Bis­[hex­aqua­magnesium(II)] pyrazine-2,3,5,6-tetra­carboxyl­ate tetrahydrate

The unit cell of the title compound, [Mg(H₂O)₆]₂(C₈N₂O₈)·4H₂O, contains two hex­aqua­magnesium(II) cations, one fully deprotonated pyrazine-2,3,5,6-tetra­carboxyl­ate [(2,3,5,6-PZTC)^4-] anion with its geometrical centre situated at the inversion centre and four solvent water mol­ecules. The magnesium(II) ion is surrounded by six water mol­ecules with their O atoms located at the apices of a fairly regular octahedron. Mg-O bond distances range from 2.036 (1) to 2.113 (1) Å (mean value 2.065 Å). The carboxyl­ate groups are inclined to the pyrazine ring by 22.8 (1)and 99.1 (1)°. A three-dimensional network of hydrogen bonds is built up between the coordinated water mol­ecules and the carboxyl­ate O atoms.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](C-C) = 0.002 Å
• H-atom completeness 25%
• R factor = 0.042
• wR factor = 0.137
• Data-to-parameter ratio = 16.4

checkCIF/PLATON results

No syntax errors found



Alert level A
CHEMW01_ALERT_1_A  The ratio of given/expected molecular weight as calculated
            from the _chemical_formula_sum lies outside
            the range 0.90 <> 1.10
            Calculated formula weight =   689.7397
            Formula weight given      =   588.9800

Alert level C
ABSMU01_ALERT_1_C  The ratio of given/expected absorption coefficient lies
               outside the range 0.99 <> 1.01
            Calculated value of mu =     0.216
            Value of mu given      =     0.206
CHEMW01_ALERT_1_C  The difference between the given and expected weight for
            compound is greater 1 mass unit. Check that all hydrogen
            atoms have been taken into account.
PLAT041_ALERT_1_C Calc. and Rep. SumFormula Strings    Differ ....          ?
PLAT042_ALERT_1_C Calc. and Rep. MoietyFormula Strings Differ ....          ?
PLAT045_ALERT_1_C Calculated and Reported Z Differ by ............       2.00 Ratio
PLAT125_ALERT_4_C No _symmetry_space_group_name_Hall Given .......          ?
PLAT152_ALERT_1_C Supplied and Calc Volume s.u. Inconsistent .....          ?
PLAT250_ALERT_2_C Large U3/U1 Ratio for Average U(i,j) Tensor ....       2.21
PLAT417_ALERT_2_C Short Inter D-H..H-D       H142   ..  H222    ..       2.10 Ang.
PLAT417_ALERT_2_C Short Inter D-H..H-D       H161   ..  H161    ..       2.12 Ang.
PLAT751_ALERT_4_C Bond    Calc     0.88297, Rep    0.883(3) ......  Senseless su
              O22  -H222    1.555   1.555
PLAT751_ALERT_4_C Bond    Calc     0.92655, Rep  0.9266(18) ......  Senseless su
              O22  -H221    1.555   1.555
PLAT752_ALERT_4_C Angle   Calc       98.89, Rep     98.9(3) ......  Senseless su
              H222 -O22  -H221    1.555   1.555   1.555
PLAT755_ALERT_4_C D-H     Calc     0.92655, Rep  0.9266(18) ......  Senseless su
              O22  -H221    1.555   1.555
PLAT755_ALERT_4_C D-H     Calc     0.92655, Rep  0.9266(18) ......  Senseless su
              O22  -H221    1.555   1.555
PLAT755_ALERT_4_C D-H     Calc     0.88297, Rep    0.883(3) ......  Senseless su
              O22  -H222    1.555   1.555
PLAT756_ALERT_4_C H...A   Calc     2.29959, Rep  2.2996(14) ......  Senseless su
              H221 -O3      1.555   1.455
PLAT756_ALERT_4_C H...A   Calc     2.48651, Rep  2.4865(18) ......  Senseless su
              H221 -O2      1.555   2.655
PLAT756_ALERT_4_C H...A   Calc     2.22330, Rep  2.2234(17) ......  Senseless su
              H222 -O16     1.555   1.655

