High‐Pressure Synthesis of Metal–Inorganic Frameworks Hf₄N₂₀⋅N₂, WN₈⋅N₂, and Os₅N₂₈⋅3 N₂ with Polymeric Nitrogen Linkers

Abstract

Polynitrides are intrinsically thermodynamically unstable at ambient conditions and require peculiar synthetic approaches. Now, a one‐step synthesis of metal–inorganic frameworks Hf4N20⋅N2, WN8⋅N2, and Os5N28⋅3 N2 via direct reactions between elements in a diamond anvil cell at pressures exceeding 100 GPa is reported. The porous frameworks (Hf4N20, WN8, and Os5N28) are built from transition‐metal atoms linked either by polymeric polydiazenediyl (polyacetylene‐like) nitrogen chains or through dinitrogen units. Triply bound dinitrogen molecules occupy channels of these frameworks. Owing to conjugated polydiazenediyl chains, these compounds exhibit metallic properties. The high‐pressure reaction between Hf and N2 also leads to a non‐centrosymmetric polynitride Hf2N11 that features double‐helix catena‐poly[tetraz‐1‐ene‐1,4‐diyl] nitrogen chains [−N−N−N=N−]∞.

Stabilization of Polynitrogen Anions in Tantalum–Nitrogen Compounds at High Pressure

“…The calculation for TaN 5 gives b (NN)=2, which agrees with the results of crystal‐chemical analysis (1 N−N bond for N3, 3 for N4 and 2 for the rest of nitrogen atoms give 2 in average). Infinite nitrogen units were previously reported in FeN 4 , [13, 14] BeN 4 , [12] ReN 8 ⋅N 2 , [20] Hf 4 N 20 ⋅N 2 , Hf 2 N 11 , WN 8 ⋅N 2 , Os 5 N 28 ⋅N 2 , [21] and MgN 4 compounds [7] . However, TaN 5 is a unique compound.…”

“…Nitrogen atoms form unprecedented branched chains infinitely expanding along the c-axis (Figure 1a and e(N) are the numbers of valence electrons of metal and nitrogen, respectively.T he calculation for TaN 5 gives b(NN) = 2, which agrees with the results of crystal-chemical analysis (1 NÀNb ond for N3, 3f or N4 and 2f or the rest of nitrogen atoms give 2inaverage). Infinite nitrogen units were previously reported in FeN 4 , [13,14] BeN 4 , [12] ReN 8 •N 2 , [20] [21] and MgN 4 compounds. [7] However, TaN 5 is au nique compound.…”

“…[15][16][17][18][19] Forexample,alkali metals form ionic pentazolate salts at high pressure, [10,11] while some transition metals were shown to form metal-inorganic frameworks with predominantly polar covalent bonding between metals and polymeric nitrogen linkers. [20,21] Thed iversity of polynitrogen species may be significantly increased in the compounds of early transition metals,which can combine both covalent and ionic components in the M À Nb onds.A mong binary transition metal nitrides,the highest thermodynamic stability is reached in groups 4a nd 5. [22] Particularly,t antalum has the largest number of different nitride compositions currently known.…”

Stabilization of Polynitrogen Anions in Tantalum–Nitrogen Compounds at High Pressure

“…TheT aN 5 compound can be well rationalized within the extended 8-N Mooser Pearson rule.According to this rule,the and e(N) are the numbers of valence electrons of metal and nitrogen, respectively.T he calculation for TaN 5 gives b(NN) = 2, which agrees with the results of crystal-chemical analysis (1 NÀNb ond for N3, 3f or N4 and 2f or the rest of nitrogen atoms give 2inaverage). Infinite nitrogen units were previously reported in FeN 4 , [13,14] BeN 4 , [12] ReN 8 •N 2 , [20] Hf 4 N 20 •N 2 ,H f 2 N 11 ,W N 8 •N 2 ,O s 5 N 28 •N 2 , [21] and MgN 4 compounds. [7] However, TaN 5 is au nique compound.…”

“…Thelatter effect is well demonstrated by acomparison of Ta with other 5d metals (W, Re,O s), which form compounds with nitrogen at circa 100 GPa, where metals achieve only the oxidation state of + 4 (WN 10 ,R eN 10 ,O s 5 N 34 )d espite the wider range of possible oxidation states of W, Re,a nd Os compared to Ta. [20,21] Secondly,t he stabilization of complex polyanions requires am etal to have large coordination numbers:T ai s1 0-fold coordinated in TaN 5 and 9-coordinated in TaN 4 ,which would not be possible for the 3d metals.Lastly,the Ta-N compounds should occupy an intermediate position between ionic alkali or alkaline-earth metal polynitrides (anions are stabilized by resonance,M À Nb onds are ionic) and polar covalent transition metal polynitrides (anions destabilized by the distortion imposed by polar covalent M À Nb onds like in FeN 4 ), which leads to novel structure types.Interestingly,wecould not find any compounds that naturally link nitrides and polynitrides, namely the compounds containing dinitrogen anions [N 2 ] xÀ . Such species are common among transition metal (OsN 2 , [47] PtN 2 , [3] RhN 2 , [48] etc.…”