Mild, reversible reaction of iridium(III) amido complexes with carbon dioxide

Unlike some other Ir(III) hydrides, the aminopyridine complex [(2-NH₂-C₅NH₄)IrH₃(PPh₃)₂] (1-PPh₃) does not insert CO₂ into the Ir-H bond. Instead 1-PPh₃ loses H₂ to form the cyclometalated species [(κ²–N,N-2-NH-C₅NH₄)IrH₂(PPh₃)₂] (2-PPh₃), which subsequently reacts with CO₂ to form the carbamato species [(κ²-O,N-2-OC(O)NH-C₅NH₄)IrH₂(PPh₃)₂] (10-PPh₃). To study the insertion of CO₂ into the Ir-N bond of the cyclometalated species, a family of compounds of the type [(κ²–N,N-2-NR-C₅NH₄)IrH₂(PR′₃)₂] (R = H, R′ = Ph (2-PPh₃); R = H, R′ = Cy (2-PCy₃); R = Me, R′ = Ph (4-PPh₃); R = Ph, R′ = Ph (5-PPh₃); R = Ph, R′ = Cy (5-PCy₃)) and the pyrimidine complex [(κ²–N,N-2-NH-C₄N₂H₃)IrH₂(PPh₃)₂] (6-PPh₃) were prepared. The rate of CO₂ 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 CO₂ to form an N-bound carbamato complex, followed by rearrangement to the O-bound species. CO₂ insertion into 1-PPh₃ is reversible in the presence of H₂ and treatment of 10-PPh₃ with H₂ regenerates 1-PPh₃, along with Ir(PPh₃)₂H₅.