Dieuropium(III) trisulfate octahydrate: a redetermination at 120 K

School of Chemical and Biotechnology, Shanmuga Arts, Science, Technology and Research Academy (SASTRA), Tirumalaisamudram, Thanjavur 613 402, India, Department of Chemistry, Bharathidasan University, Tiruchirappalli, Tamil Nadu 620 024, India, Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen AB24 3UE, Scotland, and School of Chemistry, University of St Andrews, Fife KY16 9ST, Scotland

The title compound, Eu 2 (SO 4 ) 3 Á8H 2 O, crystallizes in space group C2/c, with one of the anions lying on a twofold rotation axis and the other in a general position, and is best formulated as [Eu(H 2 O) 4/1 (SO 4 ) 3/3 (SO 4 ) 1/2 ] 2 , where one of the anions lies across a twofold axis. The coordination environment of Eu III consists of four water molecules and four sulfate ions. All the water molecules and sulfate ions are involved in hydrogenbonding interactions. The structure is similar to that previously determined at 293 K [Wei & Zheng (2003). Z. Kristallogr. New Cryst. Struct. 218, 277±278], but the cell parameters and the interatomic distances are more precise in the present determination.

Comment
Hydrated lanthanide(III) sulfates can adopt a number of different compositions, namely M 2 (SO 4 ) 3 Á9H 2 O, M 2 (SO 4 ) 3 Á-8H 2 O, M 2 (SO 4 ) 3 Á5H 2 O and M 2 (SO 4 ) 3 Á4H 2 O, 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.
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 con®rmed by bond valence sum calculations (Brown, 1992(Brown, , 2002. A total valence of 3.016 for europium was obtained using the observed EuÐO bond lengths (Table 1) and a bond valence parameter of 2.036 A Ê for europium (Trzesowska et al., 2004).

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).

Figure 3
Packing diagram of (I), viewed along the c axis.
All H atoms were located in difference maps and then allowed to ride on their parent atoms, with OÐH distances of 0.84 A Ê and with U iso (H) = 1.2U eq (O).