Determination of complex stabilities with nearly insoluble host molecules: cucurbit[5]uril, decamethylcucurbit[5]uril and cucurbit[6]uril as ligands for the complexation of some multicharged cations in aqueous solution

A new experimental method has been developed to determine the stability constants of metal complexes of nearly insoluble ligands in homogeneous solution. Cucurbit[5]uril, decamethylcucurbit[5]uril and cucurbit[6]uril are used as ligands for the complexation of doubly and triply charged transition metal cations in aqueous solution. The total concentration of the ligand in solution increases due to complex formation. Different experimental methods have been tested for the determination of the total ligand concentration in solution. The measurement of the total organic carbon content can be used to study complex formation quantitatively. The sensitivity of NMR spectroscopy is not sufficient. UV–VIS spectrophotometry is also not suitable because, the maximum absorbance of the ligands used is below 220 nm. Of the metals studied the most stable complex was formed by Cd(II) and cucurbit[6]uril. The stability constants of the complexes formed with cucurbit[5]uril are of the same order of magnitude as the complexes with cucurbit[6]uril. No stability constants can be calculated from the measurements of the total organic carbon content in the case of decamethylcucurbit[5]uril due to a decrease of this value in the presence of the metal salts. The solubilities of the complexes formed in this case are lower than that of the pure ligand.