Polymorphism of dimethylaminoborane N(CH3)2-BH2

De­hydro­coupling of the adduct of di­methyl­amine and borane, NH(CH3)2-BH3 leads to di­methyl­amino­borane with formal composition N(CH3)2-BH2. The structure of this product depends on the conditions of the synthesis; it may crystallize either as a dimer in a triclinic space group forming a four-membered ring [N(CH3)2-BH2]2 or as a trimer forming a six-membered ring [N(CH3)2-BH2]3 in an orthorhombic space group. Due to the denser packing, the six-membered ring in the trimer structure should be energetically more stable than the four-membered ring. The triclinic structure is stable at low temperatures. Heating the triclinic phase above 290 K leads to a second-order phase transition to a new monoclinic polymorph. While the crystal structures of the triclinic and orthorhombic phases were already known in the literature, the monoclinic crystal structure was determined from powder diffraction data in this study. Monoclinic di­methyl­amino­borane crystallizes in space group C2/m with the boron and nitro­gen atoms located on the mirror plane, Wyckoff position 4i, while the carbon and hydrogen atoms are on the general position 8j.