Weighting non-covalent forces in the molecular recognition of C60. Relevance of concave–convex complementarity

Abstract

The relative contributions of several weak intermolecular forces to the overall stability of the complexes formed between structurally related receptors and [60]fullerene are compared, revealing a discernible contribution from concave-convex complementarity.The construction of molecular receptors for fullerenes continues to be a very active area of research, with their purification from fullerite and the construction of self-organized electroactive nanostructures as main driving forces.1-11 To achieve these objectives, the formation of stable associates with fullerenes is a prerequisite. In this regard, the group of Kawase has recently coined the term ''concave-convex interactions '' 5-8 to denote the increase in non-covalent interactions between curved aromatic hosts and guests, and suggested these might play a distinct role in the stabilization of the complexes. A fair and quantitative comparison of the stability of the complexes formed between fullerenes and receptors based on flat or concave recognizing fragments would require a collection of receptors with enough structural similarity-that is, dissimilar only with regard to the recognizing units-to be studied under experimentally identical conditions. Herein, we investigate the relative contributions of p-p, van der Waals, electrostatic, and concave-convex interactions to the molecular recognition of C 60 by a series of related receptors.We have reported receptors that exploit the concave, electron-rich, aromatic surface of p-extended tetrathiafulvalene derivatives to associate [60]fullerene.9,10 Receptor 1 features 2-[9-(1,3-dithiol-2-ylidene)anthracen-10(9H)-ylidene]-1,3-dithiole (exTTF) as the recognizing element (Chart 1).9 Despite the lack of preorganization in its design, 1 forms stable associates with C 60 (see Table 1). Since a charge-transfer band is experimentally observed in the UV-Vis titrations (l max E 482 nm) of 1 against C 60 , up to four ''separate'' contributions to the stability of the complex can be envisaged: p-p aromatic interactions, van der Waals forces, electrostatic interactions, and concave-convex complementarity. With the aim of weighting those contributions separately, we designed and synthesized a collection of tweezer-like receptors, 1-4, in which the size, shape and electronic character of the recognizing motifs are selectively tuned. The solubility of receptors 1-4 at the concentrations employed in titration experiments (r1 mM) is sufficient to rule out solvophobic effects as a major factor in the stability of the complexes.As shown in Chart 1, receptors 1-4 consist of an isophthalic diester spacer to which two units of the corresponding recognizing moieties are attached. All the receptors were synthesized from the commercially available or previously reported methylene alcohols and isophthaloyl dichloride by standard condensation reactions in good to excellent yields, and unambiguously characterized.w Table 1 and are the average of at least two titration experiments (for details, see the Supplementary Informationw). Unfo...

Receptors for Pristine Fullerenes Based on Concave–Convex π–πInteractions

“…The order of K a values of the complexes for C 60 is 41 > 43 > 44, and no sign of association of receptor 45 was observed. The results support the perceptible contribution of concave-convex complementarity to the stabilization of supramolecular associates [72]. A concave tetrathiafulvalene-type Yoshida and coworkers form rather stable complexes with C 60 (48: K a ¼ 3410 AE 1510 M À1 and 49: K a ¼ 5430 AE 370 M À1 in toluene) [75].…”

Wraparound Hosts for Fullerenes: Tailored Macrocycles and Cages

“…However, most TTF derivatives suffer from the planarity and small surface of the electroactive unit, which results in relatively small interactions between the TTF-based host and the fullerene guest. [29] In this regard, derivative 16, a p-extended TTF derivative (exTTF) with a concave aromatic shape complementary to the convex surface of fullerenes, proved to be a remarkably successful building block for fullerene hosts ( Figure 7). [30,31] Unlike cyclotriveratrylene-based receptors 1-3, derivatives 16-20 ( Figure 7 and Figure 8) display extremely different binding abilities.…”

Buckyballs

“…Thanks to their synthetic accessibility we could access a collection of hosts in which we could investigate the specific contribution of concave-convex interactions to the molecular recognition of C 60 (46-48 in Fig. 19) [153]. All three receptors bear the same number of aromatic rings and are approximately equal in size, so the contribution of πÀπ and dispersion interactions can be considered equivalent.…”