Recent Catalytic Applications of (Cyclopentadienone)iron Complexes

Iron- and Organo-Catalyzed Borrowing Hydrogen for the Stereoselective Construction of Tetrahydropyrans

Stereocontrolled oxa-Michael additions are challenging, given the high reversibility of the process, which ultimately leads to racemization of the newly formed stereocenters. When iron-catalyzed borrowing hydrogen from allylic alcohols was combined with a stereocontrolled organocatalytic oxa-Michael addition, a wide array of chiral tetrahydropyrans were efficiently prepared. The reaction could be performed in a diastereoselective manner from pre-existing stereocenters or enantioselectively from achiral substrates. The key to success was the reactivity of the iron complex, which was selective for allylic alcohol dehydrogenation and irreversibly led the reaction to the final product.

Trapping an unprecedented octacoordinated iron(II) complex with neutral bis-tetrazolylpyridyl ligands and solvent molecules

Iron(II) can show a very rich coordination chemistry with concomitant modulation of its properties as promising functional materials. Metalation of the neutral tridentate nitrogen-donor mer-coordinating ligand 2,6-bis(2-(methyl)-2H-tetrazol-5-yl)pyridine (Me2btp) with Fe(ClO4)2·6H2O through accurate solvent polarity control enables the selective crystallization of [FeHS/LS(Me2btp)2](ClO4)2·MeCN·2.75H2O (2HS/LS·MeCN·2.75H2O) as red rods, where half of the iron(II) centres resides in the low spin (LS, S = 0) state and the other half is in the high spin (HS, S = 2) state. The red rods spontaneously convert into yellow crystals once removed from the mother liquor and exposed to air due to solvent rearrangement within the crystal packing; these new crystals can be assigned to [FeHS(Me2btp)2](ClO4)2·solvent (2HS·solvent) where all the iron(II) centres are now blocked in the HS state, as confirmed by magnetic measurements. The polarity of the crystallization solvent, together with the maintenance of the crystals within the mother liquor, are pivotal for the reactivity and interconversion of different species. Indeed, upon long standing in solution, 2HS/LS·MeCN·2.75H2O converts to another form of red crystals belonging to [FeLS(Me2btp)2][FeHS(Me2btp)(MeCN)2(H2O)](ClO4)4·MeCN (2LS·3HS·MeCN), as confirmed by single crystal X-ray diffraction data. In this co-crystal, the iron(II) in 2 resides in the LS state at all temperatures while the iron(II) in 3 is blocked in the HS state. Well-formed yellow crystals could be also isolated among the red crystals of 2HS/LS·MeCN·2.75H2O, and they could be identified as the unprecedented octacoordinated species [Fe(Me2btp)2(MeCN)(H2O)](ClO4)2·H2O (1·H2O) by single-crystal X-ray diffraction. These yellow crystals are stable in the air, but slowly convert into 2LS·3HS·MeCN if kept in the mother liquor for about one week. 1·H2O can be considered the trapped intermediate in the solid state during the conversion of [FeHS(Me2btp)2]2+ into [FeHS(Me2btp)(MeCN)2(H2O)]2+ in solution, where the two tridentate ligands in the starting species can unfold to accommodate coordinated MeCN and H2O molecules, as confirmed by theoretical calculations, and eventually one of the two Me2btp is completely replaced by the solvent.

