Ti3+-, V2+/3+-, Cr2+/3+-, Mn2+-, and Fe2+-Substituted MOF-5 and Redox Reactivity in Cr- and Fe-MOF-5

The metal nodes in metal-organic frameworks (MOFs) are known to act as Lewis acid catalysts, but few reports have explored their ability to mediate reactions that require electron transfer. The unique chemical environments at the nodes should facilitate unusual redox chemistry, but the difficulty in synthesizing MOFs with metal ions in reduced oxidation states has precluded such studies. Herein, we demonstrate that MZn3O(O2C-)6 clusters from Zn4O(1,4-benzenedicarboxylate)3 (MOF-5) serve as hosts for V(2+) and Ti(3+) ions and enable the synthesis of the first MOFs containing these reduced early metal ions, which can be accessed from MOF-5 by postsynthetic ion metathesis (PSIM). Additional MOF-5 analogues featuring Cr(2+), Cr(3+), Mn(2+), and Fe(2+) at the metal nodes can be obtained by similar postsynthetic methods and are reported here for the first time. The inserted metal ions are coordinated within an unusual all-oxygen trigonal ligand field and are accessible to both inner- and outer-sphere oxidants: Cr(2+)- converts into Cr(3+)-substituted MOF-5, while Fe(2+)-MOF-5 activates NO to produce an unusual Fe-nitrosyl complex.