Effect of Polydentate Donor Molecules on Lithium Hexamethyldisilazide Aggregation:  An X-ray Crystallographic and a Combination Semiempirical PM3/Single Point ab Initio Theoretical Study

Addition of 1 equiv of polydentate amine or ether donor solvent to hydrocarbon solutions of lithium hexamethydisilazide (LHMDS) yields a variety of complexes with different aggregation states. X-ray crystallographic analyses have been carried out on six new compounds revealing three- and four-coordinate monomers, η 1 -coordinated mono- and disolvated dimers, and polymers of dimers. PM3 calculations were able to locate minima for a variety of possible structures for the ligands N, N, N‘, N‘ -tetramethylethylenediamine and 1,2-dimethoxyethane. However, the heats of formation from these calculations are found to be unreliable in predicting the relative stabilities of the isomers. Single point ab initio calculations at the 6-31G* level on the PM3 optimized structures give energies which correspond well to the known aggregation states of LHMDS species. Deaggregation from dimers to monomers appears to be driven by a combination of steric, electronic, and chelate effects.