pH-dependent H₂-activation cycle coupled to reduction of nitrate ion by Cp^*Ir complexes

This paper reports a pH-dependent H₂-activation {H₂ (pH 1-4) → H⁺ + H^- (pH -1) → 2H⁺ + 2e^-} promoted by Cp^*Ir complexes {Cp^* = η⁵-C₅(CH₃)₅}. In a pH range of about 1-4, an aqueous HNO₃ solution of [Cp^*III(H₂O)₃]²⁺ (1) reacts with 3 equiv of H₂ to yield a solution of [(Cp^*Ir^III)₂(μ- H)₃]⁺ (2) as a result of heterolytic H₂-activation {2[1] + 3H₂ (pH 1-4) → [2] + 3H⁺ + 6H₂O}. The hydrido ligands of 2 display protonic behavior and undergo H/D exchange with D⁺: [M-(H)₃-M]⁺ + 3D⁺ ⇌ [M-(D)y₃-M]⁺ + 3H⁺ (where M = Cp^*Ir). Complex 2 is insoluble in a pH range of about -0.2 (1.6 M HNO₃/H₂O) to -0.8 (6.3 M HNO₃/H₂O). At pH -1 (10 M HNO₃/H₂O), a powder of 2 drastically reacts with HNO₃ to give a solution of [Cp^*Ir^III(NO₃)₂] (3) with evolution of H₂, NO, and NO₂ gases. D-labeling experiments show that the evolved H₂ is derived from the hydrido ligands of 2. These results suggest that oxidation of the hydrido ligands of 2 {[2] + 4NO₃^- (pH -1) → 2[3] + H₂ + H⁺ + 4e^-} couples to reduction of NO₃^- (NO₃^- → NO₂^- → NO). To complete the reaction cycle, complex 3 is transformed into 1 by increasing the pH of the solution from.-1 to 1. Therefore, we are able to repeat the reaction cycle using 1, H₂, and a pH gradient between 1 and -1. A conceivable mechanism for the H₂-activation cycle with reduction of NO₃^- is proposed.