Scandium and yttrium metallocene borohydride complexes: comparisons of (BH4)1−vs. (BPh4)1− coordination and reactivity

Abstract

The synthetically accessible borohydride complexes (C₅Me₄H)₂Ln(THF)(BH₄) and (C₅Me₅)₂Ln(THF)(BH₄) (Ln = Sc, Y) were examined as precursors alternative to the heavily-used tetraphenylborate analogs, [(C₅Me₄H)₂Ln][BPh₄] and [(C₅Me₅)₂Ln][BPh₄], employed in LnA₂A′/M reduction reactions (A = anion; M = alkali metal) that generate “LnA₂” reactivity and form reduced dinitrogen complexes [(C₅R₅)₂(THF)_xLn]₂(μ-η²:η²-N₂) (x = 0, 1). The crystal structures of the yttrium borohydrides, (C₅Me₄H)₂Y(THF)(μ-H)₃BH, 1, and (C₅Me₅)₂Y(THF)(μ-H)₂BH₂, 2, were determined for comparison with those of the yttrium tetraphenylborates, [(C₅Me₄H)₂Y][(μ-Ph)₂BPh₂], 3, and [(C₅Me₅)₂Y][(μ-Ph)₂BPh₂], 4. The complex (C₅Me₄H)₂Sc(μ-H)₂BH₂, 5, was synthesized and structurally characterized for comparison with (C₅Me₅)₂Sc(μ-H)₂BH₂, 6, [(C₅Me₄H)₂Sc][(μ-Ph)BPh₃], 7, and [(C₅Me₅)₂Sc][(μ-Ph)BPh₃], 8. Structural information was also obtained on the borohydride derivatives, (C₅Me₄H)₂Sc(μ-H)₂BC₈H₁₄, 9, and (C₅Me₅)₂Sc(μ-H)₂BC₈H₁₄, 10, obtained from 9-borabicyclo(3.3.1)nonane (9-BBN) and (C₅Me₄R)₂Sc(η³-C₃H₅), where R = H, 11; Me, 12. The preference of the metals for borohydride over tetraphenylborate binding was shown by the facile displacement of (BPh₄)¹⁻ in 3, 4, 7, and 8 by (BH₄)¹⁻ to make the respective borohydride complexes 1, 2, 5, and 6. These results are consistent with the fact that the borohydrides are not as useful as precursors in A₂LnA′/M reductions of N₂. An unusual structural isomer of [(C₅Me₄H)₂Sc]₂(μ-η²:η²-N₂), 13′, was isolated from this study that shows the variations in ligand orientation that can occur in the solid state.