Synthesis and crystal structure of a new polymorph of potassium europium(III) bis­(sulfate) mono­hydrate, KEu(SO₄)₂·H₂O

The mixed-metal sulfate, KEu(SO₄)₂·H₂O, has been obtained as a new polymorph using hydro­thermal conditions. The crystal structure is isotypic with NaCe(SO₄)₂·H₂O and shows a three-dimensional connectivity of the tetra­hedral sulfate units with Eu^III and K^I ions. Tricapped trigonal–prismatic EuO₉ units and square-anti­prismatic KO₈ units link the SO₄ tetra­hedra, building the three-dimensional structure. Topological analysis reveals the existence of two nodes with 6- and 10-connected nets. The compound was previously reported [Kazmierczak & Höppe (2010). J. Solid State Chem. 183, 2087–2094] in the monoclinic space group P2₁/c with a similar structural connectivity and coordination environments to the present compound.

1. Chemical context

The design of new solids including rare earth metal ions is an emerging field because of their potential applications in catalysis, luminescence and optoelectronics (Ramya et al., 2012; Höppe, 2009; Mahata et al., 2008; Shehee et al., 2003). In general, the discovery of new solids is a major thrust in the field of solid-state research because of their diverse topol­ogical architectures and properties. In particular for rare earth metal compounds, the connectivity within the crystal structure becomes novel and complex as the coordination numbers are higher than for transition metals. In this regard, crystal engin­eering becomes challenging with non-centrosymmetric solids as it can lead to many chiral-related applications such as enanti­oselective separation, heterogeneous chiral catalysis or non-linear optical (NLO) effects (Ramya et al., 2012; Höppe, 2009; Mahata et al., 2008; Shehee et al., 2003; Halasyamani & Poeppelmeier, 1998; Sweeting & Rheingold, 1987). Obtaining new structures with various anions such as silicates, phosphates, phosphites, carboxyl­ates, sulfates, arsenates, selenates, selenites, germanates, borates or thio­sulfates is a long-standing research area (Sweeting et al., 1992; Paul, 2016; Paul & Natarajan, 2010; Paul et al., 2009, 2010; Natarajan & Mandal, 2008; Natarajan et al., 2006; Feng et al., 2005; Hathwar et al., 2011; Held, 2014). A rare earth metal can be a better choice than a transition metal as it provides many variations arising from coordination preferences, ligand geometry and valence states. The presence of two metals in a crystal structure can introduce more structural variation along with specific properties. From earlier reports, it is obvious that the design of chiral frameworks mostly require chiral fragments or chiral ligands. The synthesis of sulfate compounds with a chiral framework is a challenging task that requires a particular strategy. Hence, the synthetic strategy was modified (piperazine was used, which is not in the product but supports the crystallization of the sulfate compound) and the resultant compound is a new polymorph of KEu(SO₄)₂·H₂O that is isotypic with trigonal NaCe(SO₄)₂·H₂O (Blackburn & Gerkin, 1995).

