Co–Ca Phosphonate Showing Humidity-Sensitive Single Crystal to Single Crystal Structural Transformation and Tunable Proton Conduction Properties

A combination of humidity-dependent single crystal to single crystal (SC–SC) structural transformation and single crystal proton conductivity measurements is essential to elucidate the underlying proton transport mechanism in metal–organic framework materials. Herein, we report a new layered Co–Ca phosphonate [CoIIICaII(notpH2)(H2O)2]ClO4·nH2O [abbreviated as CoCa·nH2O, where notpH6 = 1,4,7-triazacyclononane-1,4,7-triyl-tris(methylenephosphonic acid), C9H18N3(PO3H2)3]. CoCa·nH2O undergoes a reversible relative humidity (RH) dependent SC–SC structural transformation between CoCa·2H2O and CoCa·4H2O at room temperature. Accordingly the continuous hydrogen bond network observed in CoCa·4H2O (95% RH) is interrupted in CoCa·2H2O (40% RH), leading to a drastic decrease in proton conductivity by ∼5 orders of magnitude. The process is reversible; hence, the proton conductivity is tunable simply through humidity control. The AC impedance measurements using single crystals of CoCa·nH2O reveal that the [010] direction of H-bond extension is the preferred proton conduction pathway showing the greatest conductivity of 1.00 × 10–3 S cm–1 at 25 °C and 95% RH. Although the [20–1] direction, which involves the phosphonate oxygen atoms in the H-bond network shows the lowest conductivity of 4.35 × 10–8 S cm–1 at 25 °C and 95% RH, the ClO4– anions play a key role in not only connecting the lattice water molecules into a continuous hydrogen bond network but also assisting the proton diffusion between the lattice water molecules. This work provides a rare example of a proton conductive MOF with a well-illustrated proton conduction mechanism and is a promising humidity sensor for future applications.