f-Element complexes with benzyl and cyclohexyl substituted trihydroborates

Actinide complexes containing the simplest borohydrides (BH₄)¹⁻ and (MeBH₃)¹⁻ can exhibit remarkably highly volatility, which creates unique hazards and handling challenges, especially when making measurements on solid samples under vacuum. Here we describe efforts to prepare new actinide borohydride complexes with attenuated volatility by adding bulkier benzyl (Bn) and cyclohexyl (Cy) substituents to boron. Reactions of ThCl₄, UI₃(thf)₄, and NdI₃ with the mixed alkali metal salt Li/K(BnBH₃)(thf)_n yielded Th(BnBH₃)₄(thf)₂, U(BnBH₃)₄(thf)₂, and K[Nd(BnBH₃)₄], respectively. Notable amongst these, the reaction with UI₃(thf)₄ proceeds via oxidation of U(III) to U(IV) despite the presence of reducing borohydride ligands. Similarly, reactions of the same metal halides with four equivalents of Li(CyBH₃)(Et₂O)_n yielded Th(CyBH₃)₄, U(CyBH₃)₄(thf)₂, and [Li(Et₂O)₃][Nd(CyBH₃)₄]. Single crystal X-ray diffraction studies of the M(BnBH₃)₄(thf)₂ complexes with M = Th and U confirmed their formulations. The complexes have approximate D_2d point group symmetry and adopt bicapped hexagonal antiprismatic coordination geometries with axial thf ligands and κ³-BnBH₃ ligands bound in the equatorial plane. K[Nd(BnBH₃)₄] and [Li(Et₂O)₃][Nd(CyBH₃)₄], which were prepared for comparison to U(III) complexes that were unsuccessfully targeted, were also structurally characterized to reveal complex anions with tetrahedral arrangements of trihydroborate ligands bound to Nd(III). Crystals obtained for Th(CyBH₃)₄ and U(CyBH₃)₄(thf)₂ were not suitable for XRD studies, but ¹H and ¹¹B NMR spectra were consistent with their formulations. Collectively, these complexes represent rare examples of structurally characterized f-element trihydroborate complexes with carbon substituents other than methyl.