Supramolecular complexation studies of [60]fullerene with calix[4]naphthalenes—a reinvestigation

Abstract

Estimations of equilibrium or association constant (K_ASSOC) values reported by many other groups for the supramolecular complexation between [60]fullerene (‘C₆₀’) with different macrocyclic hosts, in solvents such as toluene or carbon disulfide, for example, is often conducted by UV–vis absorption and/or ¹H NMR spectroscopy. In this paper, the complexation behaviour of two calix[4]naphthalene hosts with C₆₀ in toluene and carbon disulfide has been re-examined, using both of these methods. An analysis is presented of the data newly obtained, in light of recent advances and understanding published by others of the limitations of, in particular, the absorption spectroscopic methods. The discussion presented is also intended to aid those who may be unfamiliar with the nuances and limitations of the analytic models involving C₆₀ supramolecular complexation. Also presented is a general mechanism for C₆₀ supramolecular complexation studies, which lay the groundwork for further experiments.

Introduction

In 1999 Diederich and Gomez-Lopez¹ reviewed advances on supramolecular fullerene chemistry which had taken place during the previous decade and pointed out the importance of inclusion complexes of [60]fullerene (‘C₆₀’) with various macrocyclic host molecules. Prominent among those host molecules reviewed were the calixarenes, for example, 1, (Fig. 1), a class of molecules whose unusual and varied properties continue to be subjects of extensive study.² In 1992 Williams and Verhoeven showed that the water-soluble calix[8]arene derivative (2) was capable of extracting C₆₀ into an aqueous phase.³ Later, Atwood⁴ and coincidentally, Shinkai,⁵ discovered that in toluene solution, p-tert-butylcalix[8]arene (3) could selectively sequester C₆₀ from a mixture containing both C₆₀ and C₇₀, by forming a precipitate of a 1:1 complex with C₆₀. An attempt to crystallize this precipitate from chloroform resulted instead in de-complexation, leading to the production of pure C₆₀. This finding among others, led to extensive studies of fullerenes with various other calixarenes and calixarene derivatives, many of which were more recently reviewed in 2001 by Shinkai et al.⁶

In 1999 we reported⁷ that calix[4]naphthalenes, for example, 4 and 5, a class of naphthalene-ring based calixarenes,⁸ also formed supramolecular complexes with C₆₀. At the time, we had reasoned that the deeper, more electron-rich cavities formed by the naphthalene subunits would facilitate binding with the electron-deficient fullerenes, as a result of the extra π–π* interactions⁹ possible when compared with the corresponding calix[4]arenes, which do not bind with C₆₀. Our initial results were consistent with this hypothesis. Those findings were based upon the observation that the pale yellow solutions of p-tert-butylcalix[4]naphthalene (5) in toluene, turned to red-brown upon the addition of C₆₀. UV–vis absorption spectroscopic studies conducted at the time were strongly suggestive of 1:1 supramolecular complexation between C₆₀ and, for example, 5, in all three solvents which were used, namely, toluene, benzene and CS₂, affording binding, or apparent association constants (K_ASSOC) of 676±28, 295±13, and 6920±330 M⁻¹, respectively. These binding constants were determined by measuring the changes of the absorbances at λ=430 nm of solutions into which microlitre aliquots of solutions (e.g., 2–10×10⁻⁴ M) of the host molecule, in the same solvent, were added to a fullerene solution (approximately 1×10⁻⁴ M). The λ=430 nm region is that upon which studies were basing their evidence for complex formation with similar systems,6, 7, 10, 11, 12, 13, 14 and as discussed below, could be misleading.

Toluene, benzene or CS₂ are solvents commonly used for such spectrophotometric-based supramolecular complexation studies with fullerenes. The solubilities¹⁵ of C₆₀ at 298 K, in benzene (1.22–2.58×10⁻³ M), toluene (2.99–4.44×10⁻³ M) or CS₂ (7.17–16.4×10⁻³ M) are sufficient enough to conveniently allow the determination of association (formation) constants for the complexation of various macrocyclic hosts with C₆₀ in these solvents, using either absorption spectroscopy or, ¹H NMR spectroscopy.¹⁶ For the ¹H NMR studies, the deuterated analogues, benzene-d₆ or toluene-d₈ are used, but for CS₂, an external deuterium atom-containing lock is required. In several examples reported more recently by Mukerjhee's group,17, 18 however, the use of tetrachloromethane, surprisingly, has also been used in complexation studies of various calixarenes with C₆₀ using both absorption and NMR studies, even though the solubility of C₆₀ in this solvent is lower (i.e., 1.4–6.2×10⁻⁴ M),¹⁵ than those of the other solvents described above.

Although ruby-red rod-like crystals of a 1:1 complex of C₆₀ and 5 (as confirmed by +FAB MS data) were produced from the slow evaporation of a toluene solution that was equimolar with respect to both C₆₀ and 5, we were unable at the time to obtain suitable single-crystal X-ray diffraction data from these crystals. During the collection of the diffraction data, the solvent diffused out from the crystal lattice resulting in the observation of only very low angle peaks. Later, however, we successfully obtained a single-crystal X-ray structure of the related p-tert-butylhexahomotrioxacalix[3]naphthalene 6,¹⁹ which revealed it to be a 2:1 ‘capsule-like’⁶ structure in which a C₆₀ molecule is encapsulated by two molecules of 6. The tert-butyl groups of each molecule of 6 surround the fullerene in a pincer-like embrace, in support with the hypothesis that π–CH₃ interactions are dominant ones. The binding, or association constants in this study were determined in both toluene-d₈ and benzene-d₆, by measuring ¹H NMR shift changes in 6 as a function of added C₆₀.

Since our initial 1999 studies with 4 and 5 were undertaken using UV–vis absorption spectroscopy only, we decided to reinvestigate their complexation properties. We have now found that while the ¹H NMR spectra of solutions of 5 in toluene-d₈ to which aliquots of C₆₀ were added, showed clear, reasonably sizeable chemical shift changes, and afforded reproducible K_ASSOC values, we could find no comparable evidence for any complexation of either 4 or 5 in CS₂ solution. In this paper, we describe the results from a reinvestigation of the supramolecular complexation properties of 4 and 5 using both ¹H NMR and absorption spectroscopy, in light of more recent research findings by us, and by other researchers, in particular, with respect to the behaviour of C₆₀ in various solvents.