supplementary materials


Acta Cryst. (2013). E69, i48-i49    [ doi:10.1107/S1600536813018588 ]

Redetermination of MgCrO4·5H2O

M. Weil

Abstract top

The CCD-data based redetermination of the crystal structure of the title compound, magnesium chromate(VI) pentahydrate, confirms in principle the previous study based on precession film data [Bertrand et al. (1971). C. R. Hebd. Seances Acad. Sci. Serie C, 272, 530-533.], but with all atoms refined with anisotropic displacement parameters and with all H atoms localized. This allowed an unambiguous assignment of the hydrogen-bonding pattern. MgCrO4·5H2O adopts the MgSO4·5H2O structure type. It contains two Mg2+ sites on special positions with site symmetry -1, one tetrahedral CrO4 group and five water molecules. Four of them coordinate to the Mg2+ cation, and one is an uncoordinating lattice water molecule. The octahedral environment of the Mg2+ cation is completed by two axial O atoms of CrO4 tetrahedra. This arrangement leads to the formation of chains parallel to [011]. Adjacent chains are linked through O-H...O hydrogen bonds (one of them bifurcated), involving both the coordinating and lattice water molecules, into a three-dimensional network.

Comment top

In the current study MgCrO4.5H2O was prepared as a precursor for preparation of anhydrous MgCrO4. The structure of MgCrO4.5H2O has already been determined from film data (precession camera) by Bertrand et al. (1971), revealing isotypism with MgSO4.5H2O, CuSO4.5H2O and other MXO4.5H2O structures (M = Mg or a divalent first row transition metal; X = S, Se, Cr). The original refinement of MgCrO4.5H2O did not report any standard uncertainties on lattice parameters and atomic coordinates. It converged with a R value of 0.095, with the displacement parameters of all atoms refined isotropically and with no H-atoms localized. The geometric parameters (bond lengths, bond angles, hydrogen bonding pattern) of the crystal structures of the three isotypic salts MgSO4.5H2O, CuSO4.5H2O and MgCrO4.5H2O were compared by Baur & Rolin (1972), using the original MgCrO4.5H2O data by Bertrand et al. (1971) under assumption of geometrically calculated hydrogen positions for the water molecules. Therefore a redetermination of the MgCrO4.5H2O structure based on modern CCD-based intensity data seemed appropriate. The current study revealed all non-H atoms with anisotropic displacement parameters and with all H atoms localized, allowing an unambiguous assignment of the hydrogen-bonding pattern, together with more accurate bond lengths.

In a crystal chemical sense, MgCrO4.5H2O is better represented by the formula [Mg(H2O)4]CrO4.H2O. Its structure contains two Mg2+ cations, each located on an inversion centre, one CrO42- anion and five water molecules. The Mg2+ cations are octahedrally surrounded by four water molecules in equatorial sites and by O atoms of CrO4 tetrahedra in axial sites. The bridging character of the CrO4 tetrahedra leads to the formation of chains extending parallel to [011] (Fig. 1). The two [MgO2(H2O)4] octahedra are slightly distorted (Table 1), with average Mg—O bond lengths of 2.070 for Mg1 and 2.061 for Mg2. The CrO4 tetrahedron is likewise slightly distorted and has a mean Cr—O bond lengths of 1.658 Å, typical for chromates(VI) with isolated CrO4 anions (1.646 (25) Å; Pressprich et al. 1988). The values of bond lengths and angles of the MgO6 octahedron and the CrO4 tetrahedron in the title structure are in the same range as in the related undecahydrate MgCrO4.11H2O (Fortes et al., 2013).

Neighbouring chains are linked through O—H···O hydrogen bonds, involving the coordinating water molecules as well as the lattice water molecule (OW4). The strength of most of the hydrogen bonds can be considered as medium-strong, with O···O separations between 2.7262 (13) and 2.7906 (14) Å. Somewhat weaker hydrogen bonds are also present, with O···O separations > 2.80 Å, and the longest O···O separation being 3.1205 (14) Å. It is interesting to note that HW2A protons are involved in a bifurcated hydrogen bond (Table 1).

The experimentally determined hydrogen bonding scheme of [Mg(H2O)4]CrO4.H2O is in good agreement with the one calculated and discussed previously by Baur & Rolin (1972).

