S_E2 reaction in noncarbon system: Metal-halide catalysis for dehydrogenation of ammonia borane

An electrophilic substitution (S_E) reaction of BN isosteres has been investigated for the dehydrogenation of ammonia borane (AB) by metal chlorides (MCl₂) using various ab initio calculations. In contrast to the typical S_E reaction occurring at the carbon atom, the nitrogen atom in AB serves as the reaction center for the S_E reaction with the boron moiety as the leaving group when the MCl₂ approaches the AB. The S_E2 backside reaction is favored as a trigger step for the dehydrogenation of AB by the MCl₂ The S_E2 reaction is found for 3d-transition-metal chlorides (e.g., FeCl₂, CoCl₂, NiCl₂, CuCl₂, and ZnCl₂), while PdCl₂ leads to the dehydrogenation of AB by a direct B-H σ-bond activation, similar to most organometallic catalysts. Interestingly, the polymerization of AB promoted by MCl₂ can be explained with the similar S_E2 mechanism, and the dehydrogenation of the BN derivative 3-methyl-1,2-BN-cyclopentane (CBN) bearing a carbon backbone ring also follows the S_E2 reaction. In particular, the experimental observation that the use of metal-chloride catalysis decreases the by-products obtained during the hydrogenation of AB can be explained by our mechanism involving the S_E2 reaction. This work is helpful for the development of novel metal-halide catalysts for practical hydrogen storage materials, including the BN moiety.