Crystal structures of Sr(ClO4)2·3H2O, Sr(ClO4)2·4H2O and Sr(ClO4)2·9H2O.

The title compounds, strontium perchlorate trihydrate {di-μ-aqua-aquadi-μ-perchlorato-strontium, [Sr(ClO4)2(H2O)3] n }, strontium perchlorate tetra-hydrate {di-μ-aqua-bis-(tri-aqua-diperchloratostrontium), [Sr2(ClO4)4(H2O)8]} and strontium perchlorate nona-hydrate {hepta-aqua-diperchloratostrontium dihydrate, [Sr(ClO4)2(H2O)7]·2H2O}, were crystallized at low temperatures according to the solid-liquid phase diagram. The structures of the tri- and tetra-hydrate consist of Sr(2+) cations coordinated by five water mol-ecules and four O atoms of four perchlorate tetra-hedra in a distorted tricapped trigonal-prismatic coordination mode. The asymmetric unit of the trihydrate contains two formula units. Two [SrO9] polyhedra in the trihydrate are connected by sharing water mol-ecules and thus forming chains parallel to [100]. In the tetra-hydrate, dimers of two [SrO9] polyhedra connected by two sharing water mol-ecules are formed. The structure of the nona-hydrate contains one Sr(2+) cation coordinated by seven water mol-ecules and by two O atoms of two perchlorate tetra-hedra (point group symmetry ..m), forming a tricapped trigonal prism (point group symmetry m2m). The structure contains additional non-coordinating water mol-ecules, which are located on twofold rotation axes. O-H⋯O hydrogen bonds between the water mol-ecules as donor and ClO4 tetra-hedra and water mol-ecules as acceptor groups lead to the formation of a three-dimensional network in each of the three structures.

Chemical context

The amount of research into perchlorates has increased considerably in the last few years, beginning with the Phoenix Mars mission (Kim et al., 2013 ▶; Kerr, 2013 ▶; Chevrier et al., 2009 ▶; Quinn et al., 2013 ▶; Davila et al., 2013 ▶; Gough et al., 2011 ▶; Navarro-González & McKay, 2011 ▶; Robertson & Bish, 2011 ▶; Schuttlefield et al., 2011 ▶; Navarro-González et al., 2010 ▶; Marion et al., 2010 ▶; Hecht et al., 2009 ▶). Important perchlorate salts in the martian regolith are Mg and Ca perchlorates. It seemed worthwhile to complete the chemical systematics in this series of alkaline-earth perchlorates. The solubility diagram of strontium perchlorate has been investigated by several authors (Pestova et al., 2005 ▶; Lilich & Djurinskii, 1956 ▶; Nicholson & Felsing, 1950 ▶; Willard & Smith, 1923 ▶) in different temperature and concentration regions. They reported the tetra­hydrate and the hexa­hydrate to be stable phases. While re-investigating the phase diagram, we found at higher temperatures the trihydrate, the tetra­hydrate at room temperature and the nona­hydrate near the eutectic temperature. The existence of the hexa­hydrate could not be confirmed.

Structural commentary

The crystal structure of strontium perchlorate trihydrate contains two crystallographically distinct Sr2+cations. Both are coordinated by five water mol­ecules and four monodentately bonding perchlorate tetra­hedra (Fig. 1 ▶). Four of the five water mol­ecules (O1, O6 and O3, O4) share edges between two Sr2+ cations, resulting in chains with alternating Sr1 and Sr2 cations. The chains extend parallel to [100] (Fig. 2 ▶). The crystal structure of strontium perchlorate tetra­hydrate is similar to the trihydrate, but different to the magnesium analogue (Robertson & Bish, 2010 ▶; Solovyov, 2012 ▶) or mercury perchlor­ate tetra­hydrate (Johansson et al., 1966 ▶). Two symmetry-related Sr2+ cations, both coordinated by five water mol­ecules and four monodentate perchlorate tetra­hedra, form dimers by sharing two water mol­ecules. In strontium perchlorate nona­hydrate, the Sr2+ cation occupies a single crystallographic site with site symmetry m2m. It is coordinated by seven water mol­ecules and two monodentate perchlorate tetra­hedra (point group symmetry ..m; Fig. 3 ▶ a) within a tricapped trigonal-prismatic oxygen coordination environment (Fig. 3 ▶ b). Thereby, the trigonal base planes are chosen such that each oxygen atom of the perchlorate anions represents a capping atom. The third cap is provided by a water oxygen atom.

