Macrocyclic Hosts for Fullerenes: Extreme Changes in Binding Abilities with Small Structural Variations.

Abstract

Exploiting the shape and electronic complementarity of C(60) and C(70) with π-extended derivatives of tetrathiafulvalene (exTTF), we have very recently reported a macrocyclic receptor featuring two exTTF recognizing units which forms 1:1 complexes with C(60) with log K(a) = 6.5 ± 0.5 in chlorobenzene at 298 K. This represents one of the highest binding constants toward C(60) reported to date and a world-record for all-organic receptors. Here, we describe our efforts to fine-tune our macrocyclic bis-exTTF hosts to bind C(60) and/or C(70), through structural variations. On the basis of preliminary molecular modeling, we have explored p-xylene, m-xylene, and 2,6-dimethylnaphthalene as aromatic spacers between the two exTTF fragments and three alkene-terminated chains of different length to achieve macrocycles of different size through ring closing metathesis. Owing to the structural simplicity of our design, all nine receptors could be accessed in a synthetically straightforward manner. A thorough investigation of the binding abilities of these nine receptors toward C(60) and C(70) has been carried out by means of UV-vis titrations. We have found that relatively small variations in the structure of the host lead to very significant changes in affinity toward the fullerene, and in some cases even in the stoichiometry of the associates. Our results highlight the peculiarities of fullerenes as guests in molecular recognition. The extreme stability of these associates in solution and the unique combination of electronic and geometrical reciprocity of exTTF and fullerenes are the main features of this new family of macrocyclic hosts for fullerenes.

Wraparound Hosts for Fullerenes: Tailored Macrocycles and Cages

“…[32] We have thoroughly investigated the recognition properties of a whole family of exTTF-based macrocycles 18-20 by systematically modifying the spacer (p-phenylene, m-phenylene, and 2,6-naphthylene) and the alkenyl chain length (n = 1, 2, 3). [33] This study allowed us to discover some remarkably efficient macrocyclic hosts for fullerenes; for instance, 20 c binds C 70 with log K a = 6.1 in chlorobenzene at room temperature. Moreover, we also showed how small structural variations lead to important changes in the binding ability.…”

Size‐Selective Encapsulation of C60 by [10]Cycloparaphenylene: Formation of the Shortest Fullerene‐Peapod

“…During studies on the synthesis of CPPs [22][23][24] by our own approach [25] and elucidation of their properties, [26,27] we found the first example of a host-guest complex of CPP and C 60 . [28] Here we report the size-selective encapsulation of C 60 by [10]CPP, which represents the shortest fullerene-peapod.…”

Sponge-like molecular cage for purification of fullerenes

“…Host platforms containing extended p-systems have been designed because of their affinity towards the sphere-like unsaturated structure of fullerenes. In this line, several covalent or supramolecular approaches pursued include: (a) macrocyclic arene-based receptors [18][19][20][21][22][23][24] , (b) macrocyclic structures containing aromatic heterocycles [25][26][27][28] , (c) p-extended TTF receptors [29][30][31][32][33] and (d) macrocyclic or jawlike structures containing porphyrin moieties 17,[34][35][36][37][38][39][40][41][42][43][44][45] . Interestingly, Fujita and co-workers reported the use of the empty channels of a metal-organic framework (MOF) for encapsulating fullerenes upon soaking MOF crystals in fullerene-containing toluene.…”