LAUSR

The magnesium N,N-dimethylaminodiboranate compound Mg[(BH3)2NMe2]2, the most volatile compound of magnesium known, serves as an excellent chemical vapor deposition (CVD) precursor for the growth of thin films such as the dielectric material MgO. To explore how the thermal stability and physical properties of magnesium aminodiboranates depend on the steric and electronic properties of the nitrogen-bound substituents, we have made a series of analogues of Mg[(BH3)2NMe2]2, in which one of the two methyl substituents on nitrogen is replaced with an ethyl, iso-propyl, or tert-butyl group. In the crystal structure of Mg[(BH3)2NMe(t-Bu)]2, the magnesium center is coordinated to two chelating (BH3)2NMe(t-Bu) ligands, each of which binds in a κ2,κ2 fashion so that the magnesium center forms eight Mg-H contacts. Unlike Mg[(BH3)2NMe2]2, however, which has a linear N···Mg···N angle and is an isolated molecule in the solid state, the N···Mg···N angle in Mg[(BH3)2NMe(t-Bu)]2 is distinctly nonlinear (149.9°) because hydrogen atoms of BH3 groups of nearby molecules form two additional Mg-H contacts with the magnesium center. When the complexes are heated in toluene solution, the (BH3)2NMeR- groups reversibly undergo B-N bond cleavage (with concomitant migration of a hydrogen atom) to release the aminoborane BH2═NMeR and form magnesium borohydride, Mg(BH4)2. For the methyl, ethyl, and iso-propyl derivatives, the equilibrium strongly favors Mg[(BH3)2NMeR]2. In contrast, for the tert-butyl derivative, the equilibrium strongly favors BH2═NMe(t-Bu) and Mg(BH4)2. The results suggest that more strongly electron-donating groups slightly strengthen the B-N bonds and disfavor B-N bond cleavage, provided that the groups are not too large. In contrast, sterically bulky ligands disfavor B-N bond reformation, thus promoting the dissociative equilibrium that involves B-N bond cleavage. Interestingly, the rates at which the complexes approach equilibrium depend only weakly on the nature of the R group, at least within the series studied. These findings are relevant to the potential use of magnesium aminodiboranates as CVD precursors to the superconducting phase MgB2.