Mild Redox Complementation Enables H2 Activation by [FeFe]-Hydrogenase Models

Mild oxidants such as [Fe(C5Me5)2]+ accelerate the activation of H2 by [Fe2[(SCH2)2NBn](CO)3(dppv)(PMe3)]+ ([1]+), despite the fact that the ferrocenium cation is incapable of oxidizing [1]+. The reaction is first-order in [1]+ and [H2] but independent of the E1/2 and concentration of the oxidant. The analogous reaction occurs with D2 and proceeds with an inverse kinetic isotope effect of 0.75(8). The activation of H2 is further enhanced with the tetracarbonyl [Fe2[(SCH2)2NBn](CO)4(dppn)]+ ([2]+), the first crystallographically characterized model for the Hox state of the active site containing an amine cofactor. These studies point to rate-determining binding of H2 followed by proton-coupled electron transfer. Relative to that by [1]+, the rate of H2 activation by [2]+/Fc+ is enhanced by a factor of 104 at 25 °C.