Iron Complexes for Hydrogen Activation and Catalytic Hydrogenation

Simona Mazza
EPFL, 2015
EPFL thesis

Abstract

Inspired by Nature several groups have developed structural and functional iron complexes mimicking the active site of the iron-hydrogenases, which show high reactivity in the H2 cleavage. Usually pendant bases have been incorporated onto families of Fe complexes in order to achieve active systems. In the view of these recent developments, in chapter two, we investigated the possibility of synthesizing novel iron (II) complexes bearing an amine internal base and providing an open site for substrate binding as catalysts for H2 activation. Pentacoordinated Fe(II) low spin complexes [(PhPNP)Fe(CO)(bdt)] (1), [(PhPNP)Fe-(CO)(Nbt)] (4), [(CyPNP)Fe(CO)(Nbt)] (5), [(dppe)Fe(CO)(Nbt)] (6) and the paramagnetic complex [(CyPNP)FeCl2] (10) have been synthesized and fully characterised. Unfortunately, when these complexes were tested as catalysts for hydrogenation reaction of a wide range of unsaturated substrates, no appreciable reactivity was observed. Same behaviour was observed in the case of complexes [(CyPNP)Fe(CO)(Cp)] (11) and [(CyPNP)Fe(CH3CN)(Cp)] (12) where the Cp ligand was installed in order to modulate the electronic and steric properties on the iron center. In chapter three, a new class of well-defined iron pincer complexes is reported. Several Fe(II) complexes supported by a 2,6-bis(phosphinito)pyridine ligand (PONOP) have been synthesized and fully characterised. In particular, the Fe-hydride complexes [(iPrPONOP)-Fe(CO)(H)Br] (14) and (iPrPONOP)Fe(CO)(H)(CH3CN) (15) could activate H2 at room temperature. Moreover, complexes 14 and 15 served as catalysts for the selective hydrogenation of aldehydes at room temperature. In presence of sodium formate as hydrogen donor, 14 and 15 showed an excellent reactivity in hydrogen transfer reaction of aldehydes. The mechanism of hydrogen activation and hydrogenation is discussed based on the observed reactivity of iron complexes. The feature of being chemoselective towards aldehydes and a broad functional-group tolerance make these iron-hydride systems remarkable in the class of the earth-abundant-metal hydrogenation catalysts. Chapter four is dedicated to the tuning of the Fe-PONOP systems reported in chapter three in order to synthesize similar iron(II) complexes exhibiting enhanced reactivity for H2 activation and hydrogenation of unsaturated substrates. As first attempt, complexes [(CyPONOP)Fe(CO)Br2] (22) and the analogous chloride [(CyPONOP)Fe(CO)Cl2] (23) bearing the stronger donor CyPONOP ligand were synthesized and tested as catalysts for hydrogenation reaction, but none of the substrates employed was reduced. As second attempt, the CO ligand was substituted with the better donor ligand tert-butyl isocyanide in presence of iPrPONOP as pincer ligand. Several Fe-PONOP complexes were synthesized and fully characterized. In particular the Fe-hydride complex [(iPrPONOP)Fe(tBuNC)(H)Br] (25) exhibited reactivity towards hydrogenation of aldehydes under the same reaction conditions reported for 14. No reaction was observed in presence of acetophenone, cyclohexene and 1-decene demonstrating that, although spectroscopically different, 25 did not exhibit enhanced reactivity relative to 14.

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Simona Mazza
EPFL, 2015
EPFL thesis

Abstract

Official source

https://infoscience.epfl.ch/record/211117?ln=en

About this result

Related concepts (20)

Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is comm

Iron is a chemical element with the symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common elem

A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen (H2), as shown below: Hydrogen uptake () is coupled to the reduction of electron acceptors such as oxygen,

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Related publications (89)

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Chemoselective Hydrogenation and Transfer Hydrogenation of Aldehydes Catalyzed by Iron(II) PONOP Pincer Complexes

