Are Two Metal Ions Better than One? Mono- and Binuclear α-Diimine-Re(CO) 3 Complexes with Proton-Responsive Ligands in CO 2 Reduction Catalysis.

Here, the reduction chemistry of mono- and binuclear α-diimine-Re(CO) ₃ complexes with proton responsive ligands and their application in the electrochemically-driven CO ₂ reduction catalysis are presented. The work was aimed to investigate the impact of 1) two metal ions in close proximity and 2) an internal proton source on catalysis. Therefore, three different Re complexes, a binuclear one with a central phenol unit, 3, and two mononuclear, one having a central phenol unit, 1, and one with a methoxy unit, 2, were utilised. All complexes are active in the CO ₂ -to-CO conversion and CO is always the major product. The catalytic rate constant k _cat for all three complexes is much higher and the overpotential is lower in DMF/water mixtures than in pure DMF (DMF=N,N-dimethylformamide). Cyclic voltammetry (CV) studies in the absence of substrate revealed that this is due to an accelerated chloride ion loss after initial reduction in DMF/water mixtures in comparison to pure DMF. Chloride ion loss is necessary for subsequent CO ₂ binding and this step is around ten times faster in the presence of water [2: k ^Cl (DMF)≈1.7 s ^-1 ; k ^Cl (DMF/H ₂ O)≈20 s ^-1 ]. The binuclear complex 3 with a proton responsive phenol unit is more active than the mononuclear complexes. In the presence of water, the observed rate constant k _obs for 3 is four times higher than of 2, in the absence of water even ten times. Thus, the two metal centres are beneficial for catalysis. Lastly, the investigation showed that the phenol unit has no impact on the rate of the catalysis, it even slows down the CO ₂ -to-CO conversion. This is due to an unproductive, competitive side reaction: After initial reduction, 1 and 3 loose either Cl ^- or undergo a reductive OH deprotonation forming a phenolate unit. The phenolate could bind to the metal centre blocking the sixth coordination site for CO ₂ activation. In DMF, O-H bond breaking and Cl ^- ion loss have similar rate constants [1: k ^Cl (DMF)≈2 s ^-1 , k ^OH ≈1.5 s ^-1 ], in water/DMF Cl ^- loss is much faster. Thus, the effect on the catalytic rate is more pronounced in DMF. However, the acidic protons lower the overpotential of the catalysis by about 150 mV.
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