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, eta(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.