Intermolecular Interactions of a Chiral Amine Borane Adduct Revealed by VCD Spectroscopy

Amine boranes feature strong hydrogen bonding acceptor and donor moieties in close proximity, leading, for instance, to dihydrogen bonding driven self-aggregation. In this work, the infrared (IR) and vibrational circular dichroism (VCD) spectra of the bulky bis(α-phenylethyl)amine borane 1 in chloroform and acetonitrile solution are reported. By comparison with calculated spectra, the VCD spectral features observed in chloroform solution can clearly be associated with the presence of monomeric species. A shift of the conformational preferences occurs when changing the solvent to acetonitrile, which can only be deduced from the VCD spectral signatures but not from the IR spectrum. Using variable-temperature IR and VCD spectroscopy, the dihydrogen bonded dimeric species is characterized experimentally at -50 °C and theoretically by means of density functional theory calculations.