The reaction of (C5H5)3UBH4 with R3B yields the corresponding (C5H5)3UH3BR compound, R = C2H5, C6H5. Vibrational spectra indicate that the BR group is connected to uranium via a triple hydrogen bridge. The compounds (C5H5)3UNCBH3 and (C5H5)3UNCB(C6H5) 3 were synthesized by the reaction of (C5H5)3UCl with the corresponding anionic borates. Vibrational spectra suggest the metal-borate linkage is of the type UN≡CB|←. Large isotropic shifts were observed in the pmr spectra of these paramagnetic U(IV) compounds. For the (C5H5)3UH3BR system it was possible to separate contact and dipolar contributions; the mechanism of unpaired spin distribution involves negative spin density on the protons bound directly to uranium. The 11B-decoupled pmr spectrum of the fluxional molecule (C5H5)3UBH4 exhibits collapse of the BH4 resonance at low temperatures. This indicates that the paramagnetism induces sufficient energy separation between exchanging sites (bridge and terminal hydrogens) to observe, for the first time in a metal tetrahydroborate, slowing of the dynamic intramolecular rearrangement process. At the estimated coalescence point (-140 ± 20°) δω = 15,000-36,000 Hz and ΔG* ≈ 5.0 ± 0.6 kcal/mol.
|Number of pages||7|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - 1975|
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