Repeated Dihydrogen Elimination from Boranes and Gallanes Stabilized by Guanidine‐Type Bases: A Quantum Chemical Study Motivated by Recent Experimental Results

Abstract

AbstractMonomeric boranes and gallanes stabilized by guanidine derivatives undergo in certain cases H2 elimination reactions leading in a first step to dinuclear E(III) hydrides (E = B or Ga) and in the second step upon elimination of further H2 to dinuclear compounds with the elements in the formal oxidation state +II featuring a direct E–E bond. The thermodynamics for such H2 elimination reactions were calculated applying several methods and basis sets. In addition the calculations provide information about possible mechanisms. Thus the calculations suggest the first step to be an intramolecular H2 elimination followed by a cycloaddition reaction. The second H2 elimination, in which E–E and H–H bonds are formed at the expense of E–H bonds, is almost energy neutral, as anticipated from the small differences in electronegativity between E and H. The reaction barriers are important indicators to answer the question if H2 elimination can be made reversible. Reversible H2 elimination and addition would make this class of compounds in principle interesting for hydrogen storage applications, although obviously the weight percentage of stored dihydrogen is in the special systems presented herein not sufficient for mobile applications. The compounds can also be applied for dehydrocoupling and transfer hydrogenation to deliver a stoichiometric amount of H2 to olefins. Finally, the possibility of elimination of a third H2 leading to a dinuclear E(I) species with an E–E double bond is discussed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)