Dibromidobis(pyrazine-2-carboxylic acid-κN⁴)mercury(II) dihydrate

The asymmetric unit of the title compound, [HgBr₂(C₅H₄N₂O₂)₂]·2H₂O, contains one half-mol­ecule and one water mol­ecule. The Hg^II ion, lying on a twofold rotation axis, is four-coordinated by two N atoms of pyrazine-2-carboxylic acid ligands and two bromide ions, forming a highly distorted tetrahedral geometry. In the crystal structure, inter­molecular O—H...O and O—H...N hydrogen bonds link the mol­ecules.

Key indicators

• Single-crystal X-ray study
• T = 294 K
• Mean [sigma](C-C) = 0.011 Å
• R factor = 0.070
• wR factor = 0.149
• Data-to-parameter ratio = 13.2

checkCIF/PLATON results

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Alert level C
PLAT250_ALERT_2_C Large U3/U1 Ratio for Average U(i,j) Tensor ....       2.12
PLAT342_ALERT_3_C Low Bond Precision on C-C Bonds (x 1000) Ang ...         11

Alert level G
PLAT860_ALERT_3_G Note: Number of Least-Squares Restraints .......          2

   0 ALERT level A = In general: serious problem
   0 ALERT level B = Potentially serious problem
   2 ALERT level C = Check and explain
   1 ALERT level G = General alerts; check

   0 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   1 ALERT type 2 Indicator that the structure model may be wrong or deficient
   2 ALERT type 3 Indicator that the structure quality may be low
   0 ALERT type 4 Improvement, methodology, query or suggestion
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S1. Comment

Functional materials built up by organic ligands and metal ions, especially transition metals, have potential applications in optics, electronics, magnetics, biology, catalyst and medicine (Zhang, 2005; O'Conner et al., 1982). Pyrazine-2,3-dicarboxylic acid, having six possible coordination sites, is a good ligand with versatile coordination types, which is widely used in the self-assembled polymeric coordination synthesis (Zou et al., 1999; Wang et al., 2007). The title compound, (I), was obtained unintentionally as the product of a hydrothermal synthesis of pyrazine-2,3-dicarboxylic acid and mercury(II) bromide. Under high temperature as 413 K and mercury(II) ion catalyst, pyrazine-2,3-dicarboxylic acid is likely to decarboxylate as 2-pyrazine carboxylic acid. We report herein the crystal structure of (I), a complex containing the ligand of 2-pyrazine carboxylic acid.

The asymmetric unit of (I), (Fig. 1), contains one half-molecule and one water molecule, in which the bond lengths and angles are within normal ranges (Allen et al., 1987). The Hg^II ion lying on a twofold rotation axis, is four -coordinated (Table 1) by two N atoms of pyrazine carboxylic acid ligands and two bromide atoms. The two pyrazine rings are oriented at a dihedral angle of 78.4 (9)°.

The Hg—N [2.528 (13) Å] bond is slightly longer, while Hg—Br [2.4234 (15) Å] bond is slightly shorter than the corresponding values [2.270 (5) Å and 2.5269 (7) Å, respectively] in [Hg(bib)Br₂]0.5THF (where bib is 1-bromo-3,5 -bis(imidazol-1-ylmethyl)benzene) (Wang et al., 2007).

In the crystal structure, intermolecular O—H···O and O—H···N hydrogen bonds (Table 2, Fig. 2) link the molecules, in which they seem to be effective in the stabilization of the structure.

Related literature

For general background, see: O'Conner et al. (1982); Zhang (2005); Zou et al. (1999). For related structure, see: Wang et al.(2007). For bond-length data, see: Allen et al. (1987).

S2. Experimental

For the preparation of the title compound, mercury(II) bromide (360 mg, 1 mmol) and 2,3-pyrazine dicarboxylic acid (168 mg, 1 mmol) were dissolved in a mixed solvent of ethanol (5 ml) and acetonitrile (5 ml). Then the mixture was added into a Teflon-lined stainless steel autoclave at 413 K for 2 d. The green crystals were obtained after cooling to room temperature and was filtrated. Elemental analysis calcd: C 19.58%, H 4.40%, N 45.60%; Found: C 19.51%, H 4.35%, N 45.53%.

