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 [Co III Ca II (notpH 2 )(H 2 O) 2 ]ClO 4 •nH 2 O [abbreviated as CoCa•nH 2 O, where notpH 6 = 1,4,7-triazacyclononane-1,4,7-triyl-tris-(methylenephosphonic acid), C 9 H 18 N 3 (PO 3 H 2 ) 3 ]. CoCa•nH 2 O undergoes a reversible relative humidity (RH) dependent SC−SC structural transformation between CoCa•2H 2 O and CoCa• 4H 2 O at room temperature. Accordingly the continuous hydrogen bond network observed in CoCa, 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•nH 2 O 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 ClO 4 − 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.

Three new layered Co-Ca phosphonates are reported, namely, [CoCa(notpH 2 )(H 2 O) 2 ]X·nH 2 O (CoCa-X·nH 2 O; X = Cl -, NO 3 -, PF 6 -), where notpH 6 is 1,4,7-triazacyclononane-1,4,7triyltris(methylenephosphonic acid) [C 9 H 18 N 3 (PO 3 H 2 ) 3 ]. The layer topology of these compounds are identical to that of [CoCa(notpH 2 )(H 2 O) 2 ]ClO 4 ·nH 2 O (CoCa-ClO 4 ·nH 2 O). The most significant difference lies in the counter-anions, which not only have different size and shape but also show different capabilities towards hydrogen bonding. Consequently, the number of lattice water molecules and the hydrogen-bond networks be- [a]

Three new layered mixed metal phosphonates [CoMg(notpH2)(H2O)2]ClO4·nH2O (CoMg·nH2O), [Co2Sr2(notpH2)2(H2O)5](ClO4)2·nH2O (CoSr·nH2O), and [CoBa(notpH2)(H2O)1.5]ClO4 (CoBa) were synthesized by reacting a tripodal metalloligand CoIII(notpH3) [notpH6 = C9H18N3(PO3H2)3] with alkaline earth metal ions. Along with an increase in the radius of the alkaline earth metal ions, the 6-coordinate {MgO6}, 7-coordinate {SrO7}, and 9-coordinate {BaO9} geometries are the distorted octahedron, capped triangular prism, and tricapped triangular prism, respectively. Consequently, the metalloligand Co(notpH2)− adopts variable coordination modes to bind the alkaline earth metal nodes, forming diverse layer topologies in the three mixed metal phosphonates. The AC impedance measurements revealed that the proton conductivities at 25 °C and 95% relative humidity (RH) follow the sequence: CoMg·nH2O > CoSr·nH2O > CoBa. As expected, CoMg·nH2O exhibits a 28-fold enhanced value for proton conductivity (4.36 × 10−4 S cm−1) compared with the previously reported isostructural compound, CoCa·nH2O, at 25 °C and 95% RH due to the greater Lewis acid strength of Mg(II) lowering the pKa of the coordinated water.