Reversible structural transformations in a Co(II)-based 2D dynamic metal-organic framework showing selective solvent uptake

Abstract

A Co(II)-based two-dimensional (2D) metal-organic framework (MOF) [Co(pca)(bdc)0.5(H2O)2] (1) {pca = pyrazine carboxylic acid, and bdc = 1,4-benzene dicarboxylic acid} was synthesized solvothermally. The compound loses the coordinated lattice water molecules on heating which is accompanied by solid-state structural transformation to yield dehydrated phase [Co(pca)(bdc)0.5] (\(\mathbf {1}^{\boldsymbol {\prime }}\)). The hydrated structure can be regained by exposing 1 \(^{\boldsymbol {\prime }}\) to water vapour (1 \(^{\boldsymbol {\prime \prime }}\)). These reversible solid-state structural transformations are accompanied by a visible colour change in the material. The dehydrated compound also shows highly selective water uptake over other solvents like MeOH, EtOH, THF. This selective water uptake can be ascribed to the high affinity of polar water molecule towards the open metal site created on heating. The present report provides important insights into the reversible structural transformations observed due to variable coordination number of the central metal ion and transformability of the framework. The selective water uptake over alcohols along with visible colour change demonstrates the potential of the present compound in bio-alcohol purification. Herein, we report a Co(II)-based 2D dynamic metal-organic framework synthesized under solvothermal conditions. The framework exhibits structural transformation accompanied by a visible colour change owing to the loss of coordinated water. The structure can be regained by selective water uptake over other solvents. The selective water uptake due to flexible framework and visible colour change demonstrates the potential of the present compound for bio-alcohol purification.