S3. Refinement

H atoms (for H₂O) were located in a difference map and refined [O—H = 0.84 (2) and 0.84 (2) Å, U_iso(H) = 1.5U_eq(O)]. The remaining H atoms were positioned geometrically, with O—H = 0.82 Å (for OH) and C—H = 0.93 Å, for aromatic H atoms and constrained to ride on their parent atoms, with U_iso(H) = xU_eq(C,O), where x = 1.2 for aromatic H and x = 1.5 for OH H atoms.

Computing details

Data collection: CAD-4 Software (Enraf–Nonius, 1989); cell refinement: CAD-4 Software (Enraf–Nonius, 1989); data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 2000); software used to prepare material for publication: SHELXTL (Bruker, 2000).

Figures

	Fig. 1. The molecular structure of the title molecule, with the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level [symmetry code A: 2 - x, y, 3/2 - z]. Hydrogen bonds are shown as dashed lines.
	Fig. 2. A packing diagram of (I). Hydrogen bonds are shown as dashed lines.

Dibromidobis(pyrazine-2-carboxylic acid-κN⁴)mercury(II) dihydrate

Crystal data

[HgBr2(C5H4N2O2)2]·2H2O	F(000) = 1192
Mr = 644.63	Dx = 2.524 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 25 reflections
a = 13.895 (1) Å	θ = 9–13°
b = 5.7176 (2) Å	µ = 13.82 mm^−1
c = 21.8753 (7) Å	T = 294 K
β = 102.544 (2)°	Block, green
V = 1696.42 (15) Å^3	0.4 × 0.2 × 0.2 mm
Z = 4

Data collection

Enraf–Nonius CAD-4 diffractometer	1287 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.062
Graphite monochromator	θmax = 25.1°, θmin = 1.9°
ω/2θ scans	h = −16→8
Absorption correction: ψ scan (North et al., 1968)	k = −6→5
Tmin = 0.048, Tmax = 0.063	l = −20→26
2775 measured reflections	3 standard reflections every 200 reflections
1480 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.070	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.150	w = 1/[σ^2(Fo^2) + (0.1015P)^2 + 4.7441P] where P = (Fo^2 + 2Fc^2)/3
S = 1.12	(Δ/σ)max < 0.001
1480 reflections	Δρmax = 2.85 e Å^−3
112 parameters	Δρmin = −3.62 e Å^−3
2 restraints	Extinction correction: SHELXL, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0045 (4)

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 F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² 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 (Ų)

	x	y	z	Uiso*/Ueq
Hg1	1.0000	1.10939 (9)	0.7500	0.0355 (4)
Br1	1.16422 (12)	1.2057 (6)	0.73701 (9)	0.0525 (6)
C1	0.8717 (5)	0.6941 (4)	0.6566 (4)	0.039 (4)
H1	0.8234	0.7561	0.6754	0.047*
C2	0.8499 (5)	0.5080 (4)	0.6166 (4)	0.041 (4)
H2	0.7871	0.4427	0.6093	0.050*
C3	1.0047 (6)	0.5069 (4)	0.6006 (6)	0.030 (3)
C4	1.0302 (6)	0.6941 (6)	0.6411 (6)	0.043 (5)
H4	1.0940	0.7538	0.6490	0.051*
C5	1.0825 (6)	0.4040 (4)	0.5701 (5)	0.029 (3)
N1	0.9628 (7)	0.7881 (7)	0.6688 (6)	0.033 (3)
N2	0.9161 (7)	0.4188 (7)	0.5885 (5)	0.034 (3)
O1	1.1642 (8)	0.4862 (6)	0.5761 (6)	0.046 (3)
O2	1.0514 (10)	0.2121 (5)	0.5379 (6)	0.060 (4)
H2A	1.0954	0.1606	0.5221	0.090*
O3	0.8315 (9)	0.0042 (3)	0.5179 (6)	0.051 (3)
H3A	0.794 (2)	−0.090 (3)	0.531 (2)	0.077*
H3B	0.857 (2)	0.123 (2)	0.537 (2)	0.077*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Hg1	0.0234 (5)	0.0443 (7)	0.0442 (6)	0.000	0.0149 (3)	0.000
Br1	0.0263 (10)	0.0587 (14)	0.0617 (13)	−0.0056 (8)	0.0225 (9)	0.0088 (10)
C1	0.029 (9)	0.045 (10)	0.047 (10)	0.007 (8)	0.017 (7)	−0.011 (8)
C2	0.031 (8)	0.050 (11)	0.045 (10)	0.000 (9)	0.014 (7)	−0.004 (9)
C3	0.027 (7)	0.044 (10)	0.021 (7)	−0.011 (7)	0.005 (6)	0.003 (7)
C4	0.029 (8)	0.051 (13)	0.027 (8)	0.004 (9)	0.020 (7)	−0.009 (8)
C5	0.035 (9)	0.032 (8)	0.033 (7)	0.001 (7)	0.012 (6)	0.006 (7)
N1	0.037 (7)	0.034 (8)	0.032 (7)	0.000 (6)	0.012 (6)	−0.012 (6)
N2	0.033 (6)	0.053 (9)	0.028 (6)	−0.012 (6)	0.002 (5)	−0.008 (6)
O1	0.030 (6)	0.054 (8)	0.051 (7)	−0.011 (6)	0.016 (5)	−0.020 (6)
O2	0.050 (8)	0.061 (9)	0.057 (9)	−0.010 (7)	0.033 (7)	−0.039 (8)
O3	0.039 (7)	0.052 (9)	0.052 (9)	−0.014 (7)	0.033 (6)	−0.030 (7)

