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 (Carb^{2,4,6-<i>i</i>Pr})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 (Carb^{2,4,6-<i>i</i>Pr})Li(η²-L) are readily replaced to afford the Li-alkane complex (Carb^{2,4,6-<i>i</i>Pr})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. (Carb^{2,4,6-<i>i</i>Pr})Li(η²-L) and (Carb^{2,4,6-<i>i</i>Pr})Li(κ²-C₆H₁₂) are transformed into η²-arene complexes upon treatment with benzene; however, a reverse reaction is not possible at room temperature.