Crystal structure of iron(III) perchlorate nona­hydrate

Since the discovery of perchlorate salts on Mars and the known occurrence of ferric salts in the regolith, there is a distinct possibility that the title compound could form on the surface of Mars. [Fe(H₂O)₆](ClO₄)₃·3H₂O was crystallized from aqueous solutions at low temperatures according to the solid–liquid phase diagram. It consists of Fe(H₂O)₆ octa­hedra (point group symmetry -3.) and perchlorate anions (point group symmetry .2) as well as non-coordinating water mol­ecules, as part of a second hydrogen-bonded coordination sphere around the cation. The perchlorate appears to be slightly disordered, with major–minor component occupancies of 0.773 (9):0.227 (9).

1. Chemical context

Since the discovery of perchlorate salts on the surface of Mars during the Phoenix expedition (Hecht et al., 2009; Davila et al., 2013; Kerr, 2013; Marion et al., 2010; Navarro-González et al., 2010), inter­est in the solubility and crystal structures of the perchlorate hydrate phases became more important (Chevrier, Hanley & Altheide, 2009; Catling et al., 2010). Based on the red color of the planet, one can expect different iron phases, such as perchlorate and sulfate, to be important constituents of the regolith (Chevrier, Ulrich & Altheide, 2009; Chevrier & Altheide, 2008; Hennings et al., 2013). While investigating the solubility of ferric perchlorate, we obtained the nona­hydrate as a stable phase in the binary salt–water system.

2. Structural commentary

The central Fe atom is situated on a threefold inversion axis and is octa­hedrally coordinated by six water mol­ecules in the first, and by six water mol­ecules as well as six perchlorate tetra­hedra in the second coordination spheres (Fig. 1). The water mol­ecules of the second coordination sphere (O4 and symmetry equivalents) are connected to perchlorate tetra­hedra (Fig. 2a) via hydrogen bonds (Table 1). Six O4-water mol­ecules form a second, larger octa­hedron outside the octa­hedron of the first coordination shell (Fig. 2b). The perchlorate anion, situated on a twofold rotation axis, appears to be slightly disordered, with major:minor component occupancies of 0.773 (9):0.227 (9).

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1—H1A⋯O2^i	0.83 (5)	1.92 (5)	2.745 (5)	174 (4)
O1—H1A⋯O2′^i	0.83 (5)	2.33 (5)	3.153 (14)	170 (4)
O1—H1A⋯O3′	0.83 (5)	2.27 (5)	2.864 (17)	129 (4)
O1—H1B⋯O4^ii	0.82 (5)	1.83 (5)	2.642 (3)	173 (4)
O4—H4⋯O2	0.84 (4)	2.39 (4)	3.073 (5)	139 (4)
O4—H4⋯O3^iii	0.84 (4)	2.13 (4)	2.796 (4)	136 (4)
O4—H4⋯O2′	0.84 (4)	2.13 (5)	2.812 (17)	138 (4)
O4—H4⋯O3′^iii	0.84 (4)	2.12 (4)	2.708 (12)	127 (4)
Symmetry codes: (i) ; (ii) ; (iii) .

Figure 1 The mol­ecular units (a) and second coordination sphere (b) of ferric perchlorate nona­hydrate. Dashed lines indicate hydrogen bonds. Displacement ellipsoids are drawn at the 50% probability limit. The minor disorder component of the ClO4 tetrahedron has been omitted. [Symmetry codes: (i) x − y, x, 1 − z; (ii) −x + y, −x, z; (iii) −x, −y, 1 − z; (iv) −y, x − y, z; (v) y, −x + y, 1 − z; (vi)  − x,  − x + y,  − z.]

Figure 2 The connection scheme of water mol­ecules of the second coordination sphere by hydrogen bonds (a) and the formation of a secondary hydration shell (yellow) around the cations (b). The minor disorder component of the ClO4 tetrahedron has been omitted for clarity. Dashed lines indicate hydrogen bonds. [Symmetry code: (i)  − x,  − x + y,  − z.]

3. Supra­molecular features

From the unit cell of ferric perchlorate nona­hydrate (Fig. 3a), it is obvious that the O4 atoms form a secondary hydration shell around the Fe(H₂O)₆ units. This becomes clearer when drawing the second octa­hedra as water coordination polyhedra (yellow, Fig. 3b). The water mol­ecules of the second coordination sphere are closer [4.143 (4) Å] to the Fe atom than the perchlorate tetra­hedra [4.271 (4) Å].

