benzene-1,3,5-tricarboxylato-2:3κ2 O 1:O 3-[tetraaquanickel(II)]]

The microwave solvothermal reaction of nickel nitrate with trimesic acid provided the title compound, [Ni3(BTC)2(H2O)12]n (BTC = benzene-1,3,5-tricarboxylate anion, C9H3O6), which is a metal coordination polymer composed of one-dimensional zigzag chains. The crystal under investigation was ramecically twinned with an approximate twin domain ratio of 1:1. In the asymmetric unit, there are two types of Ni atoms. One of the NiO6 groups (2 symmetry) is coordinated to only one carboxylate group and thus terminal, the other is bridging, forming the coordination polymer. The extended chains are connected by the organic BTC anions via μ 2-linkages. O—H⋯O hydrogen bonds and π–π interactions between the chains [centroid–centroid distance 3.58 (1) Å] induce the complex to mimic a three-dimensional structure.

The microwave solvothermal reaction of nickel nitrate with trimesic acid provided the title compound, [Ni 3 (BTC) 2 (H 2 O) 12 ] n (BTC = benzene-1,3,5-tricarboxylate anion, C 9 H 3 O 6 ), which is a metal coordination polymer composed of one-dimensional zigzag chains. The crystal under investigation was ramecically twinned with an approximate twin domain ratio of 1:1. In the asymmetric unit, there are two types of Ni atoms. One of the NiO 6 groups (2 symmetry) is coordinated to only one carboxylate group and thus terminal, the other is bridging, forming the coordination polymer. The extended chains are connected by the organic BTC anions via 2 -linkages. O-HÁ Á ÁO hydrogen bonds andinteractions between the chains [centroid-centroid distance 3.58 (1) Å ] induce the complex to mimic a three-dimensional structure.

Related literature
For background information on the solvothermal synthesis of coordination polymers with organic carboxylate ligands, see: Kitagawa et al. (2004).

Comment
The synthesis of coordination polymers has been a subject of intense research owing to their interesting structural chemistry and potential applications in gas storage, separation, catalysis, magnetism, and luminescence. A large number of these materials have been synthesized by solvothermal reactions with organic carboxyl acids (Kitagawa et al. 2004). The coordination polymers commonly adopt three-dimensional, two-dimensional, and one-dimensional structures via employed metal ions as connectors and rigid or flexible organic ligands as linkers. As a further study of such a complex, we report here the structure of the title compound, a nickel coordination polymer with one dimensional zigzag chains.
The crystal structure analysis of the title compound reveals the structure to be composed of zigzag chains. The compound has a non-centrosymmetric C2 space group and the crystal under investigation was twinned with a Flack parameter of 0.549 (12). The asymmetric unit contains two types of NiO 6 groups ( Fig. 1). The group of Ni1 is terminal and the metal atom is coordinated in a bidentate fashion to one carboxylate ligand and to four water oxygen atoms. The other nickel atom, Ni2, is coordinated in the axial positions by two monodenate carboxylate groups, and by four water molecules in the equatorial positions. All Ni-O bond lengths range from 2.021 (3) to 2.102 (3) Å. The BTC anions also have two types of coordination modes towards the NiO 6 groups. One of the BTC bridges between two Ni2 atoms via two of its carboxylate groups. The third carboxylate is protonated and not metal coordinated. The other BTC ligand bridges via two of its carboxylates between two Ni2 atoms. Its third carboxylate group coordinates to a Ni1 atom. The one-dimensional chains thus formed are further linked with each other by hydrogen bonds and π-π interactions to form a layered structure. Hydrogen bonding interactions between coordination waters are O2-H2B···O5 ix , O3-H3A···O4 viii , O4-H4B···O10, O4-H4C···O1 iii , O5-H5A···O11, O6-H6A···O11 vi , O6-H6B···O1 v , and O9-H9B···O10 iii (Fig. 2). The uncoordinated carboxylate group is involved in hydrogen bonding via O2-H2A···O12 x , O3-H3B···O12 vii , O5-H5B···O12 vi , and O9-H9A···O12 iv between nearby layers (Fig. 3,   see table 1 for numerical values and symmetry operators). π-π stacking interactions are found between aromatic rings made up of C1 to C5, C1 ii and C5 ii , and the ring defined by C2, C7, C9, C10, C10 i and C9 i (symmetry operators: (i) -x+1, y, -z; (ii) -x+2, y, -z+1). The centroid to centroid distance between Cg1 and Cg2 ix defined by the two rings is 3.58 (1) Å. The rings are slipped against each other, and the approximate interplanar distance is 3.27 Å (as defined by the distance of carbon atom C4 and Cg2 ix (symmetry operator: (ix) 1/2+x, -1/2+y, z). These π-π interactions connect nearby layers with each other (Fig. 4).

Experimental
The title complex was obtained from the reaction of 1,3,5-benzenetricarboxylic acid (C 9 H 6 O 6 , H 3 BTC, 0.421 g, 2 mmol), Ni(NO 3 ) 2 .6H 2 O (0.8724 g, 3 mmol), ethanol (5.0 ml) and H 2 O (5.0 ml) with pH value of 2.15. The reaction mixture was heated to 453 K for 20 minutes using a microwave output power of 400 W. The title compound in the form of green crystals was collected in a yield of 0.0979 g (12.2%, based on carboxylic acid reagent).

Refinement
The hydrogen atoms of benzene rings are placed in idealized positions and constrained to ride on their parent atoms, with C-H = 0.93 Å and U iso (H) = 1.2 U eq (C). The hydrogen atoms of water molecules were found in difference Fourier maps and were refined using distance constraints with O-H = 0.81 to 0.96 Å with U iso (H) = 1.2 U eq (O). Friedel pairs were not merged prior to refinement. The value of the Flack parameter and its standard uncertainty were determined by full-matrix least-squares refinement using the TWIN/BASF commands in the SHELXTL program. It refined to 0.55 (1).

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.