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

This paper reports a pH-dependent H-2-activation {H-2 (pH 1-4) --> H+ + H- (pH -1) --> 2H(+) + 2e(-)} promoted by Cp*Ir complexes {Cp* = eta(5)-C-5(CH3)(5)}. In a pH range of about 1-4, an aqueous HNO3 solution of [Cp*Ir-III(H2O)(3)](2+) (1) reacts with 3 equiv of H-2 to yield a solution of [(Cp*Ir-III)(2)(mu-H)(3)](+) (2) as a result of heterolytic H-2-activation {2[1] + 3H(2) (pH 1-4) --> [2] + 3H(+) + 6H(2)O}. The hydrido ligands of 2 display protonic behavior and undergo H/D exchange with D+: [M-(H)(3)-M](+) + 3D(+) reversible arrow [M-(D)(3)-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 H-2, NO, and NO2 gases. D-labeling experiments show that the evolved H-2 is derived from the hydrido ligands of 2. These results suggest that oxidation of the hydrido ligands of 2 {[2] + 4NO(3)(-) (pH -1) --> 2[3] + H-2 + 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, H-2, and a pH gradient between 1 and -1. A conceivable mechanism for the H-2-activation cycle with reduction of NO3- is proposed.