Janus-Type Mixed-Valent Copper-Cyanido Honeycomb Layers

The synthesis of Janus-type layers, which possess front and back sides that consist of different structures, remains a major challenge in the field of two-dimensional materials. In this study, two Janus-type layered coordination polymers, namely, Cu-II(NEtH2)(NMe2H center dot H2O)Cu-I(CN)(3) (1) and Cu-II(NMe2H)(NMe2H center dot H2O)Cu-I(CN)(3) (2), were synthesized via a simple one-pot procedure using copper(II) nitrate and sodium cyanido in mixed solutions of dimethylamine and ethylamine. Uniquely, 1 and 2 were composed of cyanido-bridged neutral layers and exhibited a (CuCuII)-Cu-I mixed-valent state. Meanwhile, using a solution of pure dimethylamine for the synthesis yielded the monovalent three-dimensional framework (NMe2H2)[Cu-I (2)(CN)(3)] (3). Results indicated that the simultaneous use of two mixed amines gave rise to the controlled reduction of Cu-II ions during the reaction. In addition, each face of the layers was coordinated by different amines on the axial positions of the Cu-II sites, resulting in anisotropic Janus layers. Furthermore, the thermal expansion behavior of 2 was investigated, demonstrating that the neutral [(CuCuII)-Cu-I(CN)(3)] layer was relatively rigid compared with the analogous anionic [Cu-I (2)(CN)(3)](-) layer. This work presents the one-pot synthesis of rare Janus-type coordination polymers consisting of copper-cyanido honeycomb layers. The resultant layers exhibited not only such structural characteristics but also a mixed-valent copper(I/II) state.