Related literature top

For the original structure determination of the title compound, see: Bertrand et al. (1971). For hydrogen-bonding pattern in the structures of MXO4.5H2O compounds (M = Mg, Cu; X = S, Cr), see: Baur & Rolin (1972). For Cr—O bond length distributions in chromates(VI), see: Pressprich et al. (1988). For bond lengths and angles in the related structure of MgCrO4.11H2O, see: Fortes et al. (2013). For standardization of structure data, see: Gelato & Parthé (1987).

Experimental top

Half-concentrated chromic acid, prepared by dissolving CrO3 in water, was neutralized with MgCO3. This solution was evaporated until dryness and the resulting solid recrystallized in water. Yellow crystals with a platy habit and edge length up to 1 mm were obtained.

Refinement top

In the original study (Bertrand et al., 1971) a non-reduced setting in space group P1 has been used, with lattice parameters a = 6.384, b = 10.702, c = 6.115 Å, α = 81.55, β = 108.75, γ = 104.333 °. For the present study the unit-cell parameters were transformed into the reduced cell using the transformation matrix (0 0 1, 1 0 0, 0 1 0). For refinement, the atomic coordinates of the original determination were used as starting parameters. They were finally standardized with STRUCTURE TIDY (Gelato & Parthé, 1987). All H atoms were discernible from difference maps. Their coordinates were refined with distance restraints of d(O—H) = 0.82 (5) Å, with individual Uiso parameters for each H atom. One reflection (0 0 1) was affected by the beam stop and was omitted from the refinement.

Computing details top

Data collection: APEX2 (Bruker, 2011); cell refinement: SAINT (Bruker, 2011); data reduction: SAINT (Bruker, 2011); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS for Windows (Dowty, 2008); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures top
[Figure 1] Fig. 1. Projection of the structure of MgCrO4.5H2O along [110]. MgO6 octahedra are blue, CrO4 tetrahedra are red, O atoms are colourless. For non-H atoms, displacement parameters are given at the 90% probability level. O—H···O hydrogen bonds are indicated by green lines.
Magnesium chromate(VI) pentahydrate top
Crystal data top
MgCrO4·5H2OZ = 2
Mr = 230.39F(000) = 236
Triclinic, P1Dx = 1.993 Mg m3
Hall symbol: -P 1Mo Kα radiation, λ = 0.71073 Å
a = 6.1467 (3) ÅCell parameters from 3623 reflections
b = 6.3742 (4) Åθ = 3.5–39.7°
c = 10.7048 (6) ŵ = 1.59 mm1
α = 75.919 (4)°T = 296 K
β = 81.603 (3)°Plate, yellow
γ = 71.134 (3)°0.10 × 0.08 × 0.01 mm
V = 383.92 (4) Å3
Data collection top
Bruker APEXII CCD
diffractometer
4019 independent reflections
Radiation source: fine-focus sealed tube3340 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.031
ω– and φ–scansθmax = 37.5°, θmin = 3.5°
Absorption correction: multi-scan
(SADABS; Bruker, 2011)
h = 1010
Tmin = 0.594, Tmax = 0.748k = 1010
10569 measured reflectionsl = 1718
Refinement top
Refinement on F2Primary atom site location: isomorphous structure methods
Least-squares matrix: fullHydrogen site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.031H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.069 w = 1/[σ2(Fo2) + (0.0263P)2 + 0.1663P]
where P = (Fo2 + 2Fc2)/3
S = 1.02(Δ/σ)max < 0.001
4019 reflectionsΔρmax = 1.17 e Å3
143 parametersΔρmin = 0.72 e Å3
10 restraints
Crystal data top
MgCrO4·5H2Oγ = 71.134 (3)°
Mr = 230.39V = 383.92 (4) Å3
Triclinic, P1Z = 2
a = 6.1467 (3) ÅMo Kα radiation
b = 6.3742 (4) ŵ = 1.59 mm1
c = 10.7048 (6) ÅT = 296 K
α = 75.919 (4)°0.10 × 0.08 × 0.01 mm
β = 81.603 (3)°
Data collection top
Bruker APEXII CCD
diffractometer
4019 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker, 2011)
3340 reflections with I > 2σ(I)
Tmin = 0.594, Tmax = 0.748Rint = 0.031
10569 measured reflectionsθmax = 37.5°
Refinement top
R[F2 > 2σ(F2)] = 0.031H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.069Δρmax = 1.17 e Å3
S = 1.02Δρmin = 0.72 e Å3
4019 reflectionsAbsolute structure: ?
143 parametersAbsolute structure parameter: ?
10 restraintsRogers parameter: ?
Special details top

