Terminal Hydride in [FeFe]-Hydrogenase Model Has Lower Potential for H2 Production Than the Isomeric Bridging Hydride

Protonation of the symmetrical tetraphosphine complexes Fe2(S2C n H2n )(CO)2(dppv)2 afforded the corresponding terminal hydrides, establishing that even symmetrical diiron(I) dithiolates undergo protonation at terminal sites. The terminal hydride [HFe2(S2C3H6)(CO)2(dppv)2]+ was found to catalyze proton reduction at potentials 200 mV milder than the isomeric bridging hydride, thereby establishing a thermodynamic advantage for catalysis operating via terminal hydride. The azadithiolate protonates to afford, [Fe2[(SCH2)2NH2](CO)2(dppv)2]+, [HFe2[(SCH2)2NH](CO)2(dppv)2]+, and [HFe2[(SCH2)2NH2](CO)2(dppv)2]2+, depending on conditions.