Ascorbate as an electron relay between an irreversible electron donor and Ru(II) or Re(I) photosensitizers.

Ascorbate acts as a reversible electron shuttle between tris(2-carboxyethyl) phosphine (TCEP) and Re(I) or Ru(II) photosensitizers. Oxidized ascorbate is recycled up to 50 times by the TCEP → TCEP[double bond, length as m-dash]O redox process which enables 30 000 TONs per WRC in photocatalytic hydrogen production, thus exceeding the performance with pure ascorbate by far.

Molecular artificial photosynthesis.

The replacement of fossil fuels by a clean and renewable energy source is one of the most urgent and challenging issues our society is facing today, which is why intense research has been devoted to this topic recently. Nature has been using sunlight as the primary energy input to oxidise water and generate carbohydrates (solar fuel) for over a billion years. Inspired, but not constrained, by nature, artificial systems can be designed to capture light and oxidise water and reduce protons or other organic compounds to generate useful chemical fuels. This tutorial review covers the primary topics that need to be understood and mastered in order to come up with practical solutions for the generation of solar fuels. These topics are: the fundamentals of light capturing and conversion, water oxidation catalysis, proton and CO2 reduction catalysis and the combination of all of these for the construction of complete cells for the generation of solar fuels.

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 [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.

A highly stable polypyridyl-based cobalt catalyst for homo- and heterogeneous photocatalytic water reduction.

Synthesis, characterization and activity in homogeneous photocatalytic hydrogen production of a cobalt polypyridyl complex are reported. TONs up to 9000 H(2)/Co could be achieved. Immobilization of the complex on a swellable resin yielded a recyclable heterogeneous catalyst.

Acyloxybutadiene tricarbonyl iron complexes as enzyme-triggered CO-releasing molecules (ET-CORMs): a structure-activity relationship study.

A series of η(4)-acyloxycyclohexadiene-Fe(CO)(3) complexes was prepared and fully characterized by spectroscopic methods including single crystal X-ray diffraction. For this purpose a new synthetic access to differently acylated 1,3- and 1,5-dienol-Fe(CO)(3) complexes was developed. The enzymatically triggered CO release from these compounds was monitored (detection of CO through GC and/or by means of a myoglobin assay) and the anti-inflammatory effect of the compounds was assessed by a cellular assay based on the inhibition of NO-production by inducible NO synthase (iNOS). It was demonstrated that the properties (rate of esterase-triggered CO release, iNOS inhibition, cytotoxicity) of the complexes strongly depend on the substitution pattern of the π-ligand and the nature of the acyloxy substituent.

Photocatalytic H2 production from water with rhenium and cobalt complexes.

Photocatalytic hydrogen production in pure water for three component systems using a series of rhenium-based photosensitizers (PS) and cobalt-based water reduction catalysts (WRC), with triethanolamine (TEOA) as an irreversible electron donor, is described. Besides the feasibility of this reaction in water, key findings are reductive quenching of the excited state of the PS by TEOA (k(q) = 5-8 × 10(7) M(-1) s(-1); Φ(cage) = 0.75) and subsequent transfer of an electron to the WRC (k(Co(III)) = 1.1 × 10(9) M(-1) s(-1)). Turnover numbers in rhenium (TON(Re), H/Re) above 500 were obtained, whereas TON(Co) (H(2)/Co) did not exceed 17. It is shown that the cobalt-based WRC limits long-term performance. Long-term performance critically depends on pH and the type of WRC used but is unaffected by the type of PS or the concentration of WRC. A quantum yield of 30% was obtained (H/photon).

A highly stable rhenium-cobalt system for photocatalytic H(2) production: unraveling the performance-limiting steps.

Increased long-term performance was found for photocatalytic H(2) production in a homogeneous combination of [Re(NCS)(CO)(3)bipy] (1; bipy = 2,2'-bipyridine), [Co(dmgH)(2)] (dmgH(2) = dimethylglyoxime), triethanolamine (TEOA), and [HTEOA][BF(4)] in N,N-dimethylformamide, achieving TON(Re) up to 6000 (H/Re). The system proceeded by reductive quenching of *1 by TEOA, followed by fast (k(1) = 1.3 x 10(8) M(-1) s(-1)) electron transfer to [Co(II)(dmgH)(2)] and subsequent protonation (K(2)) and elimination (k(3), second-order process in cobalt) of H(2). Observed quantum yields were up to approximately 90% (H produced per absorbed photon). The type of acid had a substantial effect on the long-term stability. A decomposition pathway involving cobalt is limiting the long-term performance. Time-resolved infrared (IR) spectroscopy confirmed that photooxidized TEOA generates a second reducing equivalent, which can be transferred to 1 (70%, k(2e)(-) = 3.3 x 10(8) M(-1) s(-1)) if no [Co(II)(dmgH)(2)] is present.