Iron photocatalysis via Brønsted acid-unlocked ligand-to-metal charge transfer

Reforming sustainable 3d-metal-based visible light catalytic platforms for inert bulk chemical activation is highly desirable. Herein, we demonstrate the use of a Brønsted acid to unlock robust and practical iron ligand-to-metal charge transfer (LMCT) photocatalysis for the activation of multifarious inert haloalkylcarboxylates (CnXmCOO-, X = F or Cl) to produce CnXm radicals. This process enables the fluoro-polyhaloalkylation of non-activated alkenes by combining easily available Selectfluor as a fluorine source. Valuable alkyl fluorides including potential drug molecules can be easily obtained through this protocol. Mechanistic studies indicate that the real light-harvesting species may derive from the in situ-assembly of Fe3+, CnXmCOO-, H+, and acetonitrile solvent, in which the Brønsted acid indeed increases the efficiency of LMCT between the iron center and CnXmCOO- via hydrogen-bond interactions. We anticipate that this Brønsted acid-unlocked iron LMCT platform would be an intriguing sustainable option to execute the activation of inert compounds.

Ultrafast Events of Photoexcited Iron(III) Chloride for Activation of Benzylic C-H Bonds

The usage of rare-earth-metal catalysts in the synthesis of organic compounds is widespread in chemical industries but is limited owing to its environmental and economic costs. However, recent studies indicate that abundant-earth metals like iron(III) chloride can photocatalyze diverse organic transformations using blue-light LEDs. Still, the underlying mechanism behind such activity is debatable and controversial, especially in the absence of ultrafast spectroscopic results. To address this urgent challenge, we performed femtosecond time-resolved electronic absorption spectroscopy experiments of iron(III) chloride in selected organic solvents relevant to its photocatalytic applications. Our results show that the long-lived species [Fe(II) ← Cl•]* is primarily responsible for both oxidizing the organic substrate and reducing molecular oxygen through the diffusion process, leading to the final product and regenerating the photocatalyst rather than the most widely proposed free chloride radical (Cl•). Our study will guide the rational design of efficient earth-abundant photocatalysts.

Iron-Catalyzed Csp2-Csp3 Cross-Coupling via Double Decarboxylation: One Step Synthesis of Remote Polar Alkenes

Herein, we report an efficient photoinduced iron-catalyzed strategy for cross-couplings of alkyl carboxylic and acrylic acids, which provides a powerful tool for the synthesis of a variety of alkenes with polar functional groups. This novel synthetic methodology can also be applied to the preparation of ketones by using α-keto acids. Mechanistic experiments revealed preliminary mechanistic details. Diverse functionalization could be achieved, which may help streamline the synthesis of complex analogues for drug discovery.

Modern Organometallic C-H Functionalizations with Earth-Abundant Iron Catalysts: An Update

Iron-catalyzed C-H activation has recently emerged as an increasingly powerful synthetic method for the step- and atom- economical direct C-H functionalizations of otherwise inert C-H bonds. Iron's low-cost and toxicity along with its catalytic versatility have encouraged the scientific community to elect this metal for the development of new C-H activation methodologies. Within this review, we aim to present a collection of the most recent examples of iron-catalyzed C-H functionalizations with a particular emphasis on modern synthetic strategies and mechanistic aspects.

Trading Symmetry for Stereoinduction in Tetradentate, non-C2 -Symmetric Fe(II)-Complexes for Asymmetric Catalysis

A series of stereogenic-at-metal iron complexes comprising a non-C₂ -symmetric chiral topology is introduced and applied to asymmetric 3d-transition metal catalysis. The chiral iron(II) complexes are built from chiral tetradentate N4-ligands containing a proline-derived amino pyrrolidinyl backbone which controls the relative (cis-α coordination) and absolute metal-centered configuration (Λ vs. Δ). Two chloride ligands complement the octahedral coordination sphere. The modular composition of the tetradentate ligands facilitates the straightforward incorporation of different terminal coordinating heteroaromatic groups into the scaffold. The influence of various combinations was evaluated in an asymmetric ring contraction of isoxazoles to 2H-azirines revealing that a decrease of symmetry is beneficial for the stereoinduction to obtain chiral products in up to 99 % yield and with up to 92 % ee. Conveniently, iron catalysis is feasible under open flask conditions with the bench-stable dichloro complexes exhibiting high robustness towards oxidative or hydrolytic decomposition. The versatility of non-racemic 2H-azirines was subsequently showcased with the conversion into a variety of quaternary α-amino acid derivatives.

