Understanding Light-driven H2 Evolution through the Electronic Tuning of Aminopyridine Cobalt Complexes

A new family of cobalt complexes with the general formula [CoII(OTf)2(Y,XPyMetacn)] (1R, Y,XPyMetacn= 1-[(4-X-3,5-Y-2- pyridyl)methyl]-4,7-dimethyl-1,4,7-triazacyclononane, X= CN (1CN), CO2Et (1CO2Et), Cl (1Cl), H (1H), NMe2 (1NMe2)) where Y= H, and X= OMe when Y= Me (1DMM)) is reported. We found that the electronic tuning of the Y,XPyMetacn ligand not only has an impact on the electronic and structural properties of the metal center, but also allows for a systematic water- reduction-catalytic control. In particular, the increase of the electron-withdrawing character of the pyridine moiety promotes a 20-fold enhancement of the catalytic outcome. By UV-Vis spectroscopy, luminescence quenching studies and Transient Absorption Spectroscopy (TAS), we have studied the direct reaction of the photogenerated [IrIII(ppy)2(bpy·-)] (PSIr) species to form the elusive CoI intermediates. In particular, our attention is focused on the effect of the ligand architecture in this elemental step of the catalytic mechanism. Finally, kinetic isotopic experiments together with DFT calculations provide complementary information about the rate-determining step of the catalytic cycle.