Our planet urgently needs sustainable solutions to alleviate the anthropogenic global warming and climate change. Homogeneous catalysis has the potential to play a fundamental role in this process, providing novel, efficient, and at the same time eco-friendly routes for both chemicals and energy production. In particular, pincer-type ligation shows promising properties in terms of long-term stability and selectivity, as well as allowing for mild reaction conditions and low catalyst loading. Indeed, pincer complexes have been applied to a plethora of sustainable chemical processes, such as hydrogen release, CO2 capture and conversion, N2 fixation, and biomass valorization for the synthesis of high-value chemicals and fuels. In this work, we show the main advances of the last five years in the use of pincer transition metal complexes in key catalytic processes aiming for a more sustainable chemical and energy production.

High catalytic activities of Ru–PNP [Ru = ruthenium; PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine] complexes in ionic liquids (ILs) were obtained for the reversible hydrogenation of CO2 and dehydrogenation of formic acid (FA) under exceedingly mild conditions and without sacrificial additives. The novel catalytic system relies on the synergic combination of Ru–PNP and IL and proceeds with CO2 hydrogenation already at 25 °C under a continuous flow of 1 bar of CO2/H2 (1:5), leading to 14 mol % FA with respect to the IL. A pressure of 40 bar of CO2/H2 (1:1) provides 126 mol % of FA/IL corresponding to a space-time yield (STY) of FA of 0.15 mol L–1 h–1. The conversion of CO2 contained in imitated biogas was also achieved at 25 °C. Furthermore, the Ru–PNP/IL system catalyzes FA dehydrogenation with average turnover frequencies up to 11,000 h–1 under heat-integrated conditions for proton-exchange membrane fuel cell applications (<100 °C). Thus, 4 mL of a 0.005 M Ru–PNP/IL system converted 14.5 L FA over 4 months with a turnover number exceeding 18,000,000 and a STY of CO2 and H2 of 35.7 mol L–1 h–1. Finally, 13 hydrogenation/dehydrogenation cycles were achieved with no sign of deactivation. These results demonstrate the potential of the Ru–PNP/IL system to serve as a FA/CO2 battery, a H2 releaser, and a hydrogenative CO2 converter.

High catalytic activities of Ru-PNP complexes in ionic liquid (IL) were obtained for the reversible hydrogenation of CO2 and dehydrogenation of formic acid (FA) under exceedingly mild conditions. The novel catalytic system relies on an unprecedented IL-promoted activation of CO2 and proceeds already at 25 ⁰C under a continuous flow of 1 bar of CO2/H2, leading to 14 mol% FA with respect to the IL, whereas a pressure of 40 bar of CO2/H2 (1:1) provides 126 mol% of FA/IL. The conversion of CO2 contained in imitated biogas was also achieved at 25 ⁰C. Furthermore, the Ru-PNP/IL system catalyzes FA dehydrogenation with turnover frequencies (TOF) up to 11,000 h-1 under heat-integrated conditions for proton-exchange membrane (PEM) fuel cell applications (<100 ⁰C). Thus, 4 mL of a 0.005 M Ru-PNP/IL system converted 14.5 L FA over four months with a turnover number (TON) exceeding 18,000,000. Finally, 13 hydrogenation/dehydrogenation cycles were achieved with no sign of deactivation. These results demonstrate the potential of the Ru-PNP/IL system as a Liquid Organic Hydrogen Carrier (LOHC) battery, LOHC H2 releaser, and hydrogenative CO2 converter.