Macrocycles Containing Tin. 119Sn NMR Studies of Chloride Binding by Lewis Acidic Tin Compounds. Multidentiate Effects, Macrocyclic Effects and Size Selectivity

摘要

Three macrocyclic Lewis acidic hosts containing two symmetrically disposed dichlorostanna moieties (1-3, ring sizes 18, 22, 26 atoms, respectively) have been prepared as has an acyclic di-tin model (5,5,16,16-tetrachloro-5,16-distannadocosane, 4). The complexation of 1-4 and dibutyltin dichloride (5) with chloride ion in acetonitrile was studied by 119Sn NMR spectroscopy. Rapid exchange of chloride between tin atoms occurred at all ratios of chloride to host studied. An iterative computer program was used to provide estimates of the binding constants of 1, 2, 4 and 5 with chloride. Competition experiments were performed wherein two di-tin hosts competed for limited chloride ion, and the 119Sn NMR spectra of the mixtures were analyzed to determine the relative amount of chloride bound by each host and hence the ratio of the binding constants found in the latter studies and the ratios of the absolute binding constants found in the former studies. The energies of binding were calculated. The di-tin hosts exhibited cooperative bidentate effects in binding chloride in comparison to 5 leading to an increase in binding energy over the amount predicted for a statistical correction in the number of sites. The cyclic hosts 1-3 bind chloride more strongly than acyclic model 4 (macrocyclic effect). Host 1 binds chloride somewhat more strongly than host 2 (size-selective effect). (Author).