A novel method has been developed for the determination of the complexation constants of crown ethers with alkali salts. It comprises the equilibration of crown ether (1-7) solutions in deuterochloroform with solid trichloro(ethylene)platinum(II) salts (Na Γ·, K +, Rb +, Cs +) and the PMR spectroscopic determination of the equilibrium ratio of complex to free crown ether from the relative intensities of the ethylene and crown ether protons. The solubility of uncomplexed salt was determined independently by atomic absorption spectrometry.
The major advantages of this method over others are: (i) complexation constants in apolar solvents are obtained from a direct solid-liquid transition, (ii) the cation in the salt can be varied, and (iii) a simple detection technique can be used for monitoring the complexation.
The PMR spectra indicate that there are three types of complex, depending on the ratio of the diameter of the crown ether cavity to that of the cation. If this ratio is small (< 1), the aromatic ring is almost perpendicular to the flat polyether ring. With increasing ratio (-1.0) the flat polyether ring and the aromatic ring become almost coplanar in the complex. If the ratio is large (> 1.0) the polyether ring is twisted around the cation.