A Comparison of Complexation of Li+ Ion with Macrocyclic Ligands 15-Crown-5 and Benzo-derivatives in Binary Nitromethane–Acetonitrile Mixtures by Using Lithium-7 NMR Technique and Ab Initio Calculation

Abstract

Lithium-7 nuclear magnetic resonance (NMR) measurements were used to investigate the stoichiometry and stability of Li+ complexes with 15-crown-5 (15C5), benzo-15-crown-5 (B15C5) and dibenzo-15-crown-5 (DB15C5) in a number of nitromethane (NM)–acetonitrile (AN) binary mixtures. In all cases, the exchange between the free and complexed lithium ion was fast on the NMR time scale and a single population average resonance was observed. While all crown ethers form 1:1 complexes with Li+ ion in the binary mixtures used, stepwise formation constants of the 1:1 (ligand/metal) complexes were evaluated from computer fitting of the NMR-mole ratio data to equations which relate the observed metal ion chemical shifts to formation constants. There is an inverse linear relationship between the logarithms of the stability constants and the mole fraction of AN in the solvent mixtures. The stability order of the 1:1 complexes was observed to be 15C5.Li+ > B15C5.Li+ > DB15C5.Li+. The optimized structures of the free ligands and their 1:1 complexes with the Li+ ion were predicted by ab initio theoretical calculations using the Gaussian 98 software. The results of calculations are discussed. .