Alert level G
FORMU01_ALERT_1_G  There is a discrepancy between the atom counts in the
            _chemical_formula_sum and _chemical_formula_moiety. This is
            usually due to the moiety formula being in the wrong format.
            Atom count from _chemical_formula_sum:   C8 H132 Mg2 N2 O24
            Atom count from _chemical_formula_moiety:C8 H32 Mg2 N2 O24
FORMU01_ALERT_2_G  There is a discrepancy between the atom counts in the
            _chemical_formula_sum and the formula from the _atom_site* data.
            Atom count from _chemical_formula_sum:C8 H132 Mg2 N2 O24
            Atom count from the _atom_site data:  C8 H32 Mg2 N2 O24
CELLZ01_ALERT_1_G Difference between formula and atom_site contents detected.
CELLZ01_ALERT_1_G WARNING: H atoms missing from atom site list. Is this intentional?
           From the CIF: _cell_formula_units_Z    1
           From the CIF: _chemical_formula_sum  C8 H132 Mg2 N2 O24
           TEST: Compare cell contents of formula and atom_site data

           atom    Z*formula  cif sites diff
           C          8.00      8.00    0.00
           H        132.00     32.00  100.00
           Mg         2.00      2.00    0.00
           N          2.00      2.00    0.00
           O         24.00     24.00    0.00

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

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

---

S1. Comment

The unit cell of the title compound contains two [Mg(H₂O)₆]²⁺ cations, one fully deprotonated pyrazine-2,3,5,6-tetracarboxylate [(2,3,5,6-PZTC)⁴⁻] anion and four solvation water molecules. Fig. 1 shows the molecules with the atom-numbering scheme; Fig. 2 shows the packing of the molecules. The geometrical centre of the (2,3,5,6-PZTC)⁴⁻ anion is situated at a centre of symmetry. The deprotonated carboxylic groups are inclined to the plane of the pyrazine ring by 22.8 (s.u.?) and 99.1 (s.u.?)°. The bond distances and bond angles within the (2,3,5,6-PZTC)⁴⁻ anion do not differ from those reported in structures of 3d transiton metal ion complexes with this ligand (Marioni et al., 1986, 1994; Graf et al., 1993). The magnesium ion and the surrounding six coordinated water molecules are located in general positions. The coordination polyhedron around the Mg^II ion is a fairly regular octahedron with typical bond distances and angles (Table 1). Four solvation water molecules complete the contents of the unit cell. The coordinated water molecules act as donors in a three-dimensional network of rather weak hydrogen bonds with O—H···O > 2.7 Å. The acceptors are the carboxylate O atoms of the anion molecules. Both solvation water molecules are donors for hydrogen bonds. The geometrical parameters of the hydrogen bond system are given in Table 2. Among metal complexes with pyrazine −2,3,5,6-tetracarboxylate, an ionic pattern has been reported in the Mn^II compound (Marioni et al., 1994). Its monoclinic structure contains [Mn(H₂O)₆]²⁺ cations and doubly deprotonated [H₂(2,3,5,6-PZTC)]²⁻ anions. Separate [Mg(H₂O)₆]²⁺ cations have also been observed in the structure of the copper complex [Mg(H₂O)₆]²⁺·[Cu(2,3,5,6-PZTC)(H₂O)₂]²⁻·2H₂O, in which the ligand molecules are fully deprotonated and bridge the CuII ions to form catenated polyanions (Graf et al., 1993).

S2. Experimental

A hot aqueous solution (50 ml) containing magnesium acetate tetrahydrate (2 mmol) was added with constant stirring to a hot aqueous solution (50 ml) containing pyrazine-2,3,5,6- tetracarboxylic acid (1 mmol). After boiling for 1 h, the solution was left to crystallize at room temperature. After several days, colourless crystals were found in the mother liquid.

S3. Refinement

H atoms were found in Fourier maps and refined with isotropic displacement parameters. The U values of the H atoms bonded to O16 and O22 were set to 1.2U_eq of the parent atom. The positional parameters of the H atoms bonded to O22 were not refined. The O—H distances for the H atoms bonded to O16 were restrained to 0.82 (s.u.?) Å.

Computing details

Data collection: KM-4 Software (Kuma, 1996); cell refinement: KM-4 Software; data reduction: DATAPROC (Kuma, 2001); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XP (Siemens, 1992); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. Atom-labelling scheme for the title compound. Non-H atoms are shown as 50% probability displacement ellipsoids. [Symmetry code: (i) −x + 2, −y, −z.]
	Fig. 2. Packing of molecules in the unit cell of compound (1).

