Activating\nMetal–Organic Cages by Incorporating\nFunctional M(ImPhen)<sub>3</sub> Metalloligands: From Structural Design\nto Applications

ConspectusThe emulation of ingenious biofunctions\nhas been a research focus\nfor several decades. Metal–organic cages (MOCs), as a type\nof discrete supramolecular assembly with well-defined shapes and cavities,\nhave aroused great interest in chemists to imitate natural protein\ncages or enzymes. However, to genuinely achieve tailored functionalities\nor reactivities of enzymes, the design of cage structures combining\nboth the confined microenvironment and the active site is a prerequisite.\nTherefore, the integration of functionalized motifs into MOCs is expected\nto provide a feasible approach to construct biofunctional confined\nnanospaces, which not only allows the modulation of cage properties\nfor applications such as molecular recognition, transport, and catalysis\nbut also creates unique microenvironments that promote enzymatic effects\nfor special reactivities and selectivities, thereby providing a versatile\nplatform to achieve exceptional biomimetic functions and beyond.In this Account, we specifically focus on our research toward engineering\nactive confined-nanospaces in MOCs via incorporation of M(ImPhen)<sub>3</sub> metalloligands, a typical tris-chelate coordination moiety\ncomprising imidazophenanthroline ligands and variable metal ions,\nas the principle functional units for stepwise assembly of active-MOCs.\nStarting from their structure design and merits, we describe the versatility\nof M(ImPhen)<sub>3</sub> centers for multifunctionalization of the\nconfined cage-nanospaces. By integrating different metal ions like\nRu, Os, Fe, Co, Ni, Zn, the metal ion inherent properties, e.g., redox\nactivity of Fe/Co-centers, chirality, and photoactivity of Ru-centers,\nand dynamics of Co/Zn-centers, could be integrated and tailored on\nthe cages as isostructural nanosized containers or reactors. Changing\nthe Pd or Pt cage vertices to organic clips could remarkably enhance\nacid–base stability and endow cages with flexibility and allostery.\nUtilization of ImPhen organic ligands containing imidazole groups\nintroduces proton transfer capability, which can couple with the high-positive\ncharges on the cage to create amphoteric microenvironments in the\nporous open-cage solution. Moreover, the nonplanar stereoconfiguration\nof M(ImPhen)<sub>3</sub> confers multiple peripheral pockets on the\ncage, which render multisite, high-order, and dynamics guest binding\nfor the benefit of applications such as drug delivery, molecular separation,\nand catalytic turnover.The construction of active-MOCs from\ntailorable M(ImPhen)<sub>3</sub> metalloligands provides us with a\nnew perspective on their structural\ndesign and functionalities. Merging the cage confinement with distinct\nphysicochemical properties on a supramolecular level makes it practical\nto realize synergistic and cooperative effects for functionality enhancement\nbeyond molecular components or the reactivity different from the bulky\nsolution, which could largely expand the potential of MOCs as a multirole\nplatform to wide application scenarios such as artificial photosynthesis,\nunconventional catalysis, and theranostic nanomedicine.