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

Area of Science:

• Organometallic Chemistry
• Main group chemistry
• Computational chemistry

Background:

• Lithium complexes with unsaturated hydrocarbons are key intermediates in organic synthesis.
• Understanding the nature of lithium-ligand interactions is crucial for catalyst design.

Purpose of the Study:

• To develop a synthetic route for η2-alkene lithium complexes.
• To investigate the Li-alkene and Li-alkane interactions using quantum chemical methods.
• To explore the reactivity and equilibrium dynamics of these lithium complexes.

Main Methods:

• Synthesis of η2-alkene lithium complexes using a bulky carbene ligand.
• Reaction of lithium complexes with alkanes and arenes.
• Quantum Theory of Atoms in Molecules (QTAIM) analysis.
• Diabatization of Local Pairs Natural Orbitals Coupled Cluster with Singles, Doubles, and Perturbative Triples (DLPNO-CCSD(T)) calculations.

Main Results:

• Successful synthesis of η2-alkene lithium complexes and a binuclear μ-η2:η2-diene complex.
• Characterization of electrostatic Li-alkene interactions via QTAIM.
• Demonstration of reversible alkene-alkane ligand exchange, forming Li-alkane complexes with anagostic Li···H interactions.
• Quantification of interaction energies and dispersion contributions, revealing a small energy difference between Li-alkene and Li-alkane complexes.
• Transformation of Li-alkene and Li-alkane complexes into η2-arene complexes with benzene.

Conclusions:

• A versatile synthetic approach for η2-alkene lithium complexes was established.
• The electrostatic nature of Li-alkene bonding was confirmed.
• A dynamic equilibrium exists between lithium-alkene and lithium-alkane complexes, tunable by reactant concentrations.
• The study provides fundamental insights into the bonding and reactivity of organolithium compounds.