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The new nickel(II) coordination polymer poly­[di­aqua­nickel(II)-[mu]-(pyrazine-2,3,5,6-tetra­carboxyl­ato)-tetra­aqua­nickel(II)], {[{Ni(C8N2O8)(H2O)2}Ni(H2O)4]}n, has been synthesized and characterized both spectroscopically and crystallographically, by X-ray powder diffraction analysis. In this two-dimensional coordination polymer, NiII ions are bridged by pyrazine-2,3,5,6-tetra­carboxyl­ic acid, coordinating in a bis-bidentate manner, so forming one-dimensional polymeric chains. The chains are linked by a second NiII ion, via an O atom of the coordinated carboxyl­ate group, resulting in the formation of a two-dimensional layer-like polymer. The remaining coordination sites of the two independent octahedral NiII ions are occupied by water mol­ecules. The layers are connected via hydrogen bonds involving all six coordinated water mol­ecules.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S010827010101068X/sk1451sup1.cif
Contains datablocks global, I

rtv

Rietveld powder data file (CIF format) https://doi.org/10.1107/S010827010101068X/sk1451Isup2.rtv
Contains datablock I

CCDC reference: 174797

Comment top

Pyrazine-2,3,5,6-tetracarboxylic acid (pztcH4) was first synthesized over 100 years ago (Wolff, 1887, 1893) and an alternative synthesis was later proposed by Chattaway & Humphrey (1929). More recently, it has been used to form coordination polymers with first row transition metals such as iron(II) (Marioni et al., 1986), copper(II) (Graf et al., 1993), manganese(II) (Marioni et al., 1994) and zinc(II) (Marioni et al., 1986). Here, we present the results of the reaction of pztcH4 with NiSO4 to form the title coordination polymer, (I), and describe its analysis and structure determination, carried out using laboratory X-ray powder diffraction. \sch

Compound (I) is a planar two-dimensional polymer (Fig. 1). NiII ions are bridged by the deprotonated ligand pyrazine-2,3,5,6-tetracarboxylic acid (pztc4-, C8N2O8) coordinating in a bis-bidentate manner, so forming one-dimensional polymer chains with coplanar pyrazine rings. The chains are linked by [Ni(H2O)4]2+ units, with atom Ni2 coordinated to atom O2 of the coordinated carboxylate groups in the chains, resulting in the formation of a two-dimensional sheet-like structure. Selected bond distances and angles, and possible hydrogen bond O···O distances, are given in Table 1.

The deprotonated centrosymmetric pztc4- ligand is coordinated to two crystallographically equivalent NiII ions (Ni1) through the two N atoms of the pyrazine ring and two O atoms from two symmetrically opposing carboxylate groups. Water molecules occupy the remaining coordination sites of the octahedral Ni1 ions. The metal ions occupy inversion centres and the ligand possesses Ci symmetry.

The metal-to-metal distance, Ni1···O1—C3—O2···Ni2, is 5.4126 (3) Å, which is shorter than the Ni1-pyrazine-Ni1(1 + x, y, z) distance of 6.989 (1) Å. The Ni1—N1 bond [2.082 (1) Å] is slightly longer, and the Ni2—O2 bond [1.991 (2) Å] slightly shorter, than the same distances in similar NiII complexes with the ligands pyrazine-2,3-dicarboxylic acid (mean Ni—N 2.061 and mean Ni—O 2.060 Å; Mao et al., 1996) and pyrazine-2,5-dicarboxylic acid (Ni—N 2.033 and Ni—O 2.068 Å; Ptasiewicz-Bak et al., 1995). Atom Ni2 of the bridging [Ni(H2O)4]2+ unit forms four short Ni—O bonds [1.991 (2)–1.994 (2) Å] and two long Ni—O bonds [2.002 (5) Å], while for atom Ni1, the Ni—N bonds are longer than the Ni—O distances: 2.082 (2) compared with 2.002 (5) Å.