2. Structural commentary

The asymmetric unit of trigonal KEu(SO₄)₂·H₂O contains eight non-hydrogen atoms, of which one Eu, one K and one O site (defining the water mol­ecule) are located on a twofold rotation axis, and one complete sulfate unit. The Eu^III ion is coordinated by the O atoms of six sulfate tetra­hedra (two chelating, four in a monodentate way) and one water mol­ecule in a tricapped-trigonal–prismatic environment. The Eu—O bond lengths range from 2.425 (4) to 2.518 (4) Å with an average of 2.469 Å. The resulting three-dimensional Eu/SO₄ framework is displayed in Fig. 1. The K^I ion is eight-coordin­ated by six sulfate units, again two chelating and four in a monodentate way, leading to a square-anti­prismatic KO₈ coordination polyhedron with K—O distances ranging from 2.374 (5) to 2.830 (4) Å and an average of 2.556 Å. The K^I ions form a similar three-dimensional potassium sulfate framework (Fig. 2). The sulfate ion is an almost regular tetra­hedron with S—O distances ranging from 1.456 (4) to 1.484 (4) Å and O—S—O angles of 105.2 (2)–112.4 (3)°. The overall three-dimensional connectivity between the two metal cations and the sulfate anions is given in Fig. 3. The present framework structure crystallizes isotypically with NaCe(SO₄)₂·H₂O (Blackburn & Gerkin, 1995). It should be noted that the reported structure of NaEu(SO₄)₂·H₂O (Wu & Liu, 2006) shows the same space-group type, very similar lattice parameters, and unexpectedly also very similar Na—O distances in comparison with the K—O distances of the title compound. The previously reported KEu(SO₄)₂·H₂O polymorph crystallizes in space group P2₁/c (Kazmierczak & Höppe, 2010) and in comparison shows a similar connectivity and respective coordination polyhedra.

Figure 1 Three-dimensional framework observed by connectivity between the Eu^III ions and the SO4^2− units. Green, yellow and red spheres represent Eu, S and O sites, respectively.

Figure 2 Three-dimensional framework observed by connectivity between the K^I ions and the SO4^2− units. Cyan, yellow and red spheres represent K, S and O sites, respectively.

Figure 3 Overall three-dimensional connectivity between the Eu^III ions, the K^I ions and the SO4^2− units. Green, cyan, yellow and red spheres represent Eu, K, S and O sites, respectively

3. Supra­molecular features

As the hydrogen-atom positions could not be located during the present study, hydrogen-bonding inter­actions are not discussed here. An inter­esting structural feature arises due to the formation of three kinds of helices along the 3₁ screw axes. A detailed structural analysis of the topology of the framework was performed using TOPOS (Blatov et al., 2014). The EuO₉, KO₈ and SO₄ polyhedra are considered as different nodes and represented in different colors (Fig. 4). Although the potassium and europium cations have different coordination environments, both have similar coordination behaviors, with terminal water only extra for europium. In topological terms, both form similar 10-connected nets with three-, four-, five- and six-membered rings, point symbol 3¹².4¹⁴.5¹².6⁷. The sulfate unit is associated with three-, four- and five-membered rings and forms a 6-connected net with point symbol 3⁶.4⁶.5³. The topological approach thus allows the present complex structure to be visualized in a different way by considering the node-connectivity.

Figure 4 Three-dimensional node connectivity of the title compound. Green, cyan and yellow spheres represent the Eu and K sites and the SO4 unit, respectively.

4. Synthesis and crystallization

The title compound was synthesized under hydro­thermal conditions. All chemicals were purchased from Aldrich and used without further purification. Eu(COOCH₃)₃·xH₂O (0.329 g, 1 mmol) was dissolved in 10 ml water. Then K₂SO₄ (0.348 g, 2 mmol) was added to the solution, which was stirred for 30 mins. Finally, piperazine (0.043 g, 0.5 mmol) was added to the reaction mixture and the pH was observed to be 8. The entire mixture was stirred for another 30 mins and poured into a 23 ml Teflon-lined autoclave. The autoclave was kept at 426 K for 5 d. The product was then filtered off and washed with water. The product contained some block-like single crystals accompanied with a light-yellow powder. The yield was approximately 75% based on Eu metal.

5. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. The correctness of all atom types was checked by free refinement of the occupancy. Hydrogen atoms of the lattice water mol­ecule could not be located in difference-Fourier maps. If the hydrogen atoms were included in calculated positions and refined with a riding model, the structure did not refine with suitable parameters, and therefore the hydrogen atoms were omitted in the final refinement. Except for atom O2 that was refined with an isotropic displacement parameter, all other atoms were refined with anisotropic displacement parameters.

