Potassium cerium(III) bis­(sulfate) monohydrate, KCe(SO₄)₂·H₂O

The crystal structure of potassium cerium(III) bis­(sulfate) monohydrate, KCe(SO₄)₂·H₂O, is built up from irregular independent SO₄ tetra­hedra, CeO₉ polyhedra in the form of distorted tricapped trigonal prisms and K⁺ ions. Hydrogen bonding between the free water mol­ecule and sulfate groups supplement the ionic bonds characteristic of the rest of the structure.

S1. Comment

Recently, there has been a great deal of interest in rare earth complexes because of their important physical properties, including magnetism and luminescence (Wegh et al., 2000). Luminescence studies of Ce-doped compounds have demonstrated the fact that trivalent cerium ions give good performance when compared with all other rare earth ions (Nair et al., 2000). Ce gives a different thermostimulated luminescence peak structure probably related to the Ce³⁺–Ce²⁺ redox mechanism. In previous studies (Guillou et al., 1993; Audebrand et al., 1998), it was shown that those trivalent Ce compounds could be obtained from a solution of hydrated Ce^IV sulfate or oxide in nitric acid. An alternative synthesis of a new compound under safer conditions (approximately neutral pH) was encountered while investigating the system Ce(SO₄)₂·H₂O–KSCN–H₂O. The high oxidation power of the Ce^IV ion towards the SCN⁻ group leads to the formation of the new compound, shown here to be potassium cerium(III) bis(sulfate) monohydrate, KCe(SO₄)₂·H₂O.

The structure of the title compound comprises CeO₉ polyhedra and SO₄ tetrahedra, which are linked by common edges and vertices forming a semi-infinite three-dimensional network. K⁺ ions occur in structural channels along [010] (Fig. 1).

The coordination environments of the rare earth ions consist of nine O atoms in a configuration best described as an irregular tricapped trigonal prism (Fig. 2a). The nine O atoms are associated with one water molecule and six sulfate groups, two of which share common edges with the CeO₉ polyhedra. Each sulfate group bridges three Ce polyhedra (Fig. 3). The Ce—O bond lengths, ranging from 2.432 (3) to 2.697 (4) Å (Table 1), are similar in length to the analogous distances found in NaCe(SO₄)₂·H₂O (Blackburn & Gerkin, 1995) and RbCe(SO₄)₂·H₂O (Robinson & Jasly, 1998). The mean Ce—O distance (2.549 Å) is also in good agreement with the value (2.53 Å) calculated by the bond-valence method (Brown, 1981) for Ce^III bonded to nine O atoms.

The ninefold configuration of Ce^III in KCe(SO₄)₂·H₂O is typical of the majority of monohydrated alkali metal rare earth sulfate complexes and of rare earth ions in general. For early members of this particular rare earth series, the anionic part of the structure around the rare earth ions is relatively stable, with a coordination number of nine for Ce, Pr, La and Nd (Blackburn & Gerkin, 1994, 1995; Iskhakova et al., 1985, 1988). For later members of this sulfate series, such as Gd (Sarukhanyan et al., 1984), the coordination number decreases to eight, presumably in association with the lanthanide contraction. The rare earth configuration also varies slightly for hydrated alkali metal rare earth chloride complexes involving organic cations on the anionic structure (Mackenstedt & Urland, 1994). The change of the coordination number from nine in the heptahydrates to eight in the hexahydrates of the rare earth chlorides is also relevant (Kepert et al., 1994). Furthermore, the coordination number decreases from nine (in the case of La) (Reuter & Frenzen, 1994) to eight (in the case of Ce—Tb) (Reuter et al., 1994) for the trihydrates of these chlorides.

The two crystallographically inequivalent S atoms are each surrounded by four O atoms, establishing sulfate anions that are without symmetry contraints and which consequently form slightly irregular tetrahedra. The S1- and S2-centred sulfate tetrahedra stack in planes normal to [100], which alternate with respect to each other (Fig. 4). Both S1 and S2 form two shorter, one medium and one longer S—O bond, which range from 1.460 (4) to 1.480 (4) Å and from 1.465 (4) to 1.480 (4) Å in length, respectively, for the S1 and S2 tetrahedra. The O—S—O bond angles are consistent with slightly distorted tetrahedra, ranging from 106.8 (3) to 111.6 (2)° and 105.9 (3) to 111.6 (2)°, respectively, for S1 and S2 tetrahedra (Table 1). Furthermore, the smallest O—S—O angle for both the S1 and the S2 tetrahedron corresponds to the shared O—O edges with the CeO₉ polyhedra.

