Calcium Carbodiimide Compounds Revisited – Syntheses, Single Crystal Structure Determination and Vibrational Spectra of Ca₁₁N₆[CN₂]₂, Ca₄N₂[CN₂] and Ca[CN₂]

Single crystals of Ca₁₁N₆[CN₂]₂ (dark red needles, tetragonal, P4₂/mnm (no. 136), a = 1456.22(5), and c = 361.86(2) pm, Z = 2), Ca₄N₂[CN₂] (transparent yellow needles, orthorhombic, Pnma (no. 62), a = 1146.51(11), b = 358.33(4), and c = 1385.77(13) pm, Z = 4) and Ca[CN₂] (transparent, colorless, triangular plates, rhombohedral, R3̅m (no. 166), a = 369.00(3), and c = 1477.5(3) pm, Z = 3) were obtained by the reaction of Na₂[CN₂], CaCl₂ and Ca₃N₂ (if demanded by stoichiometry) in arc-welded Ta ampoules at temperatures between 1200 - 1400 K. Their crystal structures were re-determined by means of single crystal X-ray structure analyses. Additionally, the Raman spectra were recorded on these same single crystals, whereas the IR spectra were obtained with the KBr pellet technique. The title compounds exhibit characteristic features for carbodiimide units with D∞h symmetry (d(C-N) = 121.7 - 123.8 pm and ∡ (N-C-N) = 180°). The vibrational frequencies of these units are in the expected range (Ca₁₁N₆[CN₂]₂: νs = 1230, νs = 2008; δ = 673/645/624 cm⁻¹; Ca₄N₂[CN₂]: νs = 1230, νs = 1986; δ = 672/647 cm⁻¹; Ca[CN₂]: νs = 1274, νs = 2031, δ = 668 cm⁻¹). The structural results are more precise than the previously reported data, and with the newly attained Raman spectrum of Ca₁₁N₆[CN₂]₂ we correct data reported earlier.