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
Theodor Agapie
Michael W. Day
John E. Bercaw
10.1021/om8002653.s012
https://acs.figshare.com/articles/journal_contribution/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_Benzylid/2894341
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.
2008-12-08 00:00:00
ligand systems
Tridentate LX 2 Ligands
phosphine coordination
mer fashion
Bidentate X 2
ligand geometry
DFT calculations
LX 2 frameworks
C 2v
ligands bind
salt metathesis methods
Benzene Connectors
Mechanistic Studies
Tantalum alkyl complexes
Tantalum Benzylidene
pincer complexes
diphenolate ligands
methyl group
phenol hydrogens
Ph 2CO
benzylidene species
ligand framework
pendant phenol
PMe 2Ph concentration
Insertion Chemistry
PMe 2Ph
Tantalum Complexes
mer binding mode
alkane elimination
σ bond metathesis
tantalum complexes
TaCl 2CH
complexes show
cyclometallated group