Molecular hydrogen tweezers: Structure and mechanisms by neutron diffraction, NMR, and deuterium labeling studies in solid and solution

The mechanism of reversible hydrogen activation by ansa-aminoboranes, 1-N-TMPH-CH ₂-2-[HB(C ₆F ₅) ₂]C ₆H ₄ (NHHB), was studied by neutron diffraction and thermogravimetric mass-spectroscopic experiments in the solid state as well as with NMR and FT-IR spectroscopy in solution. The structure of the ansa-ammonium borate NHHB was determined by neutron scattering, revealing a short N-H•••H-B dihydrogen bond of 1.67 Å. Moreover, this intramolecular H-H distance was determined in solution to be also 1.6-1.8 Å by ¹H NMR spectroscopic T ₁ relaxation and 1D NOE measurements. The X-ray B-H and N-H distances deviated from the neutron and the calculated values. The dynamic nature of the molecular tweezers in solution was additionally studied by multinuclear and variable-temperature NMR spectroscopy. We synthesized stable, individual isotopic isomers NDDB, NHDB, and NDHB. NMR measurements revealed a primary isotope effect in the chemical shift difference ^pΔ ¹H(D) = δ(NH) - δ(ND) (0.56 ppm), and hence supported dihydrogen bonding. The NMR studies gave strong evidence that the structure of NHHB in solution is similar to that in the solid state. This is corroborated by IR studies providing clear evidence for the dynamic nature of the intramolecular dihydrogen bonding at room temperature. Interestingly, no kinetic isotope effect was detected for the activation of deuterium hydride by the ansa-aminoborane NB. Theoretical calculations attribute this to an "early transition state". Moreover, 2D NOESY NMR measurements support fast intermolecular proton exchange in aprotic CD ₂Cl ₂ and C ₆D ₆.