¹¹B NMR studies of the paramagnetic effects of samarium(II) in tetrahydrofuran solutions on neutral and anionic borane and carborane clusters

The dependence of ¹¹B NMR spectral parameters of several neutral and anionic borane ([B₃H₈]^-, [B₅H₈]^-, and B₅H₉) and carborane (Et₂C₂B₄H₆, [Et₂C₂B₄H₅]^- and C₂B₁₀H₁₂) clusters on the concentration of the paramagnetic samarium(II) cation in THF solutions was investigated. The magnitude of the paramagnetic isotropic chemical shifts in the ¹¹B NMR spectra was found to depend solely on the samarium(II) solution concentration for all the borane clusters studied. In general, the isotropic shifts observed were significantly larger for the anionic borane and carborane clusters. The anionic cages displayed upfield isotropic shifts while the neutral cages showed only relatively small downfield isotropic shifts. In addition, the paramagnetic isotropic shifts for the nido-cage species ([B₅H₈]^- and [Et₂C₂B₄H₅]^-) were found to be much larger for the basal boron atoms than for the apical atom. The ¹¹B-¹H_tJ_BH NMR coupling constants (where H_t is a terminal BH cage proton) for the anionic clusters were observed to decrease and the spectral line widths (Δυ_1/2) increase with increasing samarium(II) concentration in solution. Finally, a linear temperature dependence of the isotropic chemical shift of the arachno-octahydrotriborate(1-) anion, [B₃H₈]^-, was observed at constant samarium(II) concentration. A mechanism is proposed to account for these observations.