Xile Hu, Simona Mazza, Rosario Scopelliti

Iron-catalyzed hydrogenation has drawn much attention, yet the scope and chemoselectivity of iron catalysis warrant further improvement. Here we report new iron pincer complexes as chemoselective hydrogenation and transfer hydrogenation catalysts. Several Fe(II) complexes supported by a 2,6-bis(phosphinito)pyridine ligand (PONOP) have been synthesized. Fe(II) hydride complexes [((PrPONOP)-Pr-i)Fe(CO)(H)Br] (2) and ((PrPONOP)-Pr-i)Fe(CO)(H)(CH3CN) (3) can activate H-2 at room temperature. Complexes 2 and 3 are hydrogenation catalysts at room temperature, and the hydrogenation is selective for aldehyde in the presence of ketone and alkene groups. 2 and 3 are also chemoselective transfer hydrogenation catalysts using sodium formate as the hydride source.

American Chemical Society2015

This thesis is concerned with supramolecular architectures assembled at metal surfaces. The investigations pursued two objectives. On the one hand, the focus was placed on the fabrication of low-dimensional surface-supported network structures by applying the concepts of conventional three-dimensional supramolecular chemistry at surfaces. Three main driving forces were explored for the construction of low-dimensional assemblies: hydrogen bonding, metal coordination and ionic interactions. The realized structures were characterized by scanning tunneling microscopy (STM) under ultra-high vacuum conditions. In particular, the interplay between the adsorbate-substrate coupling and lateral adsorbate interactions was addressed in the experiments. Further, the electronic and magnetic properties of the fabricated coordination structures were investigated. This characterization was done by x-ray absorption experiments at the synchrotron facility in Grenoble. Moreover, the response of the metal units and open cavity structures to the adsorption of large organic and small gas molecules was studied by temperature controlled STM experiments. The first part of the thesis deals with hydrogen bonded supramolecular assemblies. Simple aromatic carboxylic acids were employed to study the assembly principles and in particular effects arising from the substrate and molecular structure. The investigation of TPA (1,4-benzenedicarboxylic acid) adlayers on Cu(100) at different substrate temperatures revealed how the protonation status of the carboxyl moieties affects the topology of the assemblies. Further, the influence of the molecular backbone functionality and symmetry is addressed in comparative investigations on TPA and PDA (2,5-pyrazinedicarboxylic acid) as well as BDA (4,4'-biphenyldicarboxylic acid) and SDA (4',4" trans-ethene-l,2-diyl-bisbenzoic acid) adlayers on Cu(100). In the second part of the thesis, metal-ligand interactions were employed for the construction of supramolecular architectures at a Cu(100) surface. The concepts of conventional coordination chemistry were successfully applied at the surface. Iron atoms in conjunction with rod-like aromatic molecular linkers comprising carboxyl, pyridyl and hydroxyl functional groups assemble into a rich variety of open network structures with distinctly arranged mono- and diiron coordination centers and well-defined cavities. The results show that the adsorbate-substrate coupling considerably affects the local coordination environment of the iron units when the molecular backbone length is varied. The occurrence of structural isomerism in open network structures is addressed in a comparative study of heterofunctional and homofunctional linker molecules. It is shown that the former ligand allows only one topologically pure structure while the other linker forms two phases with different topology. The hydroxyl ligand has been used to construct distorted hexagonal coordination networks containing threefold coordinated single iron atoms. The comparison between the networks on Cu(100) and Ag(111) surfaces shows marked differences, which are attributed to adsorbate-substrate interactions. Finally, a novel design strategy for the fabrication of two-dimensional coordination structures at surfaces is introduced by using alkali metal ions in conjunction with carboxylate ligands. The interaction between the components is dominated by electrostatic forces and is of intermediate bond strength as in the case of hydrogen bonding and transition metal coordination. It is expected, that substrate symmetry and registry plays a larger role in the network formation, because the ionic interaction is not directional. The chemical as well as magnetic properties of the fabricated coordination networks are examined in the last part of the thesis. The adsorption of C60 molecules on open network structures reveals how cavity size and chemical functionality affect the bond strength to the guest molecules as well as the number of accommodated C60 molecules. Since transition metal clusters, and in particular diiron units, are the catalytic active centers in various proteins, the reactivity of various mono- and diiron coordination structures to molecular oxygen was investigated in temperature controlled STM experiments. In particular, the focus was placed on the structural relaxation upon gas adsorption and chemical reaction. At last, the magnetic properties of the metal coordination centers are addressed in XMCD experiments. The iron units are found to be paramagnetic at temperatures down to 5 K and show distinct magnetic anisotropy and magnetization loops. These experiments demonstrate that surface-supported metal coordination structures are a promising class of materials for applications in the field of heterogenous catalysis and surface magnetism.

