For rare-earth separation, selective crystallization into metal-organic frameworks (MOFs) offers new opportunities. Especially important is the development of MOF platforms with high selectivity toward target ions. Here we report a MOF platform (CPM-66) with low-coordinationnumber environment for rare-earth ions. This platform is highly responsive to the size variation of rare-earth ions and shows exceptional ion-size selectivity during crystallization. CPM-66 family are based on M 3 O trimers (M = 6-coordinated Sc, In, Er-Lu) that are rare for lanthanides. We show that the size matching between urea-type solvents and metal ions is crucial for their successful synthesis. We further show that CPM-66 enables a dramatic multi-fold increase in separation efficiency over CPM-29 with 7-coordinated ions. This work provides some insights into methods to prepare low-coordinate MOFs from large ions and such MOFs could serve as highefficiency platforms for lanthanide separation, as well as other applications.

Although many rod‐packing metal‐organic frameworks are known, few are based on ordered heterometallic rod building unit. We show here the synthesis of CPM‐76 based on an unprecedented Zn‐Mg bimetallic rod with crystallographically distinguishable metal sites. The configuration of the rod offers two types of coordination site with trigonal bipyramidal and octahedral sites selectively occupied by Zn and Mg, respectively. Also unusual is the inter‐connection mode between the rods, which is based on dual‐charged forms (−3 and −2) of the 2‐hydroxyterephthalic acid (H3OBDC) ligand. Interestingly, each metal site in CPM‐76 binds one solvent molecule, leading to a high density of solvent binding sites.

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For rare-earth separation, selective crystallization into metal-organic frameworks (MOFs) offers new opportunities. Especially important is the development of MOF platforms with high selectivity toward target ions. Here we report a MOF platform (CPM-66) with low-coordinationnumber environment for rare-earth ions. This platform is highly responsive to the size variation of rare-earth ions and shows exceptional ion-size selectivity during crystallization. CPM-66 family are based on M 3 O trimers (M = 6-coordinated Sc, In, Er-Lu) that are rare for lanthanides. We show that the size matching between urea-type solvents and metal ions is crucial for their successful synthesis. We further show that CPM-66 enables a dramatic multi-fold increase in separation efficiency over CPM-29 with 7-coordinated ions. This work provides some insights into methods to prepare low-coordinate MOFs from large ions and such MOFs could serve as highefficiency platforms for lanthanide separation, as well as other applications.