Structure of mixed alkyllithium/lithium alkoxide aggregates in ethereal solvents. Insights from combined QM/MM molecular dynamics simulations

Abstract

Mixed alkyllithium/lithium alkoxides aggregates are important species in synthetic organic chemistry, but their electronic and geometric properties have not been extensively studied yet. The main objective of this work was to analyze the structure of simple prototypical aggregates in a coordinating solvent with the help of elaborated theoretical chemistry calculations. Within this aim, we have carried out molecular dynamics simulations for MeOLi, (EtLi)(MeOLi), and (EtLi)(2)(MeOLi)(2) systems in dimethyl ether solution. We use a combined QM/MM (quantum mechanics/molecular mechanics) force field that allows an appropriate description of the aggregate structure and of its interactions with the solvent. In the simulations, the aggregates are described at the B3LYP/6-31G(d) level while the solvent is described using the classical OPLS potential. For completeness, the influence of the chemical environment on the C-O-Li bond structure has been analyzed in some detail. The discussion focuses on (1) the distinctive properties of the alkoxide C-O-Li bond pattern, (2) the coordination of solvent molecules to the aggregates (number, stability), and (3) the time fluctuations of main structural parameters (Li-C and Li-O distances). We also show that nonclassical C-H...O hydrogen bonds involving H atoms of the solvent methyl groups and the O atom of the alkoxide are formed in the solvated MeOLi monomer and (EtLi)(MeOLi) dimer. Owing to these specific interactions, the monomer exhibits a nonlinear C-O-Li bond in solution.