Geometric parameters (Å, º)

Hg1—Br1	2.4234 (15)	C3—C4	1.381 (6)
Hg1—Br1^i	2.4234 (15)	C3—C5	1.510 (7)
Hg1—N1	2.528 (13)	C4—N1	1.334 (10)
Hg1—N1^i	2.528 (13)	C4—H4	0.9300
C1—N1	1.351 (7)	C5—O1	1.209 (18)
C1—C2	1.369 (8)	C5—O2	1.327 (19)
C1—H1	0.9300	O2—H2A	0.8200
C2—N2	1.310 (6)	O3—H3A	0.84 (2)
C2—H2	0.9300	O3—H3B	0.84 (2)
C3—N2	1.303 (8)
Br1—Hg1—Br1^i	153.72 (12)	C4—C3—C5	118.4 (6)
Br1—Hg1—N1	97.8 (3)	N1—C4—C3	119.6 (7)
Br1^i—Hg1—N1	101.2 (3)	N1—C4—H4	120.2
Br1—Hg1—N1^i	101.2 (3)	C3—C4—H4	120.2
Br1^i—Hg1—N1^i	97.8 (3)	O1—C5—O2	124.8 (7)
N1—Hg1—N1^i	86.9 (6)	O1—C5—C3	123.1 (8)
N1—C1—C2	120.3 (8)	O2—C5—C3	112.1 (6)
N1—C1—H1	119.8	C1—N1—C4	118.1 (7)
C2—C1—H1	119.8	C1—N1—Hg1	118.0 (10)
N2—C2—C1	121.3 (8)	C4—N1—Hg1	123.7 (11)
N2—C2—H2	119.4	C2—N2—C3	118.7 (8)
C1—C2—H2	119.4	C5—O2—H2A	109.5
N2—C3—C4	122.0 (7)	H3A—O3—H3B	125 (4)
N2—C3—C5	119.6 (8)
N1—C1—C2—N2	−1.0 (12)	C3—C4—N1—Hg1	175.2 (12)
N2—C3—C4—N1	−0.3 (13)	Br1—Hg1—N1—C1	−174.8 (12)
C5—C3—C4—N1	−179.9 (15)	Br1^i—Hg1—N1—C1	−13.0 (13)
N2—C3—C5—O1	175.2 (15)	N1^i—Hg1—N1—C1	84.4 (13)
C4—C3—C5—O1	−4.8 (12)	Br1—Hg1—N1—C4	10.9 (13)
N2—C3—C5—O2	−7.1 (11)	Br1^i—Hg1—N1—C4	172.7 (13)
C4—C3—C5—O2	173.3 (15)	N1^i—Hg1—N1—C4	−89.9 (14)
C2—C1—N1—C4	0.3 (9)	C1—C2—N2—C3	1.8 (12)
C2—C1—N1—Hg1	−175.1 (5)	C4—C3—N2—C2	−1.1 (11)
C3—C4—N1—C1	1.2 (9)	C5—C3—N2—C2	178.5 (14)

Symmetry code: (i) −x+2, y, −z+3/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O2—H2A···O3^ii	0.82	1.75	2.56 (2)	166
O3—H3A···O1^iii	0.84 (2)	2.28 (3)	2.89 (2)	130 (2)
O3—H3B···N2	0.84 (2)	2.09 (3)	2.93 (2)	177 (3)

Symmetry codes: (ii) −x+2, −y, −z+1; (iii) x−1/2, y−1/2, z.