Disorder in the anionic part of catena-poly[[(pyrazine-2-carboxylato)copper(II)]-μ-pyrazine-2-carboxylato]

The title compound, [Cu(C5H3N2O2)0.88(C6H4NO2)1.12]n, is characterized by disorder of the anion, resulting from a statistical occupation in a 0.44 (3):0.56 (3) ratio of pyrazine-2-carboxylate and pyridine-2-carboxylate. The compound was isolated during attempts to synthesize a mixed-ligand coordination polymer by solvothermal reaction between copper(II) nitrate and equimolar mixtures of pyrazine-2-carboxylic acid and pyridine-2-carboxylic acid in a mixture of water and EtOH. The difference in the two components of the compound is due to substitutional disorder of a CH group for one of the N atoms of the pyrazine ring which share the same site in the structure. In the crystal structure, the CuII atom lies on an inversion centre and is six-coordinated in a distorted N2O4 geometry. The carboxylate group carbonyl O atoms are weakly coordinated to an equivalent CuII atom that is translated one unit cell in the a-axis direction, thus forming a polymeric chain through carboxylate bridges.

The title compound, [Cu(C 5 H 3 N 2 O 2 ) 0.88 (C 6 H 4 NO 2 ) 1.12 ] n , is characterized by disorder of the anion, resulting from a statistical occupation in a 0.44 (3):0.56 (3) ratio of pyrazine-2carboxylate and pyridine-2-carboxylate. The compound was isolated during attempts to synthesize a mixed-ligand coordination polymer by solvothermal reaction between copper(II) nitrate and equimolar mixtures of pyrazine-2carboxylic acid and pyridine-2-carboxylic acid in a mixture of water and EtOH. The difference in the two components of the compound is due to substitutional disorder of a CH group for one of the N atoms of the pyrazine ring which share the same site in the structure. In the crystal structure, the Cu II atom lies on an inversion centre and is six-coordinated in a distorted N 2 O 4 geometry. The carboxylate group carbonyl O atoms are weakly coordinated to an equivalent Cu II atom that is translated one unit cell in the a-axis direction, thus forming a polymeric chain through carboxylate bridges.

ato)copper(II)]-µ-pyrazine-2-carboxylato] Joselyn Albanez, Iván Brito, Alejandro Cárdenas and Matías López-Rodríguez Comment
The design of polymeric organic-inorganic materials with novel topologies and structural motifs is of current interest in the field of coordination chemistry, (Blake et al., 1999). This paper forms part of our continuing study of the synthesis, structural characterization and physical properties of coordination polymers (Brito et al., 2011). The title compound was isolated during attempts to synthesize a mixed-ligand coordination polymer by solvothermal reaction. Our initial model, with mixed ligands, was not considered because the compound crystallizes in the space group P1 with Cu II located at a centre of symmetry so the model was changed to considering both ligands separately . A marginally better refinement is obtained when pyridine-2-carboxylate is used versus pyrazine-2-carboxylate as the ligand, (R1 = 0.024 versus R1 = 0.025 respectively) although the bond distances are, admittedly, more consistent with pyrazine-2-carboxylate as opposed to pyridine-2-carboxylate ligand. However, we observed that modeling the ligand as pyrazine-2-carboxylate led to displacement parameter for the putative N2 atom being considerably larger than those of the neighboring carbon atoms.
Further refinements shown evidence for an H atom of partial occupancy at a distance from N2 atom which suggest this site is partially occupied by a CH group. Consequently, a crystal model containing both bis(pyridinecarboxylate)copper(II) and bis(pyridinecarboxylate)copper(II) species was used and the final value for R1is 0.0216 with the displacement parameter for the site occupied by both N2 and C2′ is now comparable to those of the adjacent atoms. The title compound, Fig.1, shows only slight variations in molecular geometry and supramolecular organization from the structure described for bis(pyridinecarboxylate)copper(II), Żurowska et al. (2007). The Cu II atom is coordinated in a bidentate fashion by two O atoms and two N atoms from symmetry-related pyrazine-2-carboxylate anions. The carboxylate group carbonyl O atoms are weakly coordinated to an equivalent copper atom that is translated one unit cell in the x direction, thus forming a polymeric one-dimensional chain through a carboxylate bridge in a slightly distorted octahedral geometry, Fig.2.

Experimental
The title compound was obtained by the solvothermal reaction of a mixture of copper(II) nitrate (1 mmol), pyrazine-2carboxylic acid (0.5 mmol), pyridine-2-carboxylic acid (0.5 mmol) in water (5 ml) and EtOH (1 ml) in an acid digestion bomb heated at 150 °C for 3 d and then cooled to room temperature. Suitable single crystals grow upon cooling of the solution to room temperature. Only a few blue single crystals were obtained due to low yield of the reaction and no spectroscopic data were recorded.

Refinement
Atom H2′ was found in difference map and positioned geometrically at a distance of 0.93 Å from the parent C2′ atom; a riding model was used during the refinement process. The remaining H atoms were located in a difference Fourier supplementary materials sup-2 Acta Cryst. (2012). E68, m498-m499 syntheses and were refinend isotropically; C-H range is 0.86 (3)-0.93 (3) Å. The C2′ and N2 atoms were refined using the same position and atomic displacement parameters (adp), due to substitutional disorder of a CH group (C2′ & H2′) for one of the N atoms with SOFs 0.56 (3), 0.44 (3) respectively.

Figure 1
A view of the molecular structure of (I) with the atom-numbering scheme. The 5-position of the aromatic ring is partly N(0.46) and partly a CH group (0.54). Displacement ellipsoids are drawn at the 30% probability level and H atoms are shown as small spheres of arbitrary radii. Symmetry codes i: -1 + x, y, z; ii:1 -x, 1 -y, 1 -z; iii:2 -x, 1 -y, 1 -z.  Part of the crystal structure showing the polymeric structure with linear chains extending parallel to [100] direction.

catena-Poly[[(pyrazine-2-carboxylato)copper(II)]-µ-pyrazine-2-carboxylato]
Crystal data [Cu(C 5  Secondary atom site location: difference Fourier map Hydrogen site location: inferred from neighbouring sites H atoms treated by a mixture of independent and constrained refinement w = 1/[σ 2 (F o 2 ) + (0.0201P) 2 + 0.1488P] where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.029 Δρ max = 0.31 e Å −3 Δρ min = −0.28 e Å −3 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.