Abstract

The rising global population and economic growth have significantly increased energy demand, with fossil fuels currently meeting about 80% of this demand, especially in transportation. This reliance has severe consequences, notably raising atmospheric CO2 levels to 427 ppm in 2024, compared to 180 ppm pre-industrialization. This highlights the need for low-carbon energy sources to mitigate environmental impact and fossil fuel depletion. Hydrogen (H2) is a promising alternative due to its high energy density of 33.3 kWh/kg and potential use in engines and fuel cells. However, effective storage and delivery methods are crucial. Formic acid (FA), with a volumetric capacity of 53 gH2/L, is a viable hydrogen carrier, easily stored in vehicles and catalytically converted into hydrogen and CO2, making it ideal for automotive applications. This research aims to develop a catalytic system that operates under mild conditions, selectively dehydrogenates formic acid, and can be integrated into hydrogen fuel cells. The ideal system should minimize the use of harmful solvents, be water-soluble, and exhibit reasonable activity and stability with high turnover numbers (TON). While noble transition metal-catalyzed FA dehydrogenation protocols have been extensively studied, 3d transition metals present a more economically efficient alternative due to their lower cost and abundance. Recent studies have shown promising results with iron-based complexes, though reports on other non-noble metals like manganese, cobalt, and nickel remain scarce. In this work, we present a highly efficient PN3P Mn(I) complex achieving a turnover frequency (TOF) of 2086 h⁻¹ and a TON of 15,200, one of the highest among known manganese-based catalysts. This reactivity and selectivity, with no CO gas detected, are attributed to the unique design principle enabling metal-ligand cooperativity (MLC) during substrate activation. Additionally, we explored an asymmetric PN3P Mn(I) counterpart, demonstrating a TOF of 1039 h⁻¹ and a TON of 11,600 for FA dehydrogenation reaction, along with notable reactivity towards reversible CO2 hydrogenation to formate, achieving a TON of up to 50,000 under a 50 bar CO2/H2 mixture. Furthermore, knowing the importance of water-soluble catalysis and formic acid dehydrogenation under aqueous condition towards the expansion to sustainable power generation application, we examined a well-defined ruthenium complex with a picolinamide-based ligand, achieving a TOF of 5211 h⁻¹ and a TON of 33,700 under aqueous conditions.