Na5[CN2]2[CN], (Li, Na)5[CN2]2[CN], and K2[CN2]: Carbodiimides from High-Pressure Synthesis

The first alkali metal carbodiimide cyanides Na5[CN2]2[CN] and (Li, Na)5[CN2]2[CN] were obtained from reaction of carbon with nitrogen gas pressure of 7 MPa in sodium melts without orwith admixture of Li3N, respectively. The crystal structures wererefined from single-crystal X-ray diffraction (Na5[CN2]2[CN]:a = 7.201(2) angstrom, c = 6.605(4) angstrom, I4/mmm, R1 = 0.028, wR2 = 0.079; (Li1.40Na3.60(3))5[CN2]2[CN]: a = 7.0034(3) angstrom, Im$\bar{3}$m, R1 = 0.038,wR2 = 0.084). Whereas in Na5[CN2]2[CN] a fully ordered arrangement of the anions is realized, in (Li, Na)5[CN2]2[CN] both the alkali metal cations and the anions are disordered, with some site preference of lithium and sodium, leading to an increase of space group symmetry from I4/mmm for the pure sodium compound to Im$\bar{3}$m for the mixed Li/Na phase. Ammonothermal synthesis at 90 MPa leads to crystal growth of carbodiimide K2[CN2] (a = 5.762(1) angstrom, b = 5.669(1) angstrom,c = 5.756(1) angstrom, beta = 108.78(3)degrees, C2/m) crystals in the cold zone of an autoclave. The crystal structure was analyzed and compared to the carbodiimide cyanides.