Figure 1

Coordination around the Sr12+ cation in Sr(ClO4)2·3H2O. Atoms O3 and O4 as well as O6 and O1 are shared between two different Sr2+ cations. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i)  − x, − + y,  − z; (ii) − + x,  − y,  + z.]

Figure 2

Formation of chains parallel [100] by sharing water mol­ecules in the structure of Sr(ClO4)2·3H2O.

Figure 3

(a) Coordination around the Sr2+ cation and (b) the resulting coordination polyhedron in the structure of Sr(ClO4)2·9H2O. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) x, y,  − z; (ii) 2 − x, y, z; (iii) 2 − x, y,  − z.]

Supra­molecular features

In strontium perchlorate trihydrate, chains are formed with alternating Sr2+ cations (Fig. 2 ▶). These zigzag chains are oriented parallel to [100] and are linked by edge-sharing with the perchlorate tetra­hedra (Fig. 4 ▶) into a layered arrangement parallel to (001), as shown in Fig. 5 ▶. Within the structure of the tetra­hydrate, each perchlorate anion coordinates to the dimeric unit of two Sr2+ cations (Fig. 6 ▶). At the same time, it also coordinates to another dimeric unit. Thus, each dimeric unit is connected pairwise by perchlorate anions with four others. This yields in (001) layers stacked along [001], as visualized in Fig. 7 ▶. The nona­hydrate structure contains additional lattice water mol­ecules, which are both donor and acceptor groups, resulting in a tetra­hedral arrangement of O—H⋯O hydrogen bonds. Two hydrogen bonds are formed towards the [SrO2(OH2)7] coordination polyhedra and two towards perchlorate tetra­hedra (Fig. 8 ▶ a, Table 1 ▶). The [SrO2(OH2)7] polyhedra additionally are linked via other O—H⋯O hydrogen bonds. The resulting arrangement can be seen in a larger section of the structure (Fig. 8 ▶ b). O—H⋯O hydrogen bonds also dominate the crystal packing in the two other structures, in each case leading to the formation of a three-dimensional network (Tables 2 ▶ and 3 ▶).

Figure 4

Perchlorate tetra­hedra in the structure of Sr(ClO4)2·3H2O linking the chains (oriented parallel to [100]) into (100) layers.

Figure 5

Zigzag chains parallel to [100] in the structure of Sr(ClO4)2·3H2O, linked by perchlorate tetra­hedra into (100) layers, as viewed along [001].

Figure 6

Formation of dimers in the structure of Sr(ClO4)2·4H2O by sharing two water mol­ecules. [Symmetry code: (i) 1 − x, 2 − y, 1 − z.]

Figure 7

Formation of layers in the structure of Sr(ClO4)2·4H2O, viewed along [100].

Figure 8

(a) Coordination of the lattice water mol­ecules in the structure of Sr(ClO4)2·9H2O by hydrogen bonds. (b) A larger section of the structure in the viewing direction [010]. Dashed lines indicate hydrogen bonds.

