Bis(phenoxy-imine) Alkaline-Earth Complexes with Competing Binding Pockets

Abstract

The synthesis and characterization of both monometallic and bimetallic alkaline-earth metal complexes supported by the phenoxy-imine ligand [2,7-(HC═NDipp)-1,8-O-C10H4 where Dipp = 2,6-iPr-C6H3]2- (DippL) have been reported. Though the ligand possesses two [═N,O-] pockets, it is the central [O-,O-] coordination site into which the oxophilic M2+ centers preferentially bind. Protonolysis allowed for the isolation of well-defined dimeric bimetallic complexes for magnesium [DippLMg(thf)]2 (1) and [HDippLMgCl]2 (2); for calcium, strontium, and barium, the analogous thf adducts exist in solution as an equilibrium mixture of the monomeric and dimeric species; DippLM(thf)x ⇋ [DippLM(thf)x]2 (M = Ca; 3(thf)2/[3(thf)2]2, M = Sr; 4(thf)/[4(thf)]2, and M = Ba; 5(thf)/[5(thf)]2). Addition of crown ether allowed for the trapping of the monometallic heavier alkaline-earth complexes (DippL)Ca(18-c-6) (3(18-c-6)), (DippL)Sr(18-c-6)(thf) (4(18-c-6)(thf)), and (DippL)Ba(18-c-6)(py) (5(18-c-6)(py)). A salt metathesis route to generate complexes was also investigated; facile deprotonation of H2DippL afforded DippLK2 (6) but subsequent reaction with alkaline earth halides reliably afforded mixtures consistent with DippLM(thf)x and [HDippLMgI]2