Lithium-7 NMR and ab initio Calculation Studies of Complexation of Li+ Ion with 12-Crown-4, Benzo-12-crown-4, 15-Crown-5, Benzo-15-crown-5, and Dibenzo-15-crown-5 in Binary Nitromethane-Acetonitrile Mixtures

Complexes of 12-crown-4 (12C4), benzo-12-crown-4 (B12C4), 15-crown-5 (15C5), benzo-15-crown-5 (B15C5) and dibenzo-15-crown-5 (DB15C5) with Li+ ion were investigated by lithium-7 NMR in a number of nitromethane (NM)-acetonitrile (AN) binary mixtures. In all cases, the exchange between the free and complexed lithium ionwas fast on the NMR time scale and a single population average resonance was observed. Both 1:1 and 2:1 (sandwich) complexeswere observed between lithium ion and 12C4 and B12C4 in pure nitromethane solution. Stepwise formation constants of the 1:1 and 1:2 (metal/ligand) 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 acetonitrile in the solvent mixtures. The stability of the complexes varies inversely with the Gutmann donor number of the solvent. The stability order of the complexes was found to be 15C5.Li+> B15C5.Li+> DB15C5.Li+> 12C4.Li+ > B12C4.Li+. The optimized structures of the free ligands and their 1:1 and 2:1 complexes with Li+ ionwere predicted by ab initio theoretical calculations using theGaussian 98 software, and the results are discussed.