Sustained, Selective, and Efficient Photochemical CO2 Reduction to Formate by Electron-Deficient Ruthenium Polypyridyl Complexes

Abstract

Metal hydrides play a significant role in a variety of reactions, including chemical, electrochemical, and photochemical CO₂ reduction. Molecular metal hydrides have the distinct advantage of allowing tunability of their hydricities by rational ligand modifications, with more electron-rich metal hydrides being in general more hydridic. We report here a new approach to generate highly hydridic metal hydrides of the type [Ru(tpy)(LL)(H)]ⁿ⁺ by introducing electron-withdrawing substituents into the backbone of the bidentate LL ligand. This strategy enables the generation of the metal hydride [Ru(tpy)(LL)(H)]⁺ at mild negative potentials and further one-electron reduction to the more hydridic [Ru(tpy)(LL)(H)]⁰ at a potential window that is redox silent for the more electron-rich metal hydride analogue [Ru(tpy)(bpy)(H)]⁺. In addition, formate release takes place from the hydride transfer adducts [Ru---HCOO)(tpy)(LL)]⁰ rather than from the corresponding formato complexes [Ru(tpy)(LL)(OCHO)]⁰, which would require further reduction to [Ru(tpy)(LL)(OCHO)]⁻ as demonstrated by IR spectroelectrochemistry. The parent [Ru(tpy)(LL)(CH₃CN)]ⁿ⁺ solvento complexes were then tested as catalysts for the reduction of CO₂ to formate in a four-component homogeneous photochemical approach driven by a Ru(II) sensitizer. The results showed selective (>88%) formate production with a record turnover number of ∼50,000 and record turnover frequency of 4.4 s^–1 when compared to other molecular catalysts.