Unveiling the Unique Roles of Metal Coordination and Modulator in the Polymorphism Control of Metal-Organic Frameworks

Graphical The extraordinary coordination versatility of Th 4+ cation and the unexpected bridging role of formate allow polymorphism control of thorium-organic frameworks (TOFs) with five distinct phases, wherein the highest degree of interpenetration and porosity for TOFs have been achieved. The resulting TOFs exhibit considerably different adsorption behaviors toward organic dyes, thus suggesting that TOFs represent an excellent platform for structure-property relationship study. Polymorphism control of metal-organic frameworks is highly desired for elucidating structure-property relationships, but remains an empirical process and is usually done in a trial-and-error approach. We adopted the rarely used actinide cation Th 4+ and a ditopic linker to construct a series of thorium-organic frameworks (TOFs) with a range of polymorphs. The extraordinary coordination versatility of Th 4+ cations and clusters, coupled with synthetic modulation, gives five distinct phases, wherein the highest degree of interpenetration (threefold) and porosity (75.9 %) of TOFs have been achieved. Notably, the O atom on the capping site of the nine-coordinated Th 4+ cation can function as a bridging unit to interconnect neighboring secondary building units (SBUs), affording topologies that are undocumented for other tetravalent-metal-containing MOFs. Furthermore, for the first time HCOOH has been demonstrated as a bridging unit of SBUs to further induce structural complexity. The resulting TOFs exhibit considerably different adsorption behaviors toward organic dyes, thus suggesting that TOFs represent an exceptional and promising platform for structure-property relationship study.