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 (Å2) top
xyzUiso*/Ueq
Mg10.00000.00000.00000.00672 (11)
Mg20.00000.50000.50000.00587 (10)
Cr10.35414 (3)0.02843 (3)0.709355 (18)0.00508 (4)
O10.17472 (14)0.28496 (15)0.66131 (9)0.00919 (15)
O20.61762 (13)0.04795 (15)0.70554 (9)0.00825 (15)
O30.64389 (14)0.14163 (16)0.38351 (9)0.00978 (16)
O40.72867 (14)0.07678 (16)0.14081 (9)0.00919 (15)
OW10.30276 (14)0.54057 (17)0.40541 (10)0.01038 (16)
HW1A0.331 (4)0.660 (3)0.378 (2)0.029 (6)*
HW1B0.413 (3)0.435 (3)0.397 (2)0.032 (6)*
OW20.16925 (14)0.70426 (17)0.12175 (9)0.01025 (16)
HW2A0.118 (4)0.675 (4)0.1938 (16)0.027 (6)*
HW2B0.305 (3)0.669 (4)0.130 (2)0.026 (5)*
OW30.16025 (16)0.18082 (17)0.06898 (10)0.01161 (17)
HW3A0.240 (3)0.115 (3)0.1307 (17)0.024 (5)*
HW3B0.221 (4)0.263 (4)0.016 (2)0.038 (7)*
OW40.65171 (16)0.54576 (17)0.13300 (10)0.01237 (17)
HW4A0.689 (4)0.414 (3)0.130 (2)0.036 (7)*
HW4B0.714 (4)0.560 (4)0.1924 (18)0.030 (6)*
OW50.03331 (14)0.23596 (17)0.41734 (10)0.01081 (16)
HW5A0.076 (3)0.201 (4)0.413 (2)0.023 (5)*
HW5B0.143 (3)0.156 (3)0.384 (2)0.029 (6)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Mg10.0071 (2)0.0069 (3)0.0059 (3)0.00218 (18)0.00012 (17)0.0010 (2)
Mg20.0065 (2)0.0051 (3)0.0058 (3)0.00195 (18)0.00013 (17)0.0007 (2)
Cr10.00505 (6)0.00478 (8)0.00506 (8)0.00132 (5)0.00015 (5)0.00072 (6)
O10.0101 (3)0.0067 (4)0.0085 (4)0.0003 (3)0.0012 (3)0.0001 (3)
O20.0072 (3)0.0092 (4)0.0093 (4)0.0038 (3)0.0002 (3)0.0021 (3)
O30.0102 (3)0.0092 (4)0.0113 (4)0.0021 (3)0.0015 (3)0.0054 (3)
O40.0100 (3)0.0084 (4)0.0071 (4)0.0023 (3)0.0017 (3)0.0004 (3)
OW10.0078 (3)0.0071 (4)0.0147 (4)0.0024 (3)0.0022 (3)0.0010 (3)
OW20.0094 (3)0.0116 (4)0.0074 (4)0.0020 (3)0.0003 (3)0.0006 (3)
OW30.0143 (4)0.0125 (4)0.0099 (4)0.0075 (3)0.0038 (3)0.0007 (3)
OW40.0140 (4)0.0081 (4)0.0141 (5)0.0029 (3)0.0001 (3)0.0018 (3)
OW50.0078 (3)0.0105 (4)0.0162 (5)0.0024 (3)0.0002 (3)0.0075 (3)
Geometric parameters (Å, º) top
Mg1—OW3i2.0505 (9)Mg2—OW12.0467 (8)
Mg1—OW32.0505 (9)Mg2—OW1vi2.0467 (8)
Mg1—OW2ii2.0656 (9)Mg2—O12.1099 (9)
Mg1—OW2iii2.0656 (9)Mg2—O1vi2.1099 (9)
Mg1—O4iv2.0952 (9)Cr1—O3vii1.6357 (9)
