B-halogenation of ammonia-borane: a nuclear magnetic resonance study

Ammonia-borane and BX3 (X = F, Cl, Br) were combined at low temperature in molar ratios from 3:1 to 1:2 with ether solvents. After warming and stirring of the mixtures at 250C for 18 h, NMR samples were taken of each reaction mixture. The 11B spectra of products from the H3N.BH3-BCl3 reactions gave evidence for the new compounds H3N.BH2Cl and H3N.BHCl2 along with H3N.BCl3, (C2H5)2O.BH2Cl, and (C2H5)O.BHCL2. In the case of the BBr3 reactions, products identified from their spectra include H3N.BH2Br, (C2H5)2O.BH2Br, and (C2H5)2O.BHBr2 along with ether cleavage products. A hydrogen-halogen exchange mechanism appeared to account satisfactorily for the products of the BCl3 and BBr3 reactions. The only major products from the H3N.BH3-BF3 reactions were H3N.BF3 and (μ-H2N)B2H5. An acid displacement followed by a hydrogen elimination mechanism is proposed for the BF3 reaction