Straightforward metal-free synthesis of an azacalix[6]arene forming a host–guest complex with fullerene C₆₀

Abstract

The straightforward metal-free synthesis of an azacalix[6]arene from simple aryl units is described together with its full characterization. This macrocycle features a rare 1,2,4,5-alternate conformation and is able to form a 1 : 1 stable host–guest complex with C60.

Azacalixquinarenes: From Canonical to (Poly-)Zwitterionic Macrocycles

“…The distance between the two oxidized ring centroids is 6.86 Å, this value being close to the one found in unsubstituted ACP (6.65 Å) and standing out from classical 1,3-alternate ACA substituted aromatic rings. 24 The angle formed between ble to the data measured for unsubstituted ACP (see T transfers and tautomeric structures of series 1a-d, azacalix [4]quinarenes 2a stand for canonical and zwitterionic, respectively. Note that only one structure for compounds containing at least one c unit is shown here but other (generally less stable) tautomers can be drawn, see Schemes S1, S2, and S3 in the ESI.…”

“…was crystallized from evaporation of a solution in acetone and stands as one of the rare example derivatives (Figure 4). 13,14,21,23,24,37,38 The structure reveals that pendent molecules in the asymmetric unit that stack together through two π-π ring interactions between, on one hand, cycles and, on the other hand, two of their aromatic cycles. Indeed the di 5 electrons subsystems are not belonging to the same plane, with an angle of 6° for the zwitterionic system and 4° for the canonical one.…”

“…5 Since seminal works in the late nineties on polyazacalixarenes, [6][7][8][9][10] this family has been growing to incorporate trypticene moieties, 11,12 pyridine units, 13,14 and covalently-bonded metallic centers conferring unusual reactivity. [15][16][17][18][19][20] Noteworthy, ACA macrocycles were valorized for carbon dioxide uptake, 21,22 fullerene or anions complexation [23][24][25] and demonstrated to be relevant as hole-and spin-containing architectures. 26,27 In 2013, the azacalixphyrin (ACP, Figure 1) was reported as the only known "azacalixarene-like" derivative introducing four oxidized tetraaminobenzene units that promote a stable bis-zwitterionic ground electronic state.…”

“…The synthesis of azacalix [6]arenes 8a was more straightforward and allowed the isolation of the macrocycle in only one step from octylamine-decorated tetraaminobenzene 5a with, however, a poor 5% yield. 24 Note that the formation of higher order macrocycles was not observed. As expected, series of azacalix [4]arenes 7a−c and azacalix [6]arene 8a could be oxidized to the corresponding azacalixquinarenes 2a−c and 3a using 2 and 3 equiv of 2,3dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), respectively.…”

“…The distance between the two oxidized ring centroids is 6.86 Å, this value being close to the one found in unsubstituted ACP (6.65 Å) and standing out from classical 1,3-alternate ACA 7a, which presents a distance of 4.74 Å between the two octyl-substituted aromatic rings. 24 The angle formed between the two planes of the quinone rings is ca. 110°, which is comparable to the data measured for unsubstituted ACP (see Table S3).…”