Dehydration of Octacyanido-Bridged Ni^II-W^IV Framework toward Negative Thermal Expansion and Magneto-Colorimetric Switching

Abstract

An inorganic three-dimensional [NiII­(H2O)2]2­[WIV­(CN)8]·​4H2O (1) framework undergoes a single-crystal-to-single-crystal transformation upon thermal dehydration, producing a fully anhydrous phase NiII2­[WIV­(CN)8] (1d). The dehydration process induces changes in optical, magnetic, and thermal expansion properties. While 1 reveals typical positive thermal expansion of the crystal lattice, greenish-yellow color, and paramagnetic behavior, 1d is the first ever reported octacyanido-based solid revealing negative thermal expansion, also exhibiting a deep red color and diamagnetism. Such drastic shift in the physical properties is explained by the removal of water molecules, leaving the exclusively cyanido-bridged bimetallic network, which is accompanied by the transformation of the octahedral paramagnetic [NiII­(H2O)2­(NC)4]2– to the square-planar diamagnetic [NiII­(NC)4]2– moieties