Bis[hexaquamagnesium(II)] pyrazine-2,3,5,6-tetracarboxylate tetrahydrate

Crystal data

[Mg(H2O)6]2(C8N2O8)·4H2O	Z = 1
Mr = 588.98	F(000) = 310
Triclinic, P1	Dx = 1.598 Mg m^−3
a = 6.7467 (13) Å	Mo Kα radiation, λ = 0.71073 Å
b = 9.1779 (18) Å	Cell parameters from 25 reflections
c = 10.731 (2) Å	θ = 6–15°
α = 102.95 (3)°	µ = 0.21 mm^−1
β = 103.29 (3)°	T = 293 K
γ = 100.02 (3)°	Rectangular plate, colourless
V = 612.2 (2) Å^3	0.30 × 0.25 × 0.20 mm

Data collection

Kuma KM-4 four-circle diffractometer	2752 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.018
Graphite monochromator	θmax = 30.1°, θmin = 2.0°
ω/2θ scans	h = 0→9
Absorption correction: analytical (CrysAlis RED; Oxford Diffraction, 2000)	k = −12→12
Tmin = 0.955, Tmax = 0.977	l = −14→14
3839 measured reflections	3 standard reflections every 200 reflections
3565 independent reflections	intensity decay: 3.1%

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.042	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.137	H atoms treated by a mixture of independent and constrained refinement
S = 1.14	w = 1/[σ^2(Fo^2) + (0.0859P)^2 + 0.0954P] where P = (Fo^2 + 2Fc^2)/3
3565 reflections	(Δ/σ)max = 0.001
217 parameters	Δρmax = 0.60 e Å^−3
7 restraints	Δρmin = −0.56 e Å^−3

