Dinitramidoborates: A Fascinating Case of Competing Oxygen and Nitrogen Donors and Tautomerism.

Reactions of the BH4- anion with equimolar amounts of HN(NO2 )2 or of BH3 ⋅THF with K[N(NO2 )2 ]- produced a mono-substituted [BH3 N(NO2 )2 ]- anion, which contains a B-N connected dinitramido ligand. The reaction of BH4- with two equivalents of HN(NO2 )2 afforded the di-substituted borate anion consisting of two isomers, one with both nitramido ligands attached to B through N and the other one with one ligand attached through N and the other one through O. The disubstituted dinitramidoborates are marginally stable under ambient conditions, and the isomer with two N-connected ligands was characterized by its crystal structure. A tri-substituted borate was tentatively identified by NMR in the reaction of BH4- with a large excess of HN(NO2 )2 . All of the anions are highly energetic. Theoretical calculations show that the energy differences between the B-N and B-O tautomers are small, explaining the formation of both.