No abstract

Two heterobimetallic Mo,M’ complexes (M’ = IrIII, RhIII) were synthesized and fully characterized. Their catalytic activity in homogeneous carbon dioxide hydrogenation to formate was studied. A pronounced synergistic effect between the two metals was found, most notably between Mo and Ir, leading to a fourfold increase in activity compared with a binary mixture of the two monometallic counterparts. This synergism can be attributed to spatial proximity of the two metals rather than electronic interactions. To further understand the nature of this interaction, the mechanism of the CO2 hydrogenation to formate by a monometallic IrIII catalyst was studied using computational and spectroscopic methods. The resting state of the reaction was found to be the metal-base adduct, whereas the rate-determining step is the inner-sphere hydride transfer to CO2. Based on these findings, the synergism in the heterobimetallic complex is beneficial in this key step, most likely by further activating the CO2.

The mechanism of metal-N-heterocyclic carbene (NHC) complex formation from imidazolium salts in the presence of weak bases was investigated through theoretical methods. Quantum chemical calculations revealed that the two bases considered here, sodium acetate and trimethylamine, both facilitate complex formation. In contrast to previous experiments, these calculations indicated a slightly lower barrier with the amine. Molecular dynamics simulations showed that the ionic nature of the [AuCl 2 ] À and imidazolium ions, as well as the sodium acetate base keep these species associated in the reaction mixture through ion pairing. This pre-association of the components produces those clusters that are essential for the metal complex formation reaction. The neutral amine, however, remains mostly separated from the other reaction partners, making it a significantly less effective base.

To form an ion pair with an imidazolium cation, the base must first “find” the substrate, like a pirate who is searching for treasure. The treasure is the acidic proton of the imidazolium cation which will need to be abstracted by the base. The ionic base can effectively approach the cation, while the non‐ionic one wanders off. More information can be found in the Research Article by O. Hollóczki and co‐workers (DOI: 10.1002/chem.202203636).

What questions prompted your research?N-Heterocyclic carbenes (NHCs) are widely used as ligands in transition metal catalysis, providing efficient routes for organic synthesis. However, their synthesis is often either somewhat limited or inefficient. By reacting azolium salts with metal sources in the presence of weak bases, the "weak-base route" offers an efficient alternative synthetic method for NHCs. The question of why some bases perform better than others in such reactions led us to conduct the present research.