Overcoming Synthetic Challenges: Solvent-Free Construction of a Cr–Phosphonate Framework with High Stability and Proton Conductivity

The sustainable, green, solvent-free synthesis offers new pathways for constructing high-performance metal-organic frameworks (MOFs) that are challenging to access via conventional solution chemistry. Herein, using this approach, we synthesize a highly stable crystalline chromium-phosphonate MOF, TGU-20, while a different counterpart, TGU-30, was obtained in the solution with the same ligand and Cr³⁺ ion. Interestingly, the three-dimensional electron diffraction technique reveals that TGU-20 adopts a monodeprotonated bidentate phosphonate coordination mode, forming ultranarrow channels densely lined with proton-donating acid P-OH groups. In contrast, TGU-30 features a mixed mono/dideprotonated coordination, resulting in large channels with sparse P-OH sites. As a result, TGU-20 displays exceptional proton conductivity of over 10⁻² S cm⁻¹ at 100% RH with a very low activation energy of 0.10 eV, reaching record-high room-temperature proton conductivity among phosphonate MOFs and 2 orders of magnitude higher than that of TGU-30. Additionally, both MOFs exhibit high stability in harsh conditions, including highly concentrated strong acid. The solvent-free synthesis not only directly yields a chromium-phosphonate MOF for the first time but also opens a mild and green avenue for selectively tuning metal-ligand coordination in reticular chemistry.