Ammonothermal Synthesis of Alkali-Alkaline Earth Metal and Alkali-Rare Earth Metal Carbodiimides: K5–xMx(CN2)2+x(HCN2)1–x (M = Sr, Eu) and Na4.32Sr0.68(CN2)2.68(HCN2)0.32

Abstract

Alkali-alkaline earth metal and alkali-rare earth metal carbodiimides, namely K5–xMx(CN2)2+x(HCN2)1–x (x = 0 – 1) (M = Sr, Eu) and Na4.32Sr0.68(CN2)2.68(HCN2)0.32, were synthesized under ammonothermal conditions in high-pressure autoclaves. The structures of the three compounds can be derived from homeotypic K5H(CN2)3 and Na5H(CN2)3 by partial substitution of K+ or Na+by Sr2+ or Eu2+. The reactions were carried out in two step syntheses (T1 = 673 K, T2 = 823 K) starting from sodium or potassium azide, dicyandiamide and strontium or Eu(NH2)2, respectively. The crystal structures were solved and refined from single-crystal X-ray diffraction data [K4.16Sr0.84(CN2)2.84(HCN2)0.16: space group Im3m (no. 229), a = 7.8304(5) Å, Z = 2, R1 = 0.024, wR2 = 0.052; K4.40Eu0.60(CN2)2.60(HCN2)0.40: space group Im3m (no. 229), a = 7.8502(6) Å, Z = 2, R1 = 0.022, wR2 = 0.049]. In contrast to the potassium carbodiimides, the sodium-strontium carbodiimide was only synthesized as microcrystalline powder. The crystal structure was determined by powder X-ray diffraction and refined by the Rietveld method [Na4.32Sr0.68(CN2)2.68(HCN2)0.32: space group Im3m (no. 229), a = 7.2412(1) Å, Z = 2, Rwp = 0.050]. The presence of hydrogencyanamide units ([HNCN]–) next to carbodiimide units ([CN2]2–) in all compounds was confirmed by FT-IR spectroscopy.