Structural Evidence of Strong Calcium−π Interactions to Aryl Substituents Stabilized by Coexistent Agostic Bonds to Alkyl Groups

Abstract

Metalation of 1-(2-furanylmethyl)- (1a) and 1-(2-pyridylmethyl)-2-tert-butyl-3-(2,6-diisopropylphenyl)-1,3-diazaprop-2-ene (1b) with n-butyllithium and potassium bis(trimethylsilyl)amide in tetrahydrofuran (THF) yields the corresponding lithium (2a,b) and potassium derivatives (3a,b), with the alkali metals binding to the furanylmethylamido and pyridylmethylamido pockets. The calcium derivatives are accessible via a metathetical approach of the potassium complexes with calcium iodide in THF. Whereas bis[1-(2-furanylmethyl)-2-tert-butyl-3-(2,6-diisopropylphenyl)-1,3-diazaallyl]calcium (4a) precipitates as a thf adduct, bis[1-(2-pyridylmethyl)-2-tert-butyl-3-(2,6-diisopropylphenyl)-1,3-diazaallyl]calcium (4b) crystallized without solvent coligands. Instead of coordinated solvent molecules, strong calcium−π interactions to an aryl group saturate the coordination sphere. Bidentate 1,2-dimethoxyethane (DME), however, is able to replace this side-on bound aryl group, leading to the dme adduct of bis[1-(2-pyridy...