S(E)2 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 (MCl(2)) 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 MCl(2) approaches the AB. The S(E)2 backside reaction is favored as a trigger step for the dehydrogenation of AB by the MCl(2). The S(E)2 reaction is found for 3d-transition-metal chlorides (e.g., FeCl(2), CoCl(2), NiCl(2), CuCl(2), and ZnCl(2)), while PdCl(2) 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 MCl(2) can be explained with the similar S(E)2 mechanism, and the dehydrogenation of the BN derivative 3-methyl-1,2-BN-cyclopentane (CBN) bearing a carbon backbone ring also follows the S(E)2 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(E)2 reaction. This work is helpful for the development of novel metal-halide catalysts for practical hydrogen storage materials, including the BN moiety.