Thermodynamics of Complexation Reactions of Lithium Ion with Dibenzo‐14‐Crown‐4 and Its Analogs in Nonaqueous Solvents

Abstract

AbstractFormation constants of Li+ complexes with 4‐substituted dibenzo‐14‐crown‐4 (DB14C4; 4‐substituted group: methyl‐, tert‐butyl‐, H‐, bromo‐, chloro‐, formyl‐, nitro‐) ligands were determined by 7Li NMR spectrometry for solutions in nitromethane (NM), acetonitrile (ACN), propylene carbonate (PC), acetone (AC), Pyridine (Py), tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), and N,N′‐dimethyl formamide (DMF). Only a 1:1 complex was formed in solvents with a small or medium donor number. The formation constants of these complexes are strongly influenced by the size of the metal ion – the effect of the size of the cavity, by the solvent and by substituent. The stability of lithium ion with different substituents on DB14C4 decreases in the order methyl‐ > tert‐butyl‐ > H‐ > bromo‐ > chloro‐ > formyl‐ > nitro‐ in various solvents. A good Hammett correlation was found by plotting log Kf vs. ∑σ in PC and AC. The extent of the substituent effect increases as the donor number of solvent decreases. The complexes were both enthalpy and entropy stabilized. The same magnitude of ΔS° value for different substituents indicates formation with a similar configuration upon complexation between crown ether and lithium ion. A slight variation in entropy contribution was observable depending on the nature of the alkyl substituent, whereas a large variation in enthalpic contribution shows a remarkable substituent effect upon complexation; the effect can reach 70% in magnitude.