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

Abstract

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 [Co(II)Br(PPy)]Br (1c) and [Co(II)Br(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.

Photo-induced redox catalysis for proton reduction to hydrogen with homogeneous molecular systems using rhodium-based catalysts

“…Some of these homogeneous photocatalytic systems can operate very efficiently (in terms of number of catalytic cycles or turnover number (TON)) in organic or mixed aqueous-organic solvents. Those reaching a turnover number versus catalyst (TON Cat ) above 100 in fully aqueous solution were rare and limited to rhodium [36][37][38][39] and platinum [40] but, since two years, several examples with cobalt [41][42][43][44][45][46][47][48][49][50][51][52][53][54], iron [55][56][57][58] nickel [59] were reported. Developing H 2 -evolving photocatalytic systems functioning in pure water is essential for their coupling with water oxidation systems in photoelectrochemical water-splitting devices.…”

Nickel Complex with Internal Bases as Efficient Molecular Catalyst for Photochemical H2 Production

“…In addition, two sharp oxidation peaks at E 1/2 = À1.14 and À0.89 V were observed, which implies that the reduced species of [1] 2 + adsorbed on the electrode surface are stripped off upon reoxidation. [28,29] The shape of the redox waves of [1] 2 + does not change with repeated sweeps, indicative of the lack of decomposition of [1] 2 + during the redox processes. The results indicate that the reduction peak observed in the CV of [1](BF 4 ) 2 in acidic aqueous solutions is not a catalytic peak for the electrolysis of water to H 2 .…”

Shaping and enforcing coordination spheres: probing the ability of tripodal ligands to favour trigonal prismatic geometry

“…The synthesis of L 1 and L 2 have recently been reported by 20 Alberto, 12,13 by the addition of the lithiated 6-bromo-2,2'-bipyridine to the appropriate ketone (2,2-dipyridylketone or 2-pyridine-6-(2,2'-bipyridyl)ketone). Alternatively, we have found these ligands may be synthesised via the bromo substituted tripodal ligands.…”