Pr4S3[AsS3]2: Praseodym(III)-Sulfid-Thioarsenat(III) mit Ketten und Schichten aus kondensierten [SPr4]-Tetraedern
Résumé
Single crystals of Pr4S3[AsS3]2 can be obtained through the reaction between the elements praseodymium, arsenic and sulfur using CsCl as fluxing agent in torch-sealed evacuated silica ampoules. The starting materials were heated for ten days at 700 °C and then cooled down slowly to room temperature. The transparent, pale greenish yellow platelets crystallize orthorhombically (a = 2910.6(2), b = 699.74(5), c = 704.51(5) pm) in space group Pbcn (no. 60) with Z = 4. The new compound Pr4As2S9 (≡ Pr4S3[AsS3]2) contains two crystallographically independent Pr3+ cations. (Pr1)3+ shows a bicapped trigonal prismatic environment of eight sulfur atoms (CN = 8), whereas (Pr2)3+ is surrounded by only seven sulfur atoms in the shape of a monocapped trigonal prism (CN = 7). The isolated pyramidal thioarsenate(III) anions [AsS3]3– are formed as ψ1-tetrahedra by the three sulfur atoms (d(As3+–S2−) = 223 – 228 pm, ∢(S–As–S) = 94 – 104°) and a non-binding electron pair (lone pair)at each As3+ cation. Two of the five crystallographically different sulfur atoms (S1 and S2) are not connected to As3+ at all, but coordinated by four Pr3+ cations each to build up slightly distorted [SPr4]10+ tetrahedra (d(S2−–Pr3+) = 281 – 289 pm, ∢(Pr–S–Pr) = 98 – 146°). Thus the crystal structure of Pr4S3[AsS3]2 exhibits endless {[(S1)Pr4/2]4+} chains and {[(S2)Pr4/4]+} layers, which consist of trans-edge condensed [(S1)Pr4]10+ tetrahedra for the chains and [(S2)Pr4]10+ tetrahedra sharing four common edges for layers. The lone pairs at the As3+ cations close around the {[(S1)Pr4/2]4+} chains running along [001]. The {[(S2)Pr4/4]+} layers spread out parallel to the (100) plane and complete the crystal structure of Pr4S3[AsS3]2 according to {[(S1)(Pr1)4/2]4+} {([(S2)(Pr2)4/4]+)2} {([As(S3)(S4)(S5)]3–)2}.
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