Table 1

Hydrogen-bond geometry (, ) for Sr(ClO4)29H2O

DHA	DH	HA	D A	DHA
O1H1BO7i	0.84(1)	2.02(2)	2.844(4)	169(5)
O1H1AO4	0.84(1)	1.98(1)	2.811(4)	170(5)
O2H2AO1ii	0.84(1)	1.99(2)	2.780(4)	156(5)
O3H3AO2i	0.84(1)	2.05(3)	2.851(5)	158(7)
O4H4AO6iii	0.84(1)	2.62(3)	3.337(2)	144(4)
O4H4AO7iv	0.84(1)	2.39(3)	3.041(4)	135(4)

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) .

Table 2

Hydrogen-bond geometry (, ) for Sr(ClO4)23H2O

DHA	DH	HA	D A	DHA
O1H1AO5i	0.84(1)	2.13(4)	2.683(4)	123(4)
O1H1BO18i	0.84(1)	2.07(2)	2.858(4)	158(4)
O2H2BO16ii	0.84(1)	2.10(1)	2.923(4)	169(4)
O2H2AO16	0.84(1)	2.17(2)	2.992(4)	167(7)
O3H3AO18iii	0.84(1)	1.96(1)	2.793(4)	172(4)
O3H3BO17iv	0.84(1)	2.06(2)	2.857(4)	159(4)
O4H4BO21v	0.84(1)	2.15(2)	2.953(4)	161(5)
O4H4AO22vi	0.84(1)	2.42(2)	3.173(4)	150(4)
O6H6AO14iii	0.84(7)	2.56(7)	3.069(4)	121(5)
O6H6AO19vii	0.84(7)	2.19(7)	2.964(4)	155(6)
O6H6BO17viii	0.92(6)	2.09(6)	2.920(4)	150(5)
O7H7AO20ix	0.84(1)	2.31(4)	3.044(4)	146(7)
O7H7AO22vi	0.84(1)	2.44(7)	2.902(4)	116(6)
O7H7BO17viii	0.84(1)	2.48(5)	2.916(4)	114(4)
O7H7BO20v	0.84(1)	2.25(2)	3.071(4)	167(5)

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) ; (vi) ; (vii) ; (viii) ; (ix) .

Table 3

Hydrogen-bond geometry (, ) for Sr(ClO4)24H2O

DHA	DH	HA	D A	DHA
O9H9AO8i	0.84(1)	2.15(2)	2.966(3)	164(4)
O9H9BO8ii	0.84(1)	2.18(2)	2.986(3)	161(5)
O10H10BO4iii	0.84(1)	2.04(2)	2.858(3)	165(6)
O10H10AO4iv	0.84(1)	2.17(2)	2.967(3)	157(5)
O11H11BO9v	0.84(1)	1.99(2)	2.809(3)	164(4)
O11H11AO8	0.84(1)	2.38(3)	3.093(3)	143(5)
O12H12AO7vi	0.84(1)	2.23(2)	2.986(3)	150(4)
O12H12AO10vii	0.84(1)	2.31(4)	2.820(3)	120(3)
O12H12BO4	0.84(1)	2.06(2)	2.875(3)	164(5)

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) ; (vi) ; (vii) .

Database survey

For crystal structures of other M(ClO4)2·3H2O phases, see: Gallucci & Gerkin (1988 ▶; M = Ba); Hennings et al. (2014a ▶; Sn). For crystal structures of other M(ClO4)2·4H2O phases, see: Robertson & Bish (2010 ▶; Mg); Hennings et al. (2014b ▶; Ca); Solovyov (2012 ▶; Mg); Johansson et al. (1966 ▶; Hg).

