Iron Cyclopentadienone Complexes: Discovery, Properties, and Catalytic Reactivity

Iron catalyzed diastereoselective hydrogenation of chiral imines

Cyclopentadienone-based iron complexes successfully catalyzed the stereoselective hydrogenation of chiral imines, leading to enantiopure pharmaceutically active compounds.

Cyclopentadienone iron complexes as efficient and selective catalysts for the electroreduction of CO2 to CO

Robust and easy-to-synthesize cyclopentadienone iron(0) complexes selectively catalyze the electrochemical conversion of CO₂ to CO. Cooperation between the metal center and the coordinated organic ligand is a key factor for activity of these novel electrocatalysts.

Iron Catalysis in Organic Synthesis: A Critical Assessment of What It Takes To Make This Base Metal a Multitasking Champion

The current status of homogeneous iron catalysis in organic chemistry is contemplated, as are the reasons why this particular research area only recently starts challenging the enduring dominance of the late and mostly noble metals over the field. Centered in the middle of the d-block and able to support formal oxidation states ranging from -II to +VI, iron catalysts hold the promise of being able to encompass organic synthesis at large. They are expected to serve reductive as well as oxidative regimes, can emulate "noble tasks", but are also able to adopt "early" transition metal character. Since a comprehensive coverage of this multidimensional agenda is beyond the scope of an Outlook anyway, emphasis is laid in this article on the analysis of the factors that perhaps allow one to control the multifarious chemical nature of this earth-abundant metal. The challenges are significant, not least at the analytical frontier; their mastery mandates a mindset that differs from the routines that most organic chemists interested in (noble metal) catalysis tend to cultivate. This aspect notwithstanding, it is safe to predict that homogeneous iron catalysis bears the chance to enable a responsible paradigm for chemical synthesis and a sustained catalyst economy, while potentially providing substantial economic advantages. This promise will spur the systematic and in-depth investigations that it takes to upgrade this research area to strategy-level status in organic chemistry and beyond.

Sustainable Pathways to Pyrroles through Iron-Catalyzed N-Heterocyclization from Unsaturated Diols and Primary Amines

Pyrroles are prominent scaffolds in pharmaceutically active compounds and play an important role in medicinal chemistry. Therefore, the development of new, atom-economic, and sustainable catalytic strategies to obtain these moieties is highly desired. Direct catalytic pathways that utilize readily available alcohol substrates have been recently established; however, these approaches rely on the use of noble metals such as ruthenium or iridium. Here, we report on the direct synthesis of pyrroles using a catalyst based on the earth-abundant and inexpensive iron. The method uses 2-butyne-1,4-diol or 2-butene-1,4-diol that can be directly coupled with anilines, benzyl amines, and aliphatic amines to obtain a variety of N-substituted pyrroles in moderate-to-excellent isolated yields.

An iron(ii) hydride complex of a ligand with two adjacent β-diketiminate binding sites and its reactivity

After lithiation of PYR-H2 (PYR = [(NC(Me)C(H)C(Me)NC6H3(iPr)2)2(C5H3N)](2-)) - the precursor of an expanded β-diketiminato ligand system with two binding pockets - with KN(TMS)2 the reaction of the resulting potassium salt with FeBr2 led to a dinuclear iron(ii) bromide complex [(PYR)Fe(μ-Br)2Fe] (1). Through treatment with KHBEt3 the bromide ligands could be replaced by hydrides to yield [PYR)Fe2(μ-H)2] (2), a distorted analogue of known β-diketiminato iron hydride complexes, as evidenced by NMR, Mößbauer and X-ray absorption spectroscopy, as well as by its reactivity: for instance, 2 reacts with the proton source lutidinium triflate via protonation of the hydride ligands to form an iron(ii) product [(PYR)Fe2(OTf)2] (4), while CO2 inserts into the Fe-H bonds generating the formate complex [(PYR)Fe2(μ-HCOO)2] (5); in the presence of traces of water partial hydrolysis occurs so that [(PYR)Fe2(μ-OH)(μ-HCOO)] (6) is isolated. Altogether, the iron(ii) chemistry supported by the PYR(2-) ligand is distinctly different from the one of nickel(ii), where both, the arrangement of the two binding pockets and the additional pyridyl donor led to diverging features as compared with the corresponding system based on the parent β-diketiminato ligand.

Iron-Catalyzed Allylic Amination Directly from Allylic Alcohols

Allylic amination, directly from alcohols, has been demonstrated without any Lewis acid activators using an efficient and regiospecific molecular iron catalyst. Various amines and alcohols were employed and the reaction proceeded through the oxidation/reduction (redox) pathway. A direct one-step synthesis of common drugs, such as cinnarizine and nafetifine, was exhibited from cinnamyl alcohol that produced water as side product.

