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·H₂O)Cu^I(CN)₃ (1) and Cu^II(NMe₂H)(NMe₂H·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 Cu^ICu^II mixed-valent state. Meanwhile, using a solution of pure dimethylamine for the synthesis yielded the monovalent three-dimensional framework (NMe₂H₂)[Cu^I₂(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 [Cu^ICu^II(CN)₃] layer was relatively rigid compared with the analogous anionic [Cu^I₂(CN)₃]^- layer.