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 MnII, FeII, CoII, NiII, CuII, and ZnII, 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 [CoIIBr­(PPy)]Br (1c) and [CoIIBr­(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 (H2/Co), up to 11 000 were achieved. Complex 2c was more active than 1c, whereas none of the other complexes showed any activity.