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

Abstract

Dinitrogen (N2) ligation is a common and well-characterized structural motif in bioinorganic synthesis. In solid-state chemistry, on the other hand, homonuclear dinitrogen entities as structural building units proved existence only very recently. High-pressure/high-temperature (HP/HT) syntheses have afforded a number of binary diazenides and pernitrides with [N2](2–) and [N2](4–) ions, respectively. Here, we report on the HP/HT synthesis of the first ternary diazenide. Li2Ca3[N2]3 (space group Pmma, no. 51, a = 4.7747(1), b = 13.9792(4), c = 8.0718(4) Å, Z = 4, wRp = 0.08109) was synthesized by controlled thermal decomposition of a stoichiometric mixture of lithium azide and calcium azide in a multianvil device under a pressure of 9 GPa at 1023 K. Powder X-ray diffraction analysis reveals strongly elongated N–N bond lengths of dNN = 1.34(2)–1.35(3) Å exceeding those of previously known, binary diazenides. In fact, the refined N–N distances in Li2Ca3[N2]3 would rather suggest the presence of [N2](3·–) radical ions. Also, characteristic features of the N–N stretching vibration occur at lower wavenumbers (1260–1020 cm(–1)) than in the binary phases, and these assignments are supported by first-principles phonon calculations. Ultimately, the true character of the N2 entity in Li2Ca3[N2]3 is probed by a variety of complementary techniques, including electron diffraction, electron spin resonance spectroscopy (ESR), magnetic and electric conductivity measurements, as well as density-functional theory calculations (DFT). Unequivocally, the title compound is shown to be metallic containing diazenide [N2](2–) units according to the formula (Li(+))2(Ca(2+))3([N2](2–))3·(e(–))2.

A First‐Principles Study on the Electronic, Vibrational, and Thermodynamic Properties of Jadeite and its Tentative Low‐Density Polymorph

“…13). Contrary to previous studies in which this procedure led to stable structures,22,23 no significant lowering of the total energy was observed. In addition, the new structure still exhibited imaginary frequencies.…”

Prediction and characterization of the marcasite phase of iron pernitride under high pressure

“…However, a similar N-N distance (1.34−1.35 Å) of N 2 2− units was found in the dubbed Li 2 Ca 3 N 6 [41].…”

Fe-N system at high pressure reveals a compound featuring polymeric nitrogen chains

“…Therefore, both refined N-N distances and compressibility suggest that Fe in FeN 2 has an oxidation state + 3. The presence of [N 2 ] 3·− radical ions is not very likely (however, not excluded) 56 , and the electron count in FeN 2 can be similar to that suggested for LaN 2 : 51 Fe 3+ + [N=N] 2− + e − , where the electron enters the conduction band.…”

“…This trend is clearly demonstrated by the experimental and theoretical studies. Metals that cannot have oxidation state larger than +2 (Ba, Sr, Ca), form diazenides M 2+ [N=N] 2− with the N=N distances in the range 1.2–1.24 Å 54 – 56 and rather low bulk moduli ( K 0 (SrN 2 ) = 65 GPa, K 0 (BaN 2 ) = 46 GPa) 51 . Metals that have stable oxidation states + 4 (Os, Ru, Ir, Ti, Pt) form pernitrides M 4+ [N–N] 4− with N–N distances ~1.4 Å and are highly incompressible with very large bulk moduli ( K 0 (OsN 2 ) =362 GPa 52 , K 0 (IrN 2 ) = 428 GPa 57 , K 0 (TiN 2 ) = 385 GPa 38 , K 0 (PtN 2 ) = 372 GPa 32 ).…”