Unveiling the Activation Pathway of the CO ₂ Reduction Catalyst $trans$(Cl)-[Ru(X,X′-dimethyl-2,2′-bipyridine)(CO)₂Cl₂] by Direct Spectroscopic Observation

We report on the activation pathway of a series of CO reduction catalysts, (Cl)-[Ru(X,X'-dimethyl-2,2'-bipyridine)(CO)Cl], with a focus on (Cl)-[Ru(6,6'-dimethyl-2,2'-bipyridine)(CO)Cl]), in the presence of the reductive quencher 1-benzyl-1,4-dihydronicotinamide and the photosensitizer Ru(bpy)Cl. Most mechanistic studies of these types of catalytic systems use spectroelectrochemistry in the IR, where the vibrational frequencies of the carbonyl vibrations report on the electron density on the metal center. However, spectroelectrochemistry may miss short-lived intermediates, while at the same time the spectra can be dominated by accumulating side-products, which may play only a minor role in the reaction cycle. Transient IR spectroscopy on all relevant time scales, from picoseconds to hundreds of milliseconds, can bridge this gap, revealing a surprisingly complex reaction pathway (in combination with NMR spectroscopy as well as DFT calculations). That is, electron transfer from the reduced photosensitizer is followed by a loss of a first chloride ligand, a replacement of the second chloride ligand by a solvent molecule, and a ligand rearrangement that releases the strain between the equatorial carbonyl ligands and the methyl group on the bpy ligand in this catalyst. These reaction steps happen on a tens of nanoseconds to tens of microseconds time scale. In the case of (Cl)-[Ru(6,6'-dimethyl-2,2'-bipyridine)(CO)Cl]), the complex is then reduced a second time from the oxidized 1-benzyl-1,4-dihydronicotinamide on a significantly slower 10-100 ms time scale, protonated and the solvent ligand is exchanged back to a chloride. The final product hence is a hydride, Ru(6,6'-dmbpy)(CO)ClH, which is stable on a minute-to-hour time scale. In case of (Cl)-[Ru(5,5'-dmbpy)(CO)Cl]), dimerization of the reduced species is possible, which eventually leads to the formation of (Cl)-[Ru(5,5'-dmbpy)(CO)Cl]. The work illustrates the power of transient IR spectroscopy to elucidate complex reaction pathways of such catalytic systems, and provides solid cornerstones for their kinetic control.