Tunable porosities and shapes of fullerene-like spheres

Abstract

The formation of reversible switchable nanostructures monitored by solution and solid-state methods is still a challenge in supramolecular chemistry. By a comprehensive solid state and solution study we demonstrate the potential of the fivefold symmetrical building block of pentaphosphaferrocene in combination with Cu(I) halides to switch between spheres of different porosity and shape. With increasing amount of CuX, the structures of the formed supramolecules change from incomplete to complete spherically shaped fullerene-like assemblies possessing an Ih -C80 topology at one side and to a tetrahedral-structured aggregate at the other. In the solid state, the formed nano-sized aggregates reach an outer diameter of 3.14 and 3.56 nm, respectively. This feature is used to reversibly encapsulate and release guest molecules in solution.

Keywords: fullerene chemistry; host-guest chemistry; molecular switch; phosphorus; supramolecular chemistry.

Figure 1

a) Scaffold of the fullerene-like supramolecules 2-Cl and 2-Br with one incorporated molecule CH₂Cl₂. A second position of the disordered molecule is shown in lighter shade b) Complete sphere of 2-Cl/2-Br (without H atoms).

Figure 2

Scheme for the synthesis of 2-Cl/2-Br and 3 and their transformations. The X-ray-derived core scaffolds of the molecular structures of the different supramolecules are depicted.

Figure 3

a) Minimal cube-like scaffold of 2-Br involving only the Cu(1) position. b) The complete fullerene-like scaffold of 2 a-Br with fully occupied Cu(1) and Cu(2) positions.

Figure 4

Formation trends and equilibria of 2b–d-Br, 2a CD₂Cl₂-Br, 2a CD₃CN-Br, and 3 in solution. ¹H and ³¹P spectra of 1 (33 mm) and increasing equivalents of CuBr in CD₂Cl₂ with 9 vol % CD₃CN (¹H NMR spectra at 298 K and 600 MHz, ³¹P NMR spectra at 300 K and 400 MHz; *=impurity of [Cp^4BnFe(η⁵-P₅)] (Cp^4Bn=tetrabenzylcyclopentadienyl)).

Figure 5

a)–m) Titration ¹H spectra of the self-assembling supramolecular switch with incorporated guests composed of 1, 2.0 equivalents CuBr, and 0.25 equivalents ferrocene in CD₃CN/CD₂Cl₂ mixtures, each at 298 K and 600 MHz. Starting from mixture a) CD₃CN, CuBr, CD₂Cl₂ or 1 were added, see arrows. Due to this procedure the relative, not the absolute signal intensities, have to be considered (see text).