Metal-organic polyhedra (MOPs) are ultra-small (typically 1 nm to 4 nm) porous coordination cages made from the self-assembly of metal ions and organic linkers and amenable to the chemical functionalization of its periphery; however, it has been challenging to implement post-synthetic functionalization due to their chemical instability. Herein, we report the use of coordination chemistries and covalent chemistries to post-synthetically functionalize the external surface of 2.5 nm stable Rh(II)based cuboctahedra through their Rh-Rh paddlewheel units or organic linkers, respectively. We demonstrate that 12 N-donor ligands, including amino acids, can be coordinated on the periphery of Rh-MOPs. We used this reactivity to introduce new functionalities (e.g. chirality) to the MOPs and to tune their hydrophilicity/hydrophobic character, which allowed us to modulate their solubility in diverse solvents such as dichloromethane and water. We also demonstrate that all 24 organic linkers can be postsynthetically functionalized with esters via covalent chemistry. In addition, we anticipate that these two types of post-synthetic reactions can be combined to yield doubly-functionalized Rh-MOPs, in which a total of 36 new functional molecules can be incorporated on their surfaces. Likewise, these chemistries could be synergistically combined to enable covalent functionalization of MOPs through new linkages such as ethers. We believe that both reported post-synthetic pathways can potentially be used to engineer Rh-MOPs as scaffolds for applications in delivery, sorption and catalysis.

Herein we report a novel, ozone-based method for post-synthetic generation of mesoporosity in metalorganic frameworks (MOFs). By carefully selecting mixed-ligand Zr-fcu-MOFs based on organic ligand pairs in which one ligand has ozone-cleavable olefin bonds and the other ligand is ozoneresistant, we were able to selectively break the cleavable ligand via ozonolysis to trigger fusion of micropores into mesopores within the MOF framework. This solid-gas phase method is performed at room-temperature and, depending on the cleavable ligand used, the resultant ligand-fragments can be removed from the ozonated MOF by either washing or sublimation. Compared to the corresponding highly-microporous starting MOFs, the highly-mesoporous product MOFs exhibit radically distinct gas sorption properties. Herein we report application of our post-synthetic strategy to selectively and quantitatively cleave and remove the organic ligands in two multivariate (MTV) 23 Zr-fcu-MOFs (Figure 1), thereby affecting their adsorption performance in gas uptake. By controlling the ozone inert/active ratio of ligands in these MOFs, we were able to control the final number of defects in their structures.

The transfer of nanoparticles between immiscible phases can be driven by externally-triggered changes in their surface composition. Interestingly, phase transfers can enhance the processing of nanoparticles and enable their use as vehicles for transporting molecular cargo. Herein we report extension of such phase transfers to encompass porous Metal-Organic Polyhedra (MOPs). We report that a hydroxyl-functionalized, cuboctahedral Rh(II)-based MOP can be transferred between immiscible phases by pH changes or by cation-exchange reactions. We demonstrate use of this MOP to transport coordinatively-bound cargo between immiscible layers, including into solvents in which the cargo is insoluble. As proof-of-concept that our phase transfer approach could be used in chemical separation, we employed Rh(II)-based MOPs to separate a challenging mixture of structurally similar cyclic aliphatic (tetrahydrothiophene) and aromatic (thiophene) compounds. We anticipate that transport of coordinatively-bound molecules will open new avenues for molecular separation based on the relative coordination affinity that the molecules have for the Rh(II) sites of MOP.

We describe solid-gas phase, single-crystal-to-single-crystal, postsynthetic modifications of a metal-organic framework (MOF). Using ozone, we quantitatively transformed the olefin groups of a UiO-66-type MOF into 1,2,4-trioxolane rings, which we then selectively converted into either aldehydes or carboxylic acids.

Herein we report that strategic use of protecting groups in coordination reactions enables directional inhibition that leads to synthesis of metal–organic polyhedra (MOPs) highly functionalized with carboxylic acid and amine groups.