The K⁺ cations are surrounded by six O atoms, including one water O atom (Fig. 2b). These atoms form an irregular KO₆ triangular prism polyhedron; in fact, the K—O distances range from 2.807 (3) to 2.980 (6) Å (Table 1). In the analogous compound LiCe(SO₄)₂·H₂O (Iskhakova et al., 1987), the monovalent cation, with coordination number four, forms LiO₄ tetrahedra. The reduction in radii between K⁺ and Li⁺ ions is associated with a structural phase change from the compact three-dimensional framework of the present KCe(SO₄)₂·H₂O structure to the layered structure of LiCe(SO₄)₂·H₂O

In contrast to other structural analogues, the water molecule of KCe(SO₄)₂·H₂O appears to be well ordered, being characterized by an O atom whose displacement parameters are consistent with those of other well ordered atoms in the structure. For the NaCe(SO₄)₂·H₂O, NaLa(SO₄)₂·H₂O (Blackburn & Gerkin, 1994, 1995) and AgCe(SO₄)₂·H₂O (Audebrand et al., 1998) [rather than 1997?] analogues, the water O atom was better modelled as disordered equally over two sites.

According to the classical geometry of hydrogen bonds, water molecules are linked to sulfate anions by O—H···O hydrogen bonds (Table 2). Each sulfate tetrahedron establishes one hydrogen bond with a water molecule via the O atom coresponding to the longer S—O bonds, namely atoms O4 and O7, respectively, for the S2 and S1 tetrahedra. O—H···O hydrogen bonds act in the same way as the ionic contacts to provide connectivity between the cerium polyhedra and the sulfate anions, thereby improving the stability of the structure.

S2. Experimental

Crystals of the title compound were synthesized at room temperature by mixing aqueous solutions of Ce(SO₄)₂·4H₂O and KSCN. The solutions were mixed until the yellow color was no longer detected. The colorless solution was allowed to evaporate at room temperature, and within a few days, colorless crystals of KCe(SO₄)₂·H₂O had formed. The compound was characterized by elemental chemical analysis and thermogravimetric studies to determine the formula, which was later confirmed by refinement of the crystal structure.

S3. Refinement

All H atoms were located from difference Fourier maps and were refined isotropically.

Computing details

Data collection: program (reference)?; cell refinement: program (reference)?; data reduction: Jana2000 (Petricek & Dusek, 2000); program(s) used to solve structure: SIR97 (Altomare et al., 1999; program(s) used to refine structure: Jana2000; molecular graphics: Diamond (Brandenburg & Berndt, 1999); software used to prepare material for publication: Jana2000.

Figures

	Fig. 1. : A projection of the structure of KCe(SO4)2·H2O along the c axis. Large circles represent K atoms.
	Fig. 2. : A view of the environment of (a) the Ce atom and (b) the K atom, showing the atom-numbering scheme.
	Fig. 3. : A projection of the structure of KCe(SO4)2·H2O along the b axis.
	Fig. 4. : A projection of the structure of KCe(SO4)2·H2O along the c axis. CeO9 polyhedra have been omitted.

potassium cerium bis(sulfate) monohydrate

Crystal data

KCe(SO4)2·H2O	F(000) = 732
Mr = 389.3	Dx = 3.258 (2) Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: -P 2ybc	Cell parameters from 3322 reflections
a = 10.1163 (2) Å	θ = 2.0–27.0°
b = 8.588 (4) Å	µ = 6.81 mm^−1
c = 10.3823 (6) Å	T = 293 K
β = 118.39 (3)°	Needle, colorless
V = 793.5 (4) Å^3	0.1 × 0.05 × 0.03 mm
Z = 4

Data collection

Oxford Instruments point-detector diffractometer	1267 reflections with I > 3σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.034
Graphite monochromator	θmax = 27.0°, θmin = 2.3°
θ/2θ scans	h = −12→11
Absorption correction: gaussian (Jana2000; Petricek and Dusek, 2000)	k = −10→10
Tmin = 0.483, Tmax = 0.595	l = 0→13
3334 measured reflections	Standard reflections: no; every no reflections
1729 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Weighting scheme based on measured s.u.'s w = 1/(σ^2(I) + 0.0004I^2)
R[F^2 > 2σ(F^2)] = 0.020	(Δ/σ)max = 0.015
wR(F^2) = 0.055	Δρmax = 1.40 e Å^−3
S = 1.18	Δρmin = −1.71 e Å^−3
1729 reflections	Extinction correction: B-C type 1 Lorentzian isotropic (Becker & Coppens, 1974)
127 parameters	Extinction coefficient: 0.064491
All H-atom parameters refined