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
Mg1	0.10048 (7)	0.38447 (5)	0.74376 (5)	0.02107 (14)
O12	−0.1200 (2)	0.38242 (14)	0.84634 (14)	0.0353 (3)
O14	0.33647 (18)	0.40732 (15)	0.65134 (13)	0.0337 (3)
O16	−0.1000 (2)	0.44752 (15)	0.59307 (12)	0.0349 (3)
H162	−0.146 (4)	0.526 (2)	0.618 (2)	0.052*
H161	−0.128 (3)	0.429 (2)	0.5161 (16)	0.052*
O13	0.22688 (19)	0.61361 (14)	0.85011 (14)	0.0355 (3)
O11	0.2736 (2)	0.29726 (17)	0.87786 (13)	0.0389 (3)
O15	−0.0305 (2)	0.16429 (13)	0.62377 (13)	0.0331 (3)
C6	1.11871 (19)	0.12801 (13)	0.09160 (12)	0.0175 (2)
N1	0.93117 (16)	0.13464 (12)	0.02238 (11)	0.0194 (2)
O4	1.33092 (17)	0.37946 (12)	0.15286 (11)	0.0274 (2)
O3	1.23274 (18)	0.29073 (13)	0.31122 (11)	0.0302 (2)
C8	1.24105 (19)	0.27660 (14)	0.19462 (13)	0.0198 (2)
C2	0.81111 (18)	0.00722 (14)	−0.06940 (13)	0.0183 (2)
C7	0.5984 (2)	0.01594 (15)	−0.14738 (14)	0.0227 (3)
O21	0.90832 (19)	0.09816 (14)	0.35451 (12)	0.0308 (2)
O1	0.57634 (17)	0.14607 (13)	−0.15418 (14)	0.0348 (3)
O2	0.46520 (18)	−0.10839 (14)	−0.19901 (15)	0.0434 (4)
O22	0.5024 (3)	0.1799 (3)	0.50952 (17)	0.0796 (7)
H222	0.6140	0.2569	0.5319	0.119*
H221	0.4601	0.1692	0.4188	0.119*
H112	0.300 (4)	0.324 (3)	0.963 (3)	0.046 (6)*
H142	0.377 (4)	0.326 (3)	0.611 (3)	0.054 (7)*
H212	0.790 (5)	0.101 (3)	0.300 (3)	0.055 (7)*
H111	0.358 (6)	0.241 (4)	0.863 (4)	0.080 (10)*
H151	−0.057 (4)	0.140 (3)	0.542 (3)	0.039 (6)*
H141	0.463 (5)	0.478 (4)	0.707 (3)	0.065 (9)*
H131	0.349 (6)	0.666 (4)	0.844 (4)	0.081 (10)*
H152	0.017 (5)	0.091 (3)	0.645 (3)	0.055 (7)*
H122	−0.202 (5)	0.302 (4)	0.850 (3)	0.074 (9)*
H132	0.179 (5)	0.673 (4)	0.905 (3)	0.067 (8)*
H121	−0.185 (5)	0.454 (3)	0.854 (3)	0.055 (7)*
H211	0.992 (5)	0.159 (3)	0.339 (3)	0.057 (8)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Mg1	0.0184 (2)	0.0217 (2)	0.0235 (2)	0.00725 (17)	0.00606 (17)	0.00488 (17)
O12	0.0357 (6)	0.0266 (6)	0.0567 (8)	0.0140 (5)	0.0289 (6)	0.0156 (5)
O14	0.0229 (5)	0.0382 (6)	0.0354 (6)	0.0033 (4)	0.0121 (4)	0.0003 (5)
O16	0.0421 (6)	0.0370 (6)	0.0270 (5)	0.0211 (5)	0.0044 (5)	0.0079 (5)
O13	0.0272 (5)	0.0260 (5)	0.0473 (7)	0.0020 (4)	0.0165 (5)	−0.0044 (5)
O11	0.0447 (7)	0.0555 (8)	0.0288 (6)	0.0355 (6)	0.0127 (5)	0.0151 (5)
O15	0.0405 (6)	0.0229 (5)	0.0329 (6)	0.0097 (4)	0.0063 (5)	0.0040 (4)
C6	0.0141 (5)	0.0150 (5)	0.0213 (5)	0.0030 (4)	0.0026 (4)	0.0034 (4)
N1	0.0147 (5)	0.0165 (5)	0.0247 (5)	0.0051 (4)	0.0021 (4)	0.0036 (4)
O4	0.0261 (5)	0.0197 (4)	0.0315 (5)	−0.0007 (4)	0.0054 (4)	0.0053 (4)
O3	0.0320 (5)	0.0291 (5)	0.0237 (5)	0.0018 (4)	0.0051 (4)	0.0026 (4)
C8	0.0148 (5)	0.0160 (5)	0.0244 (6)	0.0039 (4)	0.0011 (4)	0.0015 (4)
C2	0.0129 (5)	0.0173 (5)	0.0227 (5)	0.0046 (4)	0.0018 (4)	0.0042 (4)
C7	0.0141 (5)	0.0226 (6)	0.0289 (6)	0.0064 (4)	0.0011 (4)	0.0055 (5)
O21	0.0246 (5)	0.0329 (6)	0.0339 (6)	0.0056 (4)	0.0023 (4)	0.0139 (5)
O1	0.0193 (5)	0.0257 (5)	0.0577 (7)	0.0101 (4)	0.0009 (5)	0.0144 (5)
O2	0.0204 (5)	0.0259 (6)	0.0675 (9)	0.0016 (4)	−0.0129 (5)	0.0090 (6)
O22	0.0611 (11)	0.1247 (18)	0.0425 (9)	0.0516 (12)	−0.0014 (8)	−0.0042 (10)

Geometric parameters (Å, º)