The two non-coordinated carboxylate groups (atoms C2, O3 and O4) and the coordinated water molecules (atoms O5, O6 and O7) are directed into the interlayer space. The short O···O distances (2.56–2.81 Å; Table 1) suggest possible hydrogen bonding between the layers, which would lead to the formation of a three-dimensional network (Figure 2).

Compound (I) was first synthesized in 1986 (Marioni, 1986) and, based on the elemental analysis, which indicated a metal-to-ligand ratio of 2:1, a binuclear complex was proposed. Thermogravimetric analysis indicated the presence of six water molecules. This result supported the proposition of a binuclear NiII complex containing the pztc4- anion coordinating in a bis-tridentate manner, with the remaining coordination sites of the metal atoms being occupied by water molecules. A single-crystal X-ray analysis was not possible, as the compound was always obtained as a microcrystalline powder. Analysis of the laboratory X-ray powder diffraction data has shown that two chemically independent nickel atoms, Ni1 and Ni2, are found interconnected by a completely deprotonated ligand, pztc4-, to form a two-dimensional coordination polymer. The deprotonated ligand in the Ni-pztc-Ni chains has a formal charge of -4, which is not compensated for by the Ni12+ ions. As has been observed in other metal complexes of pztcH4, the carboxylate groups adjacent to those coordinated to the first metal are rotated by almost 50° out of the plane of the pyrazine ring. These negatively charged chains are then connected by [Ni(H2O)4]2+ ions, resulting in the formation of neutral sheets. This charge compensation behaviour, incorporating [M(H2O)n]2+ units in the structure, has been observed previously in a zinc(II) complex (Hakansson et al., 1993), a series of iron(II) complexes (Laine et al., 1995) and a trinuclear copper(II) complex (Colacio et al., 1993).

Experimental top

A solution of NiSO4 (1.03 g, 3.92 mmol) in water (20 ml) was added to an aqueous solution of pyrazine-2,3,5,6-tetracarboxylic acid (0.50 g, 1.95 mmol, 20 ml) at 323 K. The solution was stirred for 5 min at 323 K and then slowly cooled to room temperature. The light green microcrystalline solid formed was filtered off after 3 d (yield 0.66 g, 71%). Analysis calculated for C8H12N2O14Ni2: C 20.12, H 2.53, N 5.87%; found: C 19.42, H 2.54, N 5.73%; IR spectroscopic analysis (KBr, ν, cm-1): 3365 (H2O), 1645 (C O), 1443 and 1323 (CN, CC).

Refinement top

The indexing procedure (Visser, 1969) revealed a triclinic cell, which was used for the structure solution. Approximately 400 reflections were extracted from the powder diffraction pattern for use in direct methods. Two independent Ni-atom positions, both occupying special positions, and the coordinating N and O atoms, were located. The structural model was then completed by difference Fourier techniques. It was not possible to locate the water H atoms. This model was then used for Rietveld profile refinement. After the initial refinement of the scale, background and unit cell constants, the atomic positions were refined with soft constraints for the bond distances (International Tables for Crystallography, 1995, Vol. C, pp ?-?) and angles, and with a low weight. In the final cycles of refinement, the shifts in all the parameters were less than their estimated standard deviations. The final Rietveld plot is given in Fig. 3.

Computing details top

Data collection: Rigaku/AFC Diffractometer Control Software (Rigaku Corporation, 1995); cell refinement: EXPO (Altomare et al., 1997); data reduction: EXPO; program(s) used to solve structure: EXPO and SHELXL97 (Sheldrick, 1997); program(s) used to refine structure: GSAS (Larson & Von Dreele, 1994); molecular graphics: PLATON (Spek, 2001); software used to prepare material for publication: GSAS.