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

	x	y	z	Uiso*/Ueq
Ce1	0.23611 (3)	0.33680 (3)	0.46527 (3)	0.01018 (9)
K1	0.28298 (12)	0.68638 (14)	0.80556 (13)	0.0281 (4)
S1	−0.01646 (12)	0.68645 (12)	0.38915 (11)	0.0118 (4)
S2	0.45502 (11)	0.13151 (12)	0.82423 (12)	0.0120 (4)
O1	0.1973 (4)	0.0388 (4)	0.4717 (4)	0.0232 (15)
O2	−0.1656 (3)	0.6124 (4)	0.3303 (4)	0.0230 (13)
O3	0.4561 (4)	0.2443 (4)	0.9319 (3)	0.0189 (13)
O4	0.3277 (3)	0.0253 (4)	0.7909 (3)	0.0185 (13)
O5	0.5985 (4)	0.0474 (4)	0.8861 (4)	0.0228 (14)
O6	0.4348 (4)	0.2103 (4)	0.6909 (3)	0.0185 (13)
O7	−0.0276 (3)	0.8195 (4)	0.2941 (3)	0.0200 (13)
O8	0.0242 (4)	0.7395 (4)	0.5379 (3)	0.0210 (14)
O9	0.0952 (3)	0.5791 (4)	0.3888 (4)	0.0207 (14)
H1	0.214 (7)	0.015 (8)	0.555 (8)	0.05 (2)*
H2	0.107 (9)	−0.024 (9)	0.415 (8)	0.07 (2)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ce1	0.00844 (12)	0.01115 (12)	0.01038 (12)	0.00021 (10)	0.00400 (8)	0.00070 (11)
K1	0.0221 (5)	0.0349 (6)	0.0263 (6)	−0.0039 (5)	0.0107 (5)	−0.0019 (5)
S1	0.0106 (5)	0.0130 (5)	0.0120 (5)	0.0025 (4)	0.0055 (4)	0.0016 (4)
S2	0.0102 (5)	0.0157 (5)	0.0101 (5)	0.0031 (4)	0.0049 (4)	0.0035 (4)
O1	0.025 (2)	0.0197 (17)	0.024 (2)	−0.0042 (14)	0.0106 (17)	−0.0019 (15)
O2	0.0123 (16)	0.0293 (18)	0.0262 (19)	−0.0085 (13)	0.0083 (14)	−0.0108 (15)
O3	0.0196 (17)	0.0235 (17)	0.0153 (17)	−0.0016 (13)	0.0098 (14)	−0.0036 (13)
O4	0.0178 (16)	0.0193 (16)	0.0197 (18)	−0.0027 (13)	0.0099 (14)	−0.0008 (13)
O5	0.0187 (17)	0.0296 (18)	0.0221 (19)	0.0097 (14)	0.0113 (15)	0.0094 (14)
O6	0.0193 (16)	0.0242 (17)	0.0153 (16)	0.0045 (13)	0.0110 (14)	0.0067 (13)
O7	0.0212 (16)	0.0208 (16)	0.0175 (16)	0.0045 (14)	0.0088 (14)	0.0068 (14)
O8	0.0244 (18)	0.0251 (17)	0.0124 (17)	−0.0009 (14)	0.0078 (15)	−0.0033 (13)
O9	0.0160 (16)	0.0201 (17)	0.027 (2)	0.0065 (13)	0.0105 (15)	0.0025 (14)

Geometric parameters (Å, º)

