A porous metal–organic cage constructed from dirhodium paddle-wheels: synthesis, structure and catalysis

Abstract

Self-assembly of dirhodium(II) tetraacetate (Rh₂(OAc)₄) with a dicarboxylic acid 3,3′-(1,3-phenylenebis(ethyne-2,1-diyl))dibenzoic acid (H₂pbeddb) gives rise to a metal–organic cage (MOC) containing Rh–Rh bonds with the formula of [Rh₄(pbeddb)₄(H₂O)₂(DMAC)₂] (MOC-Rh-1). Single-crystal X-ray diffraction analysis reveals that MOC-Rh-1 shows a lantern-type cage structure, in which a pair of Rh₂(CO₂)₄ paddlewheels is linked by four diacid ligands. The dimensions of the inner cavity of MOC-Rh-1 are 9.5 × 14.8 Ų (atom-to-atom distances across opposite metal and phenyl groups of pbeddb²⁻). In the solid phase, the cages are aligned by π–π stacking to form one-dimensional channels (9.5 × 11.1 Ų) through cage windows. Therefore, the crystalline samples of MOC-Rh-1 are porous with the inner and outer cavities of the cages accessible under the heterogeneous condition. MOC-Rh-1 has been fully characterized by EA, TGA, PXRD, IR, UV-vis and XPS measurements. The catalytic tests disclose that activated MOC-Rh-1 is effective in the intramolecular C–H amination of vinyl, dienyl and biaryl azides, leading to the formation of indoles, pyrroles and carbazoles, respectively, and the porous catalyst can be recycled easily and used for at least nine runs without significant loss of activity. In the nine runs, the conversions were in the range of 93–99%, whereas in the tenth run, the conversion was reduced to 78%.