Synthesis of 1, 4-Bis (n-propoxy) pillar [7] arene and its host-guest chemistry

Pillar[n]arenes, as a new type of macrocyclic hosts, have attracted more and more attention in recent years. Their repeating units are connected by methylene bridges at the para-positions so they have a pillar architecture, which is different from the basket-shaped structure of meta-bridged calixarenes. Herein, 1, 4-Bis (n-propoxy) pillar [5] arene (P5), 1, 4-bis (n-propoxy) pillar [6] arene (P6) and 1, 4-bis (n-propoxy) pillar [7] arene (P7) were synthesized from the condensation of 1, 4-di (n-propoxy)-2, 5-bis (methoxymethyl) benzene using p-toluenesulfonic acid as the catalyst in dichloromethane. They were characterized by ¹H NMR, ¹³C NMR and mass spectrometry. They had different ¹H NMR spectra but similar ¹³C NMR spectra. The diameter of the internal cavity of P5 is ca. 4.6 Å, similar to those of cucurbit [6] uril (ca. 5.8 Å) and α-cyclodextrin (ca. 4.7 Å). The diameter of the internal cavity of P6 is ca. 6.7 Å, similar to those of cucurbit [7] uril (ca. 7.3 Å) and β-cyclodextrin (ca. 6.0 Å). The diameter of the internal cavity of P7 is ca. 8.7 Å, similar to those of cucurbit [8] uril (ca. 8.8 Å) and γ-cyclodextrin (ca. 7.5 Å). The host-guest binding properties of them were investigated with n-octyltriethyl ammonium hexafluorophosphate G as a model guest. P6 showed complexation with G while weak complexation was observed between G and P5, but no complexation occurred between P7 and G. The ¹H NMR spectrum of an equimolar solution of P6 and G in chloroform-d showed only one set of peaks, indicating fast-exchange complexation between P6 and G on the ¹H NMR time scale at 22°C. A mole ratio plot indicated that the complexation stoichiometry was 1:1 between P6 and G. This was further confirmed by a low resolution electrospray ionization mass spectroscopy peak at m/z 1451.7 corresponding to [P6⊃G-PF₆]⁺. The association constant of the 1:1 complex P6⊃G in chloroform-d was determined to be (740±86) L•mol^-1 by a proton NMR titration. These experiments demonstrate that G fits the cavity of P6 while it is too big for the cavity of P5 and too small for the cavity of P7.