Complexation of N- Heteroaromatic Cations with Crown Ethers and Tetraphenylborate

Study was made of host-guest complexation of neutral crown ethers with fiveand six-membered Nheteroaromatic cations and purinium cation. Complexation of tetraphenylborate with selected Nheteroaromatic cations and tropylium cation also was studied. Crown ether complexes were characterized by mass spectrometric and 1H NMR spectrometric methods and by X-ray crystallography. Fast atom bombardment mass spectrometry (FABMS) was used as a prelimary tool to characterize the complexes and electrospray ionization mass spectrometry (ESIMS) was used to confirm the complexation stoichiometry. Crystal structures were determined by X-ray crystallography to study the complexation in solid state, and stability constants were measured in acetonitrile by 1H NMR titration at 30 °C to study the complexation in solution. Mass spectrometric studies indicated preferential 1:1 complexation stoichiometry between crown ethers and N-heteroaromatic cations. Crystal structures of crown ether complexes showed that hydrogen bonding and to a lesser degree cation-π and π-π interactions stabilize the structures in solid state. The values of stability constants for crown ether complexes with N-heteroaromatic cations and purinium cation varied between 10 and 350 M-1. Stability constants were higher for complexes with sixmembered N-heteroaromatic cations and purinium cation than for complexes with five-membered cations. The values indicated that hydrogen bonding was the main interaction in solution. Tetraphenylborate formed complexes with four N-heteroaromatic cations and tropylium cation, and reacted with six N-heteroaromatic cations through the displacement of one phenyl group by Nheterocycle to form triphenylboranes. The complexes and displacement products were characterized by 1H NMR spectrometry. Four crystal structures of complexes and three crystal structures of displacement products were resolved. Stability constants of complexes were measured in methanol/ acetonitrile (1:1) solution at 30 °C by 1H NMR titration method. The values of stability constants for tetraphenylborate complexes with N-heteroaromatic cations ranged from 10 to 50 M-1. C-H•••π and N-H•••π interactions were found to stabilize the structures both in solid state and in solution.