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.