Redetermination of MgCrO₄·5H₂O

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.

Key indicators

• Single-crystal X-ray study
• T = 296 K
• Mean [sigma](Mg-O) = 0.001 Å
• R factor = 0.031
• wR factor = 0.069
• Data-to-parameter ratio = 28.1

checkCIF/PLATON results

No syntax errors found



Alert level C
PLAT910_ALERT_3_C Missing # of FCF Reflections Below Th(Min) .....          1
PLAT911_ALERT_3_C Missing # FCF Refl Between THmin & STh/L=  0.600          4
PLAT975_ALERT_2_C Positive Residual Density at 0.58A from O2     .       0.41 eA-3
PLAT975_ALERT_2_C Positive Residual Density at 0.63A from O1     .       0.41 eA-3

Alert level G
PLAT002_ALERT_2_G Number of Distance or Angle Restraints on AtSite         15
PLAT004_ALERT_5_G Info: Polymeric Structure Found with Dimension .          3
PLAT005_ALERT_5_G No _iucr_refine_instructions_details  in the CIF          ? Do !
PLAT720_ALERT_4_G Number of Unusual/Non-Standard Labels ..........         15
PLAT860_ALERT_3_G Note: Number of Least-Squares Restraints .......         10
PLAT912_ALERT_4_G Missing # of FCF Reflections Above STh/L=  0.600         23
PLAT960_ALERT_3_G Number of Intensities with I  <   - 2*sig(I) ...          1

   0 ALERT level A = Most likely a serious problem - resolve or explain
   0 ALERT level B = A potentially serious problem, consider carefully
   4 ALERT level C = Check. Ensure it is not caused by an omission or oversight
   7 ALERT level G = General information/check it is not something unexpected

   0 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   3 ALERT type 2 Indicator that the structure model may be wrong or deficient
   4 ALERT type 3 Indicator that the structure quality may be low
   2 ALERT type 4 Improvement, methodology, query or suggestion
   2 ALERT type 5 Informative message, check

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S1. Comment

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 CrO₄^2- 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

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).

S2. Experimental

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.

S3. Refinement

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 U_iso parameters for each H atom. One reflection (0 0 1) was affected by the beam stop and was omitted from the refinement.

Computing details

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

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

Crystal data

MgCrO4·5H2O	Z = 2
Mr = 230.39	F(000) = 236
Triclinic, P1	Dx = 1.993 Mg m^−3
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 6.1467 (3) Å	Cell parameters from 3623 reflections
b = 6.3742 (4) Å	θ = 3.5–39.7°
c = 10.7048 (6) Å	µ = 1.59 mm^−1
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) Å^3

Data collection

Bruker APEXII CCD diffractometer	4019 independent reflections
Radiation source: fine-focus sealed tube	3340 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.031
ω– and ϕ–scans	θmax = 37.5°, θmin = 3.5°
Absorption correction: multi-scan (SADABS; Bruker, 2011)	h = −10→10
Tmin = 0.594, Tmax = 0.748	k = −10→10
10569 measured reflections	l = −17→18

Refinement

Refinement on F^2	Primary atom site location: isomorphous structure methods
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.031	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.069	w = 1/[σ^2(Fo^2) + (0.0263P)^2 + 0.1663P] where P = (Fo^2 + 2Fc^2)/3
S = 1.02	(Δ/σ)max < 0.001
4019 reflections	Δρmax = 1.17 e Å^−3
143 parameters	Δρmin = −0.72 e Å^−3
10 restraints

