Gold(II) complexes are rare and their application for catalysis of chemical transformations is unexplored. The reason is their easy oxidation or reduction to the more stable gold(III) or gold(I) complexes, respectively. We explored the thermodynamics of the formation of the [(L)Au II X] + complexes (L = ligand, X = halogen) from their gold(III) precursors and investigated the stability and the spectral properties in the IR and VIS range of the formed gold(II) complexes in the gas phase. The results show that the best ancillary ligands L for stabilizing gaseous [(L)Au II X] + complexes are bidentate and tridentate ligands with nitrogen donor atoms. The electronic structure and spectral properties of the investigated gold(II) complexes were correlated with the quantum chemical calculations. The results show that the molecular and electronic structure of the gold(II) complexes as well as their spectroscopic properties are very similar to the analogous stable copper(II) complexes.

Efficient electrocatalytic CO 2 reduction requires developing catalysts with high selectivities and high activities, which is simultaneously difficult to achieve. Here, we present a new approach to tune the CO 2 reduction activity based on host-guest chemistry enabled by an iron porphyrin cage catalyst. The cage design allows the hosting of alkali metals in the side walls causing a change in the electrostatic potential inside the cage cavity. Density functional theory calculations show that the guest potassium ions assist the reduction of CO 2 by inverting the two-electron transfer from iron(0) to CO 2 from endothermic to exothermic. Accordingly, electrochemical experiments with the cage catalyst show that in the presence of the potassium ions, the overpotential for the CO 2 reduction decreases, and the catalytic activity increases while the high selectivity of the cage is retained. A novel coupling between the electrochemical cell and a mass spectrometer allowed the trapping of the key intermediates. Cryogenic ion spectroscopy characterization of the intermediates showed the details of the potassium ions hosting in the reduced cage and of the stabilization of the Fe-COOH intermediates by the interaction with the potassium ions at the single-molecule level.

The development of selective catalysts for the reduction of CO2 mostly focuses on electrocatalytic approaches and aims at increasing the selectivity of the reaction while keeping a high activity, which is difficult to achieve. Metalloporphyrins are good catalysts for CO2 reduction because they have favorable electronic properties and offer the possibility to make use of secondary coordination sphere effects. Here, we present a new approach to CO2 reduction, which is based on host-guest chemistry enabled by an iron porphyrin cage catalyst. When this iron porphyrin cage catalyst is immobilized on a conducting carbon support the selectivity for CO2 reduction to CO stays above 90 % in a wide range of overpotentials. The hosting of potassium ions in the cage of the catalyst decreases the overpotential of the reduction and increases the catalytical activity while retaining the high selectivity. DFT calculations show that the potassium ions assist the reduction of CO2 by making the 2-electron transfer from iron(0) to CO2 exothermic. Upon protonation, the Fe-COOH intermediates have been trapped by combining an electrochemical cell with an electrospray ionization mass spectrometer and their structure has been characterized by cryogenic ion spectroscopy.

No abstract