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

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-pyridylmethyl)-2-tert-butyl-3-(2,6-diisopropylphenyl)-1,3-diaallyl]calcium (4c). In all of these s-block metal complexes, strong agostic interactions
between the cations and the tert-butyl groups stabilize
these complexes.