Na 3 [BN 2 ] and Na 2 K[BN 2 ]: A Known and a Novel Alkali Metal Dinitridoborate Obtained via Mild Thermal Dehydrogenation

Na 3 [BN 2 ] and Na 2 K[BN 2 ] were obtained as white polycrystalline powders from the reaction of the respective binary mixtures NaNH 2 :NaBH 4 and NaNH 2 :KBH 4 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 (Na 3 [BN 2 ]) and 712 K (Na 2 K[BN 2 ]), 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 Na 3 [BN 2 ] {β‐Li 3 [BN 2 ] type; P 2 1 / 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, Na 2 K[BN 2 ] crystallizes isotypic to Na 2 KCuO 2 in the tetragonal space group I 4/ mmm (No. 139) with a = 4.2359(1) Å, c = 10.3014(2) Å and Z = 2. The crystal structure of Na 2 K[BN 2 ] is composed of linear [N–B–N] 3– anions centering elongated M 14 rhombic dodecahedra, which are formed by 8 sodium and 6 potassium atoms. The [BN 2 ]@Na 8/4 K 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 Na 3 [BN 2 ]. Both the wavenumbers, as well as the calculated valence force constants f (B–N) = 7.29 N · cm –1 (Na 3 [BN 2 ]) and 7.33 N · cm –1 (Na 2 K[BN 2 ]), respectively, are in good agreement with those of the known alkali and alkaline earth dinitridoborates.