Figure 3 The unit cell of iron(III) perchlorate nona­hydrate with coordination polyhedra of the first (a) and second (b) coordination sphere. The minor disorder component of the ClO4 tetrahedron has been omitted for clarity. Dashed lines indicate hydrogen bonds.

4. Database survey

For crystal structure determination of other perchlorate nona­hydrates, see: Davidian et al. (2012) for the Al, Ga and Sc salts and Hennings et al. (2014) for the strontium salt. For crystal structure determinations of other Fe^III salts with a high water content, see: Schmidt et al. (2013); Lindstrand (1936).

5. Synthesis and crystallization

Iron(III) perchlorate nona­hydrate crystallized from an aqueous solution of 54.41 wt% Fe(ClO₄)₃ thermostated at 263 K after 2 d. To prepare this solution, ferric perchlorate nona­hydrate (Fluka, pure) was used. The content of Fe^III ions was analysed using gravimetric analysis by precipitation with ammonia. All crystals are stable in their saturated solution over a period of at least four weeks.

The samples were stored in a freezer or a cryostat at low temperatures. The crystals were separated and embedded in perfluorinated ether for X-ray diffraction analysis

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. The H atoms were placed in the positions indicated by difference Fourier maps. No further constraints were applied.

Table 2
Experimental details

Crystal data
Chemical formula	[Fe(H2O)6](ClO4)3·3H2O
Mr	516.34
Crystal system, space group	Trigonal, Rc:H
Temperature (K)	100
a, c (Å)	16.1930 (15), 11.2421 (11)
V (Å^3)	2552.9 (5)
Z	6
Radiation type	Mo Kα
μ (mm^−1)	1.46
Crystal size (mm)	0.54 × 0.37 × 0.19
Data collection
Diffractometer	STOE IPDS 2T
Absorption correction	Integration (Coppens, 1970)
Tmin, Tmax	0.531, 0.755
No. of measured, independent and observed [I > 2σ(I)] reflections	8865, 659, 641
Rint	0.075
(sin θ/λ)max (Å^−1)	0.650
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.041, 0.092, 1.11
No. of reflections	658
No. of parameters	60
H-atom treatment	All H-atom parameters refined
Δρmax, Δρmin (e Å^−3)	0.64, −0.80
Computer programs: X-AREA and X-RED (Stoe & Cie, 2009), SHELXS97 and SHELXL2012 (Sheldrick, 2008), DIAMOND (Brandenburg, 2006) and publCIF (Westrip, 2010).

Supporting information

S1. Chemical context

Since the discovery of perchlorate salts on the surface of Mars during the Phoenix expedition (Hecht et al., 2009), inter­est in the solubility and crystal structures of the perchlorate hydrate phases became more important (Chevrier, Hanley & Altheide, 2009). Based on the red color of the planet, one can expect different iron phases, such as perchlorate and sulfate, to be important constituents of the regolith (Chevrier, Ulrich & Altheide, 2009; Chevrier & Altheide, 2008; Hennings et al., 2013). While investigating the solubility of ferric perchlorate, we obtained the nonahydrate as a stable phase in a binary salt–water system.

S2. Structural commentary

The central Fe atom is o­cta­hedrally coordinated by six water molecules in the first, and by six water molecules as well as six perchlorate tetra­hedra in the second coordination spheres. The water molecules of the second coordination sphere (O4 and symmetry equivalents) are connected to perchlorate tetra­hedra (Fig. 2a) via hydrogen bonds (Table 1). Six O4-water molecules form a second, larger o­cta­hedron outside the o­cta­hedron of the first coordination shell (Fig. 2b). The perchlorate anion appears to be slightly disordered, with major:minor component occupancies of 0.773 (9):0.227 (9).

S3. Supra­molecular features

From the unit cell of ferric perchlorate nonahydrate (Fig. 3a), it is obvious that the O4 atoms form a secondary hydration shell around the Fe(H₂O)₆ units. This becomes clearer when drawing the second o­cta­hedra as water coordination polyhedra (yellow, Fig. 3b). The water molecules of the second coordination sphere are closer [4.143 (4) Å] to the Fe atom than the perchlorate tetra­hedra [4.271 (4) Å].

S4. Database survey

For crystal structure determination of other perchlorate nonahydrates, see Davidian et al. (2012) and Hennings et al. (2014). For crystal structure determinations of other Fe^III salts with a high water content, see Schmidt et al. (2013).

S5. Synthesis and crystallization

Iron(III) perchlorate nonahydrate crystallized from an aqueous solution of 54.41 wt% Fe(ClO₄)₃ thermostated at 263 K after 2 d. To prepare this solution, ferric perchlorate nonahydrate (Fluka, pure) was used. The content of Fe^III ions was analysed using gravimetric analysis by precipitation with ammonia. All crystals are stable in their saturated solution over a period of at least four weeks.

