Intermolecular Lithium η2-Alkene and κ2-Alkane Complexes: Synthesis, Bonding, and Facile Interconversion.

In this work, an approach enabling the synthesis of η2-alkene lithium complexes (Carb2,4,6-iPr)Li(η2-L) (L = 1-octene, cyclohexene) is elaborated. For 1,5-hexadiene, the same approach results in a binuclear μ-η2:η2-diene complex. The QTAIM parameters reveal the electrostatic nature of the Li-alkene interaction. When treated with cyclohexane, alkene ligands in (Carb2,4,6-iPr)Li(η2-L) are readily replaced to afford the Li-alkane complex (Carb2,4,6-iPr)Li(κ2-C6H12) featuring anagostic Li···H interactions. The reverse reaction readily proceeds in the presence of excess alkene. The QTAIM and LED analyses performed at the DLPNO-CCSD(T) level show a small difference between the complexes in the total dispersion contribution (16.0-18.5 kcal/mol) and interaction energy for Li-alkene (∼3.5 kcal/mol) or Li-C6H12 (∼4 kcal/mol). These values suggest the presence of an equilibrium between these entities, which can be readily shifted by the presence of an excess of alkene or alkane. (Carb2,4,6-iPr)Li(η2-L) and (Carb2,4,6-iPr)Li(κ2-C6H12) are transformed into η2-arene complexes upon treatment with benzene; however, a reverse reaction is not possible at room temperature.