Synthesis and crystallization

Crystals of Sr(ClO4)2·3H2O were used as purchased (ABCR, 98%). The isolated crystals were stored in a freezer separated and embedded in perfluorinated ether to avoid contact with humidity. Sr(ClO4)2·4H2O crystallized from an aqueous solution of 75.08 wt% Sr(ClO4)2 at 273 K after two days and Sr(ClO4)2·9H2O from an aqueous solution of 60.12 wt% Sr(ClO4)2 at 238 K after one week. For preparing these aqueous solutions, strontium perchlorate trihydrate was used. The Sr2+ content was analyzed per complexometric titration with EDTA. The crystals are stable in the saturated aqueous solutions over a range of at least four weeks. The samples were stored in a freezer or a cryostat at low temperatures and were separated and embedded in perfluorinated ether for X-ray analysis.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 4 ▶. The H atoms of each structure were placed in the positions indicated by difference Fourier maps. For Sr(ClO4)2·3H2O and Sr(ClO4)2·4H2O distance restraints were applied for all water mol­ecules, with O—H and H—H distance restraints of 0.84 (1) and 1.4 (1) Å, respectively. For Sr(ClO4)2·9H2O U iso values were set at 1.2U eq(O) using a riding model approximation. Distance restraints were applied for that structure for all water mol­ecules, with O—H and H—H distance restraints of 0.84 (1) and 1.4 (1) Å, respectively.

Table 4

Experimental details

	Sr(ClO4)23H2O	Sr(ClO4)24H2O	Sr(ClO4)29H2O
Crystal data
M r	340.57	358.58	448.66
Crystal system, space group	Monoclinic, P21/n	Triclinic, P	Orthorhombic, C m c m
Temperature (K)	100	150	100
a, b, c ()	8.9787(6), 13.4870(12), 14.7875(10)	7.1571(6), 7.3942(6), 10.0231(9)	18.7808(15), 6.860(3), 11.1884(16)
, , ()	90, 95.448(5), 90	86.674(7), 86.291(7), 72.027(6)	90, 90, 90
V (3)	1782.6(2)	503.09(8)	1441.5(7)
Z	8	2	4
Radiation type	Mo K	Mo K	Mo K
(mm1)	6.70	5.94	4.20
Crystal size (mm)	0.45 0.34 0.23	0.33 0.25 0.16	0.20 0.11 0.05
Data collection
Diffractometer	Stoe IPDS 2T	Stoe IPDS 2T	Stoe IPDS 2T
Absorption correction	Integration (Coppens, 1970 ▶)	Integration (Coppens, 1970 ▶)	Integration (Coppens, 1970 ▶)
T min, T max	0.081, 0.212	0.187, 0.383	0.015, 0.085
No. of measured, independent and observed [I > 2(I)] reflections	50555, 4941, 3337	10691, 2818, 2650	6877, 1087, 993
R int	0.125	0.065	0.020
(sin /)max (1)	0.650	0.695	0.693
Refinement
R[F 2 > 2(F 2)], wR(F 2), S	0.024, 0.046, 1.09	0.028, 0.076, 1.10	0.048, 0.134, 1.16
No. of reflections	4087	2795	1087
No. of parameters	297	169	70
No. of restraints	15	12	6
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement	All H-atom parameters refined	Only H-atom coordinates refined
max, min (e 3)	0.56, 0.63	0.83, 1.15	1.27, 2.26

Computer programs: X-AREA and X-RED (Stoe Cie, 2009 ▶), SHELXS97 and SHELXL2012 (Sheldrick, 2008 ▶), DIAMOND (Brandenburg, 2006 ▶) and publCIF (Westrip, 2010 ▶).

Crystal structure: contains datablock(s) SrClO4_3H2O_100K, SrClO4_4H2O_150K, SrClO4_9H2O_100K. DOI: 10.1107/S1600536814024726/wm5080sup1.cif

Structure factors: contains datablock(s) SrClO4_3H2O_100K. DOI: 10.1107/S1600536814024726/wm5080SrClO4_3H2O_100Ksup2.hkl

Structure factors: contains datablock(s) SrClO4_4H2O_150K. DOI: 10.1107/S1600536814024726/wm5080SrClO4_4H2O_150Ksup3.hkl

Structure factors: contains datablock(s) SrClO4_9H2O_100K. DOI: 10.1107/S1600536814024726/wm5080SrClO4_9H2O_100Ksup4.hkl

Click here for additional data file.

