Double-Ligand [Fe/PNP/PP₃] and Their Hybrids [Fe/SiO₂@PNP/PP₃] as Catalysts for H₂-Production from HCOOH

Two types of iron-based catalysts, [Fe/SiO₂@^iProPNP/PP3] and [Fe/SiO₂@^tBuPNP/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/SiO₂@^iProPNP/PP3] and [Fe/SiO₂@^tBuPNP/PP3] were created by covalently grafting PNP ligands onto SiO₂ particles. The hybrid [Fe/SiO₂@^iProPNP/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/SiO₂@^iProPNP/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/SiO₂@^tBuPNP/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 ^tBuPNP and ^iProPNP 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.