11B-NMR shielding effects in the closo borane series: sensitivity of shifts and their additivity

Abstract

An Alpha Shift Correlation (ASC) method is used to assess quantitatively NMR shielding effects in a series of halogen derivatives of closo-borane dianions, [X_m-B_nH_n−m]²⁻. There is a simple linear correlation between the universal α_ref shifts and those induced by the substituent X in all positions of the closo borane cluster: Δδ(¹¹B) = k·α_ref (where Δδ = δ_s − δ_p and subscripts s and p denote the substituted and parent derivatives, respectively; k = correlation slope). The sensitivity of the chemical shift in a given cage position to X-changes is defined by the g factor (g = gradient = k × 10²). The ASC approach revealed that the α-deshielding effect in the monosubstituted anionic [X-B_nH_n−1]²⁻ series decreases in the order of n: 6 > 12 > 10 > 9, and is dependent on the cage type and electron density at the substituted vertex. The sensitivity (g) in individual cluster positions to substituent changes decreases in the order α ≫ A > β > γ and is of approximately additive character. The most important feature of the unsubstituted sphere is the antipodal (A) effect showing a g-trend inversed to that observed at the substituted α-sites. Of interest are also the remarkable pseudo-A and trans-γ shielding effects in the 9- and 10-vertex anionic series.