Table 1
Experimental details

Crystal data
Chemical formula	KEu(SO4)2·H2O
Mr	399.18
Crystal system, space group	Trigonal, P3121
Temperature (K)	293
a, c (Å)	6.9065 (2), 12.7802 (5)
V (Å^3)	527.94 (3)
Z	3
Radiation type	Mo Kα
μ (mm^−1)	10.12
Crystal size (mm)	0.14 × 0.12 × 0.08
Data collection
Diffractometer	Bruker SMART CCD area detector
Absorption correction	Multi-scan (SADABS; Sheldrick, 1996)
Tmin, Tmax	0.332, 0.498
No. of measured, independent and observed [I > 2σ(I)] reflections	3856, 848, 823
Rint	0.032
(sin θ/λ)max (Å^−1)	0.674
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.024, 0.058, 1.07
No. of reflections	848
No. of parameters	56
Δρmax, Δρmin (e Å^−3)	1.16, −0.86
Absolute structure	Flack (1983)
Absolute structure parameter	−0.02 (3)
Computer programs: SMART and SAINT (Bruker, 2000), SIR92 (Altomare et al., 1993), SHELXL97 (Sheldrick, 2008), ORTEP-3 for Windows (Farrugia, 2012), CAMERON (Watkin et al., 1993) and PLATON (Spek, 2009).

Supporting information

Computing details

Data collection: SMART (Bruker, 2000); cell refinement: SAINT (Bruker, 2000); data reduction: SAINT (Bruker, 2000); program(s) used to solve structure: SIR92 (Altomare et al., 1993); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and CAMERON (Watkin et al., 1993); software used to prepare material for publication: PLATON (Spek, 2009).

Potassium europium(III) bis(sulfate) monohydrate

Crystal data

KEu(SO4)2·H2O	Dx = 3.767 Mg m^−3
Mr = 399.18	Mo Kα radiation, λ = 0.71073 Å
Trigonal, P3121	Cell parameters from 848 reflections
Hall symbol: P 31 2"	θ = 3.4–28.6°
a = 6.9065 (2) Å	µ = 10.12 mm^−1
c = 12.7802 (5) Å	T = 293 K
V = 527.94 (3) Å^3	Block like, light yellow
Z = 3	0.14 × 0.12 × 0.08 mm
F(000) = 558

Data collection

Bruker SMART CCD area detector diffractometer	848 independent reflections
Radiation source: fine-focus sealed tube	823 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.032
φ and ω scans	θmax = 28.6°, θmin = 3.4°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −9→9
Tmin = 0.332, Tmax = 0.498	k = −8→9
3856 measured reflections	l = −16→17

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	H-atom parameters not defined
R[F^2 > 2σ(F^2)] = 0.024	w = 1/[σ^2(Fo^2) + (0.026P)^2 + 3.0791P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.058	(Δ/σ)max < 0.001
S = 1.07	Δρmax = 1.16 e Å^−3
848 reflections	Δρmin = −0.86 e Å^−3
56 parameters	Absolute structure: Flack (1983)
0 restraints	Absolute structure parameter: −0.02 (3)
Primary atom site location: structure-invariant direct methods

