Octaammine Eu^II and Yb^II Azides and Their Thermal Decompositions to the Nitrides

Abstract

The common preparation for many nitrides is the synthesis from the corresponding metals and nitrogen at quite high temperatures and/or high pressures. Here we present a route to metal nitrides by the use of ammine metal azides under relatively mild conditions. Europium(II) and ytterbium(II) azides are prepared in liquid ammonia at –36 °C in form of their temperature‐sensitive octaammine complexes. These were investigated by single‐crystal X‐ray diffraction at low temperatures, and their structures seem to be the first evidence for the existence of homoleptic ammine EuII and YbII complexes, as well as that the coordination number of these divalent cations can go beyond six with NH3 ligands. In one of the cases presented here the observed coordination polyhedron is better described as a bicapped trigonal prism (C2v), in one case better as square‐antiprismatic (D4d). Warming of these compounds to room temperature leads to the lanthanoid metal azides still containing approximately 1 equiv. of ammonia. The behaviour of these azides towards further heating was investigated: By very careful and slow decomposition, the nitrides of europium(III) and ytterbium(III) are obtained at only 230 °C at ambient pressure. This method may be suitable to obtain other metal nitrides at remarkably low temperatures and pressures.

Reactions in Anhydrous Liquid Ammonia: Syntheses and Crystal Structures of [M(NH3)8]I2 (M = Eu, Yb) with Bicapped Trigonal‐Prismatic Octaammine Lanthanoid(II) Cations

“…The Eu-N bond lengths within the trigonal prism are 2 ϫ 2.7132 15 The Yb-N bond lengths in the compound [Yb(NH 3 ) 8 ](N 3 ) 2 were observed from 2.599(12) to 2.660(15) Å within the trigonal prism, and from 2.697(10) to 2.758(17) Å for the capping ammine ligands. [8] Thus, the bond lengths are well in agreement with those of [Yb(NH 3 ) 8 ]I 2 presented here. To the best of our knowledge only very little structural research has been conducted on NH 3 complexes of divalent Eu and Yb complexes.…”

“…However, this multidentate aza-cryptand complex is quite unsuitable for comparison to our system as a bicapped trigonal antiprism with much larger Eu-N distances has been reported. [8] Other Eu II complexes like [Eu(C 12 H 8 N) 2 ], which forms onedimensional infinite chains with a coordination number of six for the metal atom, show Eu II -N distances in the range of 2.53 to 2.68 Å. [30] In our case the Eu II -N distances are 2 ϫ 2.7132 (15) complexes are present.…”

“…As we observed a bicapped trigonal-prismatic coordination sphere for the Yb cations of the compound [Yb(NH 3 ) 8 ](N 3 ) 2 , but a square antiprismatic one for its ammoniate [Yb(NH 3 ) 8 ](N 3 ) 2 •2NH 3 , it seems that the energy barrier between the two isomers is quite small. [8] The small energy difference has been reported and previously discussed in some detail. [21][22][23][24] Comparing the Eu II -N atom distances with other compounds is not simple as mononuclear complexes of Eu II in coordination number eight are not frequent.…”

“…We characterized them structurally by X-ray diffraction on single crystals. [8] Thus, it was for the first time that compounds of Eu II and Yb II were reported with coordination numbers larger than six at the metal atoms with simple ammine ligands. [8] We now have obtained analogous iodides of Eu II and Yb II that also show coordination number eight prismatic coordination polyhedron (point group S 8 ) to be energetically favored over the bicapped trigonal prism and the latter is not even a true local minimum.…”

“…[8] Thus, it was for the first time that compounds of Eu II and Yb II were reported with coordination numbers larger than six at the metal atoms with simple ammine ligands. [8] We now have obtained analogous iodides of Eu II and Yb II that also show coordination number eight prismatic coordination polyhedron (point group S 8 ) to be energetically favored over the bicapped trigonal prism and the latter is not even a true local minimum. Obviously, hydrogen bonding and eventually other weak interactions have an impact on the observed bicapped trigonal-prismatic coordination sphere of the [M(NH 3 ) 8 ] 2+ cations in the solid state.…”