Gold-Clip-Assisted Self-Assembly and Proton-Coupled Expansion-Contraction of a Cofacial FeIII -Porphyrin Cage.

A molecular cage {Au8 (μ-PAnP)4 [Fe(H2 O)2 (TPyP)]2 (OTf)2 }(OTf)8 (1) composed of two cofacial FeIII -porphyrin can be self-assembled from the gold clip [Au2 (PAnP)Cl2 ] and Fe3+ (H2 O)2 (TPyP)+ (PAnP=9,10-bis(diphenylphosphino)anthracene, TPyP=meso-tetra(4-pyridyl)porphyrinato). The height of the cage is 8.579(3) Å. The addition of a base to a solution of the cage leads to a contracted and twisted cage {[Au8 (μ-PAnP)4 [Fe2 (μ-O)(TPyP)2 ]}(OTf)8 (2), which has a height of ≈4.4 Å and porphyrin-porphyrin torsional angle of ≈20°. The contracted cage can be synthesized independently from the gold clip and Fe2 (μ-O)(TPyP)2 . The spectroscopy and crystal structure of an unclipped analog of the contracted cage, {[AuPPh3 )8 [Fe2 (μ-O)(TPyP)2 ]}(OTf)8 (3), supports the DFT-calculated structure of 2. NMR and UV/Vis titrations show that the expansion-untwisting and contraction-twisting of the cage is reversible.