A nuclear magnetic resonance study on conformational behaviour of diarylmethylenemalonaldehydes and their complex formation with magnesium and lithium perchlorates by using four-bond aldehyde proton and three-bond aldehyde carbon–proton spin–spin couplings; a strategy for studying strongly entropically controlled flexible molecular systems

Abstract

The conformational behaviour of the title compounds has been characterized by experiments based on the conformational dependence of spin–spin couplings. It is suggested that the compounds exist in two principal conformations: one with a nearly planar methylenemalonaldehyde fragment, favoured in the presence of polar, hydrogen-bonding, or complexing compounds like CDCl₃, LiClO₃, or Mg(ClO₄)₂ in CD₃CN; and the other with the formyl groups severely twisted out of the plane, favoured by non-polar or strongly protonating solvents. The non-planar conformation is energetically less stable but entropically strongly favoured. The conformational dependence of ⁴J(CHO,CHO) and of ³J(CHO,CHO) has been estimated on the basis of INDO/FPT level calculations and scaled by using the temperature- and solvent-dependence of the couplings. Complex formation with Mg(ClO₄)₂ and LiClO₄ and CD₃CN has been studied and the formation free energies are reported. The complexes are considered to be planar, with ⁴J(CHO,CHO) 3.660(10) Hz and ³J(CHO, CHO) 4.645(10) Hz. The value of ⁴J(CHO,CHO) in cyclohexane can be as low as 0.440 Hz. The strategy and the use of the computer programs employed in data fitting are discussed.