Comment

Hydrated lanthanide(III) sulfates can adopt a number of different compositions, namely M₂(SO₄)₃·9H₂O, M₂(SO₄)₃·8H₂O, M₂(SO₄)₃·5H₂O and M₂(SO₄)₃·4H₂O, and the octahydrated sulfates of lanthanides Ln^III exist as coordination polymers in which sulfate ions act as bridging bidentate and tridentate ligands; the presence of four coordinated water molecules leads to a coordination number of eight for the lanthanide ion (Wickleder, 2002). The unit-cell dimensions for hydrated europium(III) sulfate were reported many years ago (Geller, 1957), and the crystal structure, in space group C2/c, has recently been reported using data collected at 293  K (Wei & Zheng, 2003).

We report here the structure at 120  K. The similarity of the unit-cell dimensions and atomic coordinates at 293 and 120  K indicates that the same phase has been utilized in all of these studies. The aims of the present investigation are the determination of more precise metrical parameters and the determination of the extent of hydration. The structure (Table 1 and Fig. 1) indicates the presence of octacoordinate europium, with distorted square antiprismatic coordination by four water molecules, one O atom from a sulfate ion in the ₂ bonding mode and three O atoms from three different anions in the ₃ bonding mode. The triply bridging anions lie in general positions, while the doubly bridging anions lie on twofold rotation axes.

Compound (I) is, in fact, isostructural not only with yttrium(III) sulfate octahydrate (Held & Wickleder, 2003) but also with the analogous lanthanide sulfates Ln₂(SO₄)₃·8H₂O, where Ln is Ce (Junk et al., 1999), Pr (Ahmed Farag et al., 1981), Nd (Bartl & Rodek, 1983), Sm (Podberezskaya & Borisov, 1976), Dy (Junk et al., 1999), Er (Wickleder, 1999), Yb (Hiltunen & Niinistö, 1976) or Lu (Junk et al., 1999). The coordination polymer in this structure is most readily envisaged as inversion-related pairs of chains comprising alternating cations and triply bridging anions, themselves generated by translation along the [010] direction, which are then linked into sheets by the doubly bridging anions. The coordination-polymer sheets are linked by hydrogen bonds (Table 2) into a continuous three-dimensional framework structure. As noted for the yttrium analogue (Held & Wickleder, 2003), one of the S-O bonds in the triply bridging anion is significantly longer than the others (Table 1).

Some of the lanthanides, such as europium, can also exhibit lower oxidation states in sulfate salts. Thus, for example, europium(II) sulfate has been shown to be anhydrous and to crystallize in space group Pnma (Mayer et al., 1964). Accordingly, the oxidation state of europium in (I) was further confirmed by bond valence sum calculations (Brown, 1992, 2002). A total valence of 3.016 for europium was obtained using the observed Eu-O bond lengths (Table 1) and a bond valence parameter of 2.036  Å for europium (Trzesowska et al., 2004).

Figure 1 ORTEP diagram of (I), showing the coordination geometry around europium, with 50% probability ellipsoids. [Symmetry codes: (a) -x, y, ½  -  z; (b) ½  -  x, ½  -  y, 1  -  z; (c) ½  -  x,   -  y, 1  -  z.]

Figure 2 Crystal structure of (I), showing the sulfate coordination.

Figure 3 Packing diagram of (I), viewed along the c axis.

Experimental

The title compound was obtained during the attempted preparation of a complex between 2,5-diketopiperazine and europium sulfate, in which 2,5-diketopiperazine (0.228  g, 2  mmol) was heated with europium sulfate (0.736  g, 1  mmol) in water (30  ml). The latter was obtained by the action of sulfuric acid on europium oxide. The crystallization of europium sulfate from solution is facilitated in the presence of other ligands (Held & Wickleder, 2003; Wei & Zheng, 2003).

Crystal data

Eu2(SO4)3·8H2O Mr = 736.26 Monoclinic, C2/c a = 13.5029  (3)  Å b = 6.7601  (1)  Å c = 18.2628  (3)  Å = 102.2610  (13)° V = 1629.02  (5)  Å3 Z = 4 Dx = 3.002  Mg  m-3 Mo K radiation Cell parameters from 1869 reflections = 3.1-27.5° = 8.12  mm-1 T = 120  (2)  K Block, colourless 0.10 × 0.08 × 0.06  mm

Data collection

Bruker-Nonius KappaCCD diffractometer with FR591 rotating anode and scans Absorption correction: multi-scan (SADABS; Sheldrick, 2003) Tmin = 0.477, Tmax = 0.616 11  784 measured reflections 1869 independent reflections 1805 reflections with I > 2(I) Rint = 0.037 max = 27.5° h = -17 17 k = -8 8 l = -22 23

Refinement

Refinement on F2 R[F2 > 2(F2)] = 0.019 wR(F2) = 0.054 S = 1.26 1869 reflections 114 parameters H-atom parameters constrained w = 1/[2(Fo2) + (0.0218P)2 + 6.1179P] where P = (Fo2 + 2Fc2)/3 (/)max = 0.002 max = 0.71  e  Å-3 min = -1.32  e  Å-3

Table 1 Selected interatomic distances(Å) Eu1-O1 2.450  (3) Eu1-O2 2.390  (2) Eu1-O3 2.505  (2) Eu1-O4 2.350  (3) Eu1-O11 2.384  (2) Eu1-O13 2.461  (2) Eu1-O14 2.336  (2) Eu1-O21 2.339  (2) S1-O11i 1.473  (2) S1-O12 1.459  (3) S1-O13 1.499  (2) S1-O14ii 1.463  (2) S2-O21 1.470  (2) S2-O22 1.478  (2) Symmetry codes: (i) ; (ii) .

Table 2 Hydrogen-bonding geometry (Å, °) D-HA D-H HA DA D-HA O1-H11O3i 0.84 2.08 2.910  (3) 170 O1-H12O13ii 0.84 2.06 2.771  (3) 143 O2-H21O12iii 0.84 1.87 2.704  (4) 171 O2-H22O22iv 0.84 1.99 2.814  (3) 169 O3-H31O22 0.84 1.96 2.759  (3) 159 O3-H32O13iii 0.84 2.16 2.994  (3) 170 O4-H41O12v 0.84 1.93 2.748  (4) 163 O4-H42O22vi 0.84 1.97 2.788  (4) 165 Symmetry codes: (i) ; (ii) ; (iii) -x,1-y,1-z; (iv) x,1+y,z; (v) ; (vi) .

All H atoms were located in difference maps and then allowed to ride on their parent atoms, with O-H distances of 0.84  Å and with U_iso(H) = 1.2U_eq(O).

Data collection: COLLECT (Hooft, 1999); cell refinement: DENZO (Otwinowski & Minor, 1997) and COLLECT; data reduction: DENZO and COLLECT; program(s) used to solve structure: OSCAIL (McArdle, 2003) and SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: OSCAIL and SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 2003); software used to prepare material for publication: SHELXL97 and PRPKAPPA (Ferguson, 1999).