Complexation of lithium picrate with a novel 12-crown-4 ether:: spectrophotometric and quantum-chemical approach

The complexation of lithium picrate (LiPic) with the new crown ether Tetracyclohexan(b,e,h,k)(1,4,7, 10)tetraoxacyclododecane (CE) was studied spectrophotometrically in 1,2-dichloroethane and 1% 1,4-dioxane as solvating agent by means of computational methods. The spectra, obtained from the addition of a LiPic solution to a CE solution, showed remarkable overlapping absorption bands and two isosbestic points at 332 and 382 nm resulting from the formation of a LiPic ion pair complex. The complexation occurred instantaneously. The existence of two absorbent species was determined by the TRIANG program. It found experimental evidence for the presence of 1:1 CELi(+)Pic(-) ion pair complex. The formation constant value, determined by the SQUAD program, was log beta = 4.68 +/- 0.02. In addition, the results of conformational studies of this system were performed by the PM3 semiempirical method in gas phase and in the presence of one dioxane molecule. In the gas phase, the most stable complex exhibits three-fold trigonal coordination, while in the presence of one dioxane molecule, four-fold tetrahedral coordination was found. The overall results revealed that the formation of an inclusion complex between the LiPic and the CE due to the presence of the picrate appeared impossible. The PM3 calculations are in good agreement with X-ray reported data in the gas phase and with dioxane. (C) 2001 Elsevier Science B.V. All rights reserved.