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

摘要

Cyclic hosts that strongly entrap fullerenes are highly important for selective extraction and noncovalent modification of such spherical carbon nanoclusters. Since our discovery of the hosting capability of a zinc porphyrin cyclic dimer in 1999, various host molecules composed of multiple metalloporphyrin units have been reported, which can trap fullerenes in solution via van der Waals interactions. Noteworthy is rhodium(Ⅲ) porphyrin cyclic dimer 1_(Rh) (Chart 1), which is roughly 100 times more affinitive than its zinc porphyrin analogue toward fullerenes. The association constants (K_(assoc)) observed with C_(60) and C_(70) (10~7-10~8 M~(-1) in C_6H_6) have been the largest among those reported so far, suggesting a possible contribution of the metal centers of the host to the inclusion of fullerenes. Here we report an iridium version of 1_(Rh) (1_(Ir); Chart 1) as an ultimate host that can entrap fullerenes by a "bond-forming interaction" and displays even 2-3 orders of magnitude greater K_(assoc) values than 1_(Rh), This bond-forming interaction also forces C_(60) to be deformed and allows C_(70) to adopt an end-on orientation relative to the metalloporphyrin units. From restricted rotary motions of the guest fullerenes, we propose an interesting aspect of "chemical friction" on graphitic surfaces.