Na5 [CN2]2 [CN], (Li,Na) 5 [CN2]2 [CN], and K2 [CN2]: Carbodiimides from high-pressure synthesis

Abstract

The first alkali metal carbodiimide cyanides Na5 [CN 2]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 or with admixture of Li3N, respectively. The crystal structures were refined from single-crystal X-ray diffraction (Na5[CN2]2[CN]: a = 7.201(2) Å, c = 6.605(4) Å, I4/mmm, R1 = 0.028, wR2 = 0.079; (Li1.40Na3.60(3))5[CN2]2[CN]: a = 7.0034(3) Å, Im3̄m, 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 [CN 2]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 Im3̄m for the mixed Li/Na phase. Ammonothermal synthesis at 90 MPa leads to crystal growth of carbodiimide K2 [CN2] (a = 5.762(1) Å, b = 5.669(1) Å, c = 5.756(1) Å, β = 108.78(3)°, C2/m) crystals in the cold zone of an autoclave. The crystal structure was analyzed and compared to the carbodiimide cyanides. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.