Oxygen Reduction with a Bifunctional Iridium Dihydride Complex

The iridium dihydride [Ir(H)(2)(HPNP)](+) (PNP=N(CH2CH2PtBu2)(2)) reacts with O-2 to give the unusual, square-planar iridium(III) hydroxide [Ir(OH)(PNP)](+) and water. Regeneration of the dihydride with H-2 closes a quasi-catalytic synthetic oxygen-reduction reaction (ORR) cycle that can be run several times. Experimental and computational examinations are in agreement with an oxygenation mechanism via rate-limiting O-2 coordination followed by H-transfer at a single metal site, facilitated by the cooperating pincer ligand. Hence, the four electrons required for the ORR are stored within the two covalent M-H bonds of a mononuclear metal complex.