Hosting fullerenes by dynamic bond formation with an iridium porphyrin cyclic dimer: A Chemical friction for rotary guest motions

A cyclic dimer of methyliridiurn porphyrin 1(Ir) shows an extraordinary affinity toward fullerenes via a dynamic bond-forming interaction. The crystal structure of inclusion complex 1(Ir)superset of C-60 demonstrates the bond formation between the iridium centers of 1(Ir) and guest C-60, leading to an ellipsoidal deformation of C-60. According to variable-temperature C-13 NMR spectroscopy, included C-70 adopts, over a wide temperature range, an end-on orientation relative to the metalloporphyrin units, as a consequence of the sitespecific bond formation onto the C=C bonds located at the pole positions of C-70. 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 C-70 along its longer axis as a consequence of thermal slipping of the iridiurn centers from one C=C bond to the neighboring ones at the pole positions Of C-70. In sharp contrast, rhodiurn complex 1(Rh) as a reference does not show any of the above features under identical conditions.