Hosting Fullerenes by Dynamic Bond Formation with an Iridium Porphyrin Cyclic Dimer: A “Chemical Friction” for Rotary Guest Motions

A cyclic dimer of methyliridium porphyrin 1Ir shows an extraordinary affinity toward fullerenes via a dynamic bond-forming interaction. The crystal structure of inclusion complex 1Ir⊃C60 demonstrates the bond formation between the iridium centers of 1Ir and guest C60, leading to an ellipsoidal deformation of C60. According to variable-temperature 13C NMR spectroscopy, included C70 adopts, over a wide temperature range, an end-on orientation relative to the metalloporphyrin units, as a consequence of the site-specific bond formation onto the CC bonds located at the pole positions of C70. Such a bond formation is dynamic but prevails over van der Waals interactions and gives rise to a “chemical friction” for the rotary motions of the included fullerenes. Noteworthy is a restricted rotation of included C70 along its longer axis as a consequence of thermal slipping of the iridium centers from one CC bond to the neighboring ones at the pole positions of C70. In sharp contrast, rhodium complex 1Rh as a reference does not show any of the above features under identical conditions.