Effect of Aliphatic Amine Bases on the Aggregation of Alkali Metal Salts of 3,5-Di-tert-butylsemiquinone (3,5-DBSQ)

The alkali metal salts (Li+, Na+, K+) of 3,5-di-tert-butylsemiquinone anion (3,5-DBSQ) have been prepared in the presence of three amine bases, triethylamine (TEA), tetramethylethylenediamine (TMEDA), and pentamethyldiethyltriamine (PMDTA), as well as in the absence of amine in two solvents, toluene and THF. EPR spectral evidence shows that for M(3,5-DBSQ), aggregation of the lithium and sodium salts is effectively prohibited by Lewis basic amines PMDTA and TMEDA, with the former being more effective than the latter. The EPR spectra of the least Lewis acidic metal ion, K+, are unaffected by addition of amines. In the absence of amines, the coordinating solvent, THF, is effective at preventing aggregation, while substantial aggregation occurs in the noncoordinating solvent, toluene. The effectiveness of PMDTA is so great that M(3,5-DBSQ) (M = Li+ and Na+) aggregation is prevented even in toluene. The combination of solution and frozen solution spectra is best for determining the presence of aggregates due to aggregation-induced line broadening in solution spectra and fine structure in spectra of S > 1/2 species. The results of the frozen solution spectral studies indicates that, for the amines studied, PMDTA is singular in its ability to prevent aggregation and Li+ and Na+ salts are best chelated by PMDTA. Finally, we show the success of this method by preparing a solution of Na21 that is stable for several hours and is amenable to EPR spectral characterization.