Double-Ligand [Fe/PNP/PP 3 ] and Their Hybrids [Fe/SiO 2 @PNP/PP 3 ] as Catalysts for H 2 -Production from HCOOH

Two types of iron-based catalysts, [Fe/SiO 2 @ iPro PNP/PP3] and [Fe/SiO 2 @ tBu PNP/PP3], for the dehydrogenation of formic acid (FADH), were synthesized. These catalysts were developed using a double-ligand approach combining a PNP ligand and a PP3 ligand, demonstrating functionality without the need for additional cocatalysts or additives. Furthermore, hybrid catalysts [Fe/SiO 2 @ iPro PNP/PP3] and [Fe/SiO 2 @ tBu PNP/PP3] were created by covalently grafting PNP ligands onto SiO 2 particles. The hybrid [Fe/SiO 2 @ iPro PNP/PP3] exhibited enhanced recyclability, with turnover numbers (TONs) exceeding 74,000. In situ ATR-FTIR and UV-Vis spectroscopies were used to monitor the structure and dynamics of the catalysts under catalytic conditions, revealing the formation of active catalysts through the involvement of all components: [Fe (metal)/PNP (first ligand)/PP3 (second ligand)/FA (substrate)], which are crucial to FADH catalysis. An Arrhenius study revealed that the hybrid [Fe/SiO 2 @ iPro PNP/PP3] had a lower activation energy (E a = 42.5 kJ/mol) compared to its homogeneous counterpart (E a = 48.2 kJ/mol), indicating superior catalytic performance. Conversely, [Fe/SiO 2 @ tBu PNP/PP3] showed an increased activation energy (E a = 48.3 kJ/mol) compared to its homogeneous form (E a = 46.4 kJ/mol). This study discusses the differing roles of tBu PNP and iPro PNP in catalyst configuration, highlighting the potential of double-ligand catalysts to enhance the performance and recyclability of PNP ligands in FADH, offering significant implications for the development of efficient and reusable catalytic systems.