Photoactive Metal-to-Ligand Charge Transfer Excited States in 3d6 Complexes with Cr0, MnI, FeII, and CoIII

Many coordination complexes and organometallic compounds with the 4d⁶ and 5d⁶ valence electron configurations have outstanding photophysical and photochemical properties, which stem from metal-to-ligand charge transfer (MLCT) excited states. This substance class makes extensive use of the most precious and least abundant metal elements, and consequently there has been a long-standing interest in first-row transition metal compounds with photoactive MLCT states. Semiprecious copper(I) with its completely filled 3d subshell is a relatively straightforward and well explored case, but in 3d⁶ complexes the partially filled d-orbitals lead to energetically low-lying metal-centered (MC) states that can cause undesirably fast MLCT excited state deactivation. Herein, we discuss recent advances made with isoelectronic Cr⁰, Mn^I, Fe^II, and Co^III compounds, for which long-lived MLCT states have become accessible over the past five years. Furthermore, we discuss possible future developments in the search for new first-row transition metal complexes with partially filled 3d subshells and photoactive MLCT states for next-generation applications in photophysics and photochemistry.

Visible Light-Induced Unactivated δ-C(sp3 )-H Amination of Alcohols Catalyzed by Iron

An iron-catalyzed remote C(sp³ )-H amination of alcohols through 1,5-hydrogen atom transfer is developed. This protocol provides a method to generate δ-C(sp³ )-N bonds from primary, secondary, and tertiary alcohols under mild conditions. A wide substrate scope and a good functional group tolerance are presented. Mechanistic studies show that a LMCT course of an Fe-OR species and a chlorine radical-induced hydrogen abstraction of an alcohol are possible to generate the alkoxy radical intermediate.

Borylation in the Second Coordination Sphere of FeII Cyanido Complexes and Its Impact on Their Electronic Structures and Excited-State Dynamics

Second coordination sphere interactions of cyanido complexes with hydrogen-bonding solvents and Lewis acids are known to influence their electronic structures, whereby the non-labile attachment of B(C₆F₅)₃ resulted in several particularly interesting new compounds lately. Here, we investigate the effects of borylation on the properties of two Fe^II cyanido complexes in a systematic manner by comparing five different compounds and using a range of experimental techniques. Electrochemical measurements indicate that borylation entails a stabilization of the Fe^II-based t_2g-like orbitals by up to 1.65 eV, and this finding was confirmed by Mössbauer spectroscopy. This change in the electronic structure has a profound impact on the UV–vis absorption properties of the borylated complexes compared to the non-borylated ones, shifting their metal-to-ligand charge transfer (MLCT) absorption bands over a wide range. Ultrafast UV–vis transient absorption spectroscopy provides insight into how borylation affects the excited-state dynamics. The lowest metal-centered (MC) excited states become shorter-lived in the borylated complexes compared to their cyanido analogues by a factor of ∼10, possibly due to changes in outer-sphere reorganization energies associated with their decay to the electronic ground state as a result of B(C₆F₅)₃ attachment at the cyanido N lone pair.

Borylation in the second coordination sphere of two well-known Fe^II cyanido complexes leads to isocyanoborato complexes. The effects of borylation on their electronic structure and photophysical properties are thoroughly investigated with a range of experimental techniques.

Mild Amide Synthesis Using Nitrobenzene under Neutral Conditions

Amide synthesis is one of the most important transformations in organic chemistry due to the broad application in pharmaceutical drugs and organic materials. In this report, we describe a mild protocol for amide formation using the readily available nitroarenes as nitrogen sources and an inexpensive iron complex as a catalyst. Because of the use of the pH-neutral conditions and the avoidance of the strong oxidant or reductant, a wide range of aromatic and aliphatic aldehydes as well as nitroarenes with various functional groups could be tolerated well. A plausible mechanism is proposed based on the detailed studies, in which iron catalyst initiates the radical process and the solvent plays a key role as O-atom acceptor.

