Light-Induced H₂ Evolution with a Macrocyclic Cobalt Diketo-Pyrphyrin as a Proton-Reducing Catalyst

Abstract

Cobalt complexes are well-known catalysts for photocatalytic proton reduction in water. Macrocyclic tetrapyridyl ligands (pyrphyrins) and their Co complexes emerged in this context as a highly efficient class of H evolution catalysts. On the basis of this framework, a new macrocyclic Co complex consisting of two keto-bridged bipyridyl units (Co diketo-pyrphyrin) is presented. The complex is synthesized along a convenient route, is well soluble in water, and shows high activity as a water reduction catalyst (WRC). In an aqueous system containing [Ru(bpy)]Cl as a photosensitizer and NaAscO as a sacrificial electron donor, turnover numbers (TONs) of 2500 H/Co were achieved. Catalysis is terminated by a limited electron supply and decomposition of the photosensitizer but not of the WRC, highlighting the distinct stability of Co diketo-pyrphyrin.

Mechanistic insights into photocatalysis and over two days of stable H2 generation in electrocatalysis by a molecular cobalt catalyst immobilized on TiO2

“…We attribute this shape to the formation of a cobalt-TCEP adduct as observed in earlier studies and reported in the literature. 22,48 Catalytic experiments without TCEP, lacking the double peak, support this interpretation (Fig. S7 †).…”

“…4). 12,22 At low cobalt concentrations, the TONs go up to 20-25 kTONs H 2 WRC −1 , outperforming most of the cobalt-polypyridyl WRCs investigated up to now. In terms of total amounts of H 2 produced, the system performs less stable at low WRC concentrations.…”

“…In all experiments, excitation of the PS below the bandgap of TiO 2 was achieved by pumping with 532 nm instead of 355 nm, as commonly done in such systems. 22,34 Due to both the lower extinction coefficient of the PS at 532 nm (ca. 7 times smaller than at 355 nm), and the decreased transmittance of the probing light (for on-particle measurements), the PS concentration was set to 300 μM, resulting in WRC-deficient conditions.…”

“…Reported H 2 turnover frequencies as high as 100 000 s −1 spearhead the catalytic activity of all water reductive systems. 21 Amongst the most promising WRCs, one often finds cobalt-polypyridyl complexes, 22,23 which excel in reported turnover numbers (TON) as high as 80 000 H 2 WRC −1 and offer a large playground for ligand-modifications, tuning the electrochemical properties of the metal center. 24 Despite excellent properties, molecular catalysts bear some disadvantages compared to surface catalyzed systems, especially regarding long-term stability as well as applicability in a full water splitting system.…”

[Re(η6-arene)2]+ as a highly stable ferrocene-like scaffold for ligands and complexes

“…Compounds 11a + and 11b + are of special interest since Co(II) complexes of these tetra-pyridyl units represent a highly active class of catalysts for photocatalytic proton reduction. [29][30][31][32][33] To support the hypothesis that the [Re(η 6 -C 6 H 6 ) 2 ] + is a suitable scaffold, a Co(II) complex with 11a + was prepared by reaction of 20 eq. CoBr 2 in MeOH with the metal-free precursor.…”