Understanding Structure Formation In Organolithium Compounds: An Experimental And Quantum-Chemical Approach

As the reactivity of organolithium compounds is strongly connected with the structure of the involved species, the structural knowledge is a prerequisite for the understanding of reaction mechanisms. Using the example of methyllithium, we address particularly the issues of stabilizing non-classical interactions in organolithium aggregates and the relations between disaggregation and polarity, using a combination of X-ray structure analysis and quantum-chemical methods. The structure analyses of a series of MeLi adducts, among them the diethyl ether adduct (MeLi center dot Et2O)(4), exhibit varying arrangements of the alpha-hydrogen atoms relative to lithium. alpha-Hydrogen orientations differ even when changing the stereochemistry of the ligand in dimeric adducts. Computations confirm only weak stabilizing effects of agostic Li-H interactions, which compete with other aspects of the spatial situation in the formed aggregates. NPA and QTAIM analyses on different aggregates (e.g. monomer, dimer, tetramer, extended crystal environment) with different co-ligands reveal a complicated relation between the degree of aggregation and Li-C bond polarity but a more clearcut dependence on the lithium coordination number.