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	0.0000	0.0000	0.0000	0.00672 (11)
Mg2	0.0000	0.5000	0.5000	0.00587 (10)
Cr1	0.35414 (3)	0.02843 (3)	0.709355 (18)	0.00508 (4)
O1	0.17472 (14)	0.28496 (15)	0.66131 (9)	0.00919 (15)
O2	0.61762 (13)	0.04795 (15)	0.70554 (9)	0.00825 (15)
O3	0.64389 (14)	0.14163 (16)	0.38351 (9)	0.00978 (16)
O4	0.72867 (14)	0.07678 (16)	0.14081 (9)	0.00919 (15)
OW1	0.30276 (14)	0.54057 (17)	0.40541 (10)	0.01038 (16)
HW1A	0.331 (4)	0.660 (3)	0.378 (2)	0.029 (6)*
HW1B	0.413 (3)	0.435 (3)	0.397 (2)	0.032 (6)*
OW2	0.16925 (14)	0.70426 (17)	0.12175 (9)	0.01025 (16)
HW2A	0.118 (4)	0.675 (4)	0.1938 (16)	0.027 (6)*
HW2B	0.305 (3)	0.669 (4)	0.130 (2)	0.026 (5)*
OW3	0.16025 (16)	0.18082 (17)	0.06898 (10)	0.01161 (17)
HW3A	0.240 (3)	0.115 (3)	0.1307 (17)	0.024 (5)*
HW3B	0.221 (4)	0.263 (4)	0.016 (2)	0.038 (7)*
OW4	0.65171 (16)	0.54576 (17)	0.13300 (10)	0.01237 (17)
HW4A	0.689 (4)	0.414 (3)	0.130 (2)	0.036 (7)*
HW4B	0.714 (4)	0.560 (4)	0.1924 (18)	0.030 (6)*
OW5	0.03331 (14)	0.23596 (17)	0.41734 (10)	0.01081 (16)
HW5A	−0.076 (3)	0.201 (4)	0.413 (2)	0.023 (5)*
HW5B	0.143 (3)	0.156 (3)	0.384 (2)	0.029 (6)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Mg1	0.0071 (2)	0.0069 (3)	0.0059 (3)	−0.00218 (18)	−0.00012 (17)	−0.0010 (2)
Mg2	0.0065 (2)	0.0051 (3)	0.0058 (3)	−0.00195 (18)	−0.00013 (17)	−0.0007 (2)
Cr1	0.00505 (6)	0.00478 (8)	0.00506 (8)	−0.00132 (5)	−0.00015 (5)	−0.00072 (6)
O1	0.0101 (3)	0.0067 (4)	0.0085 (4)	−0.0003 (3)	−0.0012 (3)	−0.0001 (3)
O2	0.0072 (3)	0.0092 (4)	0.0093 (4)	−0.0038 (3)	−0.0002 (3)	−0.0021 (3)
O3	0.0102 (3)	0.0092 (4)	0.0113 (4)	−0.0021 (3)	−0.0015 (3)	−0.0054 (3)
O4	0.0100 (3)	0.0084 (4)	0.0071 (4)	−0.0023 (3)	0.0017 (3)	0.0004 (3)
OW1	0.0078 (3)	0.0071 (4)	0.0147 (4)	−0.0024 (3)	0.0022 (3)	−0.0010 (3)
OW2	0.0094 (3)	0.0116 (4)	0.0074 (4)	−0.0020 (3)	−0.0003 (3)	0.0006 (3)
OW3	0.0143 (4)	0.0125 (4)	0.0099 (4)	−0.0075 (3)	−0.0038 (3)	0.0007 (3)
OW4	0.0140 (4)	0.0081 (4)	0.0141 (5)	−0.0029 (3)	0.0001 (3)	−0.0018 (3)
OW5	0.0078 (3)	0.0105 (4)	0.0162 (5)	−0.0024 (3)	0.0002 (3)	−0.0075 (3)

Geometric parameters (Å, º)