Supporting information file. DOI: 10.1107/S1600536814024726/wm5080SrClO4_3H2O_100Ksup5.cml

Click here for additional data file.

Supporting information file. DOI: 10.1107/S1600536814024726/wm5080SrClO4_4H2O_150Ksup6.cml

Click here for additional data file.

Supporting information file. DOI: 10.1107/S1600536814024726/wm5080SrClO4_9H2O_100Ksup7.cml

CCDC references: 1033590, 1033589, 1033588

Additional supporting information: crystallographic information; 3D view; checkCIF report

[Sr(ClO4)2(H2O)7]·2H2O	Dx = 2.067 Mg m−3
Mr = 448.66	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Cmcm	Cell parameters from 5894 reflections
a = 18.7808 (15) Å	θ = 8.3–29.6°
b = 6.860 (3) Å	µ = 4.20 mm−1
c = 11.1884 (16) Å	T = 100 K
V = 1441.5 (7) Å3	Prism, colourless
Z = 4	0.2 × 0.11 × 0.05 mm
F(000) = 904

Stoe IPDS 2T diffractometer	1087 independent reflections
Radiation source: fine-focus sealed tube	993 reflections with I > 2σ(I)
Detector resolution: 6.67 pixels mm-1	Rint = 0.020
rotation method scans	θmax = 29.5°, θmin = 3.2°
Absorption correction: integration (Coppens, 1970)	h = −26→26
Tmin = 0.015, Tmax = 0.085	k = −9→9
6877 measured reflections	l = −15→15

Refinement on F2	6 restraints
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.048	Only H-atom coordinates refined
wR(F2) = 0.134	w = 1/[σ2(Fo2) + (0.1P)2] where P = (Fo2 + 2Fc2)/3
S = 1.16	(Δ/σ)max < 0.001
1087 reflections	Δρmax = 1.27 e Å−3
70 parameters	Δρmin = −2.26 e Å−3

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.

	x	y	z	Uiso*/Ueq
Sr1	0.5000	0.05870 (8)	0.2500	0.0096 (2)
Cl1	0.32953 (5)	−0.30996 (15)	0.2500	0.0113 (3)
O7	0.34391 (14)	−0.4247 (4)	0.3560 (2)	0.0177 (5)
O4	0.29063 (18)	0.0000	0.5000	0.0203 (7)
H4A	0.263 (2)	0.041 (7)	0.553 (3)	0.024*
O2	0.5000	−0.2257 (5)	0.4061 (3)	0.0156 (7)
H2A	0.5369 (16)	−0.215 (6)	0.448 (4)	0.019*
O1	0.40632 (13)	0.2050 (4)	0.4020 (2)	0.0147 (5)
H1A	0.3693 (15)	0.146 (6)	0.424 (4)	0.018*
H1B	0.392 (2)	0.315 (3)	0.380 (5)	0.018*
O5	0.3748 (2)	−0.1397 (5)	0.2500	0.0211 (8)
O6	0.2561 (2)	−0.2480 (6)	0.2500	0.0209 (8)
O3	0.5000	0.4459 (6)	0.2500	0.0141 (9)
H3A	0.5000	0.519 (9)	0.310 (4)	0.017*

	U11	U22	U33	U12	U13	U23
Sr1	0.0111 (3)	0.0142 (3)	0.0035 (3)	0.000	0.000	0.000
Cl1	0.0105 (5)	0.0152 (5)	0.0083 (5)	−0.0003 (3)	0.000	0.000
O7	0.0221 (11)	0.0230 (11)	0.0079 (11)	0.0032 (10)	0.0004 (9)	0.0033 (8)
O4	0.0117 (15)	0.0346 (18)	0.0147 (17)	0.000	0.000	0.0023 (16)
O2	0.0144 (14)	0.0219 (15)	0.0106 (16)	0.000	0.000	0.0009 (13)
O1	0.0143 (10)	0.0202 (10)	0.0095 (10)	−0.0006 (9)	0.0009 (8)	−0.0001 (9)
O5	0.0208 (17)	0.0181 (16)	0.0243 (19)	−0.0067 (14)	0.000	0.000
O6	0.0096 (15)	0.0243 (17)	0.029 (2)	0.0032 (12)	0.000	0.000
O3	0.021 (2)	0.0122 (19)	0.009 (2)	0.000	0.000	0.000