Cobalt(III) Carbene Complex with an Electronic Excited-State Structure Similar to Cyclometalated Iridium(III) Compounds

Many organometallic iridium(III) complexes have photoactive excited states with mixed metal-to-ligand and intraligand charge transfer (MLCT/ILCT) character, which form the basis for numerous applications in photophysics and photochemistry. Cobalt(III) complexes with analogous MLCT excited-state properties seem to be unknown yet, despite the fact that iridium(III) and cobalt(III) can adopt identical low-spin d⁶ valence electron configurations due to their close chemical relationship. Using a rigid tridentate chelate ligand (L^CNC), in which a central amido π-donor is flanked by two σ-donating N-heterocyclic carbene subunits, we obtained a robust homoleptic complex [Co(L^CNC)₂](PF₆), featuring a photoactive excited state with substantial MLCT character. Compared to the vast majority of isoelectronic iron(II) complexes, the MLCT state of [Co(L^CNC)₂](PF₆) is long-lived because it does not deactivate as efficiently into lower-lying metal-centered excited states; furthermore, it engages directly in photoinduced electron transfer reactions. The comparison with [Fe(L^CNC)₂](PF₆), as well as structural, electrochemical, and UV–vis transient absorption studies, provides insight into new ligand design principles for first-row transition-metal complexes with photophysical and photochemical properties reminiscent of those known from the platinum group metals.

Visible Light-Induced Hydrosilylation of Electron-Deficient Alkenes by Iron Catalysis

Herein, we reported a method for iron-catalyzed, visible-light-induced hydrosilylation reactions of electron-deficient alkenes to produce value-added silicon compounds. Alkenes bearing functional groups with different steric properties were suitable substrates, as were derivatives of structurally complex natural products. Mechanistic studies showed that chlorine radicals generated by iron-catalyzed ligand-to-metal charge transfer in the presence of lithium chloride promoted the formation of silyl radicals.

Visible Light-Induced Transition Metal Catalysis

In recent years, visible light-induced transition metal catalysis has emerged as a new paradigm in organic photocatalysis, which has led to the discovery of unprecedented transformations as well as the improvement of known reactions. In this subfield of photocatalysis, a transition metal complex serves a double duty by harvesting photon energy and then enabling bond forming/breaking events mostly via a single catalytic cycle, thus contrasting the established dual photocatalysis in which an exogenous photosensitizer is employed. In addition, this approach often synergistically combines catalyst-substrate interaction with photoinduced process, a feature that is uncommon in conventional photoredox chemistry. This Review describes the early development and recent advances of this emerging field.

Photoinduced Ligand-to-Metal Charge Transfer (LMCT) of Fe Alkoxide Enabled C-C Bond Cleavage and Amination of Unstrained Cyclic Alcohols

We report an alkoxy radical process for the C-C bond cleavage and functionalization of unstrained tertiary and secondary cyclic alcohols. In the absence of a chlorine atom, the readily available iron catalysts [Fe(OBu-t)₃ or Fe(acac)₃/t-BuONa] facilitate alkoxy radical formation via the direct ligand-to-metal charge transfer of Fe alkoxide and further enable the ring opening and amination of cyclic alcohols. The remote amino carbonyl compounds could be obtained with a broad scope in up to excellent yields under the mildly redox-neutral system. Light-driven electron transfer, alkoxy radical formation, and subsequent C-C bond cleavage via β-scission were the keys to the transformation.

Photochemical alknylation of hydrosilanes by iron catalysis

Alkynylsilanes is one kind of essential synthetic block in organic chemistry. The established synthetic routes remain some drawbacks regarding to harsh reaction conditions or expensive/rare metal catalyst. Herein, we report...

