Synthesis\nand Reactivity of a Bipyridyl Tantalum Complex

The bipyridyl tantalum complex (2,6-<i><sup>i</sup></i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>O)<sub>3</sub>Ta(bipy)\n(<b>2</b>) is synthesized by the reaction of (2,6-<i><sup>i</sup></i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>O)<sub>3</sub>TaCl<sub>2</sub> (<b>1</b>) and 2,2′-bipyridine in the\npresence\nof excess potassium graphite (KC<sub>8</sub>). Complex <b>2</b> coordinates readily with pyridine and 4-(dimethylamino)pyridine\n(dmap) to form Lewis base adducts (2,6-<i><sup>i</sup></i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>O)<sub>3</sub>Ta(bipy)(L)\n(L = py (<b>3</b>), dmap (<b>4</b>)), and it exhibits\nrich redox reactivity toward small molecules: (a) single electron\ntransfer (SET) occurs upon exposure of <b>2</b> to phenyl sulfide\nor selenide dimer, giving the open-shell, bipy-centered radical complexes\n(2,6-<i><sup>i</sup></i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>O)<sub>3</sub>Ta(bipy<sup>•</sup>)(PhE) (E = S (<b>5</b>), Se (<b>6</b>)). (b) Regioselective C–C σ-bond\nformation via radical coupling is observed in the SET reaction of <b>2</b> and aldehydes, ketones, or imines to furnish insertion products <b>7</b>–<b>13</b>, namely, sterically more crowded\nbenzophenone, acetophenone, 2,6-dichlorobenzaldehyde, and benzophenone\nimine couple with C6 or C6′ of bipy in <b>2</b>, respectively,\nwhereas sterically less hindered benzaldehyde, cyclohexanone, and\nbenzylideneaniline couple with C2 or C2′ of bipy, respectively.\n(c) <b>2</b> serves as a synthon for (2,6-<i><sup>i</sup></i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>O)<sub>3</sub>Ta(III)\nin contact with diazene, organic azides, and 1,4-diaza-1,3-diene and\n(conjugated)alkyne derivatives, in which tantalum imido/azido/hydrazido\ncomplexes (2,6-<i><sup>i</sup></i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>O)<sub>3</sub>Ta(NPh)(bipy<sup>0</sup>) (<b>14</b>), (2,6-<i><sup>i</sup></i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>O)<sub>3</sub>Ta(N<sub>3</sub>(2,6-<i><sup>i</sup></i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>))(bipy<sup>0</sup>) (<b>15</b>), (2,6-<i><sup>i</sup></i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>O)<sub>3</sub>Ta(NSiMe<sub>3</sub>)(bipy<sup>0</sup>) (<b>16</b>), and (2,6-<i><sup>i</sup></i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>O)<sub>3</sub>Ta(N<sub>2</sub>CPh<sub>2</sub>)(bipy<sup>0</sup>) (<b>17</b>); the tantalum σ<sup>2</sup>-enediamide complex (2,6-<i><sup>i</sup></i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>O)<sub>3</sub>Ta(η<sup>2</sup>-<i>N</i>,<i>N′</i>-<i>p</i>-MeC<sub>6</sub>H<sub>4</sub>-DAD) (<b>18</b>); and tantalum metallacyclopropenes (2,6-<i><sup>i</sup></i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>O)<sub>3</sub>Ta(η<sup>2</sup>-C<sub>2</sub>Ph<sub>2</sub>) (<b>19</b>) and (2,6-<i><sup>i</sup></i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>O)<sub>3</sub>Ta[η<sup>2</sup>-C(Ph)C(CCPh)](bipy<sup>0</sup>) (<b>20</b>) are formed via a two-electron redox process,\nrespectively.