Substituent Effects and Mechanistic Insights on the Catalytic Activities of (Tetraarylcyclopentadienone)iron Carbonyl Compounds in Transfer Hydrogenations and Dehydrogenations

(Cyclopentadienone)iron carbonyl compounds are catalytically active in carbonyl/imine reductions, alcohol oxidations, and borrowing hydrogen reactions, but the effect of cyclopentadienone electronics on their activity is not well established. A series of (tetraarylcyclopentadienone)iron tricarbonyl compounds with varied electron densities on the cyclopentadienone were prepared, and their activities in transfer hydrogenations and dehydrogenations were explored. Additionally, mechanistic studies, including kinetic isotope effect experiments and modifications to substrate electronics, were undertaken to gain insights into catalyst resting states and turnover-limiting steps of these reactions. As the cyclopentadienone electron density increased, both the transfer hydrogenation and dehydrogenation rates increased. A catalytically relevant, trimethylamine-ligated iron compound was isolated and characterized and was observed in solution under both transfer hydrogenation and dehydrogenation conditions. Importantly, it was catalytically active in both reactions. Kinetic isotope effect data and initial rates in transfer hydrogenation reactions with 4'-substituted acetophenones provided evidence that hydrogen transfer from the catalyst to the carbonyl substrate occurred during the turnover-limiting step, and NMR spectroscopy supports the trimethylamine adduct as an off-cycle resting state and the (hydroxycyclopentadienyl)iron hydride as an on-cycle resting state. In transfer dehydrogenations of alcohols, the use of electronically modified benzylic alcohols provided evidence that the turnover-limiting step involves the transfer of hydrogen from the alcohol substrate to the catalyst. The trimethylamine-ligated compound was proposed as the primary catalyst resting state in dehydrogenations.

Cyclo-penta-dienone triisocyanide iron complexes: general synthesis and crystal structures of tris-(2,6-di-methyl-phenyl isocyanide)(η4-tetra-phenyl-cyclo-penta-dienone)iron and tris-(naphthalen-2-yl iso-cyanide)(η4-tetra-phenyl-cyclo-penta-dienone)iron acetone hemisolvate

Irradiation of a toluene solution containing cyclo-penta-dienone tricarbonyl iron complexes and isocyanides with blue LEDs afforded the formation and isolation of 12 triisocyanide complexes, two of which, namely tris-(2,6-di-methyl-phenyl isocyanide)(η4-tetra-phenyl-cyclo-penatedienone)iron, [Fe(C9H9N)3(C29H20O)], and tris-(naphthalen-2-yl isocyanide)(η4-tetra-phenyl-cyclo-penatedienone)iron acetone hemisolvate, [Fe(C11H7N)3(C29H20O)]2·C3H6O, could be characterized crystallographically. The air-stable compounds were purified by column chroma-tography and were characterized by 1H NMR, 13C NMR, elemental analysis and HRMS. NMR and XRD data indicate generally more electron-rich Fe0 centers compared to the corresponding tricarbonyl compounds.

Zwitterionic Halido Cyclopentadienone Iron Complexes and Their Catalytic Performance in Hydrogenation Reactions

The zwitterionic halido cyclopentadienone iron complexes FeX(CO)₂-NMe₃ (X = Cl, Br, I) were prepared and characterized by NMR, XRD, MS, IR, and elemental analysis. Their catalytic performance in hydrogenation and transfer hydrogenation was assessed. Transfer hydrogenation in boiling ⁱPrOH with acetophenone as the test substrate showed no conversion with FeI(CO)₂-NMe₃. Hydrogenation reactions under H₂ pressure (7.5 bar) in water as solvent showed up to 93% conversion with FeI(CO)₂-NMe₃ (2.5 mol %) using acetophenone as the test substrate. The overall relative reactivity order was established to be Cl < Br < I, reflecting the relative bond strengths of the Fe-X bonds. Although the compounds presented in this study can be used as precatalysts for hydrogenation reactions in water, the need to employ high temperatures, leading to more catalyst decomposition, as evidenced by pressurized sample infusion-electrospray ionization-mass spectrometry (PSI-ESI-MS), and high catalyst loading limits their usefulness as catalysts. The limit can be circumvented in part by salt effects analogous to those in classical solvolysis chemistry.

