The gadolinium nitride selenides Gd3NSe3 and Gd23N5Se27: Three connectivity types of [NGd4]9+ tetrahedra and fivefold coordinated Gd3+ cations

The gadolinium nitride selenides Gd3NSe3 and Gd23N5Se27 are formed by the reaction of gadolinium metal with its triiodide, cesium azide and selenium along with an excess of cesium iodide as flux within seven days at 900 °C in torch-sealed evacuated silica ampoules. The dominant phase Gd3NSe3 crystallizes isotypically with Sm3NS3 in the orthorhombic space group Pnma (unit cell: a = 1256.71(9) pm, b = 398.65(3) pm, c = 1318.32(9) pm, Z = 4). Thus nitride-centered (Gd4)12+ tetrahedra are linked via two vertices each forming {[N(Gd1,2)2/1t(Gd3)2/2v]6+}∞1 chains (t = terminal, v = vertex-shared) along [010], which become interconnected by three crystallographically different Se2− anions. Beside C-type of Gd2Se3 and at least another unknown phase powder diffraction experiments show additional reflections indicating the second nitride selenide Gd23N5Se27. This new compound crystallizes non-centrosymmetrically in the monoclinic space group Pc (unit cell: a = 2854.87(19) pm, b = 1227.94(8) pm, c = 747.62(5) pm, β = 94.215(3)°, Z = 2). Here, {(Gd14Se40[NGd4]2)20–}∞2 and {([N2Gd6]4Se14)20+}∞2 layers embed the main structural features of isolated [NGd4]9+ tetrahedra and isolated congonial [N2Gd6]12+ bitetrahedra. Another particularity is the fivefold coordination of the (Gd4)3+ cations with one short contact to N3− and four relatively short bonds to four Se2− anions, respectively.

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► Three connectivity types of [NGd4]9+ tetrahedra can be observed for two gadolinium nitride selenides.
► In Gd3NSe3, {[NGd2/1tGd′2/2v]6+}∞1 chains of vertex-shared tetrahedra run parallel to the b-axis (top).
► Isolated [NGd4]9+ units and bitetrahedral [N2Gd6]12+ entities (bottom) dominate the crystal structure of Gd23N5Se27.