Regioswitchable Bingel Bis-Functionalization of Fullerene C70 via Supramolecular Masks

Abstract

Isomer-pure functionalized fullerenes are required to boost the development of fullerene chemistry in any field, but their multiple functionalization renders a mixture of regioisomers that are very difficult to purify by chromatography. For the specific case of C₇₀, its nonspherical geometry makes its regioselective functionalization more challenging than that of spherical C₆₀. In this work, the supramolecular mask approach is applied for the first time to C₇₀, which is encapsulated in two different nanocapsules to achieve the Bingel bis-cyclopropanation at α-bonds of opposite poles. Based on the tetragonal prismatic geometry imposed by the smaller supramolecular mask tested, the obtained major bis-adduct is completely reversed (major 5 o'clock) compared to bare C₇₀ functionalization (major 2 o'clock). Moreover, by further restricting the accessibility of C₇₀ using a three-shell Matryoshka mask and dibenzyl-bromomalonate, a single regiospecific 2 o'clock bis-isomer is obtained, owing to the perfect complementarity of the mask and the addend steric properties. The outcome of the reactions is fully explained at the molecular level by means of a thorough molecular dynamics (MD) study of the accessibility of the α-bonds to produce the different bis-adducts.

Figure 1

Current regio-functionalization strategies of fullerene C₆₀.

Figure 2

(A) α and α′ (6,6′) bonds of C₇₀. (B) The three possible regioisomers of a bis-α,α-C₇₀-adduct (Schlegel diagram using the Newman-type projection, and with the C₇₀ core viewed along the C₅ axis passing through the proximal (small) and distal (large) pentagons at the opposite poles of the fullerene).

Figure 3

General procedures for the functionalization of C₇₀. (A) Using bare C₇₀; (B) using C₇₀⊂4·(BArF)₈; and (C) using C₇₀⊂[10]CPP⊂6·(BArF)₈.

Figure 4

(A) Composition of Pd-based nanocapsules 4·(BArF)₈ and 6·(BArF)₈ and the Cu-based 7·(BArF)₈ nanocapsule (isostructural to 6). (B) Crystal structure of 7·(BArF)₈ obtained by synchrotron radiation, top and side views.

Figure 5

Experimental results obtained with the four bromomalonates (Et, iPr, tBu, and Bn) for (A) bare C₇₀, (B) using nanocapsule 4·(BArF)₈, and (C) C_70-Matryoshka.

Figure 6

(A) MD studies on mono-di-benzyl-malonate-C₇₀⊂4·(BArF)₈ and accessibility analysis using criteria A and B. Space-filling models of two representative structures are used to show the accessibility of the α-bonds. Probability density plots of criteria A and B obtained from five replicas of 500 ns of MD simulations (all distances are in Å) where X = 2, 5, and 12 o’clock centers of mass; Zn1 and Zn2 = the position of the zinc ions of the porphyrin rings; and W^COM = Centre of masses of nanocapsule windows. The thresholds of the accessible zones are determined based on nonmasked regions in the space filling models. (B) Representative structures of bis-dibenzyl-malonate-C₇₀⊂4·(BArF)₈ obtained from MD simulations.

Figure 7

Comparative experimental regioisomer ratios found for bare C₇₀ (left) and C₇₀⊂4·(BArF)₈ (right, along with the MD simulations of mono-di-^tBu-malonate-C₇₀⊂4·(BArF)₈ predicting full exposure of the fullerene. Color code: blue (12 o’clock), red (2 o’clock), and yellow (5 o’clock).

Figure 8

(A) MD studies on mono-di-benzyl-malonate-C₇₀ ⊂[10]CPP⊂6·(BArF)₈ and accessibility analysis using criteria A, B, and C. Space-filling models of two representative structures are used to show the accessibility of the α-bonds. Probability density plots of criteria A and B obtained from five replicas of 500 ns of MD simulations (all distances are in Å) where X = 2, 5, and 12 o’clock centers of mass; Zn1 and Zn2 are the positions of the zinc ions of the porphyrin rings; W^COM represents the centers of mass of nanocapsule windows; and C@[10]CPP is a carbon atom in the [10]CPP nanohoop. The thresholds of the accessible zones are determined based on nonmasked regions of the space-filling models. (B) Representative structures of bis-dibenzyl-malonate-C₇₀⊂[10]CPP⊂6·(BArF)₈ obtained from MD simulations.