Chemoselective α-Alkylation and α-Olefination of Arylacetonitriles with Alcohols via Iron-Catalyzed Borrowing Hydrogen and Dehydrogenative Coupling

α-Alkyl and α-olefin nitriles are very important for organic synthesis and medicinal chemistry. However, different types of catalysts are employed to achieve either α-alkylation of nitriles by borrowing hydrogen or α-olefination by dehydrogenative coupling methods. Designing and developing high-performance earth-abundant catalysts that can procure different products from the same starting materials remain a great challenge. Herein, we report an iron(0) catalyst system that achieves chemoselectivity between borrowing hydrogen and dehydrogenative coupling protocols by simply changing the base. A broad range of nitriles and alcohols, including benzylic, linear aliphatic, cycloaliphatic, heterocyclic, and allylic alcohols, were selectively and efficiently converted to the corresponding products. Mechanistic studies reveal that the reaction mechanism proceeds through a dehydrogenative pathway. This iron catalytic protocol is environmentally benign and atom-efficient with the liberation of H₂ and H₂O as green byproducts.

Redox-Switchable Complexes Based on Nanographene-NHCs

A series of rhodium and iridium complexes with a N‐heterocyclic carbene (NHC) ligand decorated with a perylene‐diimide‐pyrene moiety are described. Electrochemical studies reveal that the complexes can undergo two successive one‐electron reduction events, associated to the reduction of the PDI moiety attached to the NHC ligand. The reduction of the ligand produces a significant increase on its electron‐donating character, as observed from the infrared spectroelectrochemical studies. The rhodium complex was tested in the [3+2] cycloaddition of diphenylcyclopropenone and methylphenylacetylene, where it displayed a redox‐switchable behavior. The neutral complex showed moderate activity, which was suppressed when the catalyst was reduced.

Iodine-promoted insertion of the oxygen atom from water in η4-vinylketene[Fe(CO)3] complexes

Iodine promotes the in situ formation of iron(II) species from η⁴-vinylketene[Fe(CO)₃] (3a-h) as a key intermediate for the synthesis of 2(5H)-furanones (4a-h) by a sequential water-insertion/carbon-oxygen coupling under mild reaction conditions. Compounds 4a-h were obtained in good to excellent yields. A possible reaction pathway was also proposed by DFT calculations. This methodology can be extended to the synthesis of (5H)-pyrrol-2-ones using anilines, with moderate yields and a few limitations.

Mössbauer Spectroscopic and Computational Investigation of An Iron Cyclopentadienone Complex

(Cyclopentadienone)iron carbonyl complexes have recently received particular attention for their use as catalysts in hydrogenation or transfer hydrogenation reactions including the N-alkylation of amines with alcohols. This is due to their easy synthesis from simple and cheap materials, air and water stabilities, and the crucial metal-ligand cooperation giving rise to unique catalytic properties. Here, we report a Mössbauer spectroscopic and computational investigation of such a complex and its corresponding activated species for dehydrogenation and hydrogenation reactions. This study affords a deeper understanding of the species formed by the reaction with Me₃NO and their distribution upon the added amount of an oxidant.

(Cyclopentadienone)iron Complexes: Synthesis, Mechanism and Applications in Organic Synthesis

(Cyclopentadienone)iron tricarbonyl complexes are catalytically active, inexpensive, easily accessible and air-stable that are extensively studied as an active pre-catalyst in homogeneous catalysis. Its versatile catalytic activity arises exclusively due to the presence of a non-innocent ligand, which can trigger its unique redox properties effectively. These complexes have been employed widely in (transfer)hydrogenation (e. g., reduction of polar multiple bonds, Oppenauer-type oxidation of alcohols), C-C and C-N bond formation (e. g., reductive aminations, α-alkylation of ketones) and other synthetic transformations. In this review, we discuss the remarkable advancement of its various synthetic applications along with synthesis and mechanistic studies, until February 2021.

Cyclopentadienone–NHC iron(0) complexes as low valent electrocatalysts for water oxidation

Design and application of earth abundant iron based molecular electrocatalysts for water oxidation, an essential challenge for sustainable energy applications.