Mg1—OW3^i	2.0505 (9)	Mg2—OW1	2.0467 (8)
Mg1—OW3	2.0505 (9)	Mg2—OW1^vi	2.0467 (8)
Mg1—OW2^ii	2.0656 (9)	Mg2—O1	2.1099 (9)
Mg1—OW2^iii	2.0656 (9)	Mg2—O1^vi	2.1099 (9)
Mg1—O4^iv	2.0952 (9)	Cr1—O3^vii	1.6357 (9)
Mg1—O4^v	2.0952 (9)	Cr1—O4^vii	1.6554 (9)
Mg2—OW5	2.0265 (10)	Cr1—O1	1.6568 (9)
Mg2—OW5^vi	2.0265 (10)	Cr1—O2	1.6579 (8)
OW3^i—Mg1—OW3	180.00 (5)	OW5^vi—Mg2—O1^vi	92.35 (4)
OW3^i—Mg1—OW2^ii	90.97 (4)	OW1—Mg2—O1^vi	88.90 (4)
OW3—Mg1—OW2^ii	89.03 (4)	OW1^vi—Mg2—O1^vi	91.10 (4)
OW3^i—Mg1—OW2^iii	89.03 (4)	O1—Mg2—O1^vi	180.00 (3)
OW3—Mg1—OW2^iii	90.97 (4)	O3^vii—Cr1—O4^vii	109.01 (5)
OW2^ii—Mg1—OW2^iii	180.00 (7)	O3^vii—Cr1—O1	111.25 (5)
OW3^i—Mg1—O4^iv	91.55 (4)	O4^vii—Cr1—O1	108.90 (5)
OW3—Mg1—O4^iv	88.45 (4)	O3^vii—Cr1—O2	109.30 (4)
OW2^ii—Mg1—O4^iv	88.19 (4)	O4^vii—Cr1—O2	109.31 (4)
OW2^iii—Mg1—O4^iv	91.81 (4)	O1—Cr1—O2	109.05 (4)
OW3^i—Mg1—O4^v	88.45 (4)	Cr1—O1—Mg2	142.08 (6)
OW3—Mg1—O4^v	91.55 (4)	Cr1^vii—O4—Mg1^viii	140.12 (5)
OW2^ii—Mg1—O4^v	91.81 (4)	Mg2—OW1—HW1A	126.4 (16)
OW2^iii—Mg1—O4^v	88.19 (4)	Mg2—OW1—HW1B	121.9 (17)
O4^iv—Mg1—O4^v	180.00 (5)	HW1A—OW1—HW1B	111 (2)
OW5—Mg2—OW5^vi	180.00 (5)	Mg1^ix—OW2—HW2A	119.3 (16)
OW5—Mg2—OW1	90.91 (4)	Mg1^ix—OW2—HW2B	122.2 (15)
OW5^vi—Mg2—OW1	89.09 (4)	HW2A—OW2—HW2B	102 (2)
OW5—Mg2—OW1^vi	89.09 (4)	Mg1—OW3—HW3A	118.2 (15)
OW5^vi—Mg2—OW1^vi	90.91 (4)	Mg1—OW3—HW3B	117.0 (18)
OW1—Mg2—OW1^vi	180.0	HW3A—OW3—HW3B	111 (2)
OW5—Mg2—O1	92.35 (4)	HW4A—OW4—HW4B	109 (2)
OW5^vi—Mg2—O1	87.65 (4)	Mg2—OW5—HW5A	119.5 (16)
OW1—Mg2—O1	91.10 (4)	Mg2—OW5—HW5B	131.6 (16)
OW1^vi—Mg2—O1	88.90 (4)	HW5A—OW5—HW5B	109 (2)
OW5—Mg2—O1^vi	87.65 (4)

Symmetry codes: (i) −x, −y, −z; (ii) −x, −y+1, −z; (iii) x, y−1, z; (iv) x−1, 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)

D—H···A	D—H	H···A	D···A	D—H···A
OW1—HW1A···O2^x	0.81 (2)	1.96 (2)	2.7702 (13)	174 (2)
OW1—HW1B···O3	0.80 (2)	1.97 (2)	2.7598 (13)	169 (2)
OW2—HW2A···O1^vi	0.79 (2)	2.19 (2)	2.9014 (13)	150 (2)
OW2—HW2A···OW1	0.79 (2)	2.52 (2)	3.1008 (14)	132 (2)
OW2—HW2B···OW4	0.80 (2)	2.02 (2)	2.8173 (12)	171 (2)
OW3—HW3A···O2^vii	0.82 (2)	1.97 (2)	2.7891 (13)	170 (2)
OW3—HW3B···OW4^xi	0.81 (2)	1.99 (2)	2.7906 (14)	172 (2)
OW4—HW4B···O1^x	0.83 (2)	2.32 (2)	3.1205 (14)	163 (2)
OW4—HW4A···O4	0.80 (2)	2.06 (2)	2.8535 (14)	170 (2)
OW5—HW5B···O2^vii	0.80 (2)	1.93 (2)	2.7262 (13)	173 (2)
OW5—HW5A···O3^iv	0.79 (2)	1.96 (2)	2.7409 (12)	174 (2)

Symmetry codes: (iv) x−1, 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.