Mg1—O11	2.0359 (14)	O15—H151	0.83 (3)
Mg1—O12	2.0437 (13)	O15—H152	0.85 (3)
Mg1—O15	2.0523 (15)	C6—N1	1.3310 (16)
Mg1—O14	2.0654 (13)	C6—C2^i	1.3947 (16)
Mg1—O13	2.0748 (15)	C6—C8	1.5088 (18)
Mg1—O16	2.1130 (14)	N1—C2	1.3346 (17)
O12—H122	0.86 (4)	O4—C8	1.2550 (17)
O12—H121	0.85 (3)	O3—C8	1.2440 (18)
O14—H142	0.90 (3)	C2—C6^i	1.3947 (16)
O14—H141	0.95 (3)	C2—C7	1.5113 (17)
O16—H162	0.848 (16)	C7—O2	1.2440 (18)
O16—H161	0.776 (16)	C7—O1	1.2449 (17)
O13—H131	0.90 (4)	O21—H212	0.88 (3)
O13—H132	0.86 (3)	O21—H211	0.79 (3)
O11—H112	0.86 (3)	O22—H222	0.883 (3)
O11—H111	0.85 (4)	O22—H221	0.9266 (18)
O11—Mg1—O12	89.20 (6)	Mg1—O13—H131	121 (2)
O11—Mg1—O15	88.92 (7)	Mg1—O13—H132	129 (2)
O12—Mg1—O15	94.29 (6)	H131—O13—H132	110 (3)
O11—Mg1—O14	90.84 (6)	Mg1—O11—H112	128.1 (18)
O12—Mg1—O14	174.61 (6)	Mg1—O11—H111	127 (2)
O15—Mg1—O14	91.09 (6)	H112—O11—H111	104 (3)
O11—Mg1—O13	95.64 (7)	Mg1—O15—H151	123.9 (16)
O12—Mg1—O13	87.90 (6)	Mg1—O15—H152	119.0 (19)
O15—Mg1—O13	174.97 (6)	H151—O15—H152	104 (2)
O14—Mg1—O13	86.74 (6)	N1—C6—C2^i	121.42 (11)
O11—Mg1—O16	173.12 (6)	N1—C6—C8	114.00 (11)
O12—Mg1—O16	89.93 (6)	C2^i—C6—C8	124.57 (11)
O15—Mg1—O16	84.34 (6)	C6—N1—C2	117.87 (11)
O14—Mg1—O16	90.67 (6)	O3—C8—O4	125.59 (12)
O13—Mg1—O16	91.14 (6)	O3—C8—C6	117.64 (12)
Mg1—O12—H122	126 (2)	O4—C8—C6	116.57 (12)
Mg1—O12—H121	120.3 (19)	N1—C2—C6^i	120.71 (11)
H122—O12—H121	107 (3)	N1—C2—C7	117.45 (11)
Mg1—O14—H142	121.9 (18)	C6^i—C2—C7	121.84 (11)
Mg1—O14—H141	113.5 (18)	O2—C7—O1	127.18 (13)
H142—O14—H141	105 (3)	O2—C7—C2	116.19 (12)
Mg1—O16—H162	116.6 (17)	O1—C7—C2	116.62 (12)
Mg1—O16—H161	136.7 (19)	H212—O21—H211	102 (3)
H162—O16—H161	105 (2)	H222—O22—H221	98.9 (3)

Symmetry code: (i) −x+2, −y, −z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O11—H112···O4^ii	0.86 (3)	1.94 (3)	2.7935 (18)	174 (2)
O11—H111···O1^iii	0.85 (4)	1.86 (4)	2.7037 (18)	171 (3)
O12—H121···O4^iv	0.85 (3)	1.97 (3)	2.8074 (17)	172 (3)
O12—H122···O1^ii	0.86 (4)	1.86 (4)	2.7103 (19)	168 (3)
O13—H132···N1^iv	0.86 (3)	2.10 (3)	2.9084 (18)	156 (3)
O13—H131···O2^v	0.90 (4)	2.27 (4)	2.990 (2)	136 (3)
O13—H131···O4^vi	0.90 (4)	2.26 (4)	2.9806 (17)	136 (3)
O14—H142···O22	0.90 (3)	1.96 (3)	2.846 (2)	167 (3)
O14—H141···O4^vi	0.95 (3)	1.85 (3)	2.782 (2)	165 (3)
O15—H151···O21^vii	0.83 (3)	1.91 (3)	2.7323 (19)	174 (2)
O15—H152···O21^viii	0.85 (3)	1.89 (3)	2.7185 (18)	165 (3)
O16—H162···O3^iv	0.85 (2)	1.93 (2)	2.7739 (18)	175 (3)
O16—H161···O16^ix	0.78 (2)	2.42 (1)	2.896 (3)	121 (1)
O21—H211···O3	0.79 (3)	1.98 (3)	2.7604 (19)	171 (3)
O21—H212···O2^x	0.88 (3)	1.83 (3)	2.7135 (18)	175 (3)
O22—H221···O3^vii	0.93 (1)	2.30 (1)	2.951 (2)	127 (1)
O22—H221···O2^x	0.93 (1)	2.49 (1)	3.315 (3)	149 (1)
O22—H222···O16^xi	0.88 (1)	2.22 (1)	3.106 (3)	178 (1)

Symmetry codes: (ii) x−1, y, z+1; (iii) x, y, z+1; (iv) −x+1, −y+1, −z+1; (v) x, y+1, z+1; (vi) −x+2, −y+1, −z+1; (vii) x−1, y, z; (viii) −x+1, −y, −z+1; (ix) −x, −y+1, −z+1; (x) −x+1, −y, −z; (xi) x+1, y, z.