Synthesis and Reactivity of Tantalum Complexes Supported by Bidentate X<sub>2</sub> and Tridentate LX<sub>2</sub> Ligands with Two Phenolates Linked to Pyridine, Thiophene, Furan, and Benzene Connectors: Mechanistic Studies of the Formation of a Tantalum Benzylidene and Insertion Chemistry for Tantalum−Carbon Bonds

Using either alkane elimination or salt metathesis methods, tantalum complexes have been prepared with new ligand systems with tridentate bis(phenolate)donor (donor = pyridine, furan, and thiophene) or bidentate bis(phenolate)benzene arrangements. The ligand framework has two X-type phenolates connected to the flat heterocyclic L-type donor at the 2,6- or 2,5- positions or to the 2,6- positions of benzene via direct ring−ring (sp<sup>2</sup>−sp<sup>2</sup>) linkages. Solid-state structures of these complexes show that in all cases the ligands bind in a <i>mer</i> fashion, but with different geometries of the LX<sub>2</sub> frameworks. The pyridine-linked system binds in a <i>C</i><sub>s</sub>-fashion, the furan-linked system in a <i>C</i><sub>2v</sub>-fashion, and the thiophene-linked system in a <i>C</i><sub>1</sub>-fashion. A bis(phenolate)pyridine tantalum tribenzyl species (<b>7</b>), upon heating in the presence of dimethylphenylphosphine, generates a stable benzylidene complex by α-hydrogen abstraction with loss of toluene and PMe<sub>2</sub>Ph trapping. This process was found to be independent of PMe<sub>2</sub>Ph concentration with Δ<i>H</i><sup>⧧</sup> = 31.3 ± 0.6 kcal·mol<sup>−1</sup> and Δ<i>S</i><sup>⧧</sup> = 3 ± 2 cal·mol<sup>−1</sup>·K<sup>−1</sup>, and the kinetic isotope effect <i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> = 4.9 ± 0.4, consistent with a mechanism involving rate determining α-hydrogen abstraction with loss of toluene, followed by fast phosphine coordination to the resulting benzylidene species. An X-ray structure determination reveals that the benzylidene π-bond is oriented perpendicular to the oxygen−oxygen vector, in accord with the prediction of DFT calculations. Tantalum alkyl complexes with the benzene-linked bis(phenolate) ligand (Ta(CH<sub>3</sub>)<sub>2</sub>[(OC<sub>6</sub>H<sub>2</sub>-<i>t</i>Bu<sub>2</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>] (<b>16</b>), Ta(CH<sub>2</sub>Ph)<sub>2</sub>[(OC<sub>6</sub>H<sub>2</sub>-<i>t</i>Bu<sub>2</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>] (<b>17</b>), and TaCl<sub>2</sub>CH<sub>3</sub>[(OC<sub>6</sub>H<sub>2</sub>-<i>t</i>Bu<sub>2</sub>)<sub>2</sub>C<sub>6</sub>H<sub>4</sub>] (<b>18</b>)) are obtained with (to afford pincer complexes) or without cyclometalation at the <i>ipso</i>-position. Deuterium labeling of the phenol hydrogens and of the linking 1,3-benzene-diyl ring reveals an unexpected mechanism for the metalation of bis(phenol)benzene with TaCl<sub>2</sub>(CH<sub>3</sub>)<sub>3</sub> to generate <b>18</b>. This process involves protonolysis of a methyl group, followed by C-H/Ta-CH<sub>3</sub> σ bond metathesis leading to cyclometalation of the linking ring, and finally protonation of the cyclometallated group by the pendant phenol. TaCl<sub>2</sub>CH<sub>3</sub>[(OC<sub>6</sub>H<sub>2</sub>-<i>t</i>Bu<sub>2</sub>)<sub>2</sub>C<sub>6</sub>H<sub>4</sub>] was found to undergo σ bond metathesis at temperatures over 90 °C to give the pincer complex TaCl<sub>2</sub>[(OC<sub>6</sub>H<sub>2</sub>-<i>t</i>Bu<sub>2</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>] (<b>19</b>) and methane (Δ<i>H</i><sup>⧧</sup> = 27.1 ± 0.9 kcal·mol<sup>−1</sup>; Δ<i>S</i><sup>⧧</sup> = −2 ± 2 cal·mol<sup>−1</sup>·K<sup>−1</sup>; <i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> = 1.6 ± 0.2 at 125 °C). Ta(CH<sub>3</sub>)<sub>2</sub>[(OC<sub>6</sub>H<sub>2</sub>-<i>t</i>Bu<sub>2</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>] (<b>16</b>) was found to react with <i>t</i>BuNC to insert into the Ta-CH<sub>3</sub> bonds and generate an imino-acyl species (<b>23</b>). Reaction of <b>16</b> with Ph<sub>2</sub>CO or PhCN leads to insertion into the Ta-Ph bond to give <b>21</b> and <b>22</b>. Complexes <b>6</b>, <b>7</b>, <b>10</b>, <b>11-P</b>, <b>12</b>, <b>13</b>, <b>17</b>, <b>18</b>, <b>19-OEt</b><sub><b>2</b></sub>, <b>21</b>, <b>22</b>, and <b>23</b> have been structurally characterized by single crystal X-ray diffraction, and all show a <i>mer</i> binding mode of the diphenolate ligands, but the ligand geometry varies leading to <i>C</i><sub>2<i>v</i></sub>-, <i>pseudo</i>-<i>C</i><sub><i>s</i></sub>-, <i>pseudo</i>-<i>C</i><sub><i>2</i></sub>-, and <i>C</i><sub>1</sub>-symmetric structures.