Complexation of crown ethers and podands with tropylium cations: Determination of stability constants and crystal structure of the dibenzo-24-crown-8-tropylium cation complex

The host-guest complexation of macrocyclic hosts and aromatic carbenium cations has been studied. Three different tropylium tetrafluoroborate compounds were prepared in order to study the interaction between positively charged guests and crown ether hosts. The stability constants of complexes were measured by H-1 NMR and UV/Vis spectroscopic methods. Fast atom bombardment mass spectroscopy (FABMS) was used to screen the stoichiometry of complexes. The main interaction force was pi-pi-stacking and cation-pi interaction between aromatic units in benzene-substituted crown ethers and aromatic guests. Additionally, two diphenoxyalkane ligands were prepared to investigate the importance of the macrocyclic ring in the complexation between host and guest. The X-ray crystal structure of dibenzo-24-crown-8 (2B24C8) with tropylium tetrafluoroborate inclusion complex was determined, giving the following crystal data: 2B24C8-TrBF4 (C31H39O8 . 0.5 CH3CN . BF4), monoclinic, space group P2(1)/n (no. 14), a = 14.158(2), b = 14.791(2), c = 16.384(2) Angstrom, beta = 93.90(1), Z = 4. The crystal structure showed that the tropylium ion guest formed an inclusion complex with benzene-substituted crown ethers, and its plane is orientated nearly parallel to the planes of the aromatic nucleus of the host.