Pr4N2S3 and Pr4N2Se3:: Two non-isostructural praseodymium(III) nitride chalcogenides

The non-isostructural nitride chalcogenides of praseodymium, Pr4N2S3 and Pr4N2Se3, are formed by the reaction of the praseodymium metal with sodium azide (NaN3), praseodymium trihalide (PrX3; X = Cl, Br, I) and the respective chalcogen (sulfur or selenium) at 900degreesC in evacuated silica ampoules after seven days. Both crystallize monoclinically in space group C2/c (Pr4N2S3: a = 1788.57(9), b = 986.04(5), c = 1266.49(6) pm, beta = 134.546(7)degrees, Z = 8; Pr4N2Se3: a = 1311.76(7), b = 1017.03(5), c = 650.42(3) pm, beta = 90.114(6)degrees, Z = 4). The crystal structures of both compounds show a layered construction, dominated by N3--centred (Pr3+)(4) tetrahedra which share a common edge first. Continuing linkage of the so resulting bitetrahedral [N2Pr6](12+) units via the non-connected vertices to layers according to (2)(infinity){[N(Pr)(2/2)(e)(Pr')(2/2)(v)](3+)} forms different kinds of tetrahedral nets which can be described as layers consisting of "four- and eight-rings" for Pr4N2S3 and as layers of "six-rings" for Pr4N2Se3, Whereas the crystal structure of (PrNS3)-N-4-S-2 exhibits four different Pr3+ cations with coordination numbers of six (2X) and seven (2X) against N3- and S2-, the number of cations in the nitride selenide (Pr4N2Se3) is reduced to half (Pr1 and Pr2) also having six- and sevenfold anionic coordination spheres. Further motifs for the connection of [NM4](9+) tetrahedra in crystal structures of nitride chalcogenides and halides of the rare-earth elements with ratios of N : M = 1 : 2 are presented and discussed.