Mechanism of Hydrogenolysis of an Iridium–Methyl Bond: Evidence for a Methane Complex Intermediate

Evidence for key σ-complex intermediates in the hydrogenolysis of the iridium–methyl bond of (PONOP)­Ir­(H)­(Me)+ (1) [PONOP = 2,6-bis­(di-tert-butylphosphinito)­pyridine] has been obtained. The initially formed η2-H2 complex, 2, was directly observed upon treatment of 1 with H2, and evidence for reversible formation of a σ-methane complex, 5, was obtained through deuterium scrambling from η2-D2 in 2-d2 into the methyl group of 2 prior to methane loss. This sequence of reactions was modeled by density functional theory calculations. The transition state for formation of 5 from 2 showed significant shortening of the Ir–H bond for the hydrogen being transferred; no true Ir­(V) trihydride intermediate could be located. Barriers to methane loss from 2 were compared to those of 1 and the six-coordinate species (PONOP)­Ir­(H)­(Me)­(CO)+ and (PONOP)­Ir­(H)­(Me)­(Cl).