A new tilt and an old twist on the nickel arsenide structure-type: synthesis and characterisation of the quaternary transition-metal cyanamides A2MnSn2(NCN)6 (A = Li and Na)

Abstract

In this work, we describe the synthesis and structure of the quaternary transition-metal cyanamides Na₂MnSn₂(NCN)₆ and Li₂MnSn₂(NCN)₆. These phases crystallise isotypically in layered structures, with P1m symmetry, that comprise hexagonal close-packed arrays of NCN²⁻ anions with metal cations in 5/6 of the octahedral holes, thereby reflecting low-symmetry modifications of the hierarchical [NiAs]-type MNCN structure. The distinct coordination requirements of the metal cations template an ordered decoration across the octahedral sites with corundum-like [Sn₂(NCN)₃]²⁺ layers alternating with [A₂Mn(NCN)₃]²⁻ layers which resemble a portion of the Li₂Zr(NCN)₃ structure. This motif is also mirrored in the form of the NCN²⁻ anions which adopt N–CN²⁻ cyanamide shapes with clear single- and triple-bond character. Distortion-mode analysis reveals the importance of K₁ octahedral twist and K₂ cyanamide tilt displacements in stabilising these phases, the latter of which is only accessible because of the extended nature of the NCN²⁻ anion. These are the first examples of non-binary transition-metal cyanamides to be discovered and this study highlights how the additional flexibility of the NCN²⁻ anion affords a novel structure-type not observed in oxide chemistry.