Solid state and solution structures of alkaline-earth complexes with lariat ethers containing aniline and benzimidazole pendants

Herein we report the synthesis and structural characterization of Mg(II), Ca(II), Sr(II) and Ba(II) complexes with bibracchial lariat ethers derived from 1,7-diaza-15-crown-5 and 1,7-diaza-12-crown-4 containing aniline or benzimidazole pendant arms. The solid state structures of most of them have been determined by using single crystal X-ray crystallography. A coordination number of seven was observed for the Mg(II) complexes in the solid state, while the Ca(II), Sr(II) and Ba(II) complexes are 8-, 9- and 11-coordinate, respectively. The Ca(II), Sr(II) and Ba(II) complexes show a syn conformation, with the two pendant arms of the ligand disposed on the same side of the macrocyclic mean plane. However, the Mg(II) complex with the largest ligand derived from 1,7-diaza-15-crown-5 containing benzimidazole pendants presents an anti conformation in the solid state. 1H and 13C NMR spectroscopy reveal that this conformation is maintained in acetonitrile solution.

The conformational properties of alkaline-earth complexes with pendant-armed crown ether ligands derived from 1,7-diaza-12-crown-4 and 1,7-diaza-15-crown-5 have been investigated by using X-ray crystallography and NMR spectroscopy. In all cases, a syn conformation is found both in solution and in solid state, except for the Mg(II) complex with the 1,7-diaza-15-crown-5 derivate that presents an anti one.Figure optionsDownload as PowerPoint slideHighlights
► Lariat ethers bearing pendant arms with N donor atoms for Alkaline-Earth complexation.
► Hexa- or heptadentate binding to all M2+ ions in the solid state and in solution is observed.
► Increasing coordination numbers (7–11) are observed as the size of the M2+ ion increases.
► Solvent molecules or perchlorate anions complete the M2+ coordination sphere.