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 NH(CH3)(2)-BH3. 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 [NH(CH3)(2)-BH3](2) or as a trimer forming a six-membered ring [NH(CH3)(2)-BH3](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.

Automated summary

Dehydrocoupling of the dimethylamine and borane adduct, NH(CH3)(2)-BH3, leads to dimethylaminoborane with a composition of NH(CH3)(2)-BH3. The structure of the product depends on synthesis conditions, crystallizing either as a trimer with a six-membered ring or as a dimer with a four-membered ring. The triclinic structure is stable at low temperatures, but heating can cause a phase transition to a monoclinic polymorph, with crystal structure determined in this study.