Synthesis, vibrational spectra, and proton magnetic resonance of some boron-substituted tris(cyclopentadienyl)uranium(IV) tetrahydroborates

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.