C−H Oxidative Addition with a (PCP)Ir(III)-Pincer Complex

Treatment of 4-MeO-C6H3-2,6-(CH2PtBu2)2 with IrCl3·nH2O in i-PrOH/H2O gives a
cyclometalated pincer chlorohydrido iridium complex. A second intramolecular oxidative
addition reaction of one of the tert-butyl C−H bonds to the Ir(III) center followed by the
reductive elimination of H2 gives a novel doubly metalated compound that is stable to air
and water. X-ray crystallographic analysis revealed a chelated square-pyramidal iridium
compound with two cyclometalated five-membered rings in the basal plane and the iridium
bound methylene group of the four-membered ring in the apical position. The free
coordination site is protected by an agostic C−H bond. While a phosphorus−phosphorus
coupling constant of 351.1 Hz establishes the trans position of the phosphorus groups there
is no indication of a hydridic or weakly bound hydrogen. Electrochemical studies establish
an equilibrium between the pincer chlorohydrido compound and the doubly metalated
complex and H2. A square scheme can be used to describe the relationship between the redox
couples and solution equilibria. The unusual stability of the doubly metalated complex was
supported by DFT calculations at different levels of theory on model compounds. They show
the coordination of two hydrogen atoms best described as η2-coordinated dihydrogen and
suggest an Ir(V) oxidation state as intermediate.