Tribenzotriquinacene Receptors for C₆₀ Fullerene Rotors: Towards C₃ Symmetrical Chiral Stators for Unidirectionally Operating Nanoratchets

Abstract

The synthesis of a stereochemically pure concave tribenzotriquinacene receptor (7) for C60 fullerene, possessing C3 point group symmetry, by threefold condensation of C2 -symmetric 1,2-diketone synthons (5) and a hexaaminotribenzotriquinacene core (6) is described. The chiral diketone was synthesized in a five-step reaction sequence starting from C2h -symmetric 2,6-di-tert-butylanthracene. The highly diastereo-discriminating Diels-Alder reaction of 2,6-di-tert-butylanthracene with fumaric acid di(-)menthyl ester, catalyzed by aluminium chloride, is the relevant stereochemistry introducing step. The structure of the fullerene receptor was verified by (1)H and (13)C NMR spectroscopy, mass spectrometry and single crystal X-ray diffraction. VCD and ECD spectra were recorded, which were corroborated by ab initio DFT calculations, establishing the chiral nature of 7 with about 99.7 % ee, based on the ee (99.9 %) of the chiral synthon (1). The absolute configuration of 7 could thus be established as all-S [(2S,7S,16S,21S,30S,35S)-(7)]. Spectroscopic titration experiments reveal that the host forms 1:1 complexes with either pure fullerene (C60) or fullerene derivatives, such as rotor 1'-(4-nitrophenyl)-3'-(4-N,N-dimethylaminophenyl)-pyrazolino[4',5':1,2][60]fullerene (R). The complex stability constants of the complexes dissolved in CHCl3/CS2 (1:1 vol. %) are K([C60 ⊂7]) = 319(±156) M(-1) and K([R⊂7]) = 110(±50) M(-1). With molecular dynamics simulations using a first-principles parameterized force field the asymmetry of the rotational potential for [R⊂7] was shown, demonstrating the potential suitability of receptor 7 to act as a stator in a unidirectionally operating nanoratchet.

Almost Enclosed Buckyball Joints: Synthesis, Complex Formation, and Computational Simulations of Pentypticene‐Extended Tribenzotriquinacene

“…[12] Based on these findings we performed 1 H NMR titration experiments to determine the stoichiometry and the stability constant of the host-guest complex [C 60 &3]. This is in line with the results for a recently developed chiral tribenzotriquinacene-based host, which has a similar low [C 60 &host] stability constant of 319 AE 156 m À1 .…”

“…Owing to the C 2v point group symmetry of dione 9, the four bridgehead protons of the triptycene moiety show only one singlet at d = 5.37 ppm in the 1 H NMR spectrum ( Figure S6 f). [8,9,12] In detail, the six protons of the quinoxaline groups (a), the twelve para protons of the isolated benzene moieties (b, b'), the 18 bridgehead protons (g, h, h'), and the twelve protons of the four methyl groups at the tribenzotriquinacene moiety correspond to the singlets at d = 7.68, 7.43, 7.38, 5.26, 5.25, 5.19, 1.58, and 1.19 ppm, respectively. Note that the aromatic rings, which are annelated at the central 2.2.2-bicyclooctane moiety, are not chemically equivalent.…”

“…This was also observed in similar tribenzotriquinacene-based host systems. [8,9,12] In detail, the six protons of the quinoxaline groups (a), the twelve para protons of the isolated benzene moieties (b, b'), the 18 bridgehead protons (g, h, h'), and the twelve protons of the four methyl groups at the tribenzotriquinacene moiety correspond to the singlets at d = 7.68, 7.43, 7.38, 5.26, 5.25, 5.19, 1.58, and 1.19 ppm, respectively. Since the benzene rings that are located at the top of the cage are very close to each other, the signals of the 12 endo, endo situated protons (d, f) show a high-field shift (d = 7.11-7.09, 6.61-6.59 ppm).…”

“…This is in line with the results for a recently developed chiral tribenzotriquinacene-based host, which has a similar low [C 60 &host] stability constant of 319 AE 156 m À1 . [12] Based on these findings we performed 1 H NMR titration experiments to determine the stoichiometry and the stability constant of the host-guest complex [C 60 &3]. All investigations were performed with a solvent mixture of CDCl 3 and CS 2 (1:1), which allows us to compare results from the present study with the corresponding complex stability constants from previous investigations.…”

“…All investigations were performed with a solvent mixture of CDCl 3 and CS 2 (1:1), which allows us to compare results from the present study with the corresponding complex stability constants from previous investigations. [8,9,12] The signal of the endo, endo situated ortho protons (f, d = 6.61-6.59 ppm) can be evaluated while the other signals provide insufficient differences in the chemical shift. To take experimental artefacts of non-specific complex formation [16] such as non-linearity [17] or self-aggregation [18,19] into account, the validity of the Beer-Lambert law, which is a linear dependence of the absorbance on the solute's concentration, was tested for pure solutions of host 3 in the appropriate concentration regime within a first set of experiments, but such artefacts were not detected.…”