Visible Light-Driven Hydrogen Evolution from Water Catalyzed by A Molecular Cobalt Complex

Abstract

An approximately planar tetradentate polypyridine ligand, 8-(1″,10″-phenanthrol-2″-yl)-2-(pyrid-2'-yl)quinoline (ppq), has been prepared by two sequential Friedländer condensations. The ligand readily accommodates Co(II) bearing two axial chlorides, and the resulting complex is reasonably soluble in water. In DMF the complex shows three well-behaved redox waves in the window of 0 to -1.4 V (vs SHE). However in pH 7 buffer the third wave is obscured by a catalytic current at -0.95 V, indicating hydrogen production that appears to involve a proton-coupled electron-transfer event. The complex [Co(ppq)Cl2] (6) in pH 4 aqueous solution, together with [Ru(bpy)3]Cl2 and ascorbic acid as a sacrificial electron donor, in the presence of blue light (λmax = 469 nm) produces hydrogen with an initial TOF = 586 h(-1).

Light-Driven Proton Reduction in Aqueous Medium Catalyzed by a Family of Cobalt Complexes with Tetradentate Polypyridine-Type Ligands

“…37 Combined with [Ru(bpy) 3 ] 2+ as a photosensitizer in buffered water (pH 4.0), [Co(1)Cl 2 ] showed an initial turnover frequency (TOF) of 586 h −1 per catalyst, which is significantly greater than the TOF of [Co(bpy) 3 ] 2+ or [Co(qpy)(OH 2 ) 2 ] 2+ . Compared to bis-bipyridine and quaterpyridine, ppq has a less flexible and more conjugated backbone.…”

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.…”

Electrochemical-driven water reduction catalyzed by a water soluble cobalt(III) complex with Schiff base ligand

“…2) L 2-coordinates strongly through two nitrogen atoms and two oxygen atoms to the cobalt centre, leaving one Cl -ion in axial position. This observation strongly suggests that the presence of the Cl -ion in axial position is the key structural feature for eliciting water reduction catalysis[13,14,33].The observation that the catalytic current varied with the concentration of complex 1 indicated a homogeneous catalytic reaction(Fig. S14), which was also supported by several other pieces of evidence: 1) No heterogeneous deposits were produced and observed.…”

“…Yet, one of the key challenges in water splitting is the development of efficient catalysts for water reduction reactions with low overpotentials, good stability, and high turnover rates [7][8][9]. Therefore, a great deal of research efforts have been devoted to the development of effective catalysts based on earth-abundant metals, and molecular complexes that contain nickel [10,11], cobalt [12][13][14], molybdenum [15] and copper [16][17][18] for the reduction of water to form H2. Nevertheless, despite much progress, major improvements in several areas, including lowering overpotentials, increasing catalyst durability, and using earth-abundant elements, are needed before efficient electro-and photo-catalytic water splitting can be realized.…”