Mg1—O4v2.0952 (9)Cr1—O4vii1.6554 (9)
Mg2—OW52.0265 (10)Cr1—O11.6568 (9)
Mg2—OW5vi2.0265 (10)Cr1—O21.6579 (8)
OW3i—Mg1—OW3180.00 (5)OW5vi—Mg2—O1vi92.35 (4)
OW3i—Mg1—OW2ii90.97 (4)OW1—Mg2—O1vi88.90 (4)
OW3—Mg1—OW2ii89.03 (4)OW1vi—Mg2—O1vi91.10 (4)
OW3i—Mg1—OW2iii89.03 (4)O1—Mg2—O1vi180.00 (3)
OW3—Mg1—OW2iii90.97 (4)O3vii—Cr1—O4vii109.01 (5)
OW2ii—Mg1—OW2iii180.00 (7)O3vii—Cr1—O1111.25 (5)
OW3i—Mg1—O4iv91.55 (4)O4vii—Cr1—O1108.90 (5)
OW3—Mg1—O4iv88.45 (4)O3vii—Cr1—O2109.30 (4)
OW2ii—Mg1—O4iv88.19 (4)O4vii—Cr1—O2109.31 (4)
OW2iii—Mg1—O4iv91.81 (4)O1—Cr1—O2109.05 (4)
OW3i—Mg1—O4v88.45 (4)Cr1—O1—Mg2142.08 (6)
OW3—Mg1—O4v91.55 (4)Cr1vii—O4—Mg1viii140.12 (5)
OW2ii—Mg1—O4v91.81 (4)Mg2—OW1—HW1A126.4 (16)
OW2iii—Mg1—O4v88.19 (4)Mg2—OW1—HW1B121.9 (17)
O4iv—Mg1—O4v180.00 (5)HW1A—OW1—HW1B111 (2)
OW5—Mg2—OW5vi180.00 (5)Mg1ix—OW2—HW2A119.3 (16)
OW5—Mg2—OW190.91 (4)Mg1ix—OW2—HW2B122.2 (15)
OW5vi—Mg2—OW189.09 (4)HW2A—OW2—HW2B102 (2)
OW5—Mg2—OW1vi89.09 (4)Mg1—OW3—HW3A118.2 (15)
OW5vi—Mg2—OW1vi90.91 (4)Mg1—OW3—HW3B117.0 (18)
OW1—Mg2—OW1vi180.0HW3A—OW3—HW3B111 (2)
OW5—Mg2—O192.35 (4)HW4A—OW4—HW4B109 (2)
OW5vi—Mg2—O187.65 (4)Mg2—OW5—HW5A119.5 (16)
OW1—Mg2—O191.10 (4)Mg2—OW5—HW5B131.6 (16)
OW1vi—Mg2—O188.90 (4)HW5A—OW5—HW5B109 (2)
OW5—Mg2—O1vi87.65 (4)
Symmetry codes: (i) x, y, z; (ii) x, y+1, z; (iii) x, y1, z; (iv) x1, y, z; (v) x+1, y, z; (vi) x, y+1, z+1; (vii) x+1, y, z+1; (viii) x+1, y, z; (ix) x, y+1, z.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
OW1—HW1A···O2x0.81 (2)1.96 (2)2.7702 (13)174 (2)
OW1—HW1B···O30.80 (2)1.97 (2)2.7598 (13)169 (2)
OW2—HW2A···O1vi0.79 (2)2.19 (2)2.9014 (13)150 (2)
OW2—HW2A···OW10.79 (2)2.52 (2)3.1008 (14)132 (2)
OW2—HW2B···OW40.80 (2)2.02 (2)2.8173 (12)171 (2)
OW3—HW3A···O2vii0.82 (2)1.97 (2)2.7891 (13)170 (2)
OW3—HW3B···OW4xi0.81 (2)1.99 (2)2.7906 (14)172 (2)
OW4—HW4B···O1x0.83 (2)2.32 (2)3.1205 (14)163 (2)
OW4—HW4A···O40.80 (2)2.06 (2)2.8535 (14)170 (2)
OW5—HW5B···O2vii0.80 (2)1.93 (2)2.7262 (13)173 (2)
OW5—HW5A···O3iv0.79 (2)1.96 (2)2.7409 (12)174 (2)
Symmetry codes: (iv) x1, y, z; (vi) x, y+1, z+1; (vii) x+1, y, z+1; (x) x+1, y+1, z+1; (xi) x+1, y+1, z.

