Synthesis and Characterization
of Hypoelectronic Tantalaboranes:
Comparison of the Geometric and Electronic Structures of [(Cp*TaX)<sub>2</sub>B<sub>5</sub>H<sub>11</sub>] (X = Cl, Br, and I)
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Abstract
Mild thermolysis of tantalaborane [(Cp*Ta)<sub>2</sub>B<sub>5</sub>H<sub>11</sub>], <b>1</b> (Cp* = η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>) in presence of halogen sources affords
the
open cage clusters [(Cp*TaX)<sub>2</sub>B<sub>5</sub>H<sub>11</sub>], <b>2</b>–<b>4</b> (<b>2</b>: X = Cl; <b>3</b>: X = Br; and <b>4</b>: X = I) in good yields. In contrast,
the tetraborohydride cluster, [(Cp*Ta)<sub>2</sub>B<sub>4</sub>H<sub>9</sub>(μ-BH<sub>4</sub>)], <b>5</b>, under the same
reaction conditions forms the B–H substituted cluster [(Cp*Ta)<sub>2</sub>B<sub>4</sub>H<sub>8</sub>I(μ-BH<sub>4</sub>)], <b>6</b>. All the new metallaboranes have been characterized by mass
spectrometry, <sup>1</sup>H, <sup>11</sup>B, <sup>13</sup>C NMR spectroscopy,
and elemental analysis, and the structural types were established
by crystallographic analysis of clusters <b>3</b>, <b>4</b>, and <b>6</b>. 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 <sup>11</sup>B chemical shifts accurately reflect the experimentally measured
shifts. Clusters <b>2</b>–<b>4</b> can be viewed
as 7-sep 7-vertex <i>oblatoarachno</i> M<sub>2</sub>B<sub>5</sub> clusters which can be generated from a 7-sep 9-vertex <i>oblatocloso</i> M<sub>2</sub>B<sub>7</sub> cluster by removal
of two equatorial boron atoms. Cluster <b>6</b> can be considered
as an electron-deficient 6-sep 6-vertex <i>oblatoarachno</i> M<sub>2</sub>B<sub>4</sub> cluster derived from an 8-vertex <i>oblatocloso</i> hexagonal bipyramidal cluster, in which BH<sub>4</sub><sup>–</sup> anion is weakly bonded in a bidentate
mode