Nickel(0) Complexes of N-Alkyl-Substituted N-Heterocyclic Carbenes and Their Use in the Catalytic Carbon−Carbon Bond Activation of Biphenylene SchaubThomas BackesMarc RadiusUdo 2006 The reaction of [Ni(COD)<sub>2</sub>] with stable N-heterocyclic carbenes R<sub>2</sub>Im (R<sub>2</sub>Im = 1,3-di(R)imidazole-2-ylidene; R<sub>2</sub> = Me<sub>2</sub>, <i><sup>n</sup></i><sup></sup>Pr<sub>2</sub>, Me<i><sup>i</sup></i><sup></sup>Pr, <i><sup>i</sup></i><sup></sup>Pr<sub>2</sub>) affords homoleptic [Ni(Me<sub>2</sub>Im)<sub>3</sub>] (<b>1</b>) or dinuclear, COD-bridged complexes of the type [Ni<sub>2</sub>(R<sub>2</sub>Im)<sub>4</sub>(COD)] (R<sub>2</sub> = <i><sup>n</sup></i><sup></sup>Pr<sub>2</sub>, <b>2</b>; Me<i><sup>i</sup></i><sup></sup>Pr, <b>3</b>; <i><sup>i</sup></i><sup></sup>Pr<sub>2</sub>, <b>4</b>). Compounds <b>1</b>−<b>4</b> are suitable precursors for the synthesis of [Ni(R<sub>2</sub>Im)<sub>2</sub>]-containing complexes in solution, exemplified by the reaction with CO under atmospheric pressure, with equimolar amounts of diphenyl acetylene or with biphenylene to give carbonyl complexes [Ni(R<sub>2</sub>Im)<sub>2</sub>(CO)<sub>2</sub>] (R<sub>2</sub> = Me<sub>2</sub>, <b>5</b>; <i><sup>n</sup></i><sup></sup>Pr<sub>2</sub>, <b>6</b>; Me<i><sup>i</sup></i><sup></sup>Pr, <b>7</b>; <i><sup>i</sup></i><sup></sup>Pr<sub>2</sub>, <b>8</b>), diphenyl acetylene complexes [Ni(R<sub>2</sub>Im)<sub>2</sub>(η<sup>2</sup>-C<sub>2</sub>Ph<sub>2</sub>)] (R<sub>2</sub> = Me<sub>2</sub>, <b>9</b>; <i><sup>n</sup></i><sup></sup>Pr<sub>2</sub>, <b>10</b>; Me<i><sup>i</sup></i><sup></sup>Pr, <b>11</b>; <i><sup>i</sup></i><sup></sup>Pr<sub>2</sub>, <b>12</b>), and biphenylene complexes [Ni(R<sub>2</sub>Im)<sub>2</sub>(2,2‘-biphenyl)] (R<sub>2</sub> = Me<sub>2</sub>, <b>13</b>; <i><sup>n</sup></i><sup></sup>Pr<sub>2</sub>, <b>14</b>; Me<i><sup>i</sup></i><sup></sup>Pr, <b>15</b>; <i><sup>i</sup></i><sup></sup>Pr<sub>2</sub>, <b>16</b>). Furthermore, the reaction of <b>4</b> with 3-hexyne or 2-butyne afforded [Ni(<i><sup>i</sup></i><sup></sup>Pr<sub>2</sub>Im)<sub>2</sub>(η<sup>2</sup>-C<sub>2</sub>R<sub>2</sub>)] (R = Me, <b>18</b>; Et, <b>19</b>) in good yields. The compounds <b>1</b>, <b>11</b>, <b>13</b>, <b>17</b>, and <b>18</b> have been structurally characterized. Complexes <b>13</b>−<b>16</b> are the products of a stoichiometric carbon−carbon activation of biphenylene, and compounds <b>1</b>−<b>4</b> (as well as <b>9</b>−<b>12</b>) are efficient catalysts for the insertion of diphenyl acetylene into the C−C bond of biphenylene, a process in which the C−C activation of biphenylene is incorporated into a catalytic cycle. The reaction rate of the formation of 9,10-di(phenyl)phenanthrene depends on the nature of the carbene ligand of the catalyst; the highest was observed for [Ni<sub>2</sub>(<i><sup>i</sup></i><sup></sup>Pr<sub>2</sub>Im)<sub>4</sub>(COD)] (<b>4</b>).