A merged experimental and theoretical conformational study on alkaline-earth complexes with lariat ethers derived from 4,13-diaza-18-crown-6

Herein, we report the synthesis and structural characterization of alkaline-earth complexes with the bibracchial lariat ethers N,N′-bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L2) and N,N′-bis(benzimidazol-2ylmethyl)-4,13-diaza-18-crown-6 (L4). The X-ray crystal structures of the Ca(II) and Sr(II) complexes of L2 show the pendant arms of the ligand disposed on opposite sides of the macrocyclic mean plane, which results in an anti conformation in the solid state. A similar anti conformation is also observed for the Mg(II) complex of L4, whereas the Ca(II), Sr(II) and Ba(II) complexes of L4 adopt a syn conformation in the solid state, with the two pendant arms pointing at the same side of the crown moiety. However, a different behavior is observed in solution. Indeed, 1H and 13C NMR spectroscopy, in combination with density functional theory (DFT) calculations performed at the B3LYP level, suggests that the [M(L2)]2+ and [M(L4)]2+ (M = Ca, Sr or Ba) complexes exist in solution as a mixture of syn and anti isomers involved in a dynamic equilibrium. Our results also show that the relative abundance of the syn conformation increases as the ionic radius of the metal ion increases and, furthermore, for a given metal ion the proportion of syn isomer is always higher for L4 complexes than for L2 ones.

The conformational properties of alkaline-earth complexes with pendant-armed crown ether ligands derived for 4,13-diaza-18-crown-6 have been investigated by using X-ray crystallography, NMR spectroscopy and DFT calculations. The presence of 2-aminobenzyl pendant arms results in anti conformations of the ligand in the Ca(II), Sr(II) and Ba(II) complexes, while benzimidazol-2yl-methyl arms favor a syn conformation of these complexes in the solid state. In solution a mixture of syn and anti isomers is observed for all complexes.Figure optionsDownload as PowerPoint slideResearch highlights
► The nature of the pendant arm affects the conformation of alkaline-earth complexes of bibracchial lariat ethers.
► 2-Aminobenzyl pendant arms favor anti conformations.
► Benzimidazol-2yl-methyl pendants and larger metal ions favor syn conformations.
► DFT and NMR studies indicate an equilibrium between syn and anti isomers in solution.