Inelastic Neutron Scattering Spectroscopy of Diazenides: Detection of the NN Stretch

Abstract

Birds in a cage: The highly absorbent nitrogen‐rich compounds SrN and SrN2 hide their secrets from Raman scattering experiments, but when observed using inelastic neutron scattering, the dinitrogen can be shown to lie trapped within a cage of strontium atoms. Such a structure is shown in the graphic for SrN2, in which the dumbbell‐shaped dinitrogen can be clearly identified.

Thermochemistry of a New Class of Materials Containing Dinitrogen Pairs in an Oxide Matrix

“…The experimental setup was described earlier 2 and Raman spectroscopy confirms the previous XPS results: a single peak close to 2328 cm -1 reveals that the retained nitrogen exists as N 2 entities within a symmetric environment. The Raman peak position compares to that of 2330 cm -1 in N 2 gas, 1380 cm -1 in SrN, and 1307 cm -1 in SrN 2 ((N 2 ) 2- pairs) …”

High-pressure chemistry of nitride-based materials

“…Finally, vibrational modes could be detected by inelastic neutron scattering experiments using the spectrometers TOSCA and MARI at ISIS, and showed that the diazenide ions lie trapped within the cage of the strontium atoms. 291 The NLN stretch of the diazenide ions are assigned at 1380 (SrN) and 1307 (SrN 2 ) cm 21 , respectively.…”

High-Pressure Synthesis and Characterization of Li2Ca3[N2]3—An Uncommon Metallic Diazenide with [N2]2–Ions

“…Surprisingly, given the importance of homonuclear dinitrogen anions in biological and organo-metallic chemistry, it was not before 2001 that such ions have been observed in solid-state chemistry for the first time. − Pioneering works by Kniep et al introduced the hitherto unknown compounds Sr 4 N 3 (≡ Sr 8 N 4 [N 2 ]·(e – ) 2 ), SrN (≡ Sr 8 N 4 [N 2 ] 2 ), SrN 2 , and BaN 2 , which are the first so-called diazenides with ionic [N 2 ] 2– units. The latter show characteristic N–N bond lengths (1.22 Å) and stretching frequencies (1307 cm –1 in SrN 2 and 1380 cm –1 in SrN), which may be compared to protonated diazene N 2 H 2 (1.21–1.25 Å, 1400–1700 cm –1 ). − Only 5 years later, high-pressure/high-temperature (HP/HT) experiments revealed the existence of noble metal compounds with MN 2 stoichiometry (M = Os, Ir, Pd, and Pt) exhibiting ultrahigh hardness and bulk moduli of about 250–350 GPa. − Theoretical investigations finally concluded the presence of tetravalent metals and [N 2 ] 4– anions, with N–N bond lengths (about 1.40 Å) and stretching frequencies (700–1000 cm –1 ) similar to those of hydrazine N 2 H 4 ( d NN = 1.47 Å, ṽ NN < 1000 cm –1 ). − The latter anions are isoelectronic with peroxides [O 2 ] 2– , and so they were dubbed “pernitrides”.…”

“…Surprisingly, given the importance of homonuclear dinitrogen anions in biological and organo-metallic chemistry, it was not before 2001 that such ions have been observed in solid-state chemistry for the first time. − Pioneering works by Kniep et al introduced the hitherto unknown compounds Sr 4 N 3 (≡ Sr 8 N 4 [N 2 ]·(e – ) 2 ), SrN (≡ Sr 8 N 4 [N 2 ] 2 ), SrN 2 , and BaN 2 , which are the first so-called diazenides with ionic [N 2 ] 2– units. The latter show characteristic N–N bond lengths (1.22 Å) and stretching frequencies (1307 cm –1 in SrN 2 and 1380 cm –1 in SrN), which may be compared to protonated diazene N 2 H 2 (1.21–1.25 Å, 1400–1700 cm –1 ). − Only 5 years later, high-pressure/high-temperature (HP/HT) experiments revealed the existence of noble metal compounds with MN 2 stoichiometry (M = Os, Ir, Pd, and Pt) exhibiting ultrahigh hardness and bulk moduli of about 250–350 GPa. − Theoretical investigations finally concluded the presence of tetravalent metals and [N 2 ] 4– anions, with N–N bond lengths (about 1.40 Å) and stretching frequencies (700–1000 cm –1 ) similar to those of hydrazine N 2 H 4 ( d NN = 1.47 Å, ṽ NN < 1000 cm –1 ). − The latter anions are isoelectronic with peroxides [O 2 ] 2– , and so they were dubbed “pernitrides”. In 2012, we were able to extend the compositional range of binary diazenides by subjecting ionic azides to HP/HT conditions in a multianvil device. , We succeeded in synthesizing SrN 2 and BaN 2 , as well as the unprecedented (but theoretically predicted) CaN 2 and also Li 2 N 2 , the latter one representing the first alkali diazenide. − Crystallographic, spectroscopic, and theoretical investigations confirmed the presence of [N 2 ] 2– anions in these crystal structures. , Only very recently, LaN 2 (≡ La 3+ [N 2 ] 2– ·e – ) proved existence in shockwave experiments in accord with theoretical predictions. , Again, crystallographic studies showed the presence of diazenide anions with slightly elongated N–N bonds (1.30–1.32 Å), possibly due to the metallic character of the crystalline host.…”