The samples were stored in a freezer or a cryostat at low temperatures. The crystals were separated and embedded in perfluorinated ether for X-ray diffraction analysis

S6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2.

Related literature

For related literature, see: Chevrier &Altheide (2008); Davidian et al. (2012); Hecht et al. (2009); Hennings et al. (2013, 2014); Schmidt et al. (2013).

Computing details

Data collection: X-AREA (Stoe & Cie, 2009); cell refinement: X-AREA (Stoe & Cie, 2009); data reduction: X-RED (Stoe & Cie, 2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2012 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures

The molecular unit (a) and second coordination sphere (b) of ferric perchlorate nonahydrate. Dashed lines indicate hydrogen bonds. Displacement ellipsoids are drawn at the 50% probability limit. The minor disorder component has been omitted. [Symmetry codes: (i) x-y, -y, 1/2 - z; (ii) -y, x-y, z; (iii) x, x-y, 1/2 + z.]

The connection scheme of water molecules of the second coordination sphere by hydrogen bonds (a) and the formation of a secondary hydration shell (yellow) around the cations (b). The minor disorder component has been omitted for clarity. Dashed lines indicate hydrogen bonds.

The unit cell of iron(III) perchlorate nonahydrate with coordination polyhedra of the first (a) and second (b) coordination sphere. The minor disorder component has been omitted for clarity. Dashed lines indicate hydrogen bonds.

Iron(III) perchlorate nonahydrate

Crystal data

[Fe(H2O)6](ClO4)3·3H2O	Dx = 2.015 Mg m^−3
Mr = 516.34	Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3c:H	Cell parameters from 47287 reflections
a = 16.1930 (15) Å	θ = 7.0–29.7°
c = 11.2421 (11) Å	µ = 1.46 mm^−1
V = 2552.9 (5) Å^3	T = 100 K
Z = 6	Needle, colorless
F(000) = 1578	0.54 × 0.37 × 0.19 mm

Data collection

STOE IPDS 2T diffractometer	659 independent reflections
Radiation source: fine-focus sealed tube	641 reflections with I > 2σ(I)
Detector resolution: 6.67 pixels mm^-1	Rint = 0.075
rotation method scans	θmax = 27.5°, θmin = 2.5°
Absorption correction: integration (Coppens, 1970)	h = −20→20
Tmin = 0.531, Tmax = 0.755	k = −20→20
8865 measured reflections	l = −14→14

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.041	All H-atom parameters refined
wR(F^2) = 0.092	w = 1/[σ^2(Fo^2) + (0.0269P)^2 + 23.9134P] where P = (Fo^2 + 2Fc^2)/3
S = 1.11	(Δ/σ)max < 0.001
658 reflections	Δρmax = 0.64 e Å^−3
60 parameters	Δρmin = −0.80 e Å^−3

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Fe1	0.0000	0.0000	0.5000	0.0188 (3)
O1	0.07420 (15)	0.11666 (15)	0.3991 (2)	0.0252 (5)
O4	0.3333	0.47858 (17)	0.4167	0.0247 (6)
Cl1	0.3333	0.2540 (19)	0.4167	0.0329 (3)	0.773 (9)
O2	0.4134 (3)	0.3438 (3)	0.3808 (4)	0.0342 (8)	0.773 (9)
O3	0.3070 (3)	0.1914 (3)	0.3110 (4)	0.0481 (12)	0.773 (9)
Cl1'	0.3333	0.254 (7)	0.4167	0.0329 (3)	0.227 (9)
O2'	0.3946 (11)	0.3499 (13)	0.3527 (16)	0.0342 (8)	0.227 (9)
O3'	0.2699 (12)	0.1716 (10)	0.3602 (15)	0.0481 (12)	0.227 (9)
H1A	0.129 (3)	0.158 (3)	0.417 (4)	0.047 (12)*
H1B	0.048 (3)	0.138 (3)	0.357 (4)	0.051 (13)*
H4	0.375 (3)	0.468 (3)	0.388 (4)	0.054 (13)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Fe1	0.0157 (3)	0.0157 (3)	0.0251 (5)	0.00784 (15)	0.000	0.000
O1	0.0165 (10)	0.0195 (10)	0.0341 (11)	0.0050 (8)	−0.0004 (8)	0.0045 (8)
O4	0.0269 (15)	0.0164 (9)	0.0344 (16)	0.0135 (8)	0.0102 (12)	0.0051 (6)
Cl1	0.0221 (5)	0.0137 (4)	0.0659 (8)	0.0110 (2)	−0.0180 (5)	−0.0090 (2)
O2	0.0180 (19)	0.0381 (15)	0.051 (2)	0.0174 (13)	−0.0027 (15)	0.0098 (16)
O3	0.045 (3)	0.0345 (19)	0.067 (3)	0.0210 (19)	−0.0055 (19)	−0.0276 (19)
Cl1'	0.0221 (5)	0.0137 (4)	0.0659 (8)	0.0110 (2)	−0.0180 (5)	−0.0090 (2)
O2'	0.0180 (19)	0.0381 (15)	0.051 (2)	0.0174 (13)	−0.0027 (15)	0.0098 (16)
O3'	0.045 (3)	0.0345 (19)	0.067 (3)	0.0210 (19)	−0.0055 (19)	−0.0276 (19)