Figures top
[Figure 1] Fig. 1. PLATON (Spek, 2001) drawing of (I) showing the atom-numbering scheme.
[Figure 2] Fig. 2. A side view of (I) showing the possible intra- and inter-plane hydrogen-bonding network as dashed lines.
[Figure 3] Fig. 3. Observed (+) and calculated (-) profiles for the Rietveld refinement for (I). The bottom curve is the difference plot on the same intensity scale. The insert shows the 2θ range 40–70° magnified by a factor of 20.
poly[diaquanickel(II)-µ-(pyrazine-2,3,5,6-tetracarboxylato)- tetraaquanickel(II) top
Crystal data top
[Ni(C8N2O8)(H2O)2][Ni(H2O)4]Z = 1
Mr = 477.5F(000) = 242
Triclinic, P1Dx = 2.188 Mg m3
a = 6.9892 (3) ÅCu Kα radiation, λ = 1.542 Å
b = 7.169 (4) ÅT = 295 K
c = 8.2106 (3) ÅParticle morphology: plate-like
α = 85.922 (3)°green
β = 84.242 (4)°flat sheet, 25 × 25 mm
γ = 61.818 (3)°Specimen preparation: Prepared at 295 K
V = 360.65 (1) Å3
Data collection top
Rigaku model?
diffractometer
Data collection mode: reflection
Radiation source: X-ray tube, rotating anodeScan method: step
None monochromator2θmin = 10°, 2θmax = 70°, 2θstep = 0.01°
Specimen mounting: packed powder pellet
Refinement top
Refinement on profile intensities50 parameters
Least-squares matrix: full with fixed elements per cycle57 constraints
Rp = 0.065H-atom parameters not refined
Rwp = 0.086 1/[y(obs)]1/2
Rexp = 0.019(Δ/σ)max = 0.01
χ2 = 1.690Background function: square polynomial
6000 data pointsPreferred orientation correction: none
Profile function: pseudo-Voigt
Crystal data top
[Ni(C8N2O8)(H2O)2][Ni(H2O)4]β = 84.242 (4)°
Mr = 477.5γ = 61.818 (3)°
Triclinic, P1V = 360.65 (1) Å3
a = 6.9892 (3) ÅZ = 1
b = 7.169 (4) ÅCu Kα radiation, λ = 1.542 Å
c = 8.2106 (3) ÅT = 295 K
α = 85.922 (3)°flat sheet, 25 × 25 mm
Data collection top
Rigaku model?
diffractometer
Scan method: step
Specimen mounting: packed powder pellet2θmin = 10°, 2θmax = 70°, 2θstep = 0.01°
Data collection mode: reflection
Refinement top
Rp = 0.0656000 data points
Rwp = 0.08650 parameters
Rexp = 0.019H-atom parameters not refined
χ2 = 1.690
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Ni10.50.50.50.0442 (14)*
Ni20.00.00.00.0442 (14)*
O10.3411 (7)0.3757 (10)0.2861 (4)0.0357 (14)*
O20.0010 (8)0.1574 (10)0.1899 (6)0.0357 (14)*
O30.3159 (13)0.3524 (11)0.1033 (7)0.0357 (14)*
O40.4596 (13)0.0776 (11)0.2999 (10)0.0357 (14)*
O50.3538 (10)0.2121 (6)0.6071 (8)0.0357 (14)*
O60.2786 (7)0.2399 (7)0.0676 (9)0.0357 (14)*
O70.1629 (11)0.1344 (11)0.1298 (8)0.0357 (14)*
N10.2071 (6)0.5090 (12)0.5042 (7)0.0357 (14)*
C10.0512 (9)0.4183 (16)0.3764 (8)0.0357 (14)*
C20.1532 (9)0.4034 (16)0.3744 (8)0.0357 (14)*
C30.1347 (9)0.2982 (15)0.2850 (14)0.0357 (14)*
C40.3290 (17)0.2634 (12)0.2456 (8)0.0357 (14)*
Geometric parameters (Å, º) top
Ni1—N12.082 (1)Ni2—O6ii1.994 (2)
Ni1—N1i2.082 (1)Ni2—O72.002 (2)
Ni1—O12.002 (2)Ni2—O7ii2.002 (2)
Ni1—O1i2.002 (2)O3—O6iii2.80 (1)
Ni1—O52.002 (2)O3—O6iv2.80 (1)
Ni1—O5i2.002 (2)O4—O5v2.56 (1)
Ni2—O21.991 (2)O4—O7iii2.64 (1)
Ni2—O2ii1.991 (2)O5—O6vi2.81 (1)
Ni2—O61.994 (2)
O1—Ni1—N176.7 (1)O2ii—Ni2—O688.1 (1)
O1i—Ni1—N1103.3 (1)O2—Ni2—O691.9 (1)
O1i—Ni1—N1i76.7 (1)O2ii—Ni2—O6ii91.9 (1)
O1—Ni1—N1i103.3 (1)O2—Ni2—O6ii88.1 (1)
O5—Ni1—N187.1 (1)O2ii—Ni2—O786.5 (1)
O5i—Ni1—N1i87.1 (1)O2—Ni2—O793.5 (1)
O5—Ni1—N1i92.9 (1)O2—Ni2—O7ii86.5 (1)
O5i—Ni1—N192.9 (1)O2ii—Ni2—O7ii93.5 (1)
O5—Ni1—O189.5 (1)O6—Ni2—O790.7 (1)
O5i—Ni1—O1i89.5 (1)O6ii—Ni2—O789.3 (1)
O5—Ni1—O1i90.5 (1)O6—Ni2—O7ii89.3 (1)
O5i—Ni1—O190.5 (1)O6ii—Ni2—O7ii90.7 (1)
N1i—Ni1—N1180.0O2ii—Ni2—O2180.0
O1i—Ni1—O1180.0O6—Ni2—O6ii180.0
O5i—Ni1—O5180.0O7—Ni2—O7ii180.0
Symmetry codes: (i) x+1, y1, z1; (ii) x, y, z; (iii) x1, y, z; (iv) x, y1, z; (v) x, y, z1; (vi) x, y, z1.

