Conformational and complexational study of some maleonitrile mixed oxadithia crown ethers by NMR spectroscopy and molecular modelling

The macrocyclic ring interconversion of four maleonitrile mixed oxadithia c rown ethers of variable ring size, mn-12-S2O2, mn-15-S2O3, mn-18-S2O4 and f n-12-S2O2, were studied by H-1 and C-13 NMR spectroscopy and by molecular m odelling. The barriers to ring interconversion were estimated using variabl e temperature NMR spectroscopy and from the calculated activation energies, together with the spin-lattice relaxation times of the CH2 carbon atoms, c onclusions were drawn regarding the intramolecular flexibility of the crown ethers in both the free state as well as the complexed state incorporating either Ag-I, Bi-III, Sb-III, Pd-II or Pt-II metal cations. Furthermore, bo th the stoichiometry of the complexes and the coordination sites of the cro wn ethers to the various cations were also clearly implicated. Molecular mo delling was also utilised to ascertain the preferred conformers of the four compounds and their corresponding complexes, the results of which corrobor ated the experimental NMR results to a high degree.