Noncentrosymmetric two-dimensional (2D) coordination polymers/metal–organic frameworks (CPs/MOFs) are very rare and their functionalities have not been explored. Herein, we report the first 2D ferroelectric proton conductor based on cyanido-bridged undulating 2D CPs. [Mn(salen)]2[ReN(CN)4(MeCN)]·H2O (MnReMeCN·H2O) crystallized in the Pna21 space group and underwent in-plane symmetry breaking upon incorporation of water within the crystal layers. Thus, the dehydrated MnReMeCN layers were centrosymmetric and exhibited reversible switching of second harmonic generation induced by water vapor. The ferroelectricity of MnReMeCN·H2O was strongly coupled with ion conduction, yielding a colossal polarization of 21 mC cm−2, which was determined via positive-up–negative-down measurements at 0.005 Hz and 298 K for single crystals. Moreover, MnReMeCN exhibited anisotropic thermal expansion based on the undulation change, while the zigzag angle changes of the layers switched between a decrease and an increase at around 300 K. These transformations corroborate the characteristic relationship between the zigzag pitch and the interlayer interaction of the undulating layered structures.
