The long-term storage of renewable energies is a matter of urgency to accommodate seasonal variations. Hydrogen is a promising energy carrier for fuel, but its storage as a compressed gas is still problematic. Chemicals such as formic acid (FA) can act as liquid organic hydrogen carriers through hydrogenation and dehydrogenation cycles. The catalytic dehydrogenation of FA has found a growing interest during the past decade and several homogeneous complexes based on iridium, ruthenium and iron have been reported as catalysts for this reaction.^[1-3]

     We sought to propel FA dehydrogenation with complexes containing pyridylidene-amines (PYEs), that is, ligands which feature a unique electronic flexibility (Fig. 1a).^[4] These ligands are known for their stabilization of various transition metals and for imparting high catalytic activity in, oxidation catalysis, e.g. in olefin and water oxidation.^[4,5] We therefore developed a new bidentate ligand system comprised of synthetically simple and inexpensive O-functionalized PYE (Fig. 1b), and its corresponding iridium(III) complexes. Here we will present the extraordinary activity of these complexes in FA dehydrogenation catalysis with turnover numbers around 300,000 h^–1 and turnover numbers in the millions (Fig. 1c). We will also discuss recent investigations into the homogeneity of the process, the electronic and steric versatility of the ligand to optimize catalytic activity, the effect of additives, and the cost-effectiveness of the catalytic system to envisage industrial applications.

[1] Zhijun Wang, Sheng-Mei Lu, Jun Li, Jijie Wang, Can Li, Chem. - Eur. J. 2015, 21 (36), 12592–12595.
[2] Sayan Kar, Michael Rauch, Gregory Leitus, Yehoshoa Ben-David, David Milstein, Nat. Catal. 2021, 4 (3), 193–201.
[3] Elizabeth Bielinski, Paraskevi Lagaditis, Yuanyuan Zhang, Brandon Mercado, Christian Würtele, Wesley Bernskoetter, Nilay Hazari, Sven Schneider, J. Am. Chem. Soc. 2014, 136 (29), 10234–10237.
[4] Miquel Navarro, Mo Li, Stefan Bernhard, Martin Albrecht, Dalton Trans. 2018, 47 (3), 659–662.
[5] Kevin Salzmann, Candela Segarra, Martin Albrecht, Angew. Chem. Int. Ed. 2020, 59 (23), 8932–8936.