Ce1—O1	2.595 (4)	S1—O2	1.475 (3)
Ce1—O2^i	2.578 (5)	S1—O7	1.479 (4)
Ce1—O3^ii	2.508 (4)	S1—O8	1.469 (3)
Ce1—O4^ii	2.671 (4)	S1—O9	1.459 (4)
Ce1—O5^iii	2.449 (3)	S1—H2^vii	2.74 (8)
Ce1—O6	2.496 (3)	S2—O3	1.475 (4)
Ce1—O7^iv	2.514 (2)	S2—O4	1.479 (3)
Ce1—O8^i	2.699 (4)	S2—O5	1.468 (4)
Ce1—O9	2.432 (3)	S2—O6	1.465 (4)
Ce1—H1	2.96 (7)	S2—H1	2.88 (6)
K1—O1^v	2.981 (5)	O1—O4	2.929 (5)
K1—O2^i	2.897 (3)	O1—O6	2.815 (4)
K1—O3^vi	2.812 (3)	O1—O7^viii	2.852 (4)
K1—O4^vii	2.960 (4)	O1—O8^i	2.906 (6)
K1—O6^iii	2.845 (4)	O1—H1	0.82 (8)
K1—O8	2.805 (3)	O1—H2	0.98 (7)
K1—H2^v	2.89 (10)
O1—Ce1—O2^i	92.36 (13)	O6—Ce1—O7^iv	150.53 (11)
O1—Ce1—O3^ii	83.38 (13)	O6—Ce1—O8^i	104.56 (12)
O1—Ce1—O4^ii	124.07 (13)	O6—Ce1—O9	139.83 (12)
O1—Ce1—O5^iii	141.14 (10)	O6—Ce1—H1	56.3 (11)
O1—Ce1—O6	67.09 (10)	O7^iv—Ce1—O8^i	69.86 (10)
O1—Ce1—O7^iv	84.79 (10)	O7^iv—Ce1—O9	68.77 (11)
O1—Ce1—O8^i	66.55 (12)	O7^iv—Ce1—H1	97.6 (12)
O1—Ce1—O9	141.24 (11)	O8^i—Ce1—O9	77.68 (11)
O2^i—Ce1—O3^ii	140.52 (8)	O8^i—Ce1—H1	63.7 (15)
O2^i—Ce1—O4^ii	143.52 (11)	O9—Ce1—H1	141.4 (15)
O2^i—Ce1—O5^iii	74.32 (13)	O2—S1—O7	108.58 (17)
O2^i—Ce1—O6	73.47 (12)	O2—S1—O8	106.8 (2)
O2^i—Ce1—O7^iv	118.18 (10)	O2—S1—O9	111.56 (19)
O2^i—Ce1—O8^i	53.18 (9)	O2—S1—H2^vii	139.5 (16)
O2^i—Ce1—O9	77.01 (13)	O7—S1—O8	110.84 (19)
O3^ii—Ce1—O4^ii	54.05 (10)	O7—S1—O9	107.5 (2)
O3^ii—Ce1—O5^iii	84.66 (13)	O7—S1—H2^vii	41.4 (15)
O3^ii—Ce1—O6	68.74 (12)	O8—S1—O9	111.6 (2)
O3^ii—Ce1—O7^iv	100.56 (10)	O8—S1—H2^vii	74.2 (18)
O3^ii—Ce1—O8^i	148.83 (11)	O9—S1—H2^vii	105 (2)
O3^ii—Ce1—O9	127.83 (13)	O3—S2—O4	105.9 (2)
O4^ii—Ce1—O5^iii	75.31 (13)	O3—S2—O5	110.0 (2)
O4^ii—Ce1—O6	116.43 (11)	O3—S2—O6	111.1 (2)
O4^ii—Ce1—O7^iv	71.64 (10)	O3—S2—H1	131.5 (16)
O4^ii—Ce1—O8^i	138.62 (8)	O4—S2—O5	111.7 (2)
O4^ii—Ce1—O9	74.80 (13)	O4—S2—O6	110.44 (18)
O4^ii—Ce1—H1	136.9 (18)	O4—S2—H1	46.9 (18)
O5^iii—Ce1—O6	74.10 (11)	O5—S2—O6	107.8 (3)
O5^iii—Ce1—O7^iv	133.83 (11)	H1—O1—H2	99 (7)
O5^iii—Ce1—O8^i	123.88 (14)	O1—H1—O4	156 (6)
O5^iii—Ce1—O9	72.10 (10)	O1—H2—O7^viii	163 (9)
O5^iii—Ce1—H1	128.5 (12)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···O4	0.82 (8)	2.16 (7)	2.929 (5)	156 (6)
O1—H2···O7^viii	0.98 (7)	1.90 (7)	2.852 (4)	163 (9)

Symmetry code: (viii) x, y−1, z.