Ion pairing in alkali-metal salts of 1,3-diphenylbut-1-ene

Abstract

We have studied the optical spectra of the alkali-metal salts (DPB^–M⁺) of 1,3-diphenylbut-1-ene in a variety of ether solvents as a function of temperature and cation. In strong solvating agents we observe two species, a ‘solvated contact ion pair’ and a solvent-separated ion pair. In weak solvating agents, the wavelength of the absorption maximum of the contact ion pair is a sensitive function of cation and solvent. The most reasonable way to interpret this behaviour is to invoke the concept of two distinct contact ion pairs in different solvation states. We have determined the thermodynamic parameters ΔH° and ΔS° for the contact–solvent separated ion pair equilibrium for the system DPB^–Na⁺–2-methyltetrahydrofuran. The absorption spectrum of DPB^–Li⁺ in 2,5-dimethyl-tetrahydrofuran at –52 °C exhibits three maxima. We believe that this is the first example of spectroscopic evidence for the simultaneous existence of an ion pair in three distinct solvation states. Specific solvent–cation interactions are shown to be a predominant factor in interpreting the spectral behaviour of the DPB^–M⁺ salts in ether solution.