Special details

Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(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
Eu1	0.56354 (7)	0.56354 (7)	0.5000	0.00972 (12)
S1	0.5565 (2)	0.5452 (2)	0.25595 (8)	0.0068 (2)
K1	0.5398 (3)	0.5398 (3)	0.0000	0.0196 (4)
O4	0.3795 (8)	0.5049 (8)	0.1823 (3)	0.0156 (11)
O3	0.6104 (7)	0.7411 (7)	0.3231 (3)	0.0142 (9)
O5	0.4906 (8)	0.3578 (8)	0.3288 (3)	0.0155 (10)
O1	0.9181 (15)	0.9181 (15)	0.5000	0.068 (4)
O2	0.7533 (7)	0.5868 (7)	0.1949 (3)	0.0165 (10)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Eu1	0.01111 (18)	0.01111 (18)	0.00863 (18)	0.0068 (2)	−0.00007 (9)	0.00007 (9)
S1	0.0073 (6)	0.0090 (6)	0.0053 (5)	0.0049 (6)	0.0001 (5)	−0.0008 (4)
K1	0.0254 (8)	0.0254 (8)	0.0167 (8)	0.0191 (10)	0.0004 (4)	−0.0004 (4)
O4	0.013 (2)	0.025 (3)	0.0128 (18)	0.012 (2)	−0.0075 (17)	−0.0030 (18)
O3	0.020 (2)	0.010 (2)	0.0136 (18)	0.008 (2)	−0.0018 (16)	−0.0061 (15)
O5	0.022 (3)	0.016 (2)	0.014 (2)	0.013 (2)	0.0000 (17)	0.0023 (16)
O1	0.024 (4)	0.024 (4)	0.154 (11)	0.012 (4)	−0.017 (3)	0.017 (3)

Geometric parameters (Å, º)

