Catalytic pathwaysfor the reduction of dioxygen can either lead to the formation of water or peroxide as the reaction product. We demonstrate that the electrocatalytic reduction of O 2 by the pyridylalkylamine copper complex [Cu(tmpa)(L)] 2+ in an eutral aqueous solution follows as tepwise 4e À /4 H + pathway,inwhich H 2 O 2 is formed as adetectable intermediate and subsequently reduced to H 2 Oi nt wo separate catalytic reactions.These homogeneous catalytic reactions are shown to be first order in catalyst. Coordination of O 2 to Cu I was found to be the rate-determining step in the formation of the peroxide intermediate.F urthermore,e lectrochemical studies of the reaction kinetics revealed ah igh turnover frequency of 1.5 10 5 s À1 ,the highest reported for any molecular copper catalyst.

Hydrogen peroxide plays an important role as an intermediate and product in the reduction of dioxygen by copper enzymes and mononuclear copper complexes. The copper(II) tris(2‐pyridylmethyl)amine complex (Cu‐tmpa) has been shown to produce H2O2 as an intermediate during the electrochemical 4‐electron reduction of O2. We investigated the electrochemical hydrogen peroxide reduction reaction (HPRR) by Cu‐tmpa in a neutral aqueous solution. The catalytic rate constant of the reaction was shown to be one order of magnitude lower than the reduction of dioxygen. A significant solvent kinetic isotope effect (KIE) of 1.4 to 1.7 was determined for the reduction of H2O2, pointing to a Fenton‐like reaction pathway as the likely catalytic mechanism, involving a single copper site that produces an intermediate copper(II) hydroxo species and a free hydroxyl radical anion in the process.

Coupling the notoriously non-emissive complex [Ru(tpy)(bpy)Cl]Cl (tpy = 2,2':6',2''-terpyridine, bpy = 2,2'-bipyridine) to a C alkyl chain via an amide linker on the 4' position of the terpyridine yielded a new amphiphilic ruthenium complex showing red emission and chloride-dependent aggregation properties. This emissive complex is highly cytotoxic in A549 non-small lung cancer cells where it can be followed by confocal microscopy. Uptake occurs within minutes, first by insertion into the cellular membrane, and then by migration to the peri-nuclear region.

In this work the synthesis, photochemistry, and streptavidin interaction of new [Ru(tpy)(bpy)(SRR′)](PF6)2 complexes where the R′ group contains a free biotin ligand, are described. Two different ligands SRR′ were investigated: An asymmetric ligand 1 where the Ru‐bound thioether is a N‐acetylmethionine moiety linked to the free biotin fragment via a triethylene glycol spacer and a symmetrical ligand 2 containing two identical biotin moieties. The coordination of these two ligands to the precursor [Ru(tpy)(bpy)Cl]Cl was studied in water at 80 °C. In such conditions the coordination of the asymmetric ligand 1 occurred under thermodynamic control. After the reaction, a mononuclear and a binuclear complex were isolated. In the mononuclear complex, the ratio of methionine‐ {[6](PF6)2} vs. biotin‐bound {[7](PF6)2} regioisomer was 5.3 and the free biotin fragment of [6](PF6)2 allowed to purify it from its isomer [7](PF6)2 at small scales using avidin affinity chromatography. Coordination of the symmetrical ligand 2 afforded [Ru(tpy)(bpy)(2)](PF6)2 {[8](PF6)2} in synthetically useful scales (100 mg), good yield (82 %), and without traces of the binuclear impurity. In this complex, one of the biotin remains free whereas the second one is coordinated to ruthenium. Photochemical release of ligand 2 from [8](PF6)2 occurred upon blue light irradiation (465 nm) with a photosubstitution quantum yield of 0.011 that was independent of the binding of streptavidin to the free biotin ligand.

Invited for the cover of this issue is the group of Sylvestre Bonnet from Leiden University, The Netherlands. The cover image shows the attachment of a free biotin group to a Ru compound by a light‐sensitive coordination bond.