Rhenium Carbonyl Molecular Catalysts for CO2 Electroreduction: Effects on Catalysis of Bipyridine Substituents Mimicking Anchorage Functions to Modify Electrodes.

Heterogenization of molecular catalysts on (photo)electrode surfaces is required to design devices performing processes enabling to store renewable energy in chemical bonds. Among the various strategies to immobilize molecular catalysts, direct chemical bonding to conductive surfaces presents some advantages because of the robustness of the linkage. When the catalyst is, as it is often the case, a transition metal complex, the anchoring group has to be connected to the complex through the ligands, and an important question is thus raised on the influence of this function on the redox and on the catalytic properties of the complex. Herein, we analyze the effect of conjugated and non conjugated substituents, structurally close to anchoring functions previously used to immobilize a rhenium carbonyl bipyridyl molecular catalyst for supported CO2 electroreduction. We show that carboxylic ester groups, mimicking anchoring the catalyst via carboxylate binding to the surface, have a drastic effect on the catalytic activity of the complex toward CO2 electroreduction. The reasons for such an effect are revealed via a combined spectro-electrochemical analysis showing that the reducing equivalents are mainly accumulated on the electron-withdrawing ester on the bipyridine ligand preventing the formation of the rhenium(0) center and its interaction with CO2. Alternatively, alkyl-phosphonic ester substituents, not conjugated with the bpy ligand, mimicking anchoring the catalyst via phosphonate binding to the surface, allow preserving the catalytic activity of the complex.

Fusogenic supramolecular vesicle systems induced by metal ion binding to amphiphilic ligands.

The incorporation of lipophilic ligands into the bilayer membrane of vesicles offers the possibility to induce, upon binding of suitable metal ions, a variety of processes, in particular vesicle aggregation and fusion and generation of vesicle arrays, under the control of specific metal-ligand recognition events. Synthetic bipyridine lipoligands Bn bearing a bipyridine unit as head group were prepared and incorporated into large unilamellar vesicles. The addition of Ni2+ or Co2+ metal ions led to the formation of complexes MBn and MBn2 followed by spontaneous fusion to generate giant multilamellar vesicles. The metal ion complexation was followed by UV spectroscopy and the progressive fusion could be visualized by optical dark-field and fluorescence microscopies. Vesicle fusion occurred without leakage of the aqueous compartments and resulted in the formation of multilamellar giant vesicles because of the stacking of the lipoligands Bn. The fusion process required a long enough oligoethylene glycol spacer and a minimal concentration of lipoligand within the vesicle membrane. Metallosupramolecular systems such as the present one offer an attractive way to induce selective intervesicular processes, such as vesicle fusion, under the control of molecular recognition between specific metal ions and lipoligands incorporated in the bilayer membrane. They provide an approach to the design of artificial "tissue-mimetics" through the generation of polyvesicular arrays of defined architecture and to the control of their functional properties.