Experimental and theoretical study on complexation of the lithium cation with [Gly6]-antamanide

Abstract

ABSTRACTOn the basis of extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Li+(aq) + 1⋅Na+(nb) 1⋅Li+ (nb) + Na+(aq) occurring in the two-phase water–nitrobenzene system (1 = [Gly6]-antamanide; aq = aqueous phase, nb = nitrobenzene phase) was determined as log Kex(Li+,1·Na+) = 0.6 ± 0.1. Further, the stability constant of the 1⋅Li+ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: logβnb(1⋅Li+) = 5.8 ± 0.2. Finally, by using quantum chemical calculations, the most probable structure of the cationic complex species 1⋅Li+ was derived. In the resulting complex, the ‘central’ cation Li+ is bound by four bonding interactions to the corresponding four carbonyl oxygen atoms of the parent ligand 1. Besides, the whole 1⋅Li+ complex structure is stabilised by two intramolecular hydrogen bonds. The interaction energy of the considered 1⋅Li+ complex was found to be –553.5 kJ/mol, confirming also the formation of this cationic species.