Pd(II) Coordination Sphere Engineering: Pyridine Cages, Quinoline Bowls, and Heteroleptic Pills Binding One or Two Fullerenes

Fullerenes
and their derivatives are of tremendous technological
relevance. Synthetic access and application are still hampered by
tedious purification protocols, peculiar solubility, and limited control
over regioselective derivatization. We present a modular self-assembly
system based on a new low-molecular-weight binding motif, appended
by two palladium­(II)-coordinating units of different steric demands,
to either form a [Pd2L14]4+ cage or an unprecedented [Pd2L23(MeCN)2]4+ bowl (with L1 = pyridyl, L2 = quinolinyl
donors). The former was used as a selective induced-fit receptor for
C60. The latter, owing to its more open structure, also
allows binding of C70 and fullerene derivatives. By exposing
only a fraction of the bound guests’ surface, the bowl acts
as fullerene protecting group to control functionalization, as demonstrated
by exclusive monoaddition of anthracene. In a hierarchical manner,
sterically low-demanding dicarboxylates were found to bridge pairs
of bowls into pill-shaped dimers, able to host two fullerenes. The
hosts allow transferring bound fullerenes into a variety of organic
solvents, extending the scope of possible derivatization and processing
methodologies.