Experimental details

Crystal data
Chemical formulaMgCrO4·5H2O
Mr230.39
Crystal system, space groupTriclinic, P1
Temperature (K)296
a, b, c (Å)6.1467 (3), 6.3742 (4), 10.7048 (6)
α, β, γ (°)75.919 (4), 81.603 (3), 71.134 (3)
V3)383.92 (4)
Z2
Radiation typeMo Kα
µ (mm1)1.59
Crystal size (mm)0.10 × 0.08 × 0.01
Data collection
DiffractometerBruker APEXII CCD
diffractometer
Absorption correctionMulti-scan
(SADABS; Bruker, 2011)
Tmin, Tmax0.594, 0.748
No. of measured, independent and
observed [I > 2σ(I)] reflections
10569, 4019, 3340
Rint0.031
(sin θ/λ)max1)0.857
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.031, 0.069, 1.02
No. of reflections4019
No. of parameters143
No. of restraints10
H-atom treatmentH atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å3)1.17, 0.72

Computer programs: APEX2 (Bruker, 2011), SAINT (Bruker, 2011), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), ATOMS for Windows (Dowty, 2008), publCIF (Westrip, 2010).

Selected bond lengths (Å) top
Mg1—OW32.0505 (9)Mg2—O12.1099 (9)
Mg1—OW2i2.0656 (9)Cr1—O3iii1.6357 (9)
Mg1—O4ii2.0952 (9)Cr1—O4iii1.6554 (9)
Mg2—OW52.0265 (10)Cr1—O11.6568 (9)
Mg2—OW12.0467 (8)Cr1—O21.6579 (8)
Symmetry codes: (i) x, y1, z; (ii) x+1, y, z; (iii) x+1, y, z+1.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
OW1—HW1A···O2iv0.810 (15)1.963 (16)2.7702 (13)174 (2)
OW1—HW1B···O30.796 (16)1.973 (16)2.7598 (13)169 (2)
OW2—HW2A···O1v0.792 (15)2.187 (17)2.9014 (13)150 (2)
OW2—HW2A···OW10.792 (15)2.517 (19)3.1008 (14)131.7 (19)
OW2—HW2B···OW40.803 (15)2.021 (15)2.8173 (12)171 (2)
OW3—HW3A···O2iii0.824 (15)1.973 (16)2.7891 (13)170 (2)
OW3—HW3B···OW4vi0.809 (16)1.988 (16)2.7906 (14)172 (2)
OW4—HW4B···O1iv0.826 (16)2.323 (17)3.1205 (14)163 (2)
OW4—HW4A···O40.803 (16)2.060 (16)2.8535 (14)170 (2)
OW5—HW5B···O2iii0.798 (16)1.932 (16)2.7262 (13)173 (2)
OW5—HW5A···O3vii0.785 (15)1.960 (15)2.7409 (12)174 (2)
Symmetry codes: (iii) x+1, y, z+1; (iv) x+1, y+1, z+1; (v) x, y+1, z+1; (vi) x+1, y+1, z; (vii) x1, y, z.
Acknowledgements top

The X-ray centre of the Vienna University of Technology is acknowledged for financial support and for providing access to the single-crystal diffractometer.

references
References top

Baur, W. H. & Rolin, J. L. (1972). Acta Cryst. B28, 1448–1455.

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Dowty, E. (2008). ATOMS for Windows. Shape Software, Kingsport, Tennessee, USA.

Fortes, A. D., Wood, I. G. & Gutmann, M. J. (2013). Acta Cryst. C69, 324–329.

Gelato, L. M. & Parthé, E. (1987). J. Appl. Cryst. 20, 139–143.

Pressprich, M. R., Willett, R. D., Poshusta, R. D., Saunders, S. C., Davis, H. B. & Gard, H. B. (1988). Inorg. Chem. 27, 260–264.

Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122.

Westrip, S. P. (2010). J. Appl. Cryst. 43, 920–925.