Differences between amine- and phosphine-boranes: synthesis, photoelectron spectroscopy, and quantum chemical study of the cyclopropylic derivatives.

Borane complexes of aziridine, phosphirane, cyclopropylamine, cyclopropylphosphine, cyclopropylmethylamine, and cyclopropylmethylphosphine have been prepared by the reaction at low temperatures of a borane complex or diborane on the free phosphine or amine. The products characterized by NMR spectroscopy and mass spectrometry have then been investigated by photoelectron spectroscopy and B3LYP/aug-cc-pVTZ quantum chemical study. The complexation led to rotamers with structures similar to the ones of the corresponding free systems. The main geometry change with the complexation is the P-C bond elongation and the N-C bond shortening, which can be rationalized by the charge transfer attached to the electron donation. The calculated relative stability order of the conformers changes with the complexation only in the case of cyclopropylamine. The calculated complexation energies are higher for the amines, in accord with the differences observed in the flash vacuum thermolysis of methylamine-, methylphosphine-, and aziridine-borane. The photoelectron spectra indicate essential differences between the amines and phosphines toward borane complexation. The dative bond is more stable in the studied amine-boranes than in phosphine-boranes, while the sigma(B-H) orbitals are more stable in the latter compounds. The enthalpy of the hydrogen release reaction of aziridine-borane is almost thermoneutral, indicating the potential of this complex as recyclable hydrogen storage material.