Strong intramolecular calcium–π interactions with aryl substituents – requirements and limitations

Abstract

The amidines Dipp–NC(tBu)–N(H)–Qu (1a) (Dipp = 2,6-diisopropylphenyl, Qu = 8-quinolyl) and Dipp–NC(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.