Complexation of <em>N</em>-heteroaromatic cations with crown ethers and tetraphenylborate

Abstract

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>Study was made of host-guest complexation of neutral crown ethers with five- and six-membered <em>N</em>-heteroaromatic cations and purinium cation. Complexation of tetraphenylborate with selected <em>N</em>-heteroaromatic cations and tropylium cation also was studied.</p><p>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 <em>N</em>-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 <em>N</em>-heteroaromatic cations and purinium cation varied between 10 and 350 M⁻¹. Stability constants were higher for complexes with six-membered 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.</p><p>Tetraphenylborate formed complexes with four <em>N</em>-heteroaromatic cations and tropylium cation, and reacted with six <em>N</em>-heteroaromatic cations through the displacement of one phenyl group by <em>N</em>-heterocycle 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.</p></div>