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 (L-2) and N,N'-bis(benzimidazol- 2ylmethyl)-4,13-diaza-18-crown-6 (L-4). The X-ray crystal structures of the Ca(II) and Sr(II) complexes of L-2 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 L-4, whereas the Ca(II), Sr(II) and Ba(II) complexes of L-4 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, H-1 and C-13 NMR spectroscopy, in combination with density functional theory (DFT) calculations performed at the B3LYP level, suggests that the [M(L-2)](2+) and [M(L-4)](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 L-4 complexes than for L-2 ones. (C) 2011 Elsevier B.V. All rights reserved.