Oxidation of dihydrogen by iridium complexes of redox-active ligands

Unsaturated organoiridium complexes were prepared with amidophenolate ligands derived from 2-(2-trifluoromethyl)amino-4,6-di-tert-butylphenol (H ₂^tBA^F) and 2-tert-butylamino-4,6-di-tert- butylphenol (H₂^tBA^tBu). The following 16e complexes were characterized: Cp*M(^tBA^R) with M = Ir (1^F and 1^t-Bu), Rh (2^F), and (cymene)Ru( ^tBA^F) (3^F). These complexes undergo two 1e oxidations at potentials of about 0 and -0.25 V vs Cp₂Fe ^0/+. The magnitude of δE_1/2 is sensitive to the counteranions, and the reversibility is strongly affected by the presence of Lewis bases, which stabilize the oxidized derivatives. Crystallographic measurements indicate that upon oxidation the amidophenolate ligands adopt quinoid character, as indicated by increased alternation of the C-C bond lengths in the phenylene ring backbone and shortened C-N and C-O bonds. Unlike the charge-neutral precursors, the cationic [Cp*M(^tBA ^R)]⁺ are Lewis acidic and form well-characterized adducts with PR₃ (R = Me, Ph), CN^-, MeCN (reversibly), and CO. In the absence of competing ligands, the cations oxidize H₂. Coulommetry measurements indicate that H₂ is oxidized by the monocations [Cp*M(^tBA^R)]⁺, not the corresponding dications. Oxidation of H₂ is catalytic in the presence of a noncoordinating base at potentials required for the generation of [Cp*M(^tBA^R)]⁺. The rate decreases in the order [Cp*M(^tBA^F)]BAr^F₄ > [Cp*M(^tBA^F)]PF₆ > [Cp*M( ^tBA^t-Bu)]PF₆. The reduction of ferrocenium by H₂ is catalyzed by Cp*M(^tBA^R).