Determination of the bonding preference of borane (BH3) toward aminoarsines by multinuclear nuclear magnetic resonance spectroscopy

Abstract

The reactions of BH₃·thf (thf = tetrahydrofuran) with selected (dialkylamino)dimethylarsines, Me₂AsNR₂(R = Me, Et, Prⁿ, or Prⁱ) have been carried out as a function of temperature to determine the initial co-ordination site of boron and to elucidate the nature of any As–N, As–B, and N–B bond dissociation/formation processes in solution. The reactions were monitored by multinuclear (¹H, ¹¹B, and ¹³C) n.m.r. spectroscopy over a temperature range of –90 to 25 °C. With Me₂AsNMe₂, a N–B bonded adduct is formed at low temperature, which decomposes at 25 °C to yield (Me₂NBH₂)₂, Me₂NH·BH₃, µ-Me₂NB₂H₅, and Me₂AsAsMe₂. Equimolar amounts of As–B and N–B adducts are formed at low temperature when Me₂AsNEt₂ is reacted with BH₃·thf. With increasing temperature, the As–B adduct converts to the N–B adduct, which subsequently decomposes at room temperature to µ-Et₂NB₂H₅, Et₂NH·BH₃, (Et₂NBH₂)₂, and Me₂AsAsMe₂. The reaction of Me₂AsNPrⁿ₂ with BH₃·thf yields more of the As–B adduct than of the N–B adduct at –90 °C, but the former rearranges to the latter on warming. Only the As–B adduct is formed in the reaction of Me₂AsNPrⁱ₂ with BH₃·thf. These results suggest that the steric bulkiness of the R₂N moiety plays an important role in determining the co-ordination site of the boron in this series of compounds.