Toward Reversible Crystalline-to-Amorphous Guest-Induced Transitions in Manganese(III) Carboxylate Metal–Organic Frameworks

Metal-organic frameworks (MOFs) are an interesting class of inorganic/organic hybrid materials with a wide scope of applications. Although manganese is an abundant metal, the synthesis of Mn(III)-containing MOFs has not been widely studied due to the relative redox sensitivity of this species. We therefore investigated the self-oxidation of manganese(II) nitrate in the presence of aromatic dicarboxylic linkers in alcohols, discovering a series of new Mn(III)-MOFs. The obtained structures were analyzed through a combination of (synchrotron) X-ray diffraction and total scattering (pair distribution function analysis) experiments. In methanol, the syntheses led to new nonflexible members of the MIL-47/MIL-53 and MIL-69 families of MOFs, rare examples containing bridging methoxy anions. When using ethanol or 1-propanol, wine rack-like structures (named UcL-1 and UcL-2) with different Mn^II/Mn^III secondary building units are obtained. These demonstrate intriguing reversible crystalline-to-crystalline and crystalline-to-amorphous transitions upon guest exchange and release. The latter process involves strong correlated and noncorrelated structural distortions, paired with the creation of coordinatively unsaturated Mn^III sites. This work demonstrates that Mn(III)-containing carboxylate systems have the potential for the design of new functional MOF materials, including structures with tunable flexible behavior.