Na3[BN2] and Na2K[BN2]: A Known and a Novel Alkali Metal Dinitridoborate Obtained via Mild Thermal Dehydrogenation

Na₃BN₂] and Na₂KBN₂] were obtained as white polycrystalline powders from the reaction of the respective binary mixtures NaNH₂:NaBH₄ and NaNH₂:KBH₄ in molar ratio 2:1 at 873 K and 683 K, respectively, in an argon stream. According to the results of thermal analysis measurements, both compounds are thermally stable only up to 954 K (Na₃BN₂]) and 712 K (Na₂KBN₂]), respectively, decomposing under evolution of alkali metal and nitrogen to yield hexagonal BN as final residue, which was identified from powder patterns. The crystal structure of Na₃BN₂] {β‐Li₃BN₂] type; P2₁/c (No. 14); Z = 4} was confirmed and the unit cell parameters redetermined: a = 5.724(1) Å, b = 7.944(1) Å, c = 7.893(1) Å, β = 111.31(1)°. According to X‐ray powder data, Na₂KBN₂] crystallizes isotypic to Na₂KCuO₂ in the tetragonal space group I4/mmm (No. 139) with a = 4.2359(1) Å, c = 10.3014(2) Å and Z = 2. The crystal structure of Na₂KBN₂] is composed of linear N–B–N]^3– anions centering elongated M₁₄ rhombic dodecahedra, which are formed by 8 sodium and 6 potassium atoms. The BN₂]@Na_8/4K_6/6 polyhedra are stacked along 001] and condensed via common tetragonal faces to generate a space‐filling 3D arrangement. The B–N bond lengths for the strictly linear N–B–N]^3– units are 1.357(4) Å. Vibrational spectra of the title compounds were measured and analyzed based on D_∞h symmetry of the relevant N–B–N]^3– groups taking into account the site symmetry effects for Na₃BN₂]. Both the wavenumbers, as well as the calculated valence force constants f(B–N) = 7.29 N · cm^–1 (Na₃BN₂]) and 7.33 N · cm^–1 (Na₂KBN₂]), respectively, are in good agreement with those of the known alkali and alkaline earth dinitridoborates.