Eu1—O2^i	2.425 (4)	S1—K1^i	3.6182 (15)
Eu1—O2^ii	2.425 (5)	S1—K1^iv	3.6579 (19)
Eu1—O4^iii	2.428 (5)	K1—O5^vii	2.374 (5)
Eu1—O4^iv	2.428 (5)	K1—O5^viii	2.374 (5)
Eu1—O1	2.449 (10)	K1—O3^ix	2.479 (4)
Eu1—O3^v	2.514 (4)	K1—O3^x	2.479 (4)
Eu1—O3	2.514 (4)	K1—O4^xi	2.538 (4)
Eu1—O5	2.518 (4)	K1—O4	2.538 (4)
Eu1—O5^v	2.518 (4)	K1—O2	2.830 (4)
Eu1—S1^v	3.1210 (11)	K1—O2^xi	2.830 (4)
Eu1—S1	3.1210 (11)	K1—S1^xi	3.2726 (11)
Eu1—K1^vi	4.0356 (12)	K1—S1^vii	3.6182 (15)
S1—O4	1.456 (4)	K1—S1^viii	3.6182 (15)
S1—O2	1.465 (4)	O4—Eu1^x	2.428 (5)
S1—O5	1.470 (5)	O3—K1^iv	2.479 (4)
S1—O3	1.484 (4)	O5—K1^i	2.374 (5)
S1—K1	3.2726 (11)	O2—Eu1^vii	2.425 (4)
O2^i—Eu1—O2^ii	77.6 (2)	K1—S1—K1^i	105.93 (4)
O2^i—Eu1—O4^iii	73.28 (14)	O4—S1—K1^iv	81.0 (2)
O2^ii—Eu1—O4^iii	145.65 (16)	O2—S1—K1^iv	121.63 (18)
O2^i—Eu1—O4^iv	145.65 (17)	O5—S1—K1^iv	118.38 (17)
O2^ii—Eu1—O4^iv	73.28 (14)	O3—S1—K1^iv	29.66 (17)
O4^iii—Eu1—O4^iv	139.6 (2)	Eu1—S1—K1^iv	72.58 (2)
O2^i—Eu1—O1	141.21 (10)	K1—S1—K1^iv	105.03 (4)
O2^ii—Eu1—O1	141.21 (10)	K1^i—S1—K1^iv	143.32 (5)
O4^iii—Eu1—O1	69.82 (12)	O5^vii—K1—O5^viii	128.8 (3)
O4^iv—Eu1—O1	69.82 (12)	O5^vii—K1—O3^ix	76.98 (14)
O2^i—Eu1—O3^v	85.25 (14)	O5^viii—K1—O3^ix	153.95 (18)
O2^ii—Eu1—O3^v	124.29 (13)	O5^vii—K1—O3^x	153.95 (18)
O4^iii—Eu1—O3^v	71.15 (14)	O5^viii—K1—O3^x	76.98 (14)
O4^iv—Eu1—O3^v	96.34 (14)	O3^ix—K1—O3^x	77.63 (19)
O1—Eu1—O3^v	72.05 (9)	O5^vii—K1—O4^xi	79.99 (15)
O2^i—Eu1—O3	124.29 (13)	O5^viii—K1—O4^xi	113.79 (14)
O2^ii—Eu1—O3	85.25 (14)	O3^ix—K1—O4^xi	69.93 (14)
O4^iii—Eu1—O3	96.34 (14)	O3^x—K1—O4^xi	85.94 (15)
O4^iv—Eu1—O3	71.15 (14)	O5^vii—K1—O4	113.79 (14)
O1—Eu1—O3	72.05 (9)	O5^viii—K1—O4	79.99 (15)
O3^v—Eu1—O3	144.10 (18)	O3^ix—K1—O4	85.94 (15)
O2^i—Eu1—O5	68.88 (14)	O3^x—K1—O4	69.93 (14)
O2^ii—Eu1—O5	76.38 (15)	O4^xi—K1—O4	149.2 (2)
O4^iii—Eu1—O5	76.45 (15)	O5^vii—K1—O2	64.32 (13)
O4^iv—Eu1—O5	119.87 (14)	O5^viii—K1—O2	99.13 (14)
O1—Eu1—O5	112.47 (11)	O3^ix—K1—O2	88.93 (13)
O3^v—Eu1—O5	143.20 (14)	O3^x—K1—O2	120.92 (13)
O3—Eu1—O5	55.58 (14)	O4^xi—K1—O2	142.03 (14)
O2^i—Eu1—O5^v	76.38 (15)	O4—K1—O2	51.72 (13)
O2^ii—Eu1—O5^v	68.88 (14)	O5^vii—K1—O2^xi	99.13 (14)
O4^iii—Eu1—O5^v	119.87 (14)	O5^viii—K1—O2^xi	64.32 (13)
O4^iv—Eu1—O5^v	76.45 (15)	O3^ix—K1—O2^xi	120.92 (13)
O1—Eu1—O5^v	112.47 (11)	O3^x—K1—O2^xi	88.93 (13)
O3^v—Eu1—O5^v	55.58 (14)	O4^xi—K1—O2^xi	51.72 (13)
O3—Eu1—O5^v	143.20 (14)	O4—K1—O2^xi	142.03 (14)
O5—Eu1—O5^v	135.1 (2)	O2—K1—O2^xi	142.94 (19)
