title: Strong intramolecular calcium-π interactions with aryl substituents--requirements and limitations.
abstract: The amidines Dipp-N=C(tBu)-N(H)-Qu (1a) (Dipp = 2,6-diisopropylphenyl, Qu = 8-quinolyl) and Dipp-N=C(tBu)-N(H)-Py (1b) (Py = 2-pyridyl) are deprotonated with KN(SiMe3)2, yielding potassium N-(2,6-diisopropylphenyl)-N'-(8-quinolyl)pivalamidinate (2a) and potassium N-(2,6-diisopropylphenyl)-N'-(2-pyridyl)pivalamidinate (2b). Metalation of 1a with [(thf)2Ca{N(SiMe3)2}2] in tetrahydrofuran (thf) leads to the formation of ether-free calcium bis[N-(2,6-diisopropylphenyl)-N'-(8-quinolyl)pivalamidinate] (3a) with a strong intramolecular calcium-π interaction with one Dipp group. Furthermore, agostic bonds to one tert-butyl substituent complete the coordination sphere of the metal center and stabilize this bonding situation. The metathesis reaction of 2b with [(thf)4CaI2] yields the thf adduct of calcium bis[N-(2,6-diisopropylphenyl)-N'-(2-pyridyl)pivalamidinate] (3b). In addition to the bonds between calcium and the hard Lewis bases (oxygen of thf, nitrogen atoms of the pyridylamido moieties), anagostic Ca-H bonds to the tert-butyl substituents complete the coordination sphere. The intramolecular calcium-π interaction with an aryl group as observed in 3a requires steric shielding of the alkaline earth metal center, safely excluding the formation of a complex with more than one thf ligand.
authors.affiliation: Institute of Inorganic and Analytical Chemistry, Friedrich Schiller University Jena, Humboldtstrasse 8, D-07743 Jena, Germany. m.we@uni-jena.de.
authors.forename: Claas
authors.initials: C
authors.lastname: Loh
authors.forename: Susanne
authors.initials: S
authors.lastname: Seupel
authors.forename: Alexander
authors.initials: A
authors.lastname: Koch
authors.forename: Helmar
authors.initials: H
authors.lastname: Görls
authors.forename: Sven
authors.initials: S
authors.lastname: Krieck
authors.forename: Matthias
authors.initials: M
authors.lastname: Westerhausen