Luminescent First-Row Transition Metal Complexes

Precious and rare elements have traditionally dominated inorganic photophysics and photochemistry, but now we are witnessing a paradigm shift toward cheaper and more abundant metals. Even though emissive complexes based on selected first-row transition metals have long been known, recent conceptual breakthroughs revealed that a much broader range of elements in different oxidation states are useable for this purpose. Coordination compounds of V, Cr, Mn, Fe, Co, Ni, and Cu now show electronically excited states with unexpected reactivity and photoluminescence behavior. Aside from providing a compact survey of the recent conceptual key advances in this dynamic field, our Perspective identifies the main design strategies that enabled the discovery of fundamentally new types of 3d-metal-based luminophores and photosensitizers operating in solution at room temperature.

Iron-Catalyzed Photoinduced Remote C(sp3)-H Amination of Free Alcohols

We report a general photocatalytic protocol for the remote C(sp³)-H bond amination of free aliphatic alcohols. The electron transfer between the abundant and inexpensive catalyst FeCl₃ and simple alkanols under blue LED irradiation enables the alkoxy radical formation under mild redox-neutral conditions, with no need for additional oxidant and prefunctionalization. The subsequent selective 1,5-hydrogen atom transfer (HAT) and amination provide a simple and efficient way to access molecular complexity from readily available and bulk alcohols.

Accessing Photoredox Transformations with an Iron(III) Photosensitizer and Green Light

Efficient excited-state electron transfer between an iron(III) photosensitizer and organic electron donors was realized with green light irradiation. This advance was enabled by the use of the previously reported iron photosensitizer, [Fe(phtmeimb)₂]⁺ (phtmeimb = {phenyl[tris(3-methyl-imidazolin-2-ylidene)]borate}, that exhibited long-lived and luminescent ligand-to-metal charge-transfer (LMCT) excited states. A benchmark dehalogenation reaction was investigated with yields that exceed 90% and an enhanced stability relative to the prototypical photosensitizer [Ru(bpy)₃]²⁺. The initial catalytic step is electron transfer from an amine to the photoexcited iron sensitizer, which is shown to occur with a large cage-escape yield. For LMCT excited states, this reductive electron transfer is vectorial and may be a general advantage of Fe(III) photosensitizers. In-depth time-resolved spectroscopic methods, including transient absorption characterization from the ultraviolet to the infrared regions, provided a quantitative description of the catalytic mechanism with associated rate constants and yields.

Synthesis and Characterization of Bidentate Isonitrile Iron Complexes

The divalent iron complexes trans-[FeBr2(BINC)2], [Cp*FeCl(BINC)] (Cp* = Me5C5), and [FeBr2(CNAr3NC)2] with the chelating bis(isonitrile) ligands BINC (bis(2-isocyanophenyl)phenylphosphonate) and CNAr3NC (2,2″-diisocyano-3,5,3″,5"tetramethyl-1,1':3',1″-terphenyl) have been prepared and characterized. Their subsequent reduction yields the di- and trinuclear compounds [Fe3(BINC)6], [Cp*Fe(BINC)]2, [Fe(CNAr3NC)2]2, and [K(Et2O)]2[Fe(CNAr3NC)2]2. The molecular structures of all new species were determined by X-ray crystallography and compared to those of related iron carbonyl complexes, demonstrating that the bidentate isonitrile ligands are capable surrogates for two CO ligands with only minimal distortion of the tetrahedral or octahedral geometry of the parent complexes. The complexes were further characterized by NMR and IR spectroscopy, and the electrochemical properties of selected compounds were analyzed by UV-vis-NIR spectroelectrochemistry.

Efficient Amination of Activated and Non-Activated C(sp3 )-H Bonds with a Simple Iron-Phenanthroline Catalyst

A readily available catalyst consisting of iron dichloride in combination with 1,10-phenanthroline catalyzes the ring-closing C-H amination of N-benzoyloxyurea to form imidazolidin-2-ones in high yields. The C-H amination reaction is very general and applicable to benzylic, allylic, propargylic, and completely non-activated aliphatic C(sp3 )-H bonds, and it also works for C(sp2 )-H bonds. The surprisingly simple method can be performed under open flask conditions.

Visible light mediated functionalization of allenes

This review summarizes the visible light mediated strategies for the functionalization of allenes.