Peculiar Binding Modes of a Ligand with Two Adjacent β-Diiminato Binding Sites in Alkali and Alkaline-Earth Metal Chemistry

Reaction of the diprotic ligand [Xanthdim]H2 (a ligand system where two adjacent β-dialdimine units are linked by a xanthyl backbone) with 2 equiv of potassium hydride or benzylcesium gave access to bimetallic alkali metal complexes. These complexes were structurally characterized by X-ray diffraction, which showed that the β-diiminato units are orientated in a W-conformation. Treatment with 2 equiv of Mg[N(SiMe3)2]2(THF)2 led to the formation of the heteroleptic complex [Xanthdim][MgN(SiMe3)2(THF)]2, that crystallized as a highly strained monomer. The heteroleptic Mg complex is remarkably stable against ligand exchange but is not active in CO2/cyclohexene oxide copolymerization. Reaction with Ca[N(SiMe3)2]2(THF)2 gave the homoleptic complex [Xanthdim][Ca(THF)]. Both alkaline-earth metal complexes display considerable distortions in their solid state structure.