Exploring Polymorphism in Metal Carbodiimides from Graph Theory

Preparing new solid-state materials incorporating the molecular anion NCN^2- presents a considerable synthetic challenge given the metastable nature of many of these compounds. Hence, it is rather uncommon to observe polymorphism which restricts the possibilities of structural and functional exploration of NCN-based materials. Here we propose the concept of topological complexity from graph theory as a tool for exploring metal-carbodiimide polymorphism. Thus, we report the synthesis of a new modification of Y₂(NCN)₃ adopting a trigonal structure with <i>R</i>3̅<i>c</i> symmetry, distinct from the previously reported monoclinic structure. Likewise, a new metastable Li₂Sn(NCN)₃ modification with hexagonal <i>P</i>6₃/<i>mmc</i> symmetry has been found, the first example of polymorphism in a ternary carbodiimide. This new phase adopts a layered structure similar to FeNCN, with triangular networks of Li⁺ and Sn⁴⁺ cations in a fully disordered structure. In addition, a hexagonal to orthorhombic phase transition of Li₂Sn(NCN)₃ can be thermally induced. The structures of the new polymorphs are topologically simpler and favored by relatively low reaction temperatures (≈350 °C). These findings demonstrate the potential of topological complexity as a new alternative for the design of synthetic conditions and for the rational prediction of such NCN-based or similar materials.