Catalytic Formic Acid Dehydrogenation and CO₂ Hydrogenation Using Iron PN^RP Pincer Complexes with Isonitrile Ligands

Abstract

It has previously been demonstrated that complexes of the form (iPrPNP)­Fe­(H)­(CNR) (iPrPNP = N­(CH2CH2P­(iPr)2)2 –, R = 2,6-dimethylphenyl or 4-methoxyphenyl), which contain a pincer ligand capable of metal–ligand cooperation (MLC), are active for CO2 hydrogenation. Herein, the synthesis and catalytic activity of a second-generation of precatalysts containing a tertiary amine ligand, which cannot participate in MLC, are presented. Specifically, the complexes (iPrPNMeP)­Fe­(H)­(HBH3)­(CNR) (iPrPNMeP = MeN­(CH2CH2P­(iPr)2)2, R = 2,6-dimethylphenyl (2a), tert-butyl (2b), or adamantyl (2c)) have been prepared and crystallographically characterized. These complexes are precatalysts for both formic acid dehydrogenation and CO2 hydrogenation to formate, and give improved activity compared to first-generation systems with isonitrile ligands. The second-generation systems 2a–c, however, give inferior activity compared to the related carbonyl complexes (iPrPNP)­Fe­(H)­(CO) and (iPrPNMeP)­Fe­(H)­(HBH3)­(CO), which have been previously reported. This study demonstrates that a ligand which can participate in MLC is not universally advantageous for promoting the hydrogenation and dehydrogenation reactions studied in this work and provides guidance for the rational design of improved catalysts for reactions relevant to energy storage.

Additive‐Free Formic Acid Dehydrogenation Using a Pincer‐Supported Iron Catalyst

“…Notably, our proposed mechanism does not involve any elementary steps requiring metal‐ligand cooperation (MLC), in which both the metal and ligand are directly involved in bond activation. This is one of only a few examples of high activity in a dehydrogenation reaction catalyzed by a base metal that does not involve a MLC pathway …”

A Precious Catalyst: Rhodium‐Catalyzed Formic Acid Dehydrogenation in Water

“…[8a], Reaction solvents and additives play a crucial role in catalytic systems. Especially basic additives (triethylamine) are often employed to shift the thermodynamic equilibrium of systems . Due to their frequent application, we assess the effects with calorimetric and spectroscopic methods .…”

From Homogeneous to Heterogenized Molecular Catalysts for H2 Production by Formic Acid Dehydrogenation: Mechanistic Aspects, Role of Additives, and Co-Catalysts

“…Another challenge is the possibility to develop catalysts able to perform FA-dehydrogenation as well as CO2-hydrogenation. To this front, recently, Hazari and co-workers [48] reported three novel pincer Fe-isonitrile catalysts, which were active for FA dehydrogenation as well as CO2 hydrogenation, and achieved TOF = 2100 h -1 for FA dehydrogenation. [29], "1998" refers to the work of Puddephatt [30], "2009" refers to the work of Beller [43], "2019 refers to the work of Himeda [35].…”

“…Despite the methodological limitations of the theoretical analysis, experimental work [48] provides evidence that the energy barriers in Cycle-I and Cycle-II are influenced by the experimental parameters (type of complex, solvent, traces of H 2 O, FA deprotonation, presence of co-catalysts), which might decisively affect the contribution of each cycle to the overall H 2 production rates. In this context, we have experimentally demonstrated that silica nanoparticles can act as co-catalyst to the Fe/PPh 3 system [54] (see Figure 6) suppressing the energy barrier of the reaction from 77 kJ/mol to 36 kJ/mol [54].…”

“…The discrepancies between reference [52] and [53] indicate that the small energy differences of the two cycles and the limited capacity of the computational approaches to incorporate all factors i.e., solvent effects, ionization of FA, etc., should be considered in future refinement of theoretical quantum calculations. Also, currently, the mechanistic discussion is mainly based on theoretical Despite the methodological limitations of the theoretical analysis, experimental work [48] provides evidence that the energy barriers in Cycle-I and Cycle-II are influenced by the experimental parameters (type of complex, solvent, traces of H2O, FA deprotonation, presence of co-catalysts), which might decisively affect the contribution of each cycle to the overall H2 production rates. In this context, we have experimentally demonstrated that silica nanoparticles can act as co-catalyst to the Fe/PPh3 system [54] (see Figure 6) suppressing the energy barrier of the reaction from 77 kJ/mol to 36 kJ/mol [54].…”