Metal ligand protonation and the alleged proton-shuttling role of the azadithiolate ligand in catalytic H formation with FeFe hydrogenase model complexes.

Electron and proton transfer reactions of diiron complexes [Feadt(CO)] () and [Feadt(CO)(PMe)] (), with the biomimetic azadithiolate (adt) bridging ligand, have been investigated by real-time IR- and UV-vis-spectroscopic observation to elucidate the role of the adt-N as a potential proton shuttle in catalytic H formation. Protonation of the one-electron reduced complex, , occurs on the adt-N yielding and the same species is obtained by one-electron reduction of . The preference for ligand metal protonation in the Fe(i,0) state is presumably kinetic but no evidence for tautomerization of to the hydride was observed. This shows that the adt ligand does not work as a proton relay in the formation of hydride intermediates in the reduced catalyst. A hydride intermediate is formed only by protonation of with stronger acid. Adt protonation results in reduction of the catalyst at much less negative potential, but subsequent protonation of the metal centers is not slowed down, as would be expected according to the decrease in basicity. Thus, the adtH complex retains a high turnover frequency at the lowered overpotential. Instead of proton shuttling, we propose that this gain in catalytic performance compared to the propyldithiolate analogue might be rationalized in terms of lower reorganization energy for hydride formation with bulk acid upon adt protonation.