Compartmentalization of Alkaline-Earth Metals in Salen-Type Cu- and Ni-Complexes in Solution and in the Solid State

The precise arrangement of metal
ions in type and number by a ligand
represents an important challenge in biology as well as in materials
science. The preorganization of different metal ions such as alkaline-earth
and transition-metal ions is of particular interest for the design
of catalysts or precursors of oxides. This study is based on a Ω-shaped
salen-derived ligand comprising N2O2 and O2O2 coordination sites. The selective binding of
Cu­(II)
and Ni­(II) and alkaline-earth-metal ions is influenced by many factors
such as the size of the cation, the solvent, or the counterion. UV–vis
and 1H NMR titrations and single-crystal X-ray structures
reveal that the obtained complexes tend to adopt different structures
in solution compared to the solid state. Mainly discrete motifs with
a stoichiometry 1:1 (LM1 to alkaline-earth-metal ions)
have been shown to form in the solid state, whereas in solution, the
2:1 complexes are predominant.