Sr1—O2	2.619 (4)	Sr1—O5	2.716 (4)
Sr1—O2i	2.619 (4)	Sr1—O5ii	2.716 (4)
Sr1—O1ii	2.645 (2)	Cl1—O6	1.443 (4)
Sr1—O1i	2.645 (2)	Cl1—O5	1.445 (4)
Sr1—O1iii	2.645 (2)	Cl1—O7	1.449 (3)
Sr1—O1	2.645 (2)	Cl1—O7i	1.449 (3)
Sr1—O3	2.656 (5)
O2—Sr1—O2i	83.68 (16)	O2i—Sr1—O5	68.08 (7)
O2—Sr1—O1ii	81.60 (8)	O1ii—Sr1—O5	139.99 (5)
O2i—Sr1—O1ii	135.37 (6)	O1i—Sr1—O5	67.32 (8)
O2—Sr1—O1i	135.37 (6)	O1iii—Sr1—O5	139.99 (5)
O2i—Sr1—O1i	81.60 (8)	O1—Sr1—O5	67.33 (8)
O1ii—Sr1—O1i	135.38 (11)	O3—Sr1—O5	120.07 (8)
O2—Sr1—O1iii	135.37 (6)	O2—Sr1—O5ii	68.08 (7)
O2i—Sr1—O1iii	81.60 (8)	O2i—Sr1—O5ii	68.08 (7)
O1ii—Sr1—O1iii	80.01 (11)	O1ii—Sr1—O5ii	67.32 (8)
O1i—Sr1—O1iii	83.41 (11)	O1i—Sr1—O5ii	139.99 (5)
O2—Sr1—O1	81.60 (8)	O1iii—Sr1—O5ii	67.32 (8)
O2i—Sr1—O1	135.37 (6)	O1—Sr1—O5ii	139.99 (5)
O1ii—Sr1—O1	83.41 (11)	O3—Sr1—O5ii	120.07 (8)
O1i—Sr1—O1	80.01 (11)	O5—Sr1—O5ii	119.85 (17)
O1iii—Sr1—O1	135.38 (11)	O6—Cl1—O5	108.9 (2)
O2—Sr1—O3	138.16 (8)	O6—Cl1—O7	109.77 (14)
O2i—Sr1—O3	138.16 (8)	O5—Cl1—O7	109.23 (14)
O1ii—Sr1—O3	67.69 (6)	O6—Cl1—O7i	109.77 (14)
O1i—Sr1—O3	67.69 (6)	O5—Cl1—O7i	109.24 (14)
O1iii—Sr1—O3	67.69 (6)	O7—Cl1—O7i	109.9 (2)
O1—Sr1—O3	67.69 (6)	Cl1—O5—Sr1	156.2 (2)
O2—Sr1—O5	68.08 (7)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1B···O7iv	0.84 (1)	2.02 (2)	2.844 (4)	169 (5)
O1—H1A···O4	0.84 (1)	1.98 (1)	2.811 (4)	170 (5)
O2—H2A···O1v	0.84 (1)	1.99 (2)	2.780 (4)	156 (5)
O3—H3A···O2iv	0.84 (1)	2.05 (3)	2.851 (5)	158 (7)
O4—H4A···O6vi	0.84 (1)	2.62 (3)	3.337 (2)	144 (4)
O4—H4A···O7vii	0.84 (1)	2.39 (3)	3.041 (4)	135 (4)