Cooperative Crystallization of Heterometallic Indium–Chromium Metal–Organic Polyhedra and Their Fast Proton Conductivity

Metal–organic polyhedra (MOPs) or frameworks (MOFs) based on Cr 3+ are notoriously difficult to synthesize, especially as crystals large enough to be suitable for characterization of the structure or properties. It is now shown that the co‐existence of In 3+ and Cr 3+ induces a rapid crystal growth of large single crystals of heterometallic In‐Cr‐MOPs with the [M 8 L 12 ] (M=In/Cr, L=dinegative 4,5‐imidazole‐dicarboxylate) cubane‐like structure. With a high concentration of protons from 12 carboxyl groups decorating every edge of the cube and an extensive H‐bonded network between cubes and surrounding H 2 O molecules, the newly synthesized In‐Cr‐MOPs exhibit an exceptionally high proton conductivity (up to 5.8×10 −2  S cm −1 at 22.5 °C and 98 % relative humidity, single crystal).