A Step into an eco-Compatible Future: Iron- and Cobalt-catalyzed Borrowing Hydrogen Transformation

Living on borrowed hydrogen: Recent developments in iron- and cobalt-catalyzed borrowing hydrogen have shown that economically reliable catalysts can be used in this type of waste-free reactions. By using well-defined inexpensive catalysts, known reactions can now be run efficiently without the necessary use of noble metals; however, in addition new types of reactivity can also be discovered.

Iron(0) mediated C–H activation of 1-hexyne: a mechanistic study using time-resolved infrared spectroscopy

Photolysis of an iron tricarbonyl complex in the presence of 1-hexyne results in the formation of an intermediate Fe-(η²-1-hexyne) species which converts to the C–H activated Fe–CCR product.

Iron cyclopentadienone complexes derived from C2-symmetric bis-propargylic alcohols; preparation and applications to catalysis

The following complexes were prepared through cyclisation of a bis-propargylic alcohol and were tested as redox catalysts for hydrogen transfer reactions of alcohols and ketones.

Straightforward synthesis of iron cyclopentadienone N-heterocyclic carbene complexes

Novel iron complexes bearing both cyclopentadienone and N-heterocyclic carbene ancillary ligands were obtained by a straightforward synthesis from Fe2(CO)9. The preparation represents a rare example of silver transmetallation involving iron. The reaction is general and occurs in the presence of variously functionalized NHC and cyclopentadienones.

Hydrogenation and dehydrogenation iron pincer catalysts capable of metal-ligand cooperation by aromatization/dearomatization

The substitution of expensive and potentially toxic noble-metal catalysts by cheap, abundant, environmentally benign, and less toxic metals is highly desirable and in line with green chemistry guidelines. We have recently discovered a new type of metal-ligand cooperation, which is based on the reversible dearomatization/aromatization of different heteroaromatic ligand cores caused by deprotonation/protonation of the ligand. More specifically, we have studied complexes of various transition metals (Ru, Fe, Co, Rh, Ir, Ni, Pd, Pt, and Re) bearing pyridine- and bipyridine-based PNP and PNN pincer ligands, which have slightly acidic methylene protons. In addition, we have discovered long-range metal-ligand cooperation in acridine-based pincer ligands, where the cooperation takes place at the electrophilic C-9 position of the acridine moiety leading to dearomatization of its middle ring. This type of metal-ligand cooperation was used for the activation of chemical bonds, including H-H, C-H (sp(2) and sp(3)), O-H, N-H, and B-H bonds. This unusual reactivity likely takes place in various catalytic hydrogenation, dehydrogenation, and related reactions. In this Account, we summarize our studies on novel bifunctional iron PNP and PNN pincer complexes, which were designed on the basis of their ruthenium congeners. Iron PNP pincer complexes serve as efficient (pre)catalysts for hydrogenation and dehydrogenation reactions under remarkably mild conditions. Their catalytic applications include atom-efficient and industrially important hydrogenation reactions of ketones, aldehydes, and esters to the corresponding alcohols. Moreover, they catalyze the hydrogenation of carbon dioxide to sodium formate in the presence of sodium hydroxide, the selective decomposition of formic acid to carbon dioxide and hydrogen, and the E-selective semihydrogenation of alkynes to give E-alkenes. These catalysts feature, compared to other iron-based catalysts, very high catalytic activities which in some cases can even exceed those of state-of-the-art noble-metal catalysts. For the iron PNP systems, we describe the synthesis of the pyridine- and acridine-based PNP iron complexes and their performances and limitations in catalytic reactions, and we present studies on their reactivity with relevance to their catalytic mechanisms. In the case of the bipyridine-based PNN system, we summarize the synthesis of new complexes and describe studies on the noninnocence of the methylene position, which can be reversibly deprotonated, as well as on the noninnocence of the bipyridine unit. Overall, this Account underlines that the combination of cheap and abundant iron with ligands that are capable of metal-ligand cooperation can result in the development of novel, versatile, and efficient catalysts for atom-efficient catalytic reactions.

Iron-catalyzed hydrogenation of bicarbonates and carbon dioxide to formates

The catalytic hydrogenation of carbon dioxide and bicarbonate to formate has been explored extensively. The vast majority of the known active catalyst systems are based on precious metals. Herein, we describe an effective, phosphine-free, air- and moisture-tolerant catalyst system based on Knölker's iron complex for the hydrogenation of bicarbonate and carbon dioxide to formate. The catalyst system can hydrogenate bicarbonate at remarkably low hydrogen pressures (1-5 bar).

Organometallic intermediate-based organic synthesis: organo-di-lithio reagents and beyond

Metal-mediated organic reactions have become one of the great frontiers of organic synthesis.