Synthesis and characterization of hypoelectronic tantalaboranes: comparison of the geometric and electronic structures of [(Cp*TaX)₂B₅H₁₁] (X = Cl, Br, and I)

Abstract

Mild thermolysis of tantalaborane [(Cp*Ta)₂B₅H₁₁], 1 (Cp* = η⁵-C₅Me₅) in presence of halogen sources affords the open cage clusters [(Cp*TaX)₂B₅H₁₁], 2–4 (2: X = Cl; 3: X = Br; and 4: X = I) in good yields. In contrast, the tetraborohydride cluster, [(Cp*Ta)₂B₄H₉(μ-BH₄)], 5, under the same reaction conditions forms the B–H substituted cluster [(Cp*Ta)₂B₄H₈I(μ-BH₄)], 6. All the new metallaboranes have been characterized by mass spectrometry, ¹H, ¹¹B, ¹³C NMR spectroscopy, and elemental analysis, and the structural types were established by crystallographic analysis of clusters 3, 4, and 6. Density functional theory (DFT) calculations at the BP86/TZ2P ZORA level reveal geometries in agreement with the structure determinations, large gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in accord with their stabilities. B3LYP-computed ¹¹B chemical shifts accurately reflect the experimentally measured shifts. Clusters 2–4 can be viewed as 7-sep 7-vertex oblatoarachno M₂B₅ clusters which can be generated from a 7-sep 9-vertex oblatocloso M₂B₇ cluster by removal of two equatorial boron atoms. Cluster 6 can be considered as an electron-deficient 6-sep 6-vertex oblatoarachno M₂B₄ cluster derived from an 8-vertex oblatocloso hexagonal bipyramidal cluster, in which BH₄^– anion is weakly bonded in a bidentate mode.