High 3D Proton Conductivity of a 2D Zn(II) Metal–Organic Framework Synthesized via Water-Assisted Single-Crystal-to-Single-Crystal Phase Transformation

In the present work, a one-dimensional (1D) metal–organic framework, {[Zn₃(TBIB)₂(Cl)₆]·11H₂O}_n (1), has been synthesized under high temperature and acidic conditions by utilizing the tripodal ligand 1,3,5-tri­(1H-benzo­[d]­imidazol-1-yl)­benzene (TBIB). Interestingly, on immersing the single crystals of the synthesized compound 1 in water for ∼2 days, a single-crystal-to-single-crystal phase transformation occurs, which leads to the formation of a two-dimensional (2D) framework {[Zn₃(TBIB)₂(Cl)₆]·6H₂O}_n (2). The intense network of H-bonding interactions within the water clusters in the 1D framework was broken in water medium, followed by the formation of strong hydrogen bonds between the coordinated chloride ions and the lattice water molecules in compound 2 possessing a 2D framework. Nevertheless, both the polymers are not only decorated with water clusters but also contain metal-coordinated chloride ions (M–Cl) along with the benzimidazole N atoms in their frameworks, which are expected to assist in the formation of channels for the proton transport. The proton conductivity in both these compounds has been tested using ac impedance spectroscopy from room temperature to 80 °C under 3% H₂O–H₂ conditions. The 2D framework 2 shows an order of magnitude higher proton conductivity (∼1.1 × 10^–4 S cm^–1) than compound 1 having a 1D framework (1.2 × 10^–5 S cm^–1) at 25 °C. The significant enhancement in the conductivity is attributed to the three-dimensional proton conduction in compound 2, which is not reported earlier.