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.

Organic-inorganic hybrid materials were prepared by intercalation of iron 5,10,15,20 tetrakis(4-sulfonatophenyl)porphyrin complex into layered double hydroxides (ZnCr-FeTSPP LDH) and layered simple hydroxides (Zn-FeTSPP LSH) by coprecipitation. They have been characterized by ancillary techniques to compare their structural characteristics, their morphology and their ability to form films on conductive substrates in view of preparing modified electrodes for CO2 electroreduction reaction (CO2ERR). The electrochemical behavior of intercalated FeTSPP was studied by cyclic voltammetry (CV) showing the role of the nature and pH of aqueous electrolyte solutions on the Fe III/II signal. Evaluation of the CO2ERR of ZnCr-FeTSPP modified electrode was investigated by CV and preparative scale CO2 electrolysis in 0.1 M Na2SO4 aqueous solution. Whereas FeTSPP in the same electrolyte solution exhibits 18% of CO2 conversion to CO, once intercalated into ZnCr-LDH, there is no CO formation, only H2 (FY = 75 %). Using the ZnCr-NO3 modified electrode, without porphyrin, the same FY of H2 was obtained. The study illustrates the problematic and the complexity of an attractive approach for the development of innovative hybrid catalysts for heterogeneous CO2ERR.