Synthesis and Characterization of Hypoelectronic Tantalaboranes: Comparison of the Geometric and Electronic Structures of [(CpTaX)2B5H11] (X = Cl, Br, and I)

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.