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

Abstract

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.

Tunable Fullerene Affinity of Cages, Bowls and Rings Assembled by PdII Coordination Sphere Engineering

“…[14] We tested this methodo nb is-monodentate banana-shapedl igandsw ith ad ibenzo-2.2.2-bicyclo-octane backbonea nd showedt hat the pyridine derivative L 1 leads to the formation of atypical[Pd 2 L 1 4 ] 4 + cage while 6-quinolinyl derivative L 2 cleanly forms bowl [Pd 2 L 2 3 (MeCN) 2 ] 4 + when palladium precursor[ Pd(MeCN) 4 ](BF 4 ) 2 was mixed with L 2 in a2 :3 ratio in acetonitrile. [15] As explanation for the latter finding, we indeed identifieds teric congestion aroundt he coordination site, as observed in the single crystal X-ray structure of the bowl-shaped compound. While this situation disfavors (but not absolutely prevents) the bindingo fafourth bis-monodentate quinoline ligand, the square-planar coordination sphere of the Pd II cations is either completed by an acetonitrile molecule or ac hloride ligand.…”

“…Pleasingly, a further single crystal X‐ray structure could be obtained in the course of this work, namely the host‐guest complex [C 70 @Pd 2 L 2 4 ] 4+ of the previously reported cage based on quinoline ligand L 2 and C 70 fullerene (Figure a) . With the six structures reported in our previous work and four new structures given herein, we were able to compare the relationship between ligand chemistry, host structure and fullerene (C 60 and C 70 ) binding in a systematic and comprehensive way (Figure ).…”

“…When considering other sterically more demanding donor functionalities, we recently started to substitute pyridines with quinolines, which carry an hydrogen atom on the annelated benzene ring that causes steric congestion around the metal binding site . We tested this method on bis‐monodentate banana‐shaped ligands with a dibenzo‐2.2.2‐bicyclo‐octane backbone and showed that the pyridine derivative L 1 leads to the formation of a typical [Pd 2 L 1 4 ] 4+ cage while 6‐quinolinyl derivative L 2 cleanly forms bowl [Pd 2 L 2 3 (MeCN) 2 ] 4+ when palladium precursor [Pd(MeCN) 4 ](BF 4 ) 2 was mixed with L 2 in a 2:3 ratio in acetonitrile . As explanation for the latter finding, we indeed identified steric congestion around the coordination site, as observed in the single crystal X‐ray structure of the bowl‐shaped compound.…”

“…As we reported before, the backbone of the ligands was designed as a curved combination of two π‐surfaces to bestow the hosts with the ability to bind fullerenes and solubilize them in polar organic media . Furthermore, the bowl‐shaped compound was found to act as a supramolecular protecting group to allow the selective monofunctionalization of its bound fullerene guest.…”

“…As we reported before,t he backbone of the ligandsw as designed as ac urved combination of two p-surfacest ob estow the hosts with the ability to bind fullerenes and solubilize them in polar organic media. [15] Furthermore, the bowl-shaped compound was found to act as as upramolecular protecting group to allow the selective monofunctionalization of its bound fullerene guest. In as imilar way, the herein presented cage, bowl and ring derivatives were in part found to bind fullerenes C 60 and C 70 .I ntriguingly,t he tendency of the isomeric quinoline ligands to form bowls or cages as well as the fullerene affinities of all examined cage and bowl systems in comparisons how more variety than initially expecteda nd we herein suggest an umber of structural reasonst oe xplain these observations.…”