Node Distortion Modulation for Anisotropic Thermal Expansions of Two‐dimensional Coordination Polymers

Abstract Anisotropic negative thermal expansion is a valuable property of solid‐state materials, influencing their micromechanical and functional response. Two flexible two‐dimensional (2D) cyanide‐bridged coordination polymers (CPs) of type [Fe(salen)]₂[M(CN)₄] (M=MnN (FeMnN) and Pt (FePt)) were synthesized. Their anisotropic negative thermal expansions was strongly linked to relaxations of node distortions in [Fe(salen)]⁺ units. Comparison with the [Mn(salen)]⁺ analogues indicated shorter bond lengths around iron(III), due to the different electronic states. These changes of node structures resulted in FeMnN and FePt exhibiting lower and higher degrees of anisotropic thermal expansion behavior, respectively, than their Mn analogues.