A Polypyridyl Co(Ii) Complex-Based Water Reduction Catalyst With Double H-2 Evolution Sites

A series of Co(II) complexes, Co(X-TMPA) Cl-2 (X-TMPA = 1-(6-substituted-pyridin-2-yl)-N, N-bis(pyridin-2-ylmethyl) methane amine, X = Cl (1), Br (2), H (3), and CH3 (4)), were synthesized and fully characterized. The crystal structures of 2 and 4 show that TMPA and CH3-TMPA coordinate to the Co(II) center as tetra-dentate ligands, while Br-TMPA coordinates as a tridentate ligand, leaving the Br-substituted pyridyl group in the second coordination sphere. All of the complexes are efficient photocatalytic H-2 evolution catalysts in CH3CN/H2O (9/1) using [Ir(ppy)(2)(dtbpy)] Cl (ppy = 2-phenylpyridine, dtbpy = 4,4'-di-tert-butyl-2,2'-bipyridine) as the photosensitizer (PS) and triethylamine (TEA) as the sacrificial electron donor. During 6 h irradiation, the turnover numbers (TONs) of 1 and 2 reached 20000, remarkably higher than those of 3 and 4. These high photocatalytic activities may be attributed to the pendent Cl/Br-substituted pyridyl group, which serves as a proton and electron relay to facilitate proton reduction at the Co center. Interestingly and importantly, it was found that the Cl-substituted pyridyl group of 1 may catalyze H-2 evolution itself by electrocatalytic proton reduction reactions, endowing 1 with double catalytic sites for proton reduction. The unique coordination mode of Cl/Br-TMPA and the double catalytic H-2 evolution sites of 1 provide a new strategy to design more effective WRCs.