O2^i—Eu1—S1^v	80.93 (10)	O5^vii—K1—S1	89.74 (9)
O2^ii—Eu1—S1^v	96.53 (10)	O5^viii—K1—S1	89.12 (10)
O4^iii—Eu1—S1^v	96.44 (10)	O3^ix—K1—S1	87.27 (10)
O4^iv—Eu1—S1^v	84.68 (10)	O3^x—K1—S1	94.81 (10)
O1—Eu1—S1^v	91.61 (3)	O4^xi—K1—S1	156.52 (12)
O3^v—Eu1—S1^v	27.98 (9)	O4—K1—S1	25.19 (10)
O3—Eu1—S1^v	154.22 (9)	O2—K1—S1	26.53 (9)
O5—Eu1—S1^v	149.79 (11)	O2^xi—K1—S1	151.64 (10)
O5^v—Eu1—S1^v	27.67 (11)	O5^vii—K1—S1^xi	89.12 (10)
O2^i—Eu1—S1	96.53 (10)	O5^viii—K1—S1^xi	89.74 (9)
O2^ii—Eu1—S1	80.93 (10)	O3^ix—K1—S1^xi	94.81 (10)
O4^iii—Eu1—S1	84.68 (10)	O3^x—K1—S1^xi	87.27 (10)
O4^iv—Eu1—S1	96.44 (10)	O4^xi—K1—S1^xi	25.19 (10)
O1—Eu1—S1	91.61 (3)	O4—K1—S1^xi	156.52 (12)
O3^v—Eu1—S1	154.22 (9)	O2—K1—S1^xi	151.64 (10)
O3—Eu1—S1	27.98 (9)	O2^xi—K1—S1^xi	26.53 (9)
O5—Eu1—S1	27.67 (11)	S1—K1—S1^xi	177.34 (9)
O5^v—Eu1—S1	149.79 (11)	O5^vii—K1—S1^vii	15.34 (9)
S1^v—Eu1—S1	176.78 (5)	O5^viii—K1—S1^vii	132.36 (15)
O2^i—Eu1—K1^vi	69.92 (10)	O3^ix—K1—S1^vii	73.36 (10)
O2^ii—Eu1—K1^vi	141.97 (10)	O3^x—K1—S1^vii	144.15 (11)
O4^iii—Eu1—K1^vi	36.57 (10)	O4^xi—K1—S1^vii	64.67 (11)
O4^iv—Eu1—K1^vi	127.33 (11)	O4—K1—S1^vii	127.34 (11)
O1—Eu1—K1^vi	73.44 (2)	O2—K1—S1^vii	79.43 (9)
O3^v—Eu1—K1^vi	35.80 (9)	O2^xi—K1—S1^vii	88.26 (10)
O3—Eu1—K1^vi	129.45 (10)	S1—K1—S1^vii	104.28 (3)
O5—Eu1—K1^vi	108.36 (11)	S1^xi—K1—S1^vii	74.79 (3)
O5^v—Eu1—K1^vi	84.42 (10)	O5^vii—K1—S1^viii	132.36 (15)
S1^v—Eu1—K1^vi	59.86 (3)	O5^viii—K1—S1^viii	15.34 (9)
S1—Eu1—K1^vi	121.20 (3)	O3^ix—K1—S1^viii	144.15 (11)
O4—S1—O2	107.5 (2)	O3^x—K1—S1^viii	73.36 (10)
O4—S1—O5	112.4 (3)	O4^xi—K1—S1^viii	127.34 (11)
O2—S1—O5	111.0 (3)	O4—K1—S1^viii	64.67 (11)
O4—S1—O3	110.6 (3)	O2—K1—S1^viii	88.26 (10)
O2—S1—O3	110.1 (3)	O2^xi—K1—S1^viii	79.43 (9)
O5—S1—O3	105.2 (2)	S1—K1—S1^viii	74.79 (3)
O4—S1—Eu1	130.45 (19)	S1^xi—K1—S1^viii	104.28 (3)
O2—S1—Eu1	121.99 (18)	S1^vii—K1—S1^viii	140.69 (8)
O5—S1—Eu1	52.70 (17)	S1—O4—Eu1^x	144.3 (3)
O3—S1—Eu1	52.64 (16)	S1—O4—K1	106.9 (2)
O4—S1—K1	47.92 (18)	Eu1^x—O4—K1	108.69 (16)
O2—S1—K1	59.63 (17)	S1—O3—K1^iv	133.1 (2)
O5—S1—K1	129.05 (18)	S1—O3—Eu1	99.4 (2)
O3—S1—K1	125.43 (18)	K1^iv—O3—Eu1	107.82 (15)
Eu1—S1—K1	177.55 (5)	S1—O5—K1^i	139.4 (2)
O4—S1—K1^i	106.2 (2)	S1—O5—Eu1	99.6 (2)
O2—S1—K1^i	91.13 (18)	K1^i—O5—Eu1	117.07 (16)
O5—S1—K1^i	25.29 (17)	S1—O2—Eu1^vii	140.9 (3)
O3—S1—K1^i	128.50 (17)	S1—O2—K1	93.8 (2)
Eu1—S1—K1^i	76.13 (3)	Eu1^vii—O2—K1	104.89 (15)

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