Mild, Reversible Reaction
of Iridium(III) Amido Complexes
with Carbon Dioxide
- Publication date
- Publisher
Abstract
Unlike some other Ir(III) hydrides, the aminopyridine
complex [(2-NH<sub>2</sub>–C<sub>5</sub>NH<sub>4</sub>)IrH<sub>3</sub>(PPh<sub>3</sub>)<sub>2</sub>] (<b>1-PPh</b><sub><b>3</b></sub>) does not insert CO<sub>2</sub> into the Ir–H
bond. Instead <b>1-PPh</b><sub><b>3</b></sub> loses H<sub>2</sub> to form
the cyclometalated species [(κ<sup>2</sup>-<i>N</i>,<i>N</i>-2-NH-C<sub>5</sub>NH<sub>4</sub>)IrH<sub>2</sub>(PPh<sub>3</sub>)<sub>2</sub>] (<b>2-PPh</b><sub><b>3</b></sub>), which subsequently reacts with CO<sub>2</sub> to form the
carbamato species [(κ<sup>2</sup>-<i>O</i>,<i>N</i>-2-OC(O)NH-C<sub>5</sub>NH<sub>4</sub>)IrH<sub>2</sub>(PPh<sub>3</sub>)<sub>2</sub>] (<b>10-PPh</b><sub><b>3</b></sub>). To study the insertion of CO<sub>2</sub> into the Ir–N
bond of the cyclometalated species, a family of compounds of the type
[(κ<sup>2</sup>-<i>N</i>,<i>N</i>-2-NR-C<sub>5</sub>NH<sub>4</sub>)IrH<sub>2</sub>(PR′<sub>3</sub>)<sub>2</sub>] (R = H, R′ = Ph (<b>2-PPh</b><sub><b>3</b></sub>); R = H, R′ = Cy (<b>2-PCy</b><sub><b>3</b></sub>); R = Me, R′ = Ph (<b>4-PPh</b><sub><b>3</b></sub>); R = Ph, R′ = Ph (<b>5-PPh</b><sub><b>3</b></sub>); R = Ph, R′ = Cy (<b>5-PCy</b><sub><b>3</b></sub>)) and the pyrimidine complex [(κ<sup>2</sup>-<i>N</i>,<i>N</i>-2-NH-C<sub>4</sub>N<sub>2</sub>H<sub>3</sub>)IrH<sub>2</sub>(PPh<sub>3</sub>)<sub>2</sub>] (<b>6-PPh</b><sub><b>3</b></sub>) were prepared. The rate of CO<sub>2</sub> insertion is faster for the more nucleophilic amides. DFT studies
suggest that the mechanism of insertion involves initial nucleophilic
attack of the nitrogen lone pair of the amide on CO<sub>2</sub> to
form an <i>N</i>-bound carbamato complex, followed by rearrangement
to the <i>O</i>-bound species. CO<sub>2</sub> insertion
into <b>1-PPh</b><sub><b>3</b></sub> is reversible in
the presence of H<sub>2</sub> and treatment of <b>10-PPh</b><sub><b>3</b></sub> with H<sub>2</sub> regenerates <b>1-PPh</b><sub><b>3</b></sub>, along with Ir(PPh<sub>3</sub>)<sub>2</sub>H<sub>5</sub>