Geometric parameters (Å, º)

Fe1—O1^i	2.007 (2)	Cl1—O2	1.439 (18)
Fe1—O1^ii	2.007 (2)	Cl1—O3^vi	1.479 (17)
Fe1—O1^iii	2.007 (2)	Cl1—O3	1.479 (17)
Fe1—O1^iv	2.007 (2)	Cl1'—O3'^vi	1.37 (6)
Fe1—O1^v	2.007 (2)	Cl1'—O3'	1.37 (6)
Fe1—O1	2.007 (2)	Cl1'—O2'^vi	1.54 (7)
Cl1—O2^vi	1.439 (18)	Cl1'—O2'	1.54 (7)
O1^i—Fe1—O1^ii	180.00 (9)	O1^v—Fe1—O1	180.00 (10)
O1^i—Fe1—O1^iii	91.19 (9)	O2^vi—Cl1—O2	112 (2)
O1^ii—Fe1—O1^iii	88.81 (9)	O2^vi—Cl1—O3^vi	105.8 (3)
O1^i—Fe1—O1^iv	88.81 (9)	O2—Cl1—O3^vi	109.42 (19)
O1^ii—Fe1—O1^iv	91.19 (9)	O2^vi—Cl1—O3	109.42 (19)
O1^iii—Fe1—O1^iv	180.00 (10)	O2—Cl1—O3	105.8 (3)
O1^i—Fe1—O1^v	91.19 (9)	O3^vi—Cl1—O3	114 (2)
O1^ii—Fe1—O1^v	88.81 (9)	O3'^vi—Cl1'—O3'	106 (7)
O1^iii—Fe1—O1^v	91.19 (9)	O3'^vi—Cl1'—O2'^vi	124.0 (13)
O1^iv—Fe1—O1^v	88.81 (9)	O3'—Cl1'—O2'^vi	105.4 (10)
O1^i—Fe1—O1	88.81 (9)	O3'^vi—Cl1'—O2'	105.4 (10)
O1^ii—Fe1—O1	91.19 (9)	O3'—Cl1'—O2'	124.0 (13)
O1^iii—Fe1—O1	88.81 (9)	O2'^vi—Cl1'—O2'	94 (6)
O1^iv—Fe1—O1	91.19 (9)

Symmetry codes: (i) y, −x+y, −z+1; (ii) −y, x−y, z; (iii) x−y, x, −z+1; (iv) −x+y, −x, z; (v) −x, −y, −z+1; (vi) −x+2/3, −x+y+1/3, −z+5/6.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1A···Cl1	0.83 (5)	2.86 (5)	3.642 (2)	157 (4)
O1—H1A···O2^vi	0.83 (5)	1.92 (5)	2.745 (5)	174 (4)
O1—H1A···O2′^vi	0.83 (5)	2.33 (5)	3.153 (14)	170 (4)
O1—H1A···O3′	0.83 (5)	2.27 (5)	2.864 (17)	129 (4)
O1—H1B···O4^vii	0.82 (5)	1.83 (5)	2.642 (3)	173 (4)
O4—H4···O2	0.84 (4)	2.39 (4)	3.073 (5)	139 (4)
O4—H4···O3^viii	0.84 (4)	2.13 (4)	2.796 (4)	136 (4)
O4—H4···O2′	0.84 (4)	2.13 (5)	2.812 (17)	138 (4)
O4—H4···O3′^viii	0.84 (4)	2.12 (4)	2.708 (12)	127 (4)

Symmetry codes: (vi) −x+2/3, −x+y+1/3, −z+5/6; (vii) −x+1/3, −y+2/3, −z+2/3; (viii) y+1/3, −x+y+2/3, −z+2/3.