EPFL2005

Palladium Catalysed Intramolecular Carbo-oxygenation of Propargylic Amines to Access 1,2-Amino Alcohols and alpha-Amino Ketones via in situ Tether Formation

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a-Amino ketones and 1,2-amino alcohols are important structural motifs in organic chemistry, that can be observed in natural products, pharmaceutically and bioactive compounds. For these reasons, they constitute privileged targets for the development of new and effective synthetic methods. The dual-functionalisation of unsaturated carbon-carbon bonds is a powerful approach to access these crucial motifs. In this context, Pd catalysis has been comprehensively investigated and has consistently demonstrated its capability. Methods enabling the concomitant formation of a carbonâ heteroatom and a carbonâcarbon bond are highly valuable, but the challenges of intermolecular reactions have led to the development of tether systems that require additional steps for their introduction and removal. In situ tethering strategies have previously been explored within the Waser group to circumvent these limitations. This approach relies on a one-pot, three component, Pd catalysed carboetherification or amination reaction, leading to rapidly increase in complexity of simple substrates. So far, the in situ methodologies developed by the Waser group have focused on the functionalisation of alkenes from allylic amines and alcohols. In this regard, the goal of my PhD was to expand the in situ tethering methodologies established within the Waser group, to the synthesis of a-amino ketones and 1,2-amino alcohols. Specifically, my focus was on the formation a trifluoromethyl hemiaminal with a propargylic amine and using the resulting aminal to direct the oxyalkynylation and oxyarylation of the alkyne substrate via a Pd catalysed transformation. Upon optimisation, the reaction of a protected terminal propargylamine, trifluoroacetaldehyde ethyl hemiacetal and silylbromoacetylene using a Pd0 complex with DPEPhos as the catalytic system and Cs2CO3 as the base gave the desired oxazolidine alkenes in high yields and E:Z selectivity. Substrates with substitution on the terminal position and at the propargylic required a change of ligand and base. The transformation was broadly tolerant towards substitution on the nitrogen and the alkyne. An oxyarylative variant was also developed, using Ruphos as ligand, and worked well with a range of iodoarene partners. The obtained oxazolidines could be conveniently orthogonally deprotected to access the a-amino ketone and terminal alkyne; the hydrogenation of the unsaturated oxazolidines has allowed access to 1,2-amino alcohol scaffolds and the application of gold catalysis converted the enyne-oxazolidine system into trisubstituted silyl furans. The identification of a sideproduct of the oxyalkynylation process led to the development of a carboxy-alkynylation reaction using a "CO2" unit derived from the carbonate base. Optimisation established CsHCO3 to the best base/"CO2" source, using DPEPhos as ligand. This transformation gave convenient access to diverse library of cyclic carbamates. Work is ongoing to achieve an enantioselective oxyarylation using chiral phosphine ligands. The asymmetric synthesis of oxazolidines, followed by the diastereoselective alkene hydrogenation and subsequent tether removal would allow the stereodivergent access to interesting diarylalkyl-1,2- amino alcohols. Most recently, the product was synthesised in 66% yield, with 66%ee using commercially available Trost ligand.

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Palladium Catalysed Intramolecular Carbo-oxygenation of Propargylic Amines to Access 1,2-Amino Alcohols and alpha-Amino Ketones via in situ Tether Formation

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EPFL2005