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 H-1 NMR, C-13 NMR and mass spectrometry. They had different H-1 NMR spectra but similar 13C NMR spectra. The diameter of the internal cavity of P5 is ca. 4.6 angstrom, similar to those of cucurbit[6]uril (ca. 5.8 angstrom) and alpha-cyclodextrin (ca. 4.7 angstrom). The diameter of the internal cavity of P6 is ca. 6.7 angstrom, similar to those of cucurbit[7]uril (ca. 7.3 angstrom) and beta-cyclodextrin (ca. 6.0 angstrom). The diameter of the internal cavity of P7 is ca. 8.7 angstrom, similar to those of cucurbit[8]uril (ca. 8.8 angstrom) and gamma-cyclodextrin (ca. 7.5 angstrom). The host-guest binding properties of them were investigated with n-octyltriethyl ammonium hexafluorophosphate C 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 H-1 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 H-1 NMR time scale at 22 degrees C. A mole ratio plot indicated that the complexation stoichiometry was 1 : I 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 superset of G-PF6](+) The association constant of the 1 : 1 complex P6 superset of 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.