Experimental details

Crystal data
Chemical formula[Ni(C8N2O8)(H2O)2][Ni(H2O)4]
Mr477.5
Crystal system, space groupTriclinic, P1
Temperature (K)295
a, b, c (Å)6.9892 (3), 7.169 (4), 8.2106 (3)
α, β, γ (°)85.922 (3), 84.242 (4), 61.818 (3)
V3)360.65 (1)
Z1
Radiation typeCu Kα, λ = 1.542 Å
Specimen shape, size (mm)Flat sheet, 25 × 25
Data collection
DiffractometerRigaku model?
diffractometer
Specimen mountingPacked powder pellet
Data collection modeReflection
Scan methodStep
2θ values (°)2θmin = 10 2θmax = 70 2θstep = 0.01
Refinement
R factors and goodness of fitRp = 0.065, Rwp = 0.086, Rexp = 0.019, χ2 = 1.690
No. of data points6000
No. of parameters50
No. of restraints?
H-atom treatmentH-atom parameters not refined

Computer programs: Rigaku/AFC Diffractometer Control Software (Rigaku Corporation, 1995), EXPO (Altomare et al., 1997), EXPO and SHELXL97 (Sheldrick, 1997), GSAS (Larson & Von Dreele, 1994), PLATON (Spek, 2001), GSAS.

Selected geometric parameters (Å, º) top
Ni1—N12.082 (1)Ni2—O72.002 (2)
Ni1—O12.002 (2)O3—O6i2.80 (1)
Ni1—O52.002 (2)O4—O5ii2.56 (1)
Ni2—O21.991 (2)O4—O7i2.64 (1)
Ni2—O61.994 (2)O5—O6iii2.81 (1)
O1—Ni1—N176.7 (1)O2—Ni2—O691.9 (1)
O1—Ni1—N1iv103.3 (1)O2—Ni2—O6v88.1 (1)
O5—Ni1—N187.1 (1)O2—Ni2—O793.5 (1)
O5—Ni1—N1iv92.9 (1)O2—Ni2—O7v86.5 (1)
O5—Ni1—O189.5 (1)O6—Ni2—O790.7 (1)
O5—Ni1—O1iv90.5 (1)O6—Ni2—O7v89.3 (1)
Symmetry codes: (i) x1, y, z; (ii) x, y, z1; (iii) x, y, z1; (iv) x+1, y1, z1; (v) x, y, z.
 

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