Diferrate [Fe2 (CO)6 (µ-CO){µ-P(aryl)2 }]- as Self-Assembling Iron/Phosphor-Based Catalyst for the Hydrogen Evolution Reaction in Photocatalytic Proton Reduction-Spectroscopic Insights.

This work is focused on the identification and investigation of the catalytically relevant key iron species in a photocatalytic proton reduction system described by Beller and co-workers. The system is driven by visible light and consists of the low-cost [Fe3 (CO)12 ] as catalyst precursor, electron-poor phosphines P(R)3 as co-catalysts, and a standard iridium-based photosensitizer dissolved in a mixture of THF, water, and the sacrificial reagent triethylamine. The catalytic reaction system was investigated by operando continuous-flow FTIR spectroscopy coupled with H2 gas volumetry, as well as by X-ray absorption spectroscopy, NMR spectroscopy, DFT calculations, and cyclic voltammetry. Several iron carbonyl species were identified, all of which emerge throughout the catalytic process. Depending on the applied P(R)3 , the iron carbonyl species were finally converted into [Fe2 (CO)6 (µ-CO){µ-P(R)2 }]- . This involves a P-C cleavage reaction. The requirements of P(R)3 and the necessary reaction conditions are specified. [Fe2 (CO)6 (µ-CO){µ-P(R)2 }]- represents a self-assembling, sulfur-free [FeFe]-hydrogenase active-site mimic and shows good catalytic activity if the substituent R is electron poor. Deactivation mechanisms have also been investigated, for example, the decomposition of the photosensitizer or processes observed in the case of excessive amounts of P(R)3 . [Fe2 (CO)6 (µ-CO){µ-P(R)2 }]- has potential for future applications.

Base-free non-noble-metal-catalyzed hydrogen generation from formic acid: scope and mechanistic insights.

The iron-catalyzed dehydrogenation of formic acid has been studied both experimentally and mechanistically. The most active catalysts were generated in situ from cationic Fe(II) /Fe(III) precursors and tris[2-(diphenylphosphino)ethyl]phosphine (1, PP3 ). In contrast to most known noble-metal catalysts used for this transformation, no additional base was necessary. The activity of the iron catalyst depended highly on the solvent used, the presence of halide ions, the water content, and the ligand-to-metal ratio. The optimal catalytic performance was achieved by using [FeH(PP3 )]BF4 /PP3 in propylene carbonate in the presence of traces of water. With the exception of fluoride, the presence of halide ions in solution inhibited the catalytic activity. IR, Raman, UV/Vis, and EXAFS/XANES analyses gave detailed insights into the mechanism of hydrogen generation from formic acid at low temperature, supported by DFT calculations. In situ transmission FTIR measurements revealed the formation of an active iron formate species by the band observed at 1543 cm(-1) , which could be correlated with the evolution of gas. This active species was deactivated in the presence of chloride ions due to the formation of a chloro species (UV/Vis, Raman, IR, and XAS). In addition, XAS measurements demonstrated the importance of the solvent for the coordination of the PP3 ligand.

Water oxidation with molecularly defined iridium complexes: insights into homogeneous versus heterogeneous catalysis.

Molecularly defined Ir complexes and different samples of supported IrO(2) nanoparticles have been tested and compared in the catalytic water oxidation with cerium ammonium nitrate (CAN) as the oxidant. By comparing the activity of nano-scaled supported IrO(2) particles to the one of organometallic complexes it is shown that the overall activity of the homogeneous Ir precursors is defined by both the formation of the homogeneous active species and its conversion to Ir(IV)-oxo nanoparticles. In the first phase of the reaction the activity is dominated by the homogeneous active species. With increasing reaction time, the influence of nano-sized Ir-oxo particles becomes more evident. Notably, the different conversion rates of the homogeneous precursor into the active species as well as the conversion into Ir-oxo nanoparticles and the different particle sizes have a significant influence on the overall activity. In addition to the homogeneous systems, IrO(2)@MCM-41 has also been synthesized, which contains stabilized nanoparticles of between 1 and 3 nm in size. This latter system shows a similar activity to IrCl(3)⋅xH(2)O and complexes 4 and 5. Mechanistic insights were obtained by in situ X-ray absorption spectroscopy and scanning transmission electron microscopy.

HCOOH disproportionation to MeOH promoted by molybdenum PNP complexes.

Molybdenum(0) complexes with aliphatic aminophosphine pincer ligands have been prepared which are competent for the disproportionation of formic acid, thus representing the first example so far reported of non-noble metal species to catalytically promote such transformation. In general, formic acid disproportionation allows for an alternative access to methyl formate and methanol from renewable resources. MeOH selectivity up to 30% with a TON of 57 could be achieved while operating at atmospheric pressure. Selectivity (37%) and catalyst performance (TON = 69) could be further enhanced when the reaction was performed under hydrogen pressure (60 bars). A plausible mechanism based on experimental evidence is proposed.

Correction: HCOOH disproportionation to MeOH promoted by molybdenum PNP complexes.

[This corrects the article DOI: 10.1039/D1SC04181A.].

Highly Efficient Base-Free Dehydrogenation of Formic Acid at Low Temperature.

The ruthenium complex [RuH2 (PPh3 )4 ] is a competent catalyst for the selective dehydrogenation of formic acid (FA) at low temperature. It tolerates water and shows excellent performance (TOF up to 36 000 h-1 at 60 °C). Remarkably, no basic additives are necessary to obtain such high activity and the defined complex is stable for up to 120 days, making this system one of the most effective formic acid dehydrogenation catalysts known to date.