The magnesium compound Mg[(BH3)2NMe2]2 or Mg(DMADB)2, which was discovered in our laboratories a few years ago, is presently the most volatile compound of magnesium known. It serves as an excellent CVD precursor to the superconducting phase MgB2 and also (in the presence of water) to the valuable dielectric material MgO. In the formation of MgB2, cleavage of the B-N bonds takes place to form BH2-NMe2 and H2 as byproducts. An interesting question, which would facilitate formation of the superconducting MgB2 phase, is what factors lead to a lowering of the kinetic barrier for the B-N cleavage process. To explore this question, we have made a series of Mg[(BH3)2NMeR]2 complexes in which one of the two methyl substituents in Mg(DMADB)2 is replaced with an ethyl, iso-propyl, or tert-butyl substituent. Thermolysis studies show that in heated toluene solutions the (BH3)2NMeR- groups slowly but reversibly undergo B-N bond cleavage (and migration of a hydride atom) to form BH2-NMeR and BH4-. For the methyl, ethyl, and iso-propyl derivatives, the equilibrium strongly favors Mg[(BH3)2NMeR]2, but the tert-butyl compound is very different: the dissociation to generate BH2-NMeR2 and BH4- groups proceeds essentially to completion (H2 evolution is not seen). Interestingly, the rates at which all the complexes approach equilibrium are rather similar. The results suggest sterically bulky ligands strongly affect the dissociation equilibrium but not the barrier for B-N bond cleavage. If the steric factors are not too large, however, better electron donors slightly disfavor the dissociation, so that Mg[(BH3)2NMeEt]2 dissociates to a lesser extent than Mg[(BH3)2NMe2]2.

Controlling the conversion of magnesium aminodiboranates to superconducting MgB2

ACS Spring Meeting 2021

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