Synthesis of the New 2-Alkyl-nido-2,7,10-C3B8H11 Tricarbaborane by Protonation of [7-Alkyl-nido-7,8,10-C3B8H10]-: A Reversible Cage-Carbon Rearrangement

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Title:Synthesis of the New 2-Alkyl-nido-2,7,10-C3B8H11 Tricarbaborane by Protonation of [7-Alkyl-nido-7,8,10-C3B8H10]-: A Reversible Cage-Carbon Rearrangement
Creators:
Shedlow, Alexandra M.
Sneddon, Larry G.
Journal or Publication Title:
Collection of Czechoslovak Chemical Communications, 64, 5, pp. 865-882
Uncontrolled Keywords:<i>Ab initio</i> calculations, Boranes, Tricarbaboranes, Carboranes, NMR spectroscopy, <sup>1</sup>H, <sup>13</sup>C, <sup>11</sup>B, Carbon rearrangement, DFT calculations, GIAO calculations

Abstract

Protonation of the [7-R-<i>nido</i>-7,8,10-C<sub>3</sub>B<sub>8</sub>H<sub>10</sub>]<sup>-</sup> (where R = PhCH<sub>2</sub> (<strong>1a</strong>) or R = Me (<strong>1b</strong>)) tricarbollide anion, with concentrated H<sub>2</sub>SO<sub>4</sub> in a two-phase aqueous/CH<sub>2</sub>Cl<sub>2</sub> system, yields the new neutral tricarbaborane: 2-R-<i>nido</i>-2,7,10-C<sub>3</sub>B<sub>8</sub>H<sub>11</sub> (where R = PhCH<sub>2</sub> (<strong>2a</strong>) or R = Me (<strong>2b</strong>)). The three cage-carbons of the [7-R-<i>nido</i>-7,8,10-C<sub>3</sub>B<sub>8</sub>H<sub>10</sub>]<sup>-</sup> anion are located on the open face, but spectroscopic and DFT/GIAO/NMR studies of <strong>2a</strong> and <strong>2b</strong> show that during the protonation reaction, isomerization of the cage framework occurs to produce the neutral tricarbaborane having a 2,7,10-structure in which only two of the carbons remain on the open face. The third (R-substituted) carbon adopts a five-coordinate vertex off of the open face, thus enabling the incoming proton to adopt a bridging position on the B-B edge of the new C<sub>2</sub>B<sub>3</sub>-open face. The skeletal rearrangement is reversible, since deprotonation of <strong>2a</strong> or <strong>2b</strong> regenerates the anions <strong>1a</strong> and <strong>1b</strong>, respectively, having the 7,8,10-configuration. In agreement with the experimentally observed structures of the anionic (7,8,10-structure) and neutral (2,7,10-structure) species, DFT calculations at the B3LYP/6-311G*-level show that the [7-Me-<i>nido</i>-7,8,10-C<sub>3</sub>B<sub>8</sub>H<sub>10</sub>]<sup>-</sup> anion (<strong>1b</strong>, structure <strong>16</strong>) is 28.9 kcal/mol more stable than the [2-Me-<i>nido</i>-2,7,10-C<sub>3</sub>B<sub>8</sub>H<sub>10</sub>]<sup>-</sup> isomer (<strong>3b</strong>, structure <strong>18</strong>), while for the neutral tricarbaborane, the 2-R-<i>nido</i>-2,7,10-C<sub>3</sub>B<sub>8</sub>H<sub>11</sub> (<strong>2b</strong>, structure <strong>14</strong>) structure is more stable than any 7,8,10-structure (structures <strong>7</strong>-<strong>11</strong>) which has the added proton in an endo position on the open face. Transition state calculations at the HF/6-31G*-level yielded a simple, low-energy pathway (activation barrier of only 6.5 kcal/mol for the transition state <strong>TS18/16</strong>) for the rearrangement of [2-Me-<i>nido</i>-2,7,10-C<sub>3</sub>B<sub>8</sub>H<sub>10</sub>]<sup>-</sup> (<strong>3b</strong>, structure <strong>18</strong>) to [7-Me-<i>nido</i>-7,8,10-C<sub>3</sub>B<sub>8</sub>H<sub>10</sub>]<sup>-</sup> (<strong>1b</strong>, structure <strong>16</strong>) requiring the movement of only one cage atom, B11, from its original position in the C7-B8-B9-C10-B11 plane of <strong>3b</strong>, to the C7-C8-B9-C10-B11 plane of <strong>1b</strong>. <p>

Title:Synthesis of the New 2-Alkyl-nido-2,7,10-C3B8H11 Tricarbaborane by Protonation of [7-Alkyl-nido-7,8,10-C3B8H10]-: A Reversible Cage-Carbon Rearrangement
Creators:
Shedlow, Alexandra M.
Sneddon, Larry G.
Uncontrolled Keywords:<i>Ab initio</i> calculations, Boranes, Tricarbaboranes, Carboranes, NMR spectroscopy, <sup>1</sup>H, <sup>13</sup>C, <sup>11</sup>B, Carbon rearrangement, DFT calculations, GIAO calculations
Divisions:Life and Chemical Sciences > Institute of Organic Chemistry and Biochemistry > Collection of Czechoslovak Chemical Communications
Journal or Publication Title:Collection of Czechoslovak Chemical Communications
Volume:64
Number:5
Page Range:pp. 865-882
ISSN:0010-0765
E-ISSN:1212-6950
Publisher:Institute of Organic Chemistry and Biochemistry
Related URLs:
URLURL Type
http://dx.doi.org/10.1135/cccc19990865UNSPECIFIED
ID Code:1523
Item Type:Article
Deposited On:06 Feb 2009 17:10
Last Modified:06 Feb 2009 16:10

Citation

Shedlow, Alexandra M.; Sneddon, Larry G. (1999) Synthesis of the New 2-Alkyl-nido-2,7,10-C3B8H11 Tricarbaborane by Protonation of [7-Alkyl-nido-7,8,10-C3B8H10]-: A Reversible Cage-Carbon Rearrangement. Collection of Czechoslovak Chemical Communications, 64 (5). pp. 865-882. ISSN 0010-0765

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