3d Element Complexes of Pentadentate Bipyridine-Pyridine-Based Ligand Scaffolds: Structures and Photocatalytic Activities

The synthesis of the two penta-pyridyl type ligands pyridine-2,6-diylbis(dipyridin-2-ylmethanol) (PPy, 1) and bis-2,2 ''-bipyridine-6-yl(pyridine-2-yl)methanol (aPPy, 2) is described. Both ligands coordinate rapidly to the 3d element cations Mn-II, Fe-II, Co-II, Ni-II, Cu-II, and Zn-II, thereby yielding complexes of the general composition [MBr(1)](+) and [MBr(2)](+), respectively. Further, the X-ray structures of selected complexes with ligands 1 and 2 are described. They show metal center dependent structural features and complexes with 2 exhibiting distinctly distorted octahedral geometries. Moreover, photocatalytic water reduction with [(CoBr)-Br-II(PPy)]Br (1c) and [(CoBr)-Br-II(aPPy)]Br (2c) as water reducing catalysts (WRC) was investigated. Both complexes showed catalytic activity in water when in presence of ascorbic acid as sacrificial electron donor and [Re(py)(bpy)(CO)(3)](+) (3) as photosensitizer (PS). Turnover numbers, TONs (H-2/Co), up to 11 000 were achieved